Choosing a Fluid

Maybe you’ve been given the assignment of designing a new process and you realize that the high temperatures required for production will necessitate the use of a heat transfer fluid. Or maybe management has decided to convert that old batch distillation column into a production unit and commercialize the new R&D project, which will require high temperature process heating. In either case, the task of selecting the proper heat transfer fluid from the 90-odd fluids available worldwide and incorporating the fluid’s physical and engineering properties into your initial design is as important as it can be daunting.

With so many heat transfer fluids available, how can you initially narrow the fluids down to the best choices for the application? Here is a selection parameter that can easily eliminate many fluid choices and help to quickly make your “short list” of candidates.

Fluid Operating Range

A heat transfer fluid’s operating range is the temperature range between the pumpability point and the recommended maximum bulk fluid operating temperature. The pumpability point is roughly defined as the temperature where a fluid’s viscosity reaches 2000 centipoise. At this point the fluid becomes too viscous for centrifugal pumps to maintain sufficient fluid flow. Although heat transfer fluids technically can be used at temperatures close to their pumpability points, many fluids (especially petroleum-based fluids) lose much of their heat transfer ability and efficiency when used close to their pumpability points.

A fluid’s ability to withstand thermal cracking (thermal degradation) is the primary factor in setting its maximum bulk fluid operating temperature. This temperature is the maximum temperature the fluid manufacturer recommends the fluid can be used and still maintain an acceptable rate of degradation over time.

Typically, a good fit between a heat transfer fluid and an application happens when the required fluid temperature of the process falls right in the middle of the operating range of the heat transfer fluid. This “cushion” on either side of the operating temperature allows for good overall heat transfer efficiency and minimal fluid degradation.

One quick rule – There is no reason to consider fluids that have maximum bulk fluid operating temperatures below the bulk heat transfer fluid temperature required by your process. Cross those fluids off your list right away. And even if a fluid is used at a continuous temperature close to or right at its recommended maximum fluid operating limit, there is a point to take into consideration – The thermal degradation rate is not a linear function versus temperature. As the bulk fluid temperature reaches then exceeds the fluid’s maximum recommended temperature, the degradation rate soars asymptotically. Even when used within 15 – 20°F of the recommended maximum temperature, the degradation rate of most heat transfer fluids is significantly higher than when the application requires a temperature within 30- 50°F of the fluid’s maximum temperature. The costs associated with increased fluid make-up rates, the downtime required for heat transfer fluid-related maintenance, and lost heat transfer efficiency due to degradation by-products have to be strongly considered when choosing between a lower cost fluid that will be bumping up against its maximum recommended use temperature and a more expensive fluid that will fit nicely right in the middle of it’s operating range.

Cost Vs. Comfort Level

The answer to old car racing adage, “How fast do you want to go?” also holds true with heat transfer fluids- “How much do you want to spend?” Except with heat transfer fluids the question is “How high do you want to go?” As a general rule of thumb, the higher the maximum bulk fluid operating temperature, the more expensive the fluid. This is due to the fact that the chemistries required to achieve acceptable thermal stability and heat transfer efficiency at elevated temperatures gets more complex and expensive as the temperature increases. The two primary types of fluids used by the majority of high temperature applications are:

Aromatics: Also know as “synthetics”. These consist of benzene-based chemistries and, depending on he specific type, have a bulk fluid operating range generally from -70°F to 750°F.

Petroleum-based: Also known as “hot oils”. These consist of parafinnic and/or napthenic hydrocarbons. The bulk fluid operating range for these fluids are generally from -10 oF to 600 oF.

It seems that the majority of heat transfer fluid applications fall within the 500°F – 600°F temperature requirement range, which opens the doors to both types of fluids. However, if your process will require a heat transfer fluid to perform at 630°F, your options are fairly limited in that only the more expensive aromatic-based fluids can be used, so you’ll have to dig a little deeper. On the other hand, if your process requirement calls for only 525°F, using an high cost aromatic for an added thermal stability benefit would be overkill – your best choice here would be a petroleum-based fluid and you’ll be a hero for coming in under budget. The tough decision is when your application is in that 590°F to 610°F range, where higher cost aromatics are in their “cushion” range and you’re up against the maximum recommended top operating temperature of the hot oils. Some points to consider when the application falls in this 590 oF to 610 oF range:
Points For Petroleum-based fluids for 590°F to 610°F applications:

1) High performance, high grade petroleum-based fluids have been proven to be accurately rated to 600°F, demonstrate acceptable thermal stability up to and at 600°F, and have performed well for many years in properly designed systems operating at 600°F.

2) Petroleum-based fluids are 1/3 to ½ the cost of aromatics.

Points Against Petroleum-based fluids for 590°F to 610°F applications:

1) Do not use petroleum-based fluids if bulk fluid temperature exceeds 600°F, or if you think you might have occasional temperature excursions above 600°F.

2) The fluid make-up rate will be on average twice as high as most aromatics at 600°F

Points For Aromatic fluid for 590°F to 610°F applications:

1) Well within the aromatics’ “cushion” range- good heat transfer efficiency and minimal thermal degradation.

2) No degradation concerns should temperature excursions occur.

Some Points Against Aromatics for 590°F to 610°F applications:

1) Cost- two to three times more expensive than petroleum-based fluids.

2) Usually not as personnel-friendly as petroleum-based fluids.

Although there is no “best” answer to which type of fluid to use in the 590°F to 610°F range, you can feel comfortable using a petroleum-based fluid to 600°F, as long as there will not be any possible temperature excursions above that temperature. On the other hand, the more expensive aromatic will be on cruise control at these temperatures, since they are well within their “cushion range”.
Selecting The Fluid

Heat transfer fluid suppliers occasionally see systems using heat transfer fluids intended for applications for significantly higher temperatures. Although these systems will run smoothly (heat transfer fluid-wise) for many years, the same performance could have be achieved from a much more cost-effective fluid. Heat transfer fluid suppliers have seen the other side of the coin too- a low cost/low temperature heat transfer fluid (sometimes they are not even heat transfer fluids) put into high temperature applications. There are cases where these fluids have only lasted days before significant system trouble occurred. In both cases, it is obvious that the person specifying the fluid did not spend enough time determining the criteria important in making the proper fluid selection. And although there is no surefire method in selecting the proper fluid for an application, narrowing the field from the many choices is easy with a little thought. Once the field has been narrowed, the final selection process can begin where each individual fluid can be compared and contrasted and the final selection made. Whether the final choice is a hot oil or a synthetic, making the proper choice should lead to many years of problem-free heat transfer.

Michael Damiani
International Sales Manager
Radco Industries, Inc.

There are several physical properties that distinguish dibenzyltoluene-based heat transfer fluids (DBT) from partially hydrogenated terphenyls (PHT), however their characteristics, taken as a whole, make them both useful as heat transfer fluids in high temperature liquid phase thermal fluid heating systems .  These differences illustrate that DBT is a more consistent molecular formulation than PHT.  Furthermore, DBT is also an economic alternative to costly terphenyl-based fluids.

There is not a particular trait that defines a “good” heat transfer fluid.  The most correct heat transfer fluid for a particular application is defined by several important physical characteristics.  The sum total of these characteristics is measured in terms of efficiency (heat transfer ability) and thermal stability (resistance to thermal degradation resulting in longer life).  Radco Industries, Inc. manufactures a proprietary DBT-based formulation, called Xceltherm®HT, which has been developed to be an ideal alternative in heat transfer applications that are designed to use PHT.

DBT is a mixture of predominately dibenzyltoluene molecules with benzyltoluene isomers.  It is a stable, well-defined and pure molecular formulation.  DBT’s pumpability limit (2000cP) is approximately -35°C (-37°F), and can cold start at this temperature without heat-tracing. It has an optimal operating range approximately between 182°C and 350°C (360°F and 662°F). 
The beginning of a thermal fluid’s optimal operating range can be determined by its Reynolds number. The Reynolds number is the ratio of inertial forces on the fluid to the fluid’s viscosity when in motion.  In other words, it is a calculation of the turbulence of thermal fluid flow.  It is a dimensionless number that increases with increased turbulence.  Sufficient turbulence is required for a liquid to take full advantage of its ability to absorb and release heat.  The optimal operating range begins when the Reynolds number reaches 100,000.  However, the fluid will have a much broader functional range that is defined by its pumpability point at the lowest temperature.   The highest operating temperature is generally defined as the highest bulk temperature the fluid can withstand with a reasonable rate of degradation.  When evaluating heat transfer fluids, it should be understood that this temperature limit is subjective and defined by each manufacturer.

Dibenzyltoluene

PHT is a less defined mixture of terphenyls and quaterphenyls. (Terphenyls are three hybridized benzene rings, and quaterphenyls are four hybridized benzene rings.)   The optimal operating range of the PHT heat transfer fluid is similar to DBT, between 180°C and 345°C (356°F and 650°F).  The pumpability limit (2000cP) of PHT is -3°C (27°F).

 

Partially Hydrogenated Terphenyls

p-terphenyl

p-quaterphenyl

PHT’s are generally manufactured from the preparation of biphenyl and benzene.  However, there are a number of methods used to synthesize PHT.  The ratio of terphenyls and quaterphenyls inconsistently varies depending on the manufacturing process.

DBT differs from PHT in that it is synthesized from extremely pure molecular components.  The purity and quality control of DBT manufacture can yield very predictable physical characteristics between batches.  The inconsistent ratio of terphenyls and quaterphenyls in PHT manufacture may exhibit inconsistencies and therefore may produce undesirable results.

