Antifreeze and Your Chiller
Never use car antifreeze in a chiller system. The additives found in this type of antifreeze will foul heat exchangers and result in poor heat transfer. There are two basic types of glycol used in process chillers: ethylene and propylene. These glycols are available in temperature ranges of -60 to 350oF (-51 to 177oC) and work well for chiller applications. With glycol in the system, chilled water is protected from freezing in the heat exchanger.
Ethylene glycol tends to be the preferred coolant in most chiller applications. If your application is pharmaceutical or if contact with food or potable water is possible, a clear propylene glycol might be the right choice. Coolant solutions with dyes or special inhibitors are toxic and should not be used in these applications. Some states and local regulations do not accept ethylene glycol and require the use of propylene.
Some manufacturers offer inhibited glycol, which assists in reducing corrosion. Even with inhibited glycol, chilled water systems using iron, steel or galvanized water piping and fittings can suffer from corrosion problems. These materials should not be used in the construction of the system. Always use copper, stainless steel or schedule 80 PVC pipe and fittings. Antifreeze is a corrosive. Ethylene glycol is on the EPA list of toxic materials, and spills require a hazardous waste team to clean up.
Add in Antifreeze EffectsWhen sizing a chiller system, you should consider the performance change when glycol is introduced. BTU output is reduced as the concentration of glycol is increased. In most cases, it is not recommended to use concentrations of propylene glycol higher than 50 percent by weight or concentrations of ethylene glycol higher than 40 percent by weight.
When calculating the required chiller size for your application, the output of the chiller should be corrected to reflect the effect of the lower heat transfer properties of glycol vs. water. Pump flow rates also decrease as glycol concentrations are increased. The system's total pressure drop should be corrected for the increase of glycol and the process pumps sized accordingly (tables 1).
Water quality and system design efficiency will determine the limiting or safe low temperature freeze point. Some chillers may go as low as 47oF (8oC) while older or less efficient models may require settings to be set as high as 60oF (16oC) to keep water from freezing.
Once you have your system protected with coolants, it is important to keep it that way. It is a good idea to keep a premixed antifreeze solution on hand in 5-gal buckets or drums for use in the event of occasional system water loss. If the requirement to add solution presents itself, utilizing the premixed solution will allow maintenance of the correct limiting or safe low temperature freeze point.