Recirculating Chillers and the Refrigeration Cycle
In order to understand how these factors can impact the cooling capacity of a chiller, it is necessary to understand first how a chiller operates. A compressor-based recirculating chiller works by using the latent heat properties of a refrigerant to remove heat from a process and reject it to ambient air or to facility water. (See Figure 1.) In order to transfer this process heat to the ambient air or facility water, the refrigeration system must provide a refrigerant temperature below the temperature of the process fluid to be cooled. Later in the process, the system must raise the temperature of the refrigerant to a level above the temperature of the medium that is used for rejecting the heat.
A chiller is a complex system, but the basic components are the compressor, condenser, thermostatic expansion valve (TXV), and evaporator. Starting at the compressor, the refrigerant coming from the evaporator is compressed from a saturated gas to a high-pressure, high-temperature gas. The now hot gas is passed through the condenser, where it is cooled and condensed into a saturated liquid by rejecting the heat to cooler ambient air (air-cooled condenser) or to facility water (water-cooled condenser). The refrigerant then passes through the TXV, across which its pressure and temperature drop considerably. The refrigerant temperature is now lower than that of the process fluid and, as a result, heat is transferred from the process fluid to the refrigerant causing it to evaporate into a low-pressure gas. The cycle is once again repeated as the gas flows back to the compressor.
The condenser and evaporator are heat exchangers that transfer heat from one medium to another. In the case of an air-cooled condenser, an aluminum-finned copper tube liquid-to-air heat exchanger is typically used for rejecting heat from the hot refrigerant gas to the ambient air. A water-cooled condenser, on the other hand, uses a liquid-to-liquid heat exchanger to transfer heat from the hot refrigerant gas to the facility water. In the case of the evaporator, a liquid-to-liquid heat exchanger is typically used to transfer heat from the process fluid to the refrigerant. The performance of a heat exchanger depends on many factors, including the process fluid used, incoming fluid temperatures, flow rates, materials of construction, and design of the heat exchanger. With all other factors being equal, the driving force behind the transfer of heat from one fluid to another is the difference in incoming fluid temperatures.