Compression Refrigeration

Most common home air-conditioning systems use a compressor cycle, similar to that of a refrigerator. A compressor on the outside of the house is filled with a refrigerant, which is a fluid that can change back and forth between a liquid and a gas. As it changes, it absorbs or releases heat, so that it can carry heat from inside the house to the outside. It uses a lot of electricity to take heat from the cooler inside of the house and dump it in the warmer outside.

Air conditioners work by circulating a refrigerant through two sets of coils in one continuous loop. One set, the evaporator coils, cools the room; the other set, the condenser coils, gives off heat to the outdoors. Between them, a barrier (the expansion valve) keeps the two parts from working against each other. Near the barrier and as part of the refrigerant loop is the compressor. The two fans help transfer the heat from the air to the coils, and then to the outside air.

When the condenser coils and compressor work together to remove heat from the system, it is called the compression cycle (Fig. 25-2). The compressor circulates the refrigerant and compresses it. In the compressor, heat energy is released as the refrigerant changes from vapor to a liquid state. The released heat is absorbed by cooling water and pumped from the building, or absorbed by fan-blown air and pushed to the outdoors. Heated water may be dumped into sewers, or may release heat through evaporation and convection in an outdoor cooling tower. The cooling water is then recir

OUTDOORS Heat Rejection


OUTDOORS Heat Rejection


Figure 25-2 Cooling compression cycle.

culated to warm vapor emerging from the compressor to liquefy the refrigerant at fairly high pressures. The warm liquid refrigerant then passes through the expansion valve, and as it evaporates in the evaporator, it cools the chilled water system.

Compression refrigeration systems in small buildings transfer heat from one circulating water system (chilled water) to another (condenser water), and the system is referred to as a water-to-water system. Cooling takes place by changing a refrigerant from a liquid to a vapor. Heat can be extracted from water or from air.

The compression refrigeration process essentially pumps heat out of the chilled water system and into the condenser water system, continually repeating the cycle. Water or air cooled by the expansion (evaporator) coils is distributed throughout the building, absorbing heat from occupants, machinery, lighting, and building surfaces, and then returned for another chilling cycle.

With an air-to-air system, indoor air is cooled by passing over an evaporator coil in which refrigerant is expanding from liquid to gas. This direct expansion gives these evaporator coils the name DX coils. The compressor and condenser unit is often located outdoors, because it is noisy and hot. The cooled air is distributed through the ducts of a warm air heating system.

A refrigerant is a gas at normal temperatures and pressures, so it can vaporize at low temperatures. Freon, the most common refrigerant up to the 1990s, can escape into the atmosphere as chlorofluorocarbon (CFC)

gases. These CFCs have long atmospheric lives, up to 145 years for CFC-12, and lead to ozone depletion, so even with the worldwide ban on the production of CFCs in the 1990s, they will be around for a long time. Many existing older CFC chillers still haven't been replaced. In addition, there is currently a black market demand for the CFC-containing refrigerant Freon. This is despite the fact that ozone depletion increases the burning of skin by ultraviolet (UV) radiation from the sun, resulting in skin cancers, eye cataracts, and possible damage to crop production. Hydrochlorofluorocarbons (HCFCs) are still a threat to the environment but are less harmful than CFCs. Because they contain chlorine, which leads to ozone depletion, HCFCs are also being phased out.

Hydrofluorocarbons (HFCs) also pose some threat of global warming and have long atmospheric lifetimes, but have low toxicity and are nonflammable. Natural hydrocarbons (HCs) have a negligible effect on global warming and short atmospheric life. However, they are flammable and explosive. Lower-threat alternative refrigerants may use more energy, which may mean more fossil fuel use and more pollution. Production of refrigerants with photovoltaic energy is an alternative.

Ammonia is a refrigerant that is acutely toxic and flammable. Fortunately, its distinctive odor warns of leaks. Use of ammonia as a refrigerant requires regular maintenance, good ventilation, and good access and escape routes. Equipment must be kept in closed mechanical rooms.

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