Choosing the Heat Transfer Fluid
January 1, 2007
This year, I will be reviewing the steps normally taken to specify a new thermal fluid heat transfer system. In each of six articles, I will take a look at one aspect of the system and discuss important considerations for selecting equipment or materials that will perform and give acceptable service life.
In part 1, thermal fluid selection will be reviewed. The heat transfer fluid is the most important decision to make in specifying a system because the fluid’s properties have to be matched to the process requirements, and the equipment has to be matched to the fluid properties.
I have in my office information on approximately 70 different thermal fluids, and every time I think that I have them all, I learn about another. The variety of fluids available to the end user is staggering, and each supplier advertises his fluid as “the best.” So what fluid is best -- or at least a good choice for a particular application?
Previously, I reviewed various thermal fluids in depth. (See “Thermal Fluids: An Overview”) As a brief review, I will look at three classes of thermal fluids: petroleum-based fluids, synthetic aromatic fluids, and specialty fluids. Glycol and polyglycol fluids, along with molten salts, will not be discussed in this article but will be covered in later articles.
Petroleum-Based Fluids. These fluids are represented by the mineral oils and by the more highly refined white oils.
Mineral Oils. Sometimes referred to as “base stocks,” these heat transfer fluids are the ubiquitous “hot oils.” Hot oils come from the lube cut and are selected for viscosity (which partly defines the heat transfer properties) and stability, and are branded and marketed as heat transfer fluids. Hot oils may or may not have additional additives such as antioxidants and rust preventives mixed in.
Common brands of hot oils are Exxon-Mobil Mobiltherm 603, Shell Thermia C and Sunoco HT-21.
White Oils. White oils are petroleum-based stocks that have been chemically treated with hydrogen to make a “cleaner,” more consistent product. This process is referred to as hydrogenation, hydro-finishing or hydro-treating. The result is a pale yellow to water white fluid with improved thermal stability and improved resistance to oxidation as compared to mineral oils.
Common brand names of white oils include Calflo, Duratherm, Multitherm and Paratherm.
Synthetic Aromatic Fluids. These materials are, for the most part, modified benzene compounds and generally are thermally stable at higher temperatures than are practical for petroleum-based fluids. These chemicals are either purposely produced for heat transfer service or other uses, or they are co-produced with other chemicals. While benzene is a listed hazardous chemical and these fluids may carry benzene at part per million concentrations, the derivatives used for heat transfer fluids generally are less toxic and safe to use. It is recommended, however, that the end user review the MSDS for these products -- and all heat transfer products, for that matter -- before putting them into service.
Common brand names for synthetic aromatic fluids include Dowtherm, Marlotherm and Therminol.
Specialty Fluids. Silicone- and fluorocarbon-based fluids are two examples of specialty fluids available for highly specific uses. These fluids offer enhanced performance, depending on the specific product, in such areas as superior low temperature performance, broad temperature range, low flammability and low toxicity. They are most often selected when the special demands of a particular process justify the higher cost of these fluids.
Common trade names for specialty fluids include Duratherm S (silicone), Syltherm (silicone) and Galden (fluorocarbon).
Fluid PropertiesWhen selecting a fluid (or fluid type) for a system, it is important to have accurate data on certain intrinsic properties of the fluids being considered. These properties are used to define fluid flow rates and temperatures on the process side of the system and also for the fluid heater manufacturer’s use in sizing his equipment.
Bulk Temperature Rating. This is simply the highest temperature for which the total volume of fluid is rated. It is perhaps the most important property to consider when selecting a fluid. Most fluids will begin to degrade at an accelerated rate when exposed to temperatures above their maximum rated temperature. As in many chemical processes, an upward change of 18°F (10°C) will double the rate of fluid degradation, so it is prudent to buy a fluid with a maximum rated temperature slightly above the maximum temperature of the process. As a general rule, less expensive heat transfer fluids have lower bulk temperature ratings.
Maximum Fluid Film Temperature. The film temperature is the temperature that the fluid achieves inside the heater at the point where it touches the heat transfer surface of the heater. The film temperature rating is often about 50°F (28°C) above the maximum bulk temperature, but this is not always the case.
