Page 268 - Schaum's Outline of Theory and Problems of Applied Physics

CHAP. 21] THERMODYNAMICS 253

High-temperature reservoir Expansion valve

T 2

Heat Condenser
exhaust Q 2
Evaporator

Refrigerator Energy
input
W
Heat
input Q 1
Refrigerant vapor
Compressor
Refrigerant liquid
T 1
Low-temperature reservoir Fig. 21-10. (From Modern Technical Physics, 6th Ed., Arthur
Beiser, c 1992. Reprinted by permission of Pearson Education,
Fig. 21-9 Inc.)

3. The liquid refrigerant now goes into the expansion valve, from which it emerges at a lower pressure and
temperature.
4. In the evaporator the cool liquid refrigerant absorbs heat from the storage chamber and vaporizes.
Farther along in the evaporator the refrigerant vapor absorbs more heat and becomes warmer. The warm
vapor then goes back to the compressor to start another cycle.

In Step 4 of this cycle, heat is extracted from the storage chamber by the refrigerant. In Step 1, work
is done on the refrigerant by the compressor. In Step 2, heat from the refrigerant leaves the system. A re-
frigerator might remove two or more times as much heat from its storage chamber as the amount of work
done.
If a refrigerator absorbs heat Q 1 at the absolute temperature T 1 and ejects heat Q 2 at the absolute temperature
T 2 , its coefficient of performance (CP) is given by
heat absorbed Q 1 Q 1
CP = = =
work done W Q 2 − Q 1
In an ideal refrigerator, which is a Carnot engine run backward, Q 1 /Q 2 = T 1 /T 2 and

T 1
CP(ideal) =
T 2 − T 1

SOLVED PROBLEM 21.13
In an effort to cool a kitchen during the summer, the refrigerator door is left open and the kitchen’s door
and windows are closed. What will happen?
Since no refrigerator can be completely efficient, more heat is exhausted by the refrigerator into the kitchen
than is extracted from the kitchen. The net effect, then, is to increase the kitchen’s temperature.

SOLVED PROBLEM 21.14
A 1-kW refrigerator whose coefficient of performance is 2.0 takes heat from a freezer compartment at
◦
◦
−20 C and exhausts it at 40 C. How does its CP compare with that of an ideal refrigerator?