ENGR 2250 HW3 solutions

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Rensselaer Polytechnic Institute ENGR 2250 8-2 Refrigerant R-134a is used in a vapor-compression refrigeration cycle. Saturated vapor at 20 psia enters the compressor, which has an isentropic efficiency of 80%, and leaves at 120 psia. Saturated liquid exits the condenser, and saturated vapor exits the evaporator. The mass flow rate is 15 lbm/min. Determine: a. the co ...[Show More]

Rensselaer Polytechnic Institute
ENGR 2250

8-2 Refrigerant R-134a is used in a vapor-compression refrigeration cycle. Saturated vapor at 20 psia enters the compressor, which has an isentropic efficiency of 80%, and leaves at 120 psia. Saturated liquid exits the condenser, and saturated vapor exits the evaporator. The mass flow rate is 15 lbm/min. Determine:

a. the compressor power (in hp)

b. the refrigerating capacity (in tons)

c. the coefficient of performance (COP).

Approach:An energy balance across the compressor can be used to determine its power. Likewise, an energy balance on the evaporator can be used to calculate the input heat transfer rate.

Assumptions:1. The system is steady. 2. Potential and kinetic energy effects are negligible. 3. The compressor is adiabatic. 4. No work is done in the evaporator. 5. The subcooled liquid approximation is valid.

Solution:

a) Apply the energy and mass equations to the compressor. Assume steady, adiabatic, with negligible potential and kinetic energy effects. Therefore,

m m m 1 2 1  

and

From the R-134a tables, Appendices B-14—B-16:

and

To determinewe need to use the definition of isentropic efficiency:

W m h h in    2 1

h P h P 1 1 1 1  ,saturated vapor 101.39Btu lbm    g  

s s T 1 1 1   g   0.2227 Btu / lbm R

h2 C 2 1 1 2 C 1 2  

1 2

s /

s

h h

h h h h

h h

 



    



By interpolation in the superheat vapor region at 120 psia, knowing that s s 2 1  :

h P s 2 2 2 s  , 117.3Btu / lbm  

h2     101.39 101.39 117.3 / 0.80 121.3Btu / lbm  

15 121.3 101.39 298 lbm Btu Btu  

Win          min lbm min

Btu 60min 1hp

298 7.04hp

min 1hr 2544.5Btu / hr

   

     

    Answer

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