Microsoft Word - Cengel and Boles TOC _2-03-05_.doc

It is tempting to judge things on the basis of their quantityinstead of their
qualitysince assessing quality is much more difficult than assessing quan-
tity. However, assessments made on the basis of quantity only (the first law)
may be grossly inadequate and misleading.

6–11 ■ THE CARNOT REFRIGERATOR

AND HEAT PUMP

A refrigerator or a heat pump that operates on the reversed Carnot cycle is
called a Carnot refrigerator,or a Carnot heat pump.The coefficient of
performance of any refrigerator or heat pump, reversible or irreversible, is
given by Eqs. 6–9 and 6–11 as

where QLis the amount of heat absorbed from the low-temperature medium
and QHis the amount of heat rejected to the high-temperature medium. The
COPs of all reversible refrigerators or heat pumps can be determined by
replacing the heat transfer ratios in the above relations by the ratios of the
absolute temperatures of the high- and low-temperature reservoirs, as
expressed by Eq. 6–16. Then the COP relations for reversible refrigerators
and heat pumps become

(6–20)

and

(6–21)

These are the highest coefficients of performance that a refrigerator or a
heat pump operating between the temperature limits of TLand THcan have.
All actual refrigerators or heat pumps operating between these temperature
limits (TLand TH) have lower coefficients of performance (Fig. 6–51).

COPHP,rev

1

1 TL>TH

COPR,rev

1

TH>TL 1

COPR

1

QH>QL 1

¬and¬COPHP

1

1 QL>QH

Chapter 6 | 309

Warm environment

at TH = 300 K

Cool refrigerated space

at TL = 275 K

Reversible

refrigerator

COPR = 11

Irreversible

refrigerator

COPR = 7

Impossible

refrigerator

COPR = 13

FIGURE 6–51

No refrigerator can have a higher COP

than a reversible refrigerator operating

between the same temperature limits.

SEE TUTORIAL CH. 6, SEC. 11 ON THE DVD.

INTERACTIVE

TUTORIAL