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of the outside air (da); during this process the heat Q is exchanged. In
c
practice, of course, this same air does not enter the engine again but, since
an equivalent amount does enter, we may consider the process to be
cyclic.

2.10 The Refrigerator

An important characteristic of an ideal Carnot engine is that it is
reversible, i.e. we could drive it backward so that it removes energy from
some reservoir at a lower temperature and rejects energy to a second reservoir
at a higher temperature (Fig. 2.10.1).

A refrigerator may be considered to be a heat engine operated in reverse.
A heat engine takes in heat from a hot reservoir, converts a part of the heat
into mechanical work output, and

rejects the difference as heat to a cold
reservoir. A refrigerator (fig.2.10.1) ,
however, takes in heat Q from a cold
2
reservoir, the compressor supplies
mechanical work input, and heat Q is
1
rejected to a hot reservoir. With
reference to the ordinary home
refrigerator, the food and ice cubes

constitute the cold reservoir, work is
done by the electric motor, and the hot
reservoir is the air in the kitchen.

The operating principle of the

Figure 2.10.1 refrigerator is also used in the heat
pump, a device which heats a house by
cooling the outside air.

From an economic point of view, the best refrigeration cycle is one that
removes the greatest amount of heat from the refrigerator, for the least
expenditure of mechanical work . We, therefore, define the performance

coefficient (rather than the efficiency) of a refrigerator as such a ratio:

Q
   2 (2.10.1)
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