For the next part of the discussion, individual physical characteristics are compared and analyzed for their effect on key indicators of heat transfer fluid functionality, heat transfer efficiency and thermal stability.

Specific Gravity

There is a linear relationship between the specific gravity of a heat transfer fluid and temperature.  As the temperature increases, the specific gravity decreases in a predictably linear fashion.  In general, Xceltherm®HT has a marginally lower specific gravity than PHT (approximately 1.5% difference).

Viscosity

Viscosity is a significant factor in hear transfer calculations.  There is an inverse, exponential relationship between viscosity and temperature.  As the temperature increases, the viscosity rapidly decreases.  Viscosity effects heat transfer: the lower the viscosity values, the greater the potential for turbulence as measured by the Reynolds number and the greater the heat transfer.

Xceltherm® HT heat transfer fluid has a lower viscosity than PHT.   The viscosity values of DBT and PHT show some convergence at temperatures between 275°C (527°F) and 300°C (572°F), and at temperatures above 300°C (572°F) there is a negligible difference between kinematic viscosity values (Figure 1.1).

Thermal  Conductivity

The thermal conductivity of a fluid is also an important factor in determining heat transfer efficiency of a fluid.  Thermal conductivity describes the ability of a matierial to transfer or conduct heat.  Thermal conductivity measures the rate of heat flow across a defined area. The rate of heat flow is the energy (Joules or Btu) that travels across a sectional area (1 meter or 1 foot) with respect to time.   (The units are Watts/meters-Kelvin or Btu/hour-foot-°Farhenheit.) The larger the thermal conductivity value, the greater the heat transfer.Xceltherm®HT has 7% advantage in thermal conductivity over PHT as calculated from published data of a common PHT used as a heat transfer fluid.

Heat Transfer Coefficent

The heat transfer coefficient (or simply, heat transfer) is an important variable in system design.  The heat transfer coefficient measures the thermal energy that is transferred between a fluid and solid by convection or phase change.  The heat transfer calculation is dependent on the fluid’s viscosity, thermal conductivity, and flow-rate of the fluid through a specific pipe diameter.  The heat transfer coefficient is expressed in W/m2K.  For instance, in figure 1.2 the heat transfer values are modeled from a 52.5 mm diameter pipe with 2.44 m/s flow rate. In general, the greater the heat transfer value, the greater the capacity for thermal energy to pass from the pipe to the heat transfer fluid.  However, the efficiency of the thermal energy transfer is limited by the design and function of the heat transfer system.

The calculated heat transfer of Xceltherm®HT is greater than PHT.  However, the ranges of heat transfer values between both fluids are congruent between temperatures 200°C and 290°C (392°F and 554°F).  Trending does illustrate a convergence of heat transfer values between DBT and PHT, albeit DBT’s heat transfer remains slightly higher.

The proper design of heat transfer fluids takes into account several key properties and components that are limited by physical laws.  For instance, the heat transfer coefficient is inversely related to viscosity, and directly related to thermal conductivity.  Although it is desirable to design a fluid with the lowest viscosity and greatest thermal conductivity for maximum heat transfer, there are natural and physical limitations that prevent this formulation, including increased vapor pressure and/or decreased thermal stability.

Vapor Pressure

Xceltherm®HT exhibits a higher vapor pressure than a typical PHT as a result of combining ideal characteristics to maximize the important combination of heat transfer efficiency and thermal stability.  The above chart is derived from published information about virgin, unused material.  Later in the discussion there is information about how PHT vapor pressure increases with use.

Thermal Degradation & Heat Transfer Fluid Analysis

Thermal oils, whether petroleum based or synthetic heat transfer fluids like the aromatics DBT and PHT, experience thermal degradation or oxidation when used at high temperatures.  The use of a closed system padded by nitrogen can limit oxidation.  Routine fluid analysis is the primary preventative method for monitoring the heat transfer fluid’s condition.

The Ampoule Test is generally accepted as an industry standard for the testing of relative thermal stability between heat transfer fluids. Although there is no standard ASTM-type Ampoule Test defined, the data generated by such testing has proven reliable over the course of many years.Thermal stability can be defined as the time rate of change of the chemical composition of a fluid from its initial components to its degradation products at a given temperature. To compile this information over a range of temperatures, fluid samples are capped with nitrogen to remove oxygen and sealed. They are heated to specific temperatures over a specific period of time (two weeks in our current data) and compared. The amount of degradation is then plotted on a graph of degradation by percent of weight vs. temperature to indicate the relative ability of each fluid to withstand use at specific temperatures.

Thermal degradation (or thermal cracking) of DBT and PHT is difficult to avoid and exponentially accelerates above 316°C (600°F).  Thermal cracking is when the heat transfer fluid distills into lighter components. These light distillates have lower thermal stability than the virgin heat transfer fluid (light distillates are also called “light ends” or “low boilers”).  Accelerated thermal degradation will result if there is localized overheating at the burner tubes, especially if recommended film temperatures are exceeded.  Overheating of the thermal system’s burner tubes can be caused by flame impingement in the burner, low flow rates through mechanical failure or poor heat transfer system design, laminating or coking of the tubes due to fluid degradation, or other causes.  Cokingis the formation of hard carbon particles that may clog filters and pipes. Coking may also foul burner heating tube bundles and clog heat exchangers.  It can also reduce laminar flow from sludge formation.

The Acetone Insoluble test is used to measure the carbon content of a fluid in parts-per-million (ppm).  If the ppm value exceeds specifications (more than 300ppm), corrective action is advised.

The ampule test measures thermal degradation of heat transfer fluids in a controlled environment in the absence of oxygen to give a relative comparison between fluids in a closed thermal oil system.  The ampule test does not measure the fluids efficiency or life span in a specific system, but it is useful as a relative comparison between heat transfer fluids.

Oxidation of synthetic and petroleum-based thermal fluids also produces weak acid formation.  Weak acid formation may occur from contamination from external sources, in open vent expansion tank operation due to hot fluid contact with air and/or from thermal degradation of the fluid.  The molecular integrity of the fluid deteriorates when the acidity of the thermal fluid increases.  Furthermore, weak acids produce insoluble materials that may cause mechanical failures in seals, valves, and/or pumps.

Acetone Insolubles test measures the amount of inorganic (pipe slag, sand and other construction debris) and “hard” carbon (coke) carried by the fluid. High amounts of carbon indicate thermal degradation of the fluid and a probable coking/sludge problem on the heat exchanger/heater surfaces or fluid oxidation. Coke and sludge can adversely affect a system’s heat transfer efficiency by heat transfer surface fouling. level of 50mg solids /100ml fluids can sometimes indicate problems. 

The Total Acid Number Test (Neutralization Number) is an acid-base titration that measures weak acids present in a heat transfer fluid.  If the acid number is out of specification, it is strongly recommended that the heat transfer fluid be replaced. The average acid number for new material is between 0.00 and 0.10.  If the acid number exceeds 0.50, corrective action may be necessary.

PHT and DBT degrade into different components, and require different system maintenance procedures.  PHT tends to produce more light distillates than DBT.  Light distillates increase the vapor pressure of PHT.  Therefore, PHT heat transfer systems are periodically vented as the light distillates rise out of the expansion tank, and release through a pressure relief valve.    These releases need to be monitored because upon disposal, PHT may be a hazardous waste as defined by the Resource Conservation and Recovery Act (RCRA), 40 CFR 261.24, due to its toxicity characteristic and should be tested for benzene.

An accumulation of light distillates can create air pockets that may lead to pressure drops within the system or pump cavitation. A decrease in liquid volume follows due to the loss of evaporated material, which will have to made-up with a charge of heat transfer fluid.

Total Acid Number (Neutralization Number) test is an acid/base titration detects minute amounts of strong and weak acids in the fluid. Acids usually are formed in heat transfer fluid from contamination from material outside the system. Acids usually are formed in heat transfer fluid from contamination from material outside the system. Acid number increase usually is associated with open vent expansion tank operation, heat transfer fluid oxidation or heat transfer fluid degradation, producing weak acids. Process material containing oxidizing agents can contribute strong or weak acids.Acids are harmful in two ways. First, acids tend to accelerate the molecular breakdown of the heat transfer fluid. Secondly, they tend to form insoluble solids that accelerate mechanical deterioration of seals, valves, pumps, etc. Most heat transfer fluids have an initial total acid number at 0.00 to 0.10. The maximum value in used fluid should not exceed 0.50.

Light distillates also decrease the flash point.  The flash point is the minimum temperature the vapor released from a liquid will ignite in the presence of an external ignition source and oxygen.  Virgin PHT has a comparatively higher flash point than virgin DBT, 184 °C (363 °F) and 160°C (320°) respectively.  Since PHT primarily produces light distillates, thermal cracking occur can offset the higher flash point advantage of virgin material.

Light distillates that don’t evaporate reform to create heavier compounds (“high boilers”) that remain within the fluid.  This is common with DBT and can still occur with PHT, though to a lesser degree.  This physical property is demonstrated when there is an increase in specific gravity.

These compounds continue to be soluble if the concentration of DBT is greater than 60%.  DBT needs to be periodically drained and topped off with fresh fluid when this occurs.  If the DBT falls below 60% concentration, the heavier compounds begin to polymerize very rapidly due to high viscosity and low turbulent flow, preventing efficient heat transfer. Polymerized fluids decrease laminar flow, creating a laminate coating(or coking) on the interior of the pipe.  The laminate coating drastically decreases heat transfer, and reduces the efficiency of the heater tubes to dissipate thermal energy.  The resulting increase in film temperature then leads to further fluid degradation.

