The refrigerant vapor cools inside the condenser and changes into a liquid, rejecting a large amount of heat in the process. This heat is absorbed and borne away by the water or air flowing around it. Thirdly, the refrigerant liquid proceeds through an expansion valve that decreases the pressure and allows for evaporation to take place. The flash evaporation that occurs at the expansion valve cools the liquid drastically. Lastly, the cold fluid moves into the heat exchanger or evaporator, where the remaining liquid refrigerant evaporates as it absorbs heat from the process (direct cooling) or process coolant (indirect cooling). In recirculating chillers, the refrigerant vapor is then drawn into the first compression stage to start the cycle over again.
Chillers come in a variety of types. Absorption chillers use a heat source to drive the refrigeration cycle, while vapor-compression chillers use a compressor to drive the cycle. The advantage of the absorption model is that it requires much less electricity, and can be powered by heat sources that might otherwise be wasted, such as heat-producing machinery or solar rays. On the other hand, vapor-compression is a readily-accessible and time-tested technology with more versatility and easier installation. There are a number of different types of vapor-compression chillers according to the type of gas compressor they use, the most common being centrifugal compressors, screw compressors and scroll compressors. Portable chillers usually use scroll compressors because they are the most compact and quiet. Another distinction between chillers has to do with the condenser. Condensers can be air cooled, water cooled or evaporation cooled. Air cooled chillers facilitate condensation of the refrigerant by blowing ambient air over the condenser coil or tubes and exhausting the hot air into the atmosphere, or in some cases using it to help heat the facility during the winter. Evaporation cooled chillers operate in the same way as air cooled chillers, except they introduce a mist of water in the air, the evaporation of which makes the heat transfer more efficient. Water cooled chillers employ a flow of water to take heat from the refrigerant in the condenser. This is the most effective condensing method, but also requires a constant source of cool water, and in most cases also necessitates an outdoor cooling tower and a pump to get the heated water there.
When installing a chiller system, there are a number of important considerations. Foremost is cooling capacity. Industrial chillers are measured by their cooling capacity in terms of tons, each ton being roughly equivalent to the heat of fusion of one ton of ice, or 12,000 Btu/h. Capacities range from portable chillers with fractions of a ton to permanent multi-unit “plants” with cooling capacities of thousands of tons. Another significant decision is the sort of refrigerant; this will mostly depend on the range of temperatures the chiller will face. Common refrigerant choices include water, ammonia, carbon dioxide, sulfur dioxide, alcohol, brine and methane. Fluorocarbons, especially chlorofluorocarbons (CFCs) have also been used widely as refrigerants, but they are decreasingly common because of their ozone depletion effects. Other specifications to look at include condenser and evaporator flow rates, power source, cooling capacity, efficiency, location, compressor type and compressor horsepower. Most chillers also come with a local and/or remote control panel with temperature and pressure indicators and emergency alarms. When configured properly, chillers can provide simple and effective solutions for many process cooling and industrial air conditioning applications.