ME321 (Fall 07) - Take-home Quiz 2 ---- Name________________________

We wish to do a preliminary thermodynamic evaluation of a proposed 500W input power home air conditioner/hot water heater system using refrigerant R134a. Consider the following system flow diagram

This unique combined air conditioning / hot water heating system is designed to absorb heat from the 32°C air flowing through the insulated duct in order to pump heat into the hot water heating tank. The fan provides enough air flow over the evaporator to cool the air to 15°C at the outlet of the duct, and the hot water is heated to a maximum of 50°C. In this analysis we neglect the power provided to the fan. We also assume that both the duct and the hot water tank are adiabatic.

Neatly sketch the complete cycle above on the P-h diagram provided below, showing clearly all six processes {(1) - (1x) - (2) - (3) - (3x) - (4) - (1)} and use the R134a refrigerant property tables in order to determine the following:

• .Assuming that the compressor is adiabatic, determine the mass flow rate of the refrigerant in the system.
  
• Determine the heat rejected by the condenser. Assuming that all this heat is absorbed by the water in the hot water tank, determine the time taken for 100 liters of water at 32°C to reach the required temperature of 50°C.
  
• Determine the heat transferred in the internal heat exchanger, assuming it to be externally adiabatic, and the temperature of the subcooled liquid entering the throttle (3x).
  
• Determine the heat absorbed by the evaporator. Assuming that all this heat is absorbed from the air in the duct and neglecting the fan power, determine the required mass flow rate of the inlet air at station (5) in order reduce the air temperature from 32°C to 15°C at the outlet of the duct.
  
• Determine the Coefficient of Performance of the hot water heater (COPHW) (defined as the heat rejected by the condenser divided by the work done on the compressor).
  
• Determine the Coefficient of Performance of the air conditioner (COPAC) (defined as the heat absorbed by the evaporator divided by the work done on the compressor).
  
• Consider the compressor only in the diagram above. Using the conditions shown, neatly sketch the compression process (1x) - (2) on the h-s diagram provided below. Show also the equivalent isentropic process on the sketch, and indicate the actual compressor specific work as well as the isentropic compressor specific work on the h-s diagram. Using R134a tables, determine the compressor adiabatic efficiency hC.
  
• Is this a suitable and practical system for hot climates? Discuss.

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Declaration of Honesty - I hereby declare that this take-home quiz is my own work, and that I did not collaborate with or receive help from anyone else while doing it.

(Signed)___________________________ (Date)______________________