If either heat transfer fluid is neglected, it may lead to mechanical wear and other problems.  Routine fluid analysis is necessary to diagnose thermal degradation.    Radco Industries, Inc. provides its customers with a regular thermal fluid analysis at no additional cost.

It is important to recognize that both fluids require periodic replacement of fluid lost by venting or a required draining of the system.  Off-spec fluid can be recovered by reprocessing it and a virgin heat transfer fluid “top off” of the system replenishes the fluid to acceptable use as an optimal thermal fluid.

Compatibility of Xceltherm® HT with Partially Hydrated Terphenyls

Radco Industries, Inc. has completed two sets of tests to confirm the physical and chemical compatibility of Modified Terphenyl and Radco’s Xceltherm®HT heat transfer fluids: the Chemical Compatibility Test and Acetone Insoluble Test.  These tests confirm thatXceltherm®HT and Modified Terphenyl are completely compatible and can be co-mingled in high temperature liquid-phase heat transfer systems.

The two fluids were tested for physical compatibility, defined as the ability of two fluids to form a mixture at various dilutions with no physical fluid separation.  Chemical compatibility was also tested.  When mixed, no reaction (exothermic or endothermic) will occur to yield a precipitate.

Chemical Compatibility Test: Federal Test Method 791 B, Part 3403.2

Test Summary: Virgin Xceltherm®HT and virgin Modified Terphenyl sample mixtures of 100%, 90%/10%, 50%/50%, 10%/90%, and 100% were heated to 315oC for 72 hours, and then centrifuged.  Sediment amounts (if any) were then measured.  A second series of identical tests were performed with mixtures of virgin Xceltherm HT and “used” Modified Terphenyl. Sediment amounts (if any) were measured.

Conclusion: No precipitate was created through chemical reaction in the various dilutions of Xceltherm®HT and Modified Terphenyl.  No separation was noted after the heating period.

Acetone Insolubles Test: ASTM D-893

Summary: To confirm if precipitates were formed when mixed, 100ml samples of virgin Xceltherm®HT and “used” Modified Terphenyl samples were heated to 315oC for 72 hours, then vacuumed through a 2.5 micron filter membrane.  The membrane was then flushed with acetone, then oven dried for 24 hours.  Sediment amounts were then measured.

Conclusion:  No separation of the combined fluids was noted after heating.  No increase of insoluble material (large molecular weighted sediments) was noted in the dilution samples as compared to the 100% Modified Terphenyl.  Predictable dilution factors and sediment amounts were observed, indicating that no new precipitates were formed through chemical reaction.

Summary

The physical characteristics and formulations of DBT-based Xceltherm® HT and PHT are somewhat different, but the combinations of characteristics that define a heat transfer fluid allow them to be used in similar applications.  Xceltherm® HT and PHT are also fully miscible and can be mixed in any combination.  Depending on the heat transfer system design,Xceltherm® HTcan provide improved heat transfer efficiency compared to PHT.  The improved heat transfer efficiency ofXceltherm® HTis largely derived from the fact that it has greater thermal conductivity values than PHT.  Furthermore, Xceltherm® HTand PHT have comparable thermal stability at temperatures below 290°C (554°F).  Both are suitable for high temperature liquid phase operation in applications exceeding 315°C (600°F), the typical threshold for petroleum based thermal oils.

Defining “food grade” and a new set of regulations from NSF International make it easier for process engineers in food manufacturing to choose a heat transfer fluid.

heat transfer fluids for food processing applications

Until recently, the term “food grade” has been used loosely to describe heat transfer fluids that are suitable for food processing applications. It has not been a term that specifically describes heat transfer fluids in their stated use but was an inference made from other regulations. There was an enthusiastic salesperson who would take a drink of fluid to prove to his customers the validity of this inference. While possibly valid, it’s not recommended.

The Food and Drug Administration (FDA), Washington, under a wide range of regulations, defines the food additive status of a number of products, but not specifically heat transfer fluid. One can refer to Title 21 of the Code of Federal Regulations (CFR) online (see sidebar “See for Yourself”). These chemicals are listed under part 170 subsections. It covers additives designed for indirect food additives, secondary food additives, indirect food additives of a specific chemical family, and so on. Because the same chemicals can be part of a heat transfer fluid product line, this became one of the standards called “food grade.”

Likewise, until recently, the United States Department of Agriculture (USDA), Washington, had its own regulations, which included mineral oils used as machinery lubricants and release agents to prevent meat from sticking to grills. The reference, H1 status, was used to indicate that the oil was acceptable for use in meat and poultry establishments and could have incidental food or feed contact. Again, these same mineral oils can be part or all of what is used as a “hot oil” for heat transfer.

Founded in 1944 as the National Sanitation Foundation, NSF International, Ann Arbor, Mich., is known for the development of standards, product testing and certification services in the areas of public health, safety and protection of the environment. NSF became involved when the USDA decided that it would no longer maintain its list. At first, NSF acted as record keeper to ensure the list was available for review. Now, the organization has taken an active role to refine the list. NSF created the Non-Foods Compound Group to define the use of heat transfer fluid, fluid additives and lubricants, as well as other materials, separately for their specific applications.

Kenji Yano, program manager of the nonfood compounds registration program explains, “The term ‘food-grade’ is not used. …Instead ‘incidental contact’ (HT1) is used as opposed to ‘nonfood contact’ (HT2) compounds.” NSF keeps this list online, on CD and in its “White Book,” which is published annually.

Qualifying Fluids and Specific Uses

type of heat transfer fluids
When to use which type of heat transfer fluid depends on the requirements of the application.

Whether you choose FDA or NSF as your current guide, there are many fluids from which to choose. As an example, within the following chemistries are select fluids that are safe for “incidental food contact” — brine, potassium formate (KF), polyalphaolephin (PAO) and highly refined, severely hydrogenated paraffinic white oils. Both PAO and white oil chemistries are heat transfer fluids sometimes referred to as “hot oils.” When to use which one depends on the requirements of the application; each has its own advantages and temperature ratings (table 1).

When HT1 must be specified is easy to understand. If there is any chance of incidental food contact, it should be a requirement, even if not regulated, as a matter of good manufacturing practice. It is good economics, too. For example, a packaging line heats adhesive with heat transfer fluid. The adhesive is used to seal frozen food packaging. If a leak occurs in the heat exchanger, it could go undetected for a long time, and any premium paid for “incidental food contact (HT1)” fluid wouldn’t seem like much compared to a hundred thousand units of contaminated packages. There also is the matter of storage and overall production standards. The heat transfer fluid used should complement the environment where it is required.

When the temperature requirement of the application exceeds 600oF (316oC) or surface temperatures on the heat exchange surface exceeds 650oF (343oC), the type of heat transfer fluid rated for incidental food contact should not be specified. At this point, the white oils, the highest temperature rated product of the group, do not have the thermal stability required for a long life. (There is some debate on this exact temperature, but 600oF bulk operating temperature is a good conservative number to work with.) In this case, aromatic/synthetic fluids or other high temperature fluids are the only choice.

Beyond Food Use

Not every application needs to be in food processing to use HT1 fluids. A good example is the die-cast industry. The molds are heated, cooled or held at steady temperatures using heat transfer fluid. When the molds are opened or changed, some heat transfer fluid typically leaks from the inner chamber. The leaking fluid is captured for disposal, but this involves employee exposure to the fluid. HT1 fluids made from white oils are easy to handle because they are nontoxic and nonirritating to the skin. The lack of odor also results in fewer employee complaints.

Due to the efforts by NSF International, process engineers, specifying engineers, maintenance engineers and technicians now can take advantage of a simple and straightforward guide for the type of fluid they want. There is no need to rely on a sales pitch or to sort through the voluminous FDA regulations and guess if a fluid really qualifies. 

Sidebar: Heat Transfer Fluids and Kosher Foods

Another specifying consideration in food processing is the need for kosher heat transfer fluid in applications that manufacture kosher products or products to be used in kosher foods. Even though heat transfer fluid is not put into the food directly, it must be kosher under the kosher laws that refer to heating a product through a common wall (in this case, a heat exchanger).

Rabbi Sholem Fishbane of the Chicago Rabbinical Council (CRC), a kosher overseer agency, explains that this is an important requirement. Kosher laws state that this type of heating causes a mixture of flavors. Under kosher law the “spiritual flavor” will go through the metal and affect the liquid on the other side. If the kosher product is heated by a fluid that is not kosher, explains Rabbi Fishbane, this spiritual flavor will contaminate the kosher product.

The concept of the mixing of flavors can lead to some interesting engineering issues. If a plant is broiling kosher chickens with a heat transfer fluid system and using the same fluid to process animal fats that contain pork in another part of the plant, under kosher law, the process has been contaminated. Likewise, in plants that produce both kosher and non-kosher or even dairy and pareve (non-dairy), engineers must carefully review details like returned condensate.

The CRC makes frequent random inspections and is diligent in educating themselves to understand plant engineering, ensuring that kosher laws are not broken. This is of great service to the food processor by giving assistance to their process planning and provides a third-party certification of product quality. Rabbi Fishbane emphasizes that this requires a technical education supported through “constant attendance of seminars and lectures” and “not every agency’s approval is acknowledged by the CRC.” A specifying engineer needs to determine which agencies have the ability to make the types of inspections required for the specific application. Each heat transfer fluid manufacturer has the information on which agency they use.

