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COURSE:

PROBLEM SET:

ME351: Engineering Thermodynamics

Class Example 01

DATE:

PREPARED BY:

03/23/11

Richard Ayala

An ideal vapor-compression refrigeration cycle operates at steady state with refrigerant 134a as the working fluid. Saturated vapor enters the compressor at 2 bar, and saturated liquid exits the condenser at 8 bar. The mass flow rate of refrigerant is 7 kg/min. Determine (a) (b) (c)

The compressor power, in kW The refrigerating capacity, in tons The coefficient of performance

Solution: States: 1

At the compressor inlet, P1 = 2 bar, saturated vapor (x = 1.00) From Table A-11: h1 = 241.30 kJ/kg,

2

At the compressor exit, P2 = 8 bar and for isentropic flow through the compressor, s2 = s1 = 0.9253 kJ/kg-K Noting that s2 > sg @ 8 bar, the refrigerant exiting the compressor is superheated From Table A-12: Ta = 31°C ha = 264.15 kJ/kg sa = 0.9066 kJ/kg-K ° Ta = 40 C hb = 273.66 kJ/kg sb = 0.9374 kJ/kg-K Interpolating:

h2 s  ha  ( s2  sa ) 3

s1 = 0.9253 kJ/kg-K

hb  ha = 269.92 kJ/kg sb  sa

At the exit of the condenser, the working fluid is saturated liquid at the condenser pressure From Table A-11: h3 = hf = 93.42 kJ/kg

4

Across the throttling valve the enthalpy is constant: h4 = h3 = 93.42 kJ/kg

Cycle Analysis: (a)

Compressor power:  ( h2 s  h1 ) = 3.34 kW W c  m

(b)

Refrigeration capacity:  m  ( h1  h4 ) = 4.91 tons Q in

(c)

The coefficient of performance for the ideal cycle is

Q h h   in  1 4 = 5.17 Wc h2 s  h1

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