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

11–1 ■ REFRIGERATORS AND HEAT PUMPS

We all know from experience that heat flows in the direction of decreasing

temperature, that is, from high-temperature regions to low-temperature ones.

This heat-transfer process occurs in nature without requiring any devices.

The reverse process, however, cannot occur by itself. The transfer of heat

from a low-temperature region to a high-temperature one requires special

devices called refrigerators.

Refrigerators are cyclic devices, and the working fluids used in the refrig-

eration cycles are called refrigerants.A refrigerator is shown schematically

in Fig. 11–1a. Here QLis the magnitude of the heat removed from the refrig-

erated space at temperature TL,QHis the magnitude of the heat rejected to

the warm space at temperature TH,and Wnet,inis the net work input to the

refrigerator. As discussed in Chap. 6,QLand QHrepresent magnitudes and

thus are positive quantities.

Another device that transfers heat from a low-temperature medium to a

high-temperature one is the heat pump.Refrigerators and heat pumps are

essentially the same devices; they differ in their objectives only. The objec-

tive of a refrigerator is to maintain the refrigerated space at a low tempera-

ture by removing heat from it. Discharging this heat to a higher-temperature

medium is merely a necessary part of the operation, not the purpose. The

objective of a heat pump, however, is to maintain a heated space at a high

temperature. This is accomplished by absorbing heat from a low-temperature

source, such as well water or cold outside air in winter, and supplying this

heat to a warmer medium such as a house (Fig. 11–1b).

The performance of refrigerators and heat pumps is expressed in terms of

the coefficient of performance(COP), defined as

(11–1)

(11–2)

These relations can also be expressed in the rate form by replacing the

quantities QL,QH, and Wnet,inby Q

.

L,Q

.

H, and W

.

net,in, respectively. Notice that

both COPRand COPHPcan be greater than 1. A comparison of Eqs. 11–1

and 11–2 reveals that

(11–3)

for fixed values of QLand QH. This relation implies that COPHP
1 since

COPRis a positive quantity. That is, a heat pump functions, at worst, as a

resistance heater, supplying as much energy to the house as it consumes. In

reality, however, part of QHis lost to the outside air through piping and

other devices, and COPHPmay drop below unity when the outside air tem-

perature is too low. When this happens, the system normally switches to the

fuel (natural gas, propane, oil, etc.) or resistance-heating mode.

The cooling capacityof a refrigeration system—that is, the rate of heat

removal from the refrigerated space—is often expressed in terms of tons of

refrigeration.The capacity of a refrigeration system that can freeze 1 ton

(2000 lbm) of liquid water at 0°C (32°F) into ice at 0°C in 24 h is said to be

COPHPCOPR 1

COPHP

Desired output

Required input



Heating effect

Work input



QH

Wnet,in

COPR

Desired output

Required input



Cooling effect

Work input



QL

Wnet,in

608 | Thermodynamics

WARM

house

WARM

environment

COLD

refrigerated

space

COLD

environment

(a) Refrigerator (b) Heat pump

QH

(desired

output)

R HP

QH

QL

(desired

output)

QL

Wnet,in

(required

input)

Wnet,in

(required

input)

FIGURE 11–1

The objective of a refrigerator is to
remove heat (QL) from the cold
medium; the objective of a heat pump
is to supply heat (QH) to a warm
medium.

SEE TUTORIAL CH. 11, SEC. 1 ON THE DVD.

INTERACTIVE

TUTORIAL