A note on kosher fluids is that the fluid also should be rated as kosher/pareve, which means it can be used for foods and additives intended for consumption on Passover and used all year.

When the temperature requirement of the application exceeds 600oF (316oC) or surface temperatures on the heat exchange surface exceeds 650oF (343oC), a heat transfer fluid rated for incidental food contact should not be specified. What about kosher? Kosher law allows for a non-kosher fluid heating medium if that medium is made so foul that any contamination would make the food or additive that is heated inedible. By their nature, aromatic chemistries used for higher temperatures have a strong, pungent odor, especially when heated, and qualify readily for this exemption.

 

By: Michael R. Damiani Radco Industries, Inc.

High temperature heat transfer fluids are used in process applications where their optimum bulk fluid operating temperatures of 300°F to 750°F are safer and more efficient than steam, electrical, or direct fire heating methods. Selecting the proper heat transfer fluid for a new system while in the design phase or for possible process fluid improvements during an upcoming retrofit will assure sufficient and uniform BTU delivery (or removal). The properly selected heat transfer fluid will also minimize potential production loss and downtime due to required design changes, mechanical problems, or fluid failure. The process of selecting the optimum heat transfer fluid should begin once the energy transfer required by the process and the planned/actual service ratings of the mechanical components of the heat transfer system have been calculated and thoroughly researched. Since there are a number companies specializing in heat transfer fluids and a wide range of fluid products available, the knowledge of this key element of the system’s operating requirements can help to create a set of criteria that can be used to compare various fluids and allow rapid elimination of fluids that are not best suited for the application. However, before comparing and contrasting various individual fluids, much time and effort in the selection process can be saved by comparing and contrasting the various chemistries of the fluids. Once a fluid chemistry is selected that best meets the performance properties and other criteria required by the application, the resultant list of potential fluids becomes significantly more manageable for more detailed apples-to-apples comparisons.

Fluid Chemistry. High temperature heat transfer fluids can be categorized by chemical structure into three primary groups:

Synthetics
Hot Oils
“Others” including silicones
The synthetics, also referred to as “aromatics”, consist of benzene-based structures and include the diphenyl oxide/biphenyl fluids, the diphenylethanes, dibenzyltoluenes, and terphenyls. Depending on the specific product, the overall bulk fluid temperature operating range of the synthetics is from -70°F to 750°F.

The hot oils are petroleum-based and most consist of paraffinic and/or napthenic hydrocarbons. The overall bulk fluid temperature operating range of the petroleum-based fluids is from -10°F to 600°F, with the high-grade hydrogenated white oils strongly recommended for applications requiring bulk fluid temperatures in the 575°F to 600°F range.

Silicone-based fluids, and to a greater extent hybrid glycol fluids, are used primarily in specialized applications requiring process/product compatibility should a heat exchanger leak occur. This group’s performance and cost factor disadvantages in the comparative temperature ranges of the synthetics and hot oils make silicone-based and other specialty fluids unlikely choices for most process applications.

Fluids and System Types. Hot oils and synthetics are used in a multitude of heat processing applications. The type of system design used in the process is a major consideration on the choice of a specific fluid chemistry. Processes utilizing heat transfer fluids can be categorized into three system types:

Non-pressurized liquid phase systems.
Pressurized liquid phase systems.
Pressurized pumped or natural circulation vapor-phase systems.
Non-pressurized liquid phase systems are generally the simplest to design and operate. Both hot oils and synthetics can be used equally well in this type of system as long as the operating temperature of the heat transfer fluid is below its boiling range. Major components in these systems consist of the heater, heat exchanger, vented expansion tank, and circulating pump. The expansion tank need not have inert gas applied in these types of systems in order to keep positive pressure on the circulating pump. To reduce the probability of fluid oxidation, a baffled expansion tank design is preferred to assure the fluid is below 350°F at the fluid/atmosphere interface.

Pressurized liquid phase systems using both hot oils and synthetics are similar in design to non-pressurized systems except that inert gas is applied through the expansion tank when the required operating temperature of the heat transfer fluid is above its boiling range. The pressurized inert gas (nitrogen) is used to maintain the heat transfer fluid as a liquid. The inert gas also acts as a buffer in the expansion tank between the hot fluid surface and the atmosphere, eliminating fluid oxidation. With the exception of the multi-phase fluids like the diphenyl oxide/biphenyl-type, most of the liquid phase synthetics and all of the hot oils do not require inert gas pressurization to maintain the liquid phase at their top-end recommended operating temperatures.

Pressurized vapor-phase systems utilize only a handful of synthetic fluids, most notably the diphenyl oxide/biphenyl-type. A simple vapor phase system can be designed using hydrostatic pressure to gravity return the condensate from the user to the vaporizer, eliminating the need for a condensate pump. More complex systems require a flash tank, condensate return tank, and a condensate return pump. The disadvantage of the added capital equipment cost and the complexity of vapor phase systems is offset by the increased BTUs delivered per pound of vapor versus liquid, and increased temperature control at the user- important in heat sensitive processes.

Criteria For Selecting The Best Fluid Chemistry. If a existing system or a system on the design board is a vapor phase system and requires a high temperature fluid, the fluid options are extremely limited. Only a handful of high temperature vapor phase synthetic fluids are available from different heat transfer fluid manufacturers. Hot oils cannot be used in the vapor phase. Therefore, non-pressurized or pressurized liquid phase systems allow the greatest variance in potential end-use fluids, both synthetic and petroleum-based. Especially in existing systems, the wide range of new liquid phase fluid technology available offers possibilities of increased system performance and energy savings with a minimum of downtime and cost. Whether researching a potential fluid upgrade in an existing system or specifying the proper fluid type and fluid into a new design, the following basic criteria should be considered:

Thermal Stability. Thermal stability is simply defined as the inherent ability of a heat transfer fluid to withstand molecular cracking from heat stress. Relative thermal stability testing of heat transfer fluids measures a particular fluid’s molecular bond strength at a specific temperature versus another particular heat transfer fluid at the same temperature and under identical testing conditions. ‘Relative’ is the key word- since the tests are run under ideal laboratory conditions and do not factor in real-world fluid stresses such as mechanical malfunctions, design flaws, oxidation, etc., the data generated is useful for comparative purposes only. Accurate predictions of fluid life in actual processing applications should not be implied from thermal stability data.

A fluid’s thermal stability is the primary factor in determining its maximum bulk fluid operating temperature. This is the maximum temperature the fluid manufacturer recommends the fluid can be used and still maintain an acceptable level of thermal stability. Since fluid degradation rates are closely tied to temperature, continuous use above the manufacturer’s recommend maximum bulk fluid operating temperature will increase degradation rates exponentially. Potential system problems caused by excessive degradation and the subsequent formation of degradation by-products include increased coking and fouling, mechanical difficulties, and decreased heat transfer efficiency. Therefore, in selecting a fluid chemistry, the first step in the selection process is to determine the maximum bulk operating temperature required by the process. Most hot oils have a recommended maximum bulk fluid temperature of 550°F – 600°F, while the aromatics have recommended maximum bulk fluid temperatures between 600°F- 750°F, depending on the fluid. Since the molecular structures of the aromatics are significantly more thermally stable than the hot oils above 600°F, in applications above this temperature aromatic-based heat transfer fluids are strongly recommended. Process applications requiring bulk fluid temperatures from 300°F to 600°F can specify either synthetic or petroleum-based fluids. Within this temperature range relative thermal stability data supplied from fluid manufacturers is available to compare individual fluids at specific temperatures.

Heat Transfer Efficiency. Heat transfer efficiency comparisons between heat transfer fluids are made using heat transfer coefficients. At a specific temperature, a fluid’s overall heat transfer coefficient can be calculated using its density, viscosity, thermal conductivity and specific heat at a determined flow velocity and pipe diameter. The resultant heat transfer coefficients may be then evaluated and compared. At a given temperature, the heat transfer coefficients of the fluid types may differ as much as 30%. Depending on the thermal resistance factors of the other components in the system, a fluid with a substantial heat transfer coefficient advantage may allow a reduction in sizing of system equipment. Replacing existing heat transfer fluid with a more efficient heat fluid may significantly increase production output and/or reduce energy costs. Most of the aromatic-based fluids have a significant advantage in heat transfer efficiency over hot oils from 300°F to 500°F. Above this temperature range (up to 600°F) petroleum fluids narrow the difference somewhat with a select number of highly refined paraffinic/napthenic white oils having a slight efficiency advantage over the mid-range aromatics.

Keep in mind the heat transfer coefficient is calculated using virgin fluid properties. Fluid that has been in service for an extended period of time and has undergone thermal degradation may have a significantly lower coefficient due to fluid viscosity changes and the presence of less efficient fluid degradation by-products. Therefore, a fluid’s thermal stability plays an important role in maintaining its thermal efficiency over time.

Pumpability Point. Pumpability point, not freeze point, is the true low-end temperature a heat transfer fluid can operate in heat process applications. The pumpability point is defined as the temperature at which the viscosity of the fluid reaches a point (typically 2000 cP) where centrifugal pumps can no longer circulate the fluid. Although most high temperature process applications run at bulk temperatures well above hot oil and aromatic pumpability points, system designs that might encounter cold weather during emergency shutdowns, maintenance shutdowns, or operate a batch process in a cold climate, should take into account fluid pumpability points. Generally, most of the hot oils offer adequate start-up protection down to the 0°F to +25°F range. The mid-temperature aromatics (with 650°F maximum bulk temperatures) offer protection down to -70°F to -20°F, while the top-end temperature aromatics (with 700°F- 750°F maximum bulk temperatures) are at +40°F to +60°F. Processes using a fluid that potentially may have start-up problems in cold weather will need to be heat traced.

