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

The regenerative gas cycle is shown in Fig. 11–19. Regenerative cooling

is achieved by inserting a counter-flow heat exchanger into the cycle. With-

out regeneration, the lowest turbine inlet temperature is T 0 , the temperature

of the surroundings or any other cooling medium. With regeneration, the

high-pressure gas is further cooled to T 4 before expanding in the turbine.

Lowering the turbine inlet temperature automatically lowers the turbine exit

temperature, which is the minimum temperature in the cycle. Extremely low

temperatures can be achieved by repeating this process.

630 | Thermodynamics

1

5 4 WARM

environment

Compressor

Wnet,in

Turbine

Q

Heat

exchanger

Heat

exchanger

2

3

6

Regenerator

4

2

1

T

s

5

QH

QL

3

6

COLD

refrigerated space

FIGURE 11–19

Gas refrigeration cycle with regeneration.

EXAMPLE 11–5 The Simple Ideal Gas Refrigeration Cycle

An ideal gas refrigeration cycle using air as the working medium is to maintain

a refrigerated space at 0°F while rejecting heat to the surrounding medium at

80°F. The pressure ratio of the compressor is 4. Determine (a) the maximum

and minimum temperatures in the cycle, (b) the coefficient of performance,

and (c) the rate of refrigeration for a mass flow rate of 0.1 lbm/s.

Solution An ideal gas refrigeration cycle using air as the working fluid is

considered. The maximum and minimum temperatures, the COP, and the

rate of refrigeration are to be determined.

Assumptions 1 Steady operating conditions exist. 2 Air is an ideal gas with

variable specific heats. 3 Kinetic and potential energy changes are negligible.

Analysis The T-s diagram of the gas refrigeration cycle is shown in

Fig. 11–20. We note that this is an ideal gas-compression refrigeration

cycle, and thus, both the compressor and the turbine are isentropic, and the

air is cooled to the environment temperature before it enters the turbine.

3

2

1

T, °F

s

4

QH

QL

Tmax

Tmin

80

0

·

·

FIGURE 11–20

T-sdiagram of the ideal-gas
refrigeration cycle described in
Example 11–5.