When 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 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 18oF (10oC) 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. This is the temperature that the fluid achieves at the point where it touches the heat transfer surface of a heater. The film temperature rating is often about 50oF (28oC) 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. The specific gravity of a thermal fluid 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.
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.
To learn more about heat transfer fluids, read “Specifying a Thermal Fluid System: A 6-Part Series” by Jay Hudson, P.E.