Fluid Serviceability. Fluid replacement, reprocessing, or filtration may be required from time to time due to unexpected temperature excursions, system upsets, or contamination. Because of the relatively low cost of petroleum-based fluids, very few suppliers offer reprocessing services for hot oils or a credit program for the off-spec material that can be applied toward the cost of a new charge. Most synthetics are composed of a limited number of aromatic components and have a narrow boiling range, allowing easy identification of degradation by-products and/or contaminants. Reprocessing synthetics using fractional distillation is an economical alternative to disposal and replacement; hence, most synthetic fluid suppliers offer this service at a nominal cost. Some synthetic fluid suppliers also offer credit for off-spec material- this credit is then applied to the purchase cost of the replacement charge. These programs eliminate fluid turnaround and reprocessing time, since the new fluid can be charged in immediately after the off-spec fluid has been drained. This is especially useful if the system downtime is unscheduled or a short maintenance period has been planned.

Filtration versus reprocessing or fluid replacement is a cost effective method of removing carbon and coke suspended in the heat transfer fluid. Most fluid suppliers recommend slipstream filter loops permanently installed and closely maintained on both hot oil and synthetic systems. Fluid filtering companies with portable units are also available to setup on-site and remove carbon from the fluid while the unit is still operating. Almost all suppliers of both synthetic and petroleum-based heat transfer fluids offer analytical testing of fluid condition at no-charge. This important service monitors fluid condition over time and gives an early warning should action need to be taken to replace, reprocess, or filter the fluid.

Environmental. Comparing environmental and personnel guidelines is just as important when selecting a heat transfer fluid chemistry as comparing fluid performance. In general, all heat transfer fluids do not present an appreciable health hazard when used in accordance with acceptable manufacturing practices. However, the petroleum-based fluids offer substantial advantages in ease of handling, reprocessing, shipping and disposal as compared to the synthetics. For one, most hot oils do not have a reportable spill quantity, and in the cases of the white mineral oils, meet the FDA and USDA criteria for ‘incidental food contact’. Also, the petroleum-based fluids do not form hazardous degradation by-products, therefore most spent hot oils can be sent to a local oil/lube recycler for disposal. Finally, the hot oils tend to warrant no special handling precautions, are not DOT regulated, and require no special storage requirements. From a personnel standpoint, the hot oils are extremely user-friendly. Most have a non-discernible odor and are non-toxic both in contact with skin and ingestion.

Because of the aromatic-based chemistry of most of the synthetics, some fluids can form hazardous degradation by-products that require special permits, handling and shipping precautions. Some synthetics and their vapors may cause skin and eye irritation after prolonged exposure, and emit pungent odors. In some cases, spills of synthetic fluids require reporting under the Superfund Amendments and Re-authorization Act. Since there is a wide range of chemistries available within the aromatic group, not all fluids have similar properties and environmental/personnel concerns. Regulations and precautions vary from fluid to fluid.

Cost. As a general rule, the higher the bulk fluid temperature a fluid is rated, the higher the cost of the fluid. The synthetics rated for use above 650°F are two to three times more expensive than the average hot oil rated to 600°F, while aromatics rated from 600°F to 650°F are one and a half to two times the cost of the average hot oil.

Which Chemistry Is The Best? Chances are one fluid chemistry is not superior to the other in every criteria required by a new process or retrofit. Both fluid chemistries have advantages- the aromatics offer superior heat transfer efficiency and stability at elevated temperatures coupled with serviceability and adequate pumpability, while the hot oils have a significant cost and environmental/personnel advantage. Identification of the primary criteria required by a new process or the main improvement goal desired in a retrofit will prioritize the criteria by importance. Is the goal more output and/or shorter production runs? A personnel/environmentally friendly system? Longer period of time between shutdown and fluid replacement? By first selecting the fluid chemistry that best solves the big picture, comparisons of individual fluids within the group should solve the little ones.

System Design

When selecting the proper pump for heat transfer systems, there are several factors to consider. The pump must accommodate for a system’s temperature, pressure, and fluid properties. If a pump is chosen incorrectly, it can result in inefficient system performance or even lead to pump malfunction, such as pump seal damage and leakage. Choosing the best pump for your application can be a daunting task, but knowing which types of pumps are suitable for certain situations can make the decision a lot easier.

There are two main types of pumps used in high heat transfer systems. Positive displacement pumps displace liquid by creating a cavity between the moving components, into which fluid fills. The fluid is then forced out when the mechanism closes those gaps. Please note this article only refers to rotary positive displacement pumps. Reciprocating positive displacement pumps are not designed for use with heat transfer fluid. Centrifugal pumps use a rotating impeller, motor or turbine driven, to create kinetic energy, which increases the static fluid pressure. Fluid enters the pump through the impeller along its rotating axis, and discharges radially to the outlet. Magnetic driven pumps are a unique sealless option, in which integrated magnets drive each other to turn a canister-enclosed shaft. Magnetic pumps are similar to centrifugal pumps in that the driving magnets are also motor driven. Each type of pump has its advantages and disadvantages, which are discussed in further detail.

Table 1 Xceltherm® 600 viscosity at select temperatures
°F
°C
Viscosity
(cP)
50
10.0
75.697
100
37.8
15.489
150
65.6
5.574
200
93.3
2.703
250
121.1
1.642
300
148.9
1.085

One of the most important system design points to address is fluid viscosity. If this is properly considered, you can eliminate which pumps would not perform proficiently. Keep in mind that heat transfer fluid viscosity increases considerably at low temperatures. See table 1.

A heat transfer fluid’s operating range is the temperature range between thepumpability point and the recommended maximum fluid operating temperature. The pumpability point is defined as the temperature where a fluid’s viscosity reaches 2000 centipoises. At this point, the fluid becomes too viscous for centrifugal pumps to maintain fluid flow. It is important to note that the pumpability of the fluid is usually only a factor at startup. Although heat transfer fluids technically can be used at temperatures close to their pumpability points, many fluids (especially petroleum-based fluids) lose much of their heat transfer efficiency if used close to their pumpability point.

Centrifugal pumps operate best with low viscosity liquids, typically ranging up to 550 cP. In this range, centrifugal pumps are capable of handling essentially most heat transfer fluids on the market. However, since they operate at motor speed, pump efficiency and flow rate drop significantly as viscosity increases. This is due to increased frictional losses within the pump’s mechanism. Positive displacement pumps excel in this category. They can effectively operate in a wide range of viscosities, and even more exceptionally at high viscosities (some can operate up to 1,000,000 cP!) Highly viscous fluid fills up clearances within the pump cavities, consequently improving pump operation.

positive displacement pump

The next considerable aspect of pump selection is system capacity. The main advantage of centrifugal pumps is their ability to transfer large volumes of liquid (up to 120,000 gallons per minute.) Processes can even be designed with several centrifugal pumps in parallel to maximize fluid discharge.

Conversely, positive displacement pumps, such as the internal gear pump shown in Figure 1, cannot dispel fluid in great quantities. Instead, positive displacement pumps are capable of delivering a constant, pulse-free flow through the system, independent of variations in system pressure. Centrifugal pumps can operate proficiently under certain pressures, but their efficiency significantly drops as system pressure increases. Most heat transfer systems are designed for operation under 50 psi, in which case, both centrifugal and positive displacement pumps can be used. Centrifugal pumps can be sized for head pressures up to 55 psi (125 ft.)

Net Positive Suction Head, more commonly referred to as “system pressure,” is the sum of several factors determined by system design. The NPSH is determined by the following:

*NPSH = HA ± HZ – HF + HV – HVP

Term Definition Notes
HA The absolute pressure on the surface of the liquid in the supply tank Typically atmospheric pressure (vented supply tank), but can be different for closed tanks. Don’t forget that altitude affects atmospheric pressure (HA in Denver, CO will be lower than in Miami, FL). Always positive (may be low, but even vacuum vessels are at a positive absolute pressure)
HZ The vertical distance between the surface of the liquid in the supply tank and the centerline of the pump Can be positive when liquid level is above the centerline of the pump (called static head) Can be negative when liquid level is below the centerline of the pump (called suction lift.) Always be sure to use the lowest liquid level allowed in the tank.
HF Friction losses in the suction piping Piping and fittings act as a restriction, working against liquid as it flows towards the pump inlet.
HV Velocity head at the pump suction port Often not included as it’s normally quite small.
HVP Absolute vapor pressure of the liquid at the pumping temperature Must be subtracted in the end to make sure that the inlet pressure stays above the vapor pressure. Remember, as temperature goes up, so does the vapor pressure.
*Table and equation courtesy of www.pumpschool.com

After calculating NPSH available (NPSHA), a pump with the appropriate NPSH required (NPSHR) can be chosen. NPSHA must be greater than NPSHR to avoid pump cavitation during system operation. Cavitation occurs when a fluid’s liquid pressure drops below its vapor pressure, causing the liquid to boil. Vapor bubbles produce pump noise and vibration, pitting damage to the impeller, and a sharp reduction in pump head and discharge. If a pump with a proper NPSH rating is selected, cavitation can be prevented.

