The Vapor Air Conditioning Cycle

While a number of ingenious ways have been devised to reverse the natural flow of heat, one means in particular has proven over many years to be the most commercially effective. This method is made up of continuously repeating sets of processes (known as loops or cycles) that use the flow of a sealed refrigerant that converts back and forth from liquid to gas -- called "phase changes" -- to transport heat from lower temperatures to higher ones. This cyclic (repeating) set of processes is known as the "vapor cycle."

As shown in Figure 1, the basic vapor cycle air conditioning system is quite simple in concept and can be achieved, in addition to the refrigerant that actually transports the heat, with only four physical components. These components are the:

* Compressor: - a rotating machine that causes the refrigerant to continuously circulate through the vapor cycle loop;
* Evaporator: - a heat exchanger that accepts heat from the area being cooled and transfers this heat to the refrigerant;
* Condenser: - a heat exchanger that rejects heat contained in the refrigerant to the outside (ambient);
* Expansion Valve: - a small metering orifice that causes the circulating condensed (liquid) refrigerant contained at high pressure in the Condenser to drop and flow into the low pressure in the Evaporator.

When the Compressor is running, refrigerant vapor is drawn from the Evaporator. This refrigerant vapor has already completed its cooling by absorbing heat through evaporation. The Compressor then compresses this low pressure refrigerant to a much higher pressure. This compression process, which also adds energy to the refrigerant, causes its temperature to increase considerably above the ambient outside temperature. As this now hot, high pressure compressed refrigerant flows through the Condenser, its available heat is transferred to the relatively cooler outside air. During this heat transfer process, the hot refrigerant vapor flow transforms to a relatively warm liquid through condensation and cooling, but remains at high pressure in the Condenser. (The condensing refrigerant undergoes precisely the same physical process that atmospheric water vapor does when it is cooled sufficiently and turns into rain.)

Next, the high pressure condensed refrigerant (which has rejected its heat to the environment) flows through the Expansion Valve and then into the Evaporator which, again, operates at low pressure. The significant drop in pressure experienced by the liquid refrigerant as it flows through the Expansion Valve causes it to "flash" into a low pressure mixture of cold gas and liquid. It is during this process, known as the Joule-Thompson effect, that the refrigerant actually becomes cold.

By operating the Evaporator at a sufficiently low pressure, the temperature of the refrigerant in the Evaporator will drop well below the temperature of the conditioned air space surrounding it. Therefore, the cold refrigerant contained within the Evaporator receives heat from the relatively cool (but warmer) surrounding conditioned air. This surrounding heat, although at relatively low temperature, causes the even colder low pressure liquid refrigerant to actually boil into vapor form in the Evaporator. Having absorbed heat from the cool air conditioned space, the low pressure refrigerant gas then re-enters the Compressor where it is again compressed. This continuously repeating set of processes (the "cycle") literally pumps heat from a lower temperature to a higher temperature thus achieving air conditioning.

Note, however, that this thermal pumping nonetheless relies upon heat flowing in its natural direction: from hot to cold in both heat exchangers. For example, the Evaporator is caused to be colder than the heat contained within the space being air conditioned. Therefore, heat must flow from the warmer conditioned interior to the colder refrigerant in the Evaporator. Likewise, during condensation, the Condenser is made to operate at a temperature sufficiently high to enable heat contained within it to naturally flow to the lower temperature ambient (outside) air.

On the other hand, it is the Compressor that actually causes the heat to be transported from lower temperature air conditioned area to the higher temperature environment. This is achieved by increasing the refrigerant temperature above ambient so its heat can be transferred to the outside and, simultaneously, physically moving the refrigerant through the loop. The Compressor is thus unique among the vapor cycle components: it is the only one that actually receives motive power. The remaining components are passive and have no moving parts. -- this is one reason the Compressor is considered to be the heart of an air conditioner.