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

636 | Thermodynamics

EXAMPLE 11–6 Cooling of a Canned Drink

by a Thermoelectric Refrigerator

A thermoelectric refrigerator that resembles a small ice chest is powered by

a car battery and has a COP of 0.1. If the refrigerator cools a 0.350-L

canned drink from 20 to 4°C in 30 min, determine the average electric

power consumed by the thermoelectric refrigerator.

Solution A thermoelectric refrigerator with a specified COP is used to cool

canned drinks. The power consumption of the refrigerator is to be determined.

Assumptions Heat transfer through the walls of the refrigerator is negligible

during operation.

Properties The properties of canned drinks are the same as those of water

at room temperature, r1 kg/L and c4.18 kJ/kg · °C (Table A–3).

Analysis The cooling rate of the refrigerator is simply the rate of decrease of

the energy of the canned drinks,

Then the average power consumed by the refrigerator becomes

Discussion In reality, the power consumption will be larger because of the

heat gain through the walls of the refrigerator.

W

#

in

Q

#

cooling

COPR



13 W

0.10

130 W

Q

#

cooling

Qcooling

¢t



23.4 kJ

30 60 s

0.0130 kW13 W

Qcoolingmc ¢T 1 0.350 kg 21 4.18 kJ>kg#°C 2120  42 °C23.4 kJ

mrV 1 1 kg>L 21 0.350 L 2 0.350 kg

The transfer of heat from lower temperature regions to higher

temperature ones is called refrigeration. Devices that produce

refrigeration are called refrigerators,and the cycles on which

they operate are called refrigeration cycles. The working flu-

ids used in refrigerators are called refrigerants. Refrigerators

used for the purpose of heating a space by transferring heat

from a cooler medium are called heat pumps.

The performance of refrigerators and heat pumps is

expressed in terms of coefficient of performance(COP),

defined as

COPHP

Desired output

Required input



Heating effect

Work input



QH

Wnet,in

COPR

Desired output

Required output



Cooling effect

Work input



QL

Wnet,in

SUMMARY

The standard of comparison for refrigeration cycles is the

reversed Carnot cycle. A refrigerator or heat pump that oper-

ates on the reversed Carnot cycle is called a Carnot refrigera-

toror a Carnot heat pump, and their COPs are

The most widely used refrigeration cycle is the vapor-

compression refrigeration cycle. In an ideal vapor-compression

refrigeration cycle, the refrigerant enters the compressor as a

saturated vapor and is cooled to the saturated liquid state in

the condenser. It is then throttled to the evaporator pressure

and vaporizes as it absorbs heat from the refrigerated space.

COPHP,Carnot

1

1 TL >TH

COPR,Carnot

1

TH>TL 1

• 
  
  
  
  • 
    
  
  
  
  

Heat
rejected

Heat

absorbed

FIGURE 11–27

When a current is passed through the

junction of two dissimilar materials,

the junction is cooled.

WARM

environment

Refrigerated

space

Hot plate

Cold plate

QH

QL

• 
  
  
  
  • I
  
  
  
  

pn pn pn

FIGURE 11–28

A thermoelectric refrigerator.