1. In this problem you will design a heat pump for residential use. It will include an evaporator, a...

Question

1. In this problem you will design a heat pump for residential use. It will include an evaporator, a compressor, a condenser and a throttling valve. Begin by assuming that the groundwater temperature in Indiana (about 175 feet below the surface) is 56 F year around. Recall that most heat exchangers require about 15 F temperature difference to operating efficiently. Since groundwater will serve as the energy source for the heat pump working fluid that sets an upper bound on the working fluid temperature of 40 F. Now assume that the warm air flowing through back into rooms within the house will be at 90 F. Since the warm air receives energy from the heat pump working fluid while in the condenser that working fluid will have to be at a temperature no lower than 105 F. Go to the thermodynamic tables you have and find the saturation pressures for R22, R134a and NH3 at the condenser and evaporator temperatures. Which refrigerant would you choose? Why? Now assume that the compressor in your system is isentropic, that it’s inlet is a saturated vapor, and that the condenser outlet is a saturated liquid. Compute the COP for your system. Finally, assume that your compressor actually has an isentropic efficiency of 90%. Recompute the COP. Now compute the entropy generation in the actual compressor and compare it to that of the throttling valve. What do you conclude about the choice of throttling valve instead of turb-expander?
	
Document Preview: ME 200 Summer 2012 
Homework 24 
Assigned Monday 23 Jul 2012 
Due Tuesday 24 Jul 2012 @730 am in Room 1130 ME 
 
1. In this problem you will design a heat pump for residential use.  It will include an evaporator, 
a compressor, a condenser and a throttling valve. 
 
Begin by assuming that the ground water temperature in Indiana (about 175 feet below the 
surface) is 56 F year around.  Recall that most heat exchangers require about 15 F 
temperature difference to operate efficiently.  Since ground water will serve as the energy 
source for the heat pump working fluid that sets and upper bound on the working fluid 
temperature of 40 F. 
 
Now assume that the warm air flowing through back into rooms within the house will be at 
90 F.  Since the warm air receives energy from the heat pump working fluid while in the 
condenser that working fluid will have to be at a temperature no lower than 105 F. 
 
Go to the thermodynamic tables you have and find the saturation pressures for R22, R134a 
and NH at the condenser and evaporator temperatures.  Which refrigerant would you 
3
choose?  Why? 
 
Now assume that the compressor in your system is isentropic, that it’s inlet is a saturated 
vapor, and that the condenser outlet is a saturated liquid.  Compute the COP for your 
system. 
 
Finally, assume that your compressor actually has an isentropic efficiency of 90%.  
Recompute the COP.  Now compute the entropy generation in the actual compressor 
and compare it to that of the throttling valve.  What do you conclude about the choice 
of throttling valve instead of turb-expander?

Robert · Accepted Answer

Solution:
We have, 
Evaporator temperature = 40 F 
Condenser temperature = 105 F 
Saturated pressures at the given evaporator and condenser temperatures can be found from the 
thermodynamic table for different refrigerants as: 
R22:
R134a:
NH3:
56 F 
90 F 
105 F  40 F 
We should choose R134a as the refrigerant because it has the lowest pressure difference (100 psia) for the 
same temperature difference between the evaporator and condenser. As a result of low pressure 
difference, we will need less compressor power to maintain the desired processes in the heat pump. Also, 
from the definition of coefficient of performance of heat pump,

1. In this problem you will design a heat pump for residential use. It will include an evaporator, a compressor, a condenser and a throttling valve. Begin by assuming that the groundwater temperature...

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