Insulation thickness and sealants have a significant impact on maintaining consistent fluid supply temperature and avoiding excessive compressor cycling. There are numerous manufacturers and products available for heat transfer fluids, so consultation with each fluid manufacturer’s application engineering documentation is necessary to identify specific brand characteristics. While viscosity, thermal conductivity and volumetric expansion of a heat transfer fluid are important, other factors such as polymer compatibility, corrosiveness, toxicity and flammability also should be considered. It is recommended to allow for this expansion range by means of an expansion vessel or pressure-relief device. Yet, in facilities where low temperature chillers are employed, it would not be uncommon to place the system into a standby mode throughout the weekend, which means the heat transfer fluid in the system could realistically reach ambient temperature on a consistent basis. For example, a potassium-blend heat transfer fluid can expand as much as 3 to 5 percent when the fluid temperature is increased from -45☏ (-43☌) to an ambient temperatures of 80☏ (26.6☌). When a fluid is stored at an operating temperature of -45☏ (-43☌), one must account for the potential volumetric expansion percentage of the fluid.
The viscosity and thermal conductivity values for each type are compared. The three most common types of heat transfer fluids for process chillers are propylene glycol, potassium blends and silicone blends. Silicone-based heat transfer fluids have a viscosity of 244.3 centipoise and a thermal conductivity of 0.084. By comparison, potassium-blended products have a viscosity of 28.3 centipoise at -50☏ (-45.5☌) and a thermal conductivity of 0.615.