(a) To determine The power input required to the refrigerator. Explanation Sketch the T − s diagram for the refrigeration system as in Figure (1). Write the relation between specific enthalpies at inlet and exit of a throttling process. h 3 = h 4 (I) Here, the specific enthalpy at the inlet of throttling is h 3 and the specific enthalpy at the exit of throttling is h 4 . Write the formula to calculate the mass flow rate of the refrigerant from volume flow rate. m ˙ = V ˙ 1 v 1 (II) Here, volume flow rate of refrigerant at compressor inlet is V ˙ 1 , and the specific volume of refrigerant at compressor inlet is v 1 . Write the formula to calculate the power input to the compressor ( W ˙ in ) from isentropic efficiency of the compressor. W ˙ in = m ˙ ( h 2 s − h 1 ) η C (III) Here, the specific enthalpy of refrigerant at the isentropic compressor exit is h 2 s , specific enthalpy of refrigerant compressor inlet is h 1 , and the isentropic efficiency of compressor is η C . Conclusion: From the Table A-13 of “Superheated refrigerant R-134a”, obtain the properties of refrigerant at compressor inlet pressure ( P 1 ) of 100 kPa and compressor inlet temperature ( T 1 ) of − 20 ° C as follows: h 1 = 239.52 kJ / kg s 1 = 0.97215 kJ / kg ⋅ K v 1 = 0.19841 m 3 / kg Here, the specific entropy of the refrigerant at compressor inlet is s 1 . The specific entropy at the end of isentropic compression ( s 2 s ) is calculated as s 2 s = s 1 = 0.97215 kJ / kg ⋅ K Refer to Table A-13, “Superheated R-134a”, and obtain the values of R-134a at pressure of 0 .8 MPa and specific entropy of 0.97215 kJ / kg ⋅ K by using interpolation method. Write the formula of interpolation method of two variables. y 2 = ( x 2 − x 1 ) ( y 3 − y 1 ) ( x 3 − x 1 ) + y 1 (IV) Here, the variables denoted by x and y are specific entropy and specific enthalpy. Show temperature and enthalpy values from the Table A-13. Entropy ( s 2 ) , in kJ / kg ⋅ K Enthalpy ( h 2 ) , in kJ / kg 0.9481 276.46 0.97215 ? 0.9803 285.71 Substitute 0.9481 for x 1 , 0.97215 for x 2 , 0.9803 for x 3 , 276.46 for y 1 , and 285 (b) To determine The heat removal rate from the refrigerated space. (c) To determine The pressure drop between the evaporator and the compressor. The rate at which heat gained between the evaporator and the compressor.

8th Edition

ISBN: 8220102809444

Author: CENGEL

Publisher: YUZU

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Question

Chapter 11.10, Problem 19P

(a)

To determine

The power input required to the refrigerator.

(b)

To determine

The heat removal rate from the refrigerated space.

(c)

To determine

The pressure drop between the evaporator and the compressor.

The rate at which heat gained between the evaporator and the compressor.

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Chapter 11.10, Problem 18P

Chapter 11.10, Problem 21P

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Chapter 11 Solutions

EBK THERMODYNAMICS: AN ENGINEERING APPR

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