Heating and Air Conditioning

Heating and Air Conditioning

Clausius Statement of the Second LawAccording to the second law of thermodynamics, many thermodynamic processes proceed naturally in one direction but not the opposite. For example, when a temperature difference does exist heat flows spontaneously from the warmer system to the colder system, never the reverse. In fact, such heat flow (from a colder body to a warmer system) would not violate the first law of thermodynamics, i.e. energy would be conserved. But it doesn’t happen in nature.

Directions of thermodynamic processes are subject of the second law of thermodynamics, especially of the Clausius Statement of the Second Law, which states:

“It is impossible to construct a device which operates on a cycle and whose sole effect is the transfer of heat from a cooler body to a hotter body”.

Heat cannot spontaneously flow from cold system to hot system without external work being performed on the system. This is exactly what refrigerators and heat pumps accomplish. In a refrigerator, heat pump or air conditioner, heat flows from cold to hot, but only when forced by an external work, these devices are driven by electric motors requiring work from their surroundings to operate. There is no difference between thermodynamics of refrigerators and heat pumps. Both work by moving heat from a cold space to a warm space.

See also: What is Temperature

Heat Pump – Operating Principle

Heat Pump - Heating and Air Conditioning

The term heat pump is usually reserved for a device that can heat a house in winter by using an electric motor that does work W to take heat Qcold from the outside at low temperature and delivers heat Qhot to the warmer inside of the house.

The operating principle of refrigeratorsair conditioners, and heat pumps is the same and it is just the reverse of a heat engine. In general, a heat pump is a device that transfers heat energy from a heat source to a “heat sink“, but in this case the transfer occurs in the opposite direction of spontaneous heat transfer by absorbing heat from a cold space and releasing it to a warmer one. As diagrammed in the figure, by doing external work W, heat is taken from a low-temperature region (heat source) and a greater amount of heat is exhausted at a higher temperature (heat sink).

The most widely used thermodynamic cycle or method for heating, air-conditioning, refrigerators and heat pumps is the vapor compression cycle.

See also:

Heating and Air Conditioning