Once the correct pump is selected for an application, several shaft sealing options can be considered. One of the earliest forms of shaft seals is packing, which is made up of braided or formed rings compressed in the stuffing box of a pump. This type of sealing requires lubrication, either by the circulating system fluid, or externally. The main advantage of packing is that it rarely fails catastrophically. It is most effectively used in applications with thick, non-abrasive liquids. Elastomeric lip seals are also ideal for similar applications. While traditionally used for low pressure applications, technological advancements in newer seals allow for operation in high pressure systems (150-psi or greater) as well. The drawback to using a lip seal is the possibility of catastrophic failure, which could trigger more serious pump problems. Mechanical seals share this same disadvantage.

Basically, mechanical seals consist of faces sliding against one another to form a seal. Similar to seal packing, mechanical seal faces are typically lubricated by the circulating fluid or other external methods. The most prominent benefit of mechanical seals is the wide variety of designs to accommodate a broad range of liquids, viscosities, pressures, and temperatures. Moreover, they are designed to be easily replaced or repaired. As mentioned earlier in this article, sealless magnetic driven pumps are becoming a popular option for hard-to-contain liquid applications. Although sealless pumps are a more costly alternative, they offer exceptional dependability and absolutely no leakage.

In conclusion, there are several factors to consider when deciding which pump is most ideal for your heat transfer application. Although there is an extensive variety of pump options to choose from, knowing the capabilities of your system can help you narrow down the field.

An essential component of thermal systems utilizing liquid phase thermal fluids is an expansion tank. This type of thermal system requires an expansion tank for two reasons. First, the expansion tank serves as the safe outlet for the increase in thermal fluid volume due to thermal expansion. Second, the expansion tank provides a mechanism for venting water, incondensibles, degradation

 

therman system expansion tank design
View/Download PDF * Download DWG File Format

 

by-products and entrained air during startup and operation. Of equal concern in thermal systems operating at 300°F or above is a design that will maintain the temperature of the thermal fluid/atmosphere interface below 300°F to minimize fluid oxidation.

In liquid phase thermal systems there are three types of expansion tank designs:

1. Baffled tanks 
2. Straight Leg Tanks 
3. Flow Through (or “Surge” Tanks)

Surge tank designs essentially are tanks serving as a receiver for thermal fluid from the user with the main circulating pump drawing thermal fluid from the bottom of the tank for return to the heater. This design offers no oxidation protection and in fact increases oxidation due to the aeration of the thermal fluid in the tank itself. This is a poor design for extended thermal fluid use but does provide excellent thermal system venting capabilities during startup and extended operation.

The baffled tank has three chambers or baffles. As the thermal fluid is heated, expansion of the fluid occurs from the first chamber into the second, and then into the main tank. In a properly sized expansion tank, when a thermal system is properly filled with cold thermal fluid there will be a significant level of fluid in the main baffle. With this type of design, thermal fluid in the main baffle rarely, if ever, co-mingles with hot fluid in the primary thermal system heat transfer loop because of inherent system fluid pressures. The fluid in the main baffle is significantly cooler than in the other baffles, forming a thermal blanket over the fluid in the primary thermal system heat transfer loop materially decreasing degradation of the thermal fluid due to oxidation.

As the thermal system is brought up to operating temperature, the straight leg design allows the expanded thermal fluid to enter directly into the bottom of the main expansion tank by means of a small diameter pipe bypassing the flow-through valves under the expansion tank on the pump suction side of the tank Since the expansion tank is located at the highest point of the thermal system, the bulk of the thermal fluid will flow directly to the heater. Similar inherent fluid pressures as seen in the baffled tank keep thermal fluid in the expansion tank from co-mingling freely with fluid in the primary thermal system heat transfer loop. This type of design does not offer the same oxidation protection as the baffled design and, depending on system temperatures, an inert gas blanket may be required. Excellent thermal system venting may be accomplished by configuring the main line tank bypass valving to run through the expansion tank.

As a general rule, in liquid phase thermal systems the expansion tank volumes should be approximately 26% – 30% of the total estimated volume of thermal fluid in the thermal system. The properly sized expansion tank should be approximately 1/4 full at startup temperatures and 3/4 full at operating temperatures.

HTF System Filter Recommendation

For most heat transfer fluid systems, Radco recommends a slipstream filter configuration. Compared to a direct “in-line” filter system, a slipstream unit does not require complete system shutdown and cooling for general maintenance.

Since most heat transfer systems do not have a filter system installed during initial construction, residual inorganics may be present and/or high levels of carbon (or “coke”) may have accumulated over time. The initial start up of the filter unit will require continuous monitoring and frequent changing of filter cartridges/bags, making the “in-line” filter system impractical. The slipstream filter design offers an inexpensive and a maintenance- friendly alternative to scheduled fluid replacement or reprocessing. The installation of a slipstream filter unit for preventative maintenance will also save system wear and tear on system components and greatly extend the operating life of the heat transfer fluid.

Particulates in heat transfer fluids are caused either by polymerization of degradation/oxidation by-products, or are introduced during system construction and consist of inorganic material such as pipe slag, grit, sand, etc.

SLIPSTREAM FILTER RECOMMENDATION FOR HEAT TRANSFER FLUID SYSTEM
Click here for enlargement

All heat transfer fluids degrade over time due to thermal stress. These degradation by-products, referred to by most fluid manufacturers as “high boilers” and “low boilers”, are molecular fragments formed when the heat transfer fluid’s molecular bonds are broken. These fragments can polymerize with other fragments, creating extremely large molecules which in turn lead to the formation of high molecular weight carbon particulates. These particulates are commonly referred to “coke” or “sludge”.

High and low boilers, carbon particulates, and inorganics have very little of the heat transfer efficiency of the original heat transfer fluid. The presence of high levels of carbonized and inorganic material can cause mechanical problems in the system such as seal and gasket leakage and/or failure, heater tube fouling (decreasing the amount of BTUs transferred to the fluid), and increased pressure drop (due to coking on pipe surfaces). Carbonization in the fluid also specifically affects the fluid’s heat transfer properties such as viscosity and density, lowering the heat transfer coefficient. Therefore, the elimination and/or reduction of carbon and organics in the heat transfer fluid will lead to longer periods of time between fluid change-outs, higher fluid efficiency, and decreased downtime required for system maintenance.

Filter Piping Design/Placement

A basic slipstream filter design essentially is a side loop to and from the main heat transfer fluid system return line containing the filter unit. The filter unit (or housing) contains either cartridge-type filter elements or a bag-type filter element. The filter is isolated from the return line by two gate valves, one between the return line and the intake side of the filter, and the other between the downstream side of the filter and the main return line.

Located on the main return line, between the filter loop gate valves is a pressure gauge and a throttle valve. A typical filter housing with clean cartridges/bag will create approximately 5 psi system pressure drop. Most “dirty” filter elements create close to 20 psi pressure drop. To maintain consistent fluid flow rates, set the throttle valve to the acceptable level of pressure drop (in this case 20 psi). The filter element should be replaced before the return line pressure gauge reads the maximum acceptable level of pressure, assuring adequate flow through the filter.

The slipstream filter unit should be located between the main circulating pump and the heater. This placement will assure adequate pressure and consistent fluid flow through the filter unit. The bulk fluid temperature will be at the lowest temperature at this location, decreasing cooling time required before the filter elements can be replaced.

Piping to and from the filter unit should be consistent with the intake and discharge of the filter housing. Most filter housings designed for use in liquid phase high temperature (400oF-600oF) heat transfer systems utilize 1 1/2″ or 2″ flanges.

The slipstream unit should be designed to handle from 1% – 5% of the main return line flow. The filter housing and the filter elements should be capable of handling the appropriate system’s maximum bulk fluid temperature, even though actual fluid temperature on the pre-heater, post-heat exchanger will be substantially lower. This will assure adequate filter housing/element safety should a temperature excursion occur. We recommend that initially cartridges or bags of 150-200 micron size should be used, gradually reducing to 10-20 microns as conditions permit.

 

Maintenance of Thermal Fluid

By: Michael R. Damiani
International Sales Manager
Radco Industries, Inc.
By Utilizing Your Fluid Manufacture’s Product Support Services, You Can Extend the Time Between Fluid Replacements, Lower Your Fluid Make-Up Costs, And Improve System PerformanceHigh temperature heat transfer fluid manufacturers design fluids to operate within the recommended temperature range safely and efficiently for many years with minimal required maintenance. Occasionally, a system upset will occur, drastically affecting the fluid’s heat transfer efficiency and/or thermal stability. System upsets can be caused by a wide assortment of maladies, including heat exchanger leaks, sudden temperature excursions, failed circulation pumps, the addition of a mislabeled drum of fluid to the system, etc. More commonly, over a period of time many heat transfer systems slowly lose their ability to efficiently transfer heat due the fluid’s gradual thermal and oxidative degradation. This usually leads to gradually increasing production times that even the most astute process engineer can miss. In either case, utilizing your heat transfer fluid supplier’s product support services can solve a system problem cost effectively and quickly, and more often than not, without having to drain the system and replace the whole fluid charge. Taking full advantage of a good product support program can save unit downtime, lower fluid make-up rates, and improve overall system performance. Many of these services are on a “no-cost” basis or are relatively cost-effective as compared to the cost of replacing the fluid charge and/or new fluid make-up.