Specific Gravity (Density). The specific gravity is the weight of a known volume of the fluid as compared to that of water. Specific gravity of thermal fluids varies greatly from ambient temperature to operating temperature, so the user should know the specific gravity across the entire temperature range of the fluid. This property is used in sizing the circulating pump(s) and also determining how large the expansion tank needs to be.
Specific Heat. This is simply how many BTUs it takes to change the temperature of one pound of fluid one degree Fahrenheit, and it is the measure of the heat-carrying capacity of the fluid. Specific gravity, specific heat and the change in temperature in the process or heater determine the fluid flow rate required to achieve the desired process conditions.
Heat Transfer Coefficient. This number describes how easily heat can move into or out of the fluid. This is a calculated value that depends upon other factors. Equations are available in heat transfer textbooks. If the heat transfer coefficient of the process is relatively low, then the impact of the fluid heat transfer coefficient on the overall heat transfer coefficient is relatively low. However, if the system performance is driven by the heat transfer coefficient of the fluid, then more consideration is warranted.
Viscosity. As with specific gravity, most heat transfer fluids exhibit large changes in viscosity between ambient and operating temperature. It is important to understand the changes in viscosity, particularly if the system has to be started in cold conditions or if the fluid is used for process cooling as well as heating. Fluids that exhibit high viscosity at lower temperatures can cause problems in cold startup conditions or in processes where heating and cooling are critical.
Vapor Pressure. As with most liquids, as temperature increases, the vapor pressure of the fluid increases. When the vapor pressure of a liquid equals the pressure of the surrounding gas, the fluid boils. Vapor pressure is very important to know when specifying the circulating pump(s) and in designing pump suction piping. (More information on this topic will be covered in later installments of this series.)
Flashpoint. Almost all heat transfer fluids are combustible liquids, and most are operated well in excess of the flashpoint. Considering flashpoint will help to determine how the system is sited and operated, as will be seen in later installments.
Fluid Selection CriteriaIn determining the best fluid for a new application, there are a number of factors that the owner should consider in narrowing his choices.
Fluid Properties That Match the Unique Requirements of the System. A careful technical review of the potential fluid’s intrinsic properties, along with considerations for thermal and oxidative stability, and the process requirements, will help to determine the best choice for the particular application. The fluid properties first must be compatible with the process and also meet the heat transfer, safety, operability, etc., requirements of the process. As mentioned at the beginning of this article, fluid selection is a critical first step: after this decision is made, the heating (and cooling) equipment is specified and selected to meet the requirements of the system. In addition, the operating characteristics of the fluid can impact both initial and operating costs.
Cost. The purchase of the initial system fill is a significant cost and needs to be considered carefully. This is not to say that the lowest cost fluid with the correct bulk temperature rating should be selected. Buying the cheapest fluid can often be false economy. The cheapest fluid may not have the appropriate properties to offer the owner a long and reasonably trouble-free life.
Life of the Fluid. The useful life expectancy of the fluid can help in predicting ongoing costs of fluid top-up and replacement. System design techniques (which will be discussed in subsequent parts of this series) can help protect the fluid from cracking (overheating) and oxidation. It also is important to select a fluid with excellent thermal stability for its temperature range.
Technical Service. Ongoing fluid testing helps to track the “health” of the fluid. Periodic top-up or freshening of the fluid can extend fluid life and potentially eliminate the costly practice of a complete change-out. Always ask your prospective fluid supplier if he offers free periodic fluid testing. Certain fluid suppliers even offer a “trade in” program, where used fluid can be returned for partial credit for new top-up material.
If the end user is unfamiliar with the various fluids, there are experienced consultant resources available to assist in determining the most appropriate fluid to use.
The fluid choices available are numerous, and there is no one ideal fluid for all systems and applications. Careful consideration of the properties of various fluids can help the end user make an informed choice to narrow the choices to one of performance and value for the particular application.
SidebarUse the links at the bottom of the page to continue reading this six-part series on specifying a thermal fluid heating system. You've just finished:
Specifying a Thermal Fluid System: A 6-Part Series
Part 1: Choosing the Thermal Fluid
Other parts in this series include:
Part 2: Fired Thermal Fluid Heaters
Part 3: Electric Thermal Fluids Heaters
Part 4: Thermal Fluid Pumps
Part 5: The Expansion Tank
Part 6: Piping Materials, Valves and Insulation