Consider a household refrigerator that uses HFC-134a as

Chapter 4, Problem 30P

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PROBLEM 30P

Consider a household refrigerator that uses HFC-134a as the refrigerant, operating between the pressures of 1.0 bar and 10 bars.

(a) The compression stage of the cycle begins with saturated vapour at 1 bar and ends at 10 bars. Assuming that the entropy is constant during compression, find the approximate temperature of the vapour after it is compressed. (You’ll have to do an interpolation between the values given in below Table 1)

(b) Determine the enthalpy at each of the points 1, 2, 3, and 4, and calculate the coefficient of performance. Compare to the COP of a Carnot refrigerator operating between the same reservoir temperatures. Does this temperature range seem reasonable for a household refrigerator? Explain briefly.

(c) What fraction of the liquid vaporizes during the throttling step.

Table 1: Properties of superheated (gaseous) refrigerant HFC-134a. All values are for 1 kg fluid, and are measured relative to the same reference state as in Table 2. Excerpted from Moran and Shapiro 1995).

Table 2: Properties of the refrigerant HFC-134a under saturated conditions (at its boiling point for each pressure). All values are for 1 kg of fluid, and are measured relative to an arbitrarily chosen refrence state, the saturated liquid at –40°C. Excerpted from Moran and Shapiro (1995).

P (bar)

T (°C)

Hliquid (kJ)

Hgas (kJ)

Sliquid (kJ/K)

Sgas (kJ/K)

1.0

−26.4

16

231

0.068

0.940

1.4

−18.8

26

236

0.106

0.932

2.0

−10.1

37

241

0.148

0.925

4.0

   8.9

62

252

0.240

0.915

6.0

  21.6

79

259

0.300

0.910

8.0

  31.3

93

264

0.346

0.907

10.0

  39.4

105

268

0.384

0.904

12.0

  46.3

116

271

0.416

0.902

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