Whether it is the case of a sudden system upset or the realization that the system has lost heat transfer fluid efficiency over time, the first phone call made should be to the fluid manufacturer to discuss the situation and review options. Most major fluid manufacturers offer a wide assortment of fluid support programs to aid in maximizing the fluid user’s investment. These programs usually fall under two categories- preventative maintenance services and fluid performance improvement services.

Preventative Maintenance Services- The First Step In Extending Fluid Life

Fluid Analysis: Consistent fluid analysis is by far the most important service that should be utilized by every heat transfer fluid user. This service is especially important for newly commissioned systems so that the fluid baseline analysis be established for future comparisons. Consistently taking a periodic representative system sample for analysis is invaluable in many ways- it allows both the user and the fluid manufacturer to track, or “trend” the fluid’s degradation rate (the molecular breakdown due to thermal stress) over time. Evaluating this data can give an accurate prediction of when any action, such as fluid reprocessing or replacement, is required to maintain heat transfer efficiency. An accurate forecast from sample analysis data allows more than adequate time to plan for any needed heat transfer fluid work to be completed during a scheduled shutdown or turnaround, and not on an unexpected or emergency basis. Periodically sampling the system also detects any mechanical problems such as product contamination through leaking or ruptured heat exchanger interfaces. In almost every case, the product that leaks into the heat transfer fluid is not as thermally stable or is as heat transfer efficient. Not only will this lower overall heat transfer system efficiency, in many cases the contaminant can increase the degradation rate of the heat transfer fluid. Sample analysis can catch even minute process leaks into the heat transfer system. Early detection and remedy of a process leak or other outside contamination can save major system problems and heat transfer fluid replacement costs down the road.

Most fluid manufacturers recommend that a system sample be analyzed every 6 months. Sample analysis is usually on a “no-charge” basis and many fluid manufactures will supply a sampling kit with written instructions for taking a representative sample, packaging the sample, and returning the kit to the lab. A written report and recommendation is then sent to the user with the results within a week of the lab receiving the sample. Generally, the tests performed include high/low boilers (by both gas chromatography and atmospheric boiling tests), density, moisture, acidity, residual carbon (insolubles), and fire properties (flash point, fire point, and auto-ignition temperature). Correctly interpreted, the data generated from these tests are extremely accurate in revealing overall fluid condition and level of heat transfer efficiency.

Technical/Engineering Support: In general, major fluid manufactures’ technical support teams consist of graduate engineers that not only specialize in heat transfer fluids and heat transfer theory, but most aspects of heat transfer system components, design, and system troubleshooting. Many solutions to questions and problems that seem unique and perplexing to a specific heat transfer fluid user can be readily answered with one quick phone call to the fluid manufacturer, or a plant visit by a heat transfer fluid specialist.

Advice regarding component specifications- such as fluid compatibilities, pump sizing, expansion tank sizing, seals/gaskets, and instrumentation- for specific applications, temperature ranges, and heat transfer fluids can be also quickly answered. Many heat transfer fluid manufactures also list and recommend quality suppliers of heat transfer system components, which can greatly assist in the design and equipment specification stage of a new project. Finally, most fluid manufactures’ technical support people have years of experience in solving heat transfer fluid-related problems ranging from inherent system/equipment design flaws to accelerated fluid degradation and outside contamination. These technical support teams have the experience to quickly identify the problem and recommend a solution that will correct the situation while minimizing system downtime and heat transfer fluid costs.

Fluid Improvement Services- When Something Goes Wrong Or The Fluid Needs to Be Replaced

Replacement/Reprocessing Programs: Most fluid manufacturers recommend fluid replacement when the aggregate high/low boiler (or contamination) level reaches an upper limit of 15%. Even at 10% level, a significant drop in system heat transfer efficiency occurs. If the system charge needs to be replaced either due to catastrophic system upset or from years of degradation or oxidation, a number of options are available from fluid suppliers to minimize downtime. A very common program is the “Fluid Credit Program” where the user drains the existing charge of fluid and returns it to the fluid manufacturer. The recoverable yield is determined (generally no more than 80% is given, even if the yield is higher) and the value of the yield (less a reprocessing charge) is applied against the purchase price of the new fluid cost. This type of program allows for very quick turnaround times. In just about every case, the new charge of fluid can be on-site even before the old fluid is drained from the system. This type of program is especially useful when the scheduled shutdown window is small or the change-out needs to be done on an emergency basis.

Generally, a “toll reprocessing” program is much more cost effective for system change-outs as compared to a fluid credit program. Since most high temperature heat transfer fluids have a limited number of components and a defined boiling range, manufacturers with toll reprocessing services can use fractional distillation techniques to easily separate and remove contaminants and high/low boiling degradation by-products from the heat transfer fluid product. Reprocessed fluid many times meets new fluid specifications. With this type of program, the off-spec fluid is removed from the system and returned to the fluid manufacturer for toll reprocessing. The fluid is reprocessed and the user than receives back the actual recovered yield of heat transfer fluid (in many cases the actual yield returned is greater than the maximum 80% allowed in the typical credit program).

Both types of replacement/reprocessing programs offer significant new fluid cost savings versus disposal and purchase of a completely new charge. Additional cost savings can be achieved by returning small drum quantities of material generated by leaking pumps, equipment swap outs, system vents, etc. for either credit or reprocessing.

Filtration Programs: Filtration is a cost-effective method of removing carbon (coke) and metallic particles suspended in the heat transfer fluid without draining the system and sending the fluid off-site. While permanent filters rated for high temperature duty are recommended for most heat transfer fluid systems, some fluid suppliers offer on-site filtration services with portable high temperature units. In many cases these portable units are able to filter out the carbon and particulates while the system is still in operation, eliminating system downtime.

Like all companies that use and depend on high temperature heat transfer fluids, fluid manufacturers are acutely aware of the importance of heat transfer fluids in production units (and also their relatively high cost). Service programs are offered by the manufacturers so that the user can maximize both the performance properties of the fluid and the return on the initial fluid investment. If not already familiar with the fluid support programs and services, the fluid user should ask their fluid supplier for a detailed explanation of all their programs. Chances are, one call to the fluid supplier will lead to extended fluid life and improved system performance.

Cost Effective Alternatives to Changing Thermal System Fluid

By Michael R. Damiani, Radco Industries Inc.

During normal operation, heat transfer fluids can degrade due to thermal stress or oxidation. Fluids can also become contaminated through heat exchanger leaks. Often, degradation and contamination lead to a significant decrease in heat transfer fluid efficiency, increasing production time and costs. In many cases a full fluid change-out may be required to restore system performance, but many times these other cost-effective alternatives listed below can extend fluid life and maximize system performance:

Fluid Reprocessing. Degraded or contaminated fluid is removed from the system and sent to the fluid manufacturer for reprocessing. Low boilers and high boilers are separated by distillation and the recovered heat transfer fluid is sent back to the original user. Reprocessed fluid will usually meet new fluid specifications. Partial fluid volumes can be drained from the system and sent in for reprocessing, allowing continuous operation and eliminating a system shut-down. Reprocessing costs are based on the total quantity shipped and offer significant savings over the purchase of a new charge.

Fluid Filtration. The formation of hard carbon or coke particles can lead to the fouling of heat transfer surfaces. By using portable filtration units brought on site, a service team from a fluid manufacturer can remove these particles either while the system is in operation or during a shut-down. On-site filtration saves transportation costs and fluid drainage time and expense.

Partial Fluid Change-Out. Replacement of a percentage of the initial fluid charge with new fluid can improve performance by diluting degradation by-products and/or contaminates to within acceptable limits. Often the off-spec fluid that was removed from the system can be returned to the fluid manufacturer for credit, or can be reprocessed, returned, and kept on-site for future make-up requirements.

These options can save system downtime and reduce overall heat transfer fluid costs, while maximizing system performance.

Check your fluid before it checks production

By Robert A. Damiani, Radco Industries Inc.

Preventive maintenance analysis of high temperature heat transfer fluids can play a major role in maintaining production rates, minimizing unscheduled fluid-related downtime, predicting and defining mechanical component failure, and reducing overall heat transfer fluid costs. High temperature heat transfer fluids are defined as having a useful bulk fluid operating temperature of approximately 150 to 750ºF (66 to 399ºC). These fluids include aromatics, petroleum-based fluids, polyglycols and silicones.

Proper preventive maintenance analysis starts by taking a fluid sample at predetermined time intervals. This time interval may be determined based on past experience with the system. However, in the case of no experience or a new system startup, samples should be taken quarterly. The sampling time interval then can be adjusted based on an interpretation of the data with the first sample serving as a baseline. Samples should be taken from the discharge side of the main heat transfer system pump or from any main line flowing in the system to ensure a sample representative of the fluid in the system. If not already present, provisions should be made for safe sampling, including adequate cooling of the sample. The best arrangement is a sampling bomb that allows sample flow through, sample isolation and cooling. This procedure prevents accumulation of solids in stagnant sample lines.

Once a sample is taken, it generally is sent to the heat transfer fluid supplier for analysis. Many suppliers offer this analysis free of charge with a turnaround time of one to six weeks. The data is then transmitted to the user. In some cases, the interpretation is left to the user. However, often the heat transfer fluid supplier will interpret the sample results. In cases where the sample analysis is outside the used fluid limit determined by experience, the heat transfer fluid supplier will provide data interpretation and heat transfer system recommendations. In other cases where the fluid sample results are within the used fluid limits, the fluid is satisfactory for continued use. System problems occurring after successful startup, when the heat transfer fluid is within the used fluid limits, generally are operational in nature and not related to the heat transfer fluid.

