Heat Pump Cycle

As far as heat pump cycles are concerned it is basically the conceptual and mathematical models of heat pumps and refrigerators. Heat pumps are basically devices that move heat from a lower pressure zone to a high pressure zone with the help of a mechanical work or a high temperature heat source. In a typical HVAC unit, a heat pump usually uses reversible vapor-compression refrigeration cycle for thermal energy transfer. However, in case the heat pumps stops functioning, it is important to have an idea of its operating cycles both in the heating and cooling stages. This is because it helps the mechanic to detect the exact problem and fix it quickly.

During the cooling cycle, the operation of the heat pump is similar to that of a standard cooling system. The steps involved include the following:

• The compressor pumps out the high-pressure, superheated refrigerant vapor

• The vapor then leaves the compressor and passes through the reversing valve. The vapor then flows through the outdoor vapor line to the finned outdoor coil. Air from the outdoor fan removes heat from the refrigerant vapor.

• Then the warm high pressure liquid leaves the outdoor coil and passes through the copper refrigerant liquid line. At the end of the liquid line the refrigerant passes through a metering device reducing its pressure and temperature in the process.

• As the liquid, which is under reduced pressure enters the indoor coil service it expands and absorbs heat from the indoor air passing over the finned surface. Heat from the indoor air causes the low pressure liquid to evaporate, cooling the indoor air in the process.

It needs to be mentioned here that the electrical cooling cycle is also similar to the standard cooling system.

On the other hand, the heating cycle of heat pumps is more or less similar to that of the cooling cycles, the only difference lies in the position of the reversing valve that reverses the refrigerant flow.

It is important to note here that setting the thermostat to heating mode automatically powers the solenoid valve in the reversing valve. The other processes are similar to that of the above mentioned cooling cycle the only difference being it makes the indoor air warm. The heating electrical system is similar to the cooling cycle.

Now let us have a look at the mechanical mode of the defrost cycle. In the heating mode, the outdoor coil acts as the evaporator. Moisture from the outside air condenses on the cooler coil and normally runs off. While during the colder part of the heating season, the moisture freezes and blocks air movement through the coil and the frost gets removed in the defrost cycle.

On the other hand in the electrical defrost cycle, a defrost control must be present to detect the present of ice on the outdoor coil and when the ice will be removed. There are different kinds of defrost controls available and the method of detection is different from one another. However, the action they take in case of ice accumulation is the same. It is important that these defrost control determines when the ice is gone and terminates the process of defrosting.




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