Pumps & Systems, November 2008
Seven considerations for selecting the proper heat transfer fluid.
1. Service Temperature
Consider the service temperature of the fluid, since the thermal stability must match it. Typically given by the temperature of the fluid leaving the heater, it will be the highest bulk fluid temperature. The bulk oil temperature is relevant, but what confirms the suitability of a fluid is the skin film temperature of the heating application compared to that of the fluid.
2. Additional Service Temperature
If a system is used for both heating and cooling, the fluid will have a second service temperature. The second critical temperature is that of the fluid as it leaves the heat sink in the system. Thermal stability is usually not an issue in this case, but the heat transfer properties of the fluid (mainly related to its viscosity) at the lower temperature is relevant.
3. Low Temperature Start-Up Conditions
Low temperature start-up conditions can sometimes be an issue. Organic heat transfer fluids do not freeze with volume expansion like water, but the viscosities of these fluids can increase by orders of magnitude once the temperature falls below 32-deg F (0-deg C). It is estimated that a fluid will be pumpable in a typical centrifugal pump until it reaches a viscosity of 2,000 centistokes.
4. Physical Properties that Affect Performance
The physical properties that affect heat transfer fluid performance are viscosity, density, thermal conductivity and specific heat. In simple terms, a fluid can be considered a conveyor belt for heat or as an endless conveyor belt of buckets, all containing units of heat. If the buckets are relatively large, the belt does not have to travel as fast to transfer the same amount of heat. The product of density and specific heat corresponds to bucket size, so high values of density and specific heat are desirable for moving heat from the source to the process vessels.
At the heat source and at the process vessel, the fluid must perform another function-transferring the heat across an interface. The ability of a fluid to transfer heat across an interface depends on the fluid properties of viscosity, thermal conductivity, specific heat and density. For good heat transferability, the viscosity should be low, while the other three properties should be high. Fluid viscosity often has an overwhelming effect, since it is a strong function of temperature.
When comparing the capability of different fluids to transfer heat across an interface, the properties of the process fluid on the other side of the interface must be considered. If the process fluid is also a low-viscosity, water-like fluid, it will not present much resistance to heat transfer, and the properties of the heat transfer fluid will be important. On the other hand, if the process fluid is viscous, such as a polymer melt, or perhaps a gas, it will present such a high resistance to heat transfer that the properties of the heat transfer fluid will not make much difference.
5. Fluid Supplier
Consider the fluid supplier. A reputable supplier should be able to relieve the customer of much of the burden of maintaining the fluid, including advising a suitable fluid for the application, providing change-out advice or assistance, establishing a fluid monitoring program and providing regular sample analyses with comments on the fluid condition and any corrective actions.
In addition to fluid data sheets, fluid suppliers often provide information on the physical properties of their products on company websites or specialized software. These typically allow calculations and printouts of physical properties in a variety of unit systems over any specified temperature range.
The software also allows the calculation of inside pipe heat transfer coefficients, h, and fluid pressure drops in a variety of pipe sizes. Some will allow heat transfer coefficients and pressure drops to be calculated for the fluid from any supplier, provided the appropriate physical property data is entered. This is a useful feature to have when comparing fluids from different suppliers since it ensures that the same correlation is used in each case. However, because of possible reformulations from competitors, we advise that updated data on competitive oils be obtained from the respective manufacturer.
6. Fluid Measurements Related to Fire Safety
The important fluid measurements relating to fire safety are the flash point and auto-ignition temperature. The flash point is the temperature at which the vapor over the surface of the fluid will burn, or "flash," if a source of ignition is present. The auto-ignition temperature is the temperature at which the appropriate mixture of air and the fluid vapor will self-ignite without requiring an ignition source.
The higher these two temperatures, the less chance of a fire, but having a system with the piping and seals in good condition is probably equally important in avoiding fires. If the fluid has limited or no contact with air, there is little chance of a fire.
It is common for systems to be operated at temperatures above the flash point of the fluid without mishap. Auto-ignition temperatures tend to be 212-deg F to 392-deg F (100-deg C to 200-deg C) higher than flash points, and are in the temperature range where fluid decomposition is a problem. It is the vapor in the air space above the fluid that ignites, not the fluid. A low vapor pressure at the operating temperature is, therefore, a desirable characteristic for avoiding fires.