What to Check?
Prior to discussing specific tests indicative of system performance, a review of tests commonly run on heat transfer fluids is useful. In addition, data interpretation guidelines for each test are provided. The commonly performed tests are:

Specific Gravity
This test provides a measure of fluid density relative to gravity and water. Each heat transfer fluid has a specific gravity range over a temperature use range. Any variance from this range indicates the presence of either high/low boilers (degradation products) or fluid contamination with a substance of different specific gravity.

Moisture
This test detects the presence of water in the fluid. Water has very low solubility in most high temperature heat transfer fluids (with the exception of glycol). The presence of free water in a heat transfer system can cause volatility problems such as two-phase flow, pump cavitation and excessive pressurization, especially during system startups.

Total Acid Number (Neutralization Number)
This test is an acid/base titration detects minute amounts of strong and weak acids in the fluid. Acids usually are formed in heat transfer fluid from contamination from material outside the system. Acids usually are formed in heat transfer fluid from contamination from material outside the system. Acid number increase usually is associated with open vent expansion tank operation, heat transfer fluid oxidation or heat transfer fluid degradation, producing weak acids. Process material containing oxidizing agents can contribute strong or weak acids.

Acids are harmful in two ways. First, acids tend to accelerate the molecular breakdown of the heat transfer fluid. Secondly, they tend to form insoluble solids that accelerate mechanical deterioration of seals, valves, pumps, etc. Most heat transfer fluids have an initial total acid number at 0.00 to 0.10. The maximum value in used fluid should not exceed 0.50

Insolubles
This test measures the amount of inorganic (pipe slag, sand and other construction debris) and “hard” carbon (coke) carried by the fluid. High amounts of carbon indicate thermal degradation of the fluid and a probable coking/sludge problem on the heat exchanger/heater surfaces or fluid oxidation. Coke and sludge can adversely affect a system’s heat transfer efficiency by heat transfer surface fouling. An insolubles level of 50mg solids /100ml fluids can sometimes indicate problems.

High and Low Boilers
High and low boilers are formed when heat transfer fluids are heated to a high temperature and certain molecular bonds begin to break or thermally degrade. Some of the new materials that form have a lower molecular weight and typically a lower boiling point than the original fluid: These are low boilers. Other compounds resulting from thermal degradation will polymerize into higher molecular weight and higher boiling point molecules than the original fluid: These are high boilers. High and low boilers seen in components may not have the heat transfer efficiency and thermal stability of the original heat transfer fluid molecules.

This measure of the charge in chemical composition to determine the presence of high and low boilers can be determined by two tests performed for cross reference. The first is an atmospheric or vacuum distillation; the second is a simulated distillation using gas chromatography (GC). The atmospheric or vacuum distillation test accurately detects high boilers, and in this test, the fluid is completely distilled. By comparison, the GC completes the run at a specific final temperature. GC analysis is extremely accurate detecting low boilers, especially aromatics, up to the initial boiling temperature of the fluid.

Viscosity
This test measures certain fluid flow characteristics per unit time. It can be used as an indicator of thermal degradation since low boilers will tend to reduce viscosity while high boilers will increase it. Viscosity changes also affect the overall heat transfer capabilities of the fluid.

GC Scan
This test gives a signature of the components of heat transfer fluids, degradation products (high\low boilers) and often can detect contamination. This information is cross-referenced with the atmospheric boiling range test to confirm levels of high and low boilers as well as the presence or absence of outside contaminants.

Flashpoint (Cleveland Open Cup)
This test provides a means of determining the fire/flashpoint of a fluid. Low boilers created as a result of thermal degradation will lower of the flashpoint. Outside contaminants may have a similar effect. As such, a low flashpoint serves to indicated the presence of thermal degradation products/outside contaminants.

Putting the tests to use
Preventive maintenance analysis of heat transfer fluids can aid in maintaining high level heat transfer system performance at minimum cost through proper interpretation of the sample data. Some of the common tests serve as primary indicators of fluid condition while others serve to confirm these primary indicators. The primary tests are:

Moisture [ASTM D1744 (Karl Fishner)] Percentage of high/low boilers (ASTM D86 or ASTM D1160 and ASTM D2887)
Insolubles [ASTM 893 (Modifies)] Each of these tests defines a major area of fluid quality and system performance. The secondary tests – specific gravity, acid number, viscosity and flashpoint – then can be used to confirm the interpretations reached with the data generated by the primary tests.

The moisture content, as defined by ASTM D1744 (Karl Fisher), generally is a primary indicator of the integrity of a water/fluid interface. Two examples of this interface would be water-cooled heat exchangers or reactor jackets in which water is a component of the product being processed. Although some moisture from humid air may be drawn into the fluid through improperly blanketed expansion tanks in liquid-phase systems or during the cooling phase of sample collection, moisture levels in excess of 500 ppm (0.05%) or the presence of free water in a sample generally indicate a failure at a water/fluid interface.

Initially, such failures usually are not dramatic. Generally, the leak is of the “pinhole” variety, and in fact, depending on the type of fluid in use, system pressures may not be noticeably affected at the onset. Sample frequency should be increased to confirm and monitor the situation, and plans should be made to examine or repair the appropriate heat exchange surface at the next scheduled shutdown. Should moisture levels in subsequent samples increase, or the system begin to excessively pressurize, more immediate action may be required.

High/low boiler content as defined by ASTM D86 or ASTM D1160 and ASTM D2887 serve as an indicator of heat transfer fluid thermal degradation or outside contamination. ASTM D86 and D1160 are initial- and final-boiling point determination distillations. ASTM D1160, a vacuum distillation, is used when the boiling points are too elevated to make atmospheric distillation practical (d86) or the product being tested thermally degrades at its atmospheric boiling point/range. ASTM D2887 is a gas chromatography-simulated distillation, which also defines the initial and final boiling points of products being tested. In each case, once the initial and final boiling points of virgin products are known, used fluid may be tested to determine if high/low boilers are present.

Heat transfer fluids gradually degrade during normal use with the rate of thermal degradation increasing as the fluid approaches the high end of its bulk fluid operating temperature. Experience indicates that under normal conditions, most fluids with 5% low boilers or 10% high boiler levels call for reprocessing or replacement to maintain heat transfer capabilities. In the case of a dramatic increase in high or low boiler levels from the previous sample, and with the fluid still operating within its maximum bulk operating temperatures, either outside contamination, heater malfunction or decreased fluid flow rate may be assumed to be the cause.

The presence of other outside contaminants can be determined by an examination of the ASTM D2887 GC trace of the sample and a comparison of that with the virgin product trace. One caution: With petroleum-based fluids, the contaminant, depending on molecular weight, may sometimes be masked within the characteristic bell-shaped petroleum CG trace. However, the contaminant usually will thermally degrade, and its components may be seen as high or low boilers. In either case, the product being processed by the system should be the first candidate as the outside contaminant. If the identity of the contaminant is confirmed to be product under process, a leaking product/heat transfer fluid interface should be suspected. The solution is similar to that of high moisture: Increase sample frequency to monitor the situation while making provision for repair. In some cases, the nature of the contaminant or the size of the leak may require immediate action.

A rapid increase in high/low boiler also may be indicative of heater malfunction. Either flame impingement on a tube and/or excessive tube carboning can cause abnormally high film temperatures, resulting in increased thermal degradation. A low fluid flow rate due to circulation pump wear or a restriction to flow in the system may contribute to the problem, especially in systems operating at maximum bulk fluid/film temperature. Again, increased sampling is warranted, the process pump flow rate and heater should be checked and provision made for heater tube wall examination during the next scheduled shutdown.

Insolubles, as defined by ASTM D893 (modified) primarily measures pipe slag, metal fines, hard carbon and other inorganic contaminants. Insoluble solids measurements in excess of 50mg solids/100ml fluids, and especially in excess of 250mg solids/100ml fluids of particles above 10pm, call for installation of slip-stream filtration in the system where a modest pressure drop is available to filter a low flow slip stream (less than 1% of the main flow rate). This will minimize wear on mechanical components such as pumps and valves. The presence of metal fines may indicate process pump, mechanical seal, or valve wear. Pipe slag, bits of welding beads, sand, etc. are commonly found in new systems after systems after startup or in older systems that have not been filtered.

The presence of hard carbon may confirm heater tube fouling, degraded process material for contamination and
often, insoluble fluid oxidation products. Of note, the presence of hard carbon also may indicate heat exchange surface fouling that will decrease production capabilities. A simple ashing test or solvent-washing and vacuum-drying solids collected from the system will determine if the solids are inorganic (mill scale, sand, etc.) or organic (fluid oxidation products or carbon from over heating) in origin. High ash content indicates the solids are inorganic in nature while a low ash content indicates the origin of the solids was from an organic material. These changes in solids content may lead to heat exchange surface fouling and result in increases in production times of only a few seconds per day. However, over the course of years, a significant reduction of production capability may occur almost unnoticed. As a result, if excessive hard carbon is present, heat exchange surfaces should be checked periodically to determine if fouling and its resultant production loss is present.

Preventive maintenance analysis of high temperature heat transfer fluids greatly reduces the unpredictability of heat transfer system performance. Proper interpretation of the data allows early identification of potential system problems. Planned maintenance based on the used fluid analysis reduces unscheduled system downtime caused by system upsets. In addition to these cost reductions, fluid costs are reduced since fluid changeouts or reprocessing can occur on a scheduled basis, maximizing fluid life and heat transfer capabilities.