When is it necessary to add glycol in your coolant?
Aavid recommends using a 30/70 glycol-water mixture with its recirculating chillers whenever the coolant temperature set point is below 10°C (48°F). Glycol lowers the freezing point of the mixture *(Figure 1).
In a recirculating chiller, the liquid coolant (usually water) flows through the application, removing excess heat and in doing so, raising the temperature of the liquid. This coolant then needs to be returned to set point temperature by flowing through a heat exchanger called the evaporator. Please refer to "The Basics of Compressor-Based Refrigeration - Applications Note
" for more information on how a refrigeration system works.
The evaporator is a heat exchanger. It allows the transfer of heat between the liquid coolant and the system's refrigerant gas. The refrigerant's temperature must be lower than the temperature of coolant liquid in order for heat to flow and for coolant temperature to be effectively returned to set point.
The temperature of the refrigerant is typically 5°C to 10°C lower than coolant temperature to allow heat to flow. Consequently, if the temperature set point is below 10°C (48°F), the refrigerant's temperature can be close to, or even below the freezing point of water. If the coolant freezes, the evaporator can become obstructed, preventing water flow. Water expands as it freezes and this can cause permanent damage to the evaporator.
Adding glycol to your coolant reduces the freezing point of the coolant to around -34°C, preventing any risk of damage to your chiller caused by freezing.
Glycol does not transfer heat as well as pure water (Fig. 2 & 3). It is therefore preferable to use 100% water where there is no risk of freezing. However, when the set point is below 10°C (48°F) there is a risk of freezing and Glycol should be added to water. The slight decrease in performance is a necessary trade-off to safely allow the lower temperature set point.
*Dowcal 10, by Dow Chemical Co. is 94% Ethylene Glycol. Data courtesy of Dow Chemical Co.