ME321 (Spring 08) - Take-home Quiz 2 ---- Name________________________
Recall Take-home-Quiz
1 in which we evaluated an air-conditioning system for
a very warm environment that was required to reduce the temperature
of air flowing through an insulated duct from 35°C to 10°C.
Encouraged by the results of this evaluation the designers of
this system decided to evaluate the increased capacity that this
sytem could attain by inserting an internal heat exchanger as
shown in the following flow diagram:
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:
- .Determine the mass flow rate of the refrigerant
in the system. Note that the volumetric flow rate of the refrigerant
(0.6 liters/minute) is measured at the outlet of the condenser.
This value should be the same as that obtained previously.
- Using this value of mass flow rate determine
the power required to drive the compressor [kW]. Notice that
the inlet and outlet temperatures of the compressor have substantially
increased due to the addition of the internal heat exchanger.
- 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 transferred from the air
blowing through the insulated cooling duct to the evaporator
[kW]. Compare this value to that obtained without the internal
heat exchanger, and determine the percentage increase in air-conditioning
capacity enabled by adding the internal heat exchanger.
- 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).
- 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 35°C to 10°C at the outlet
of the duct.
- 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.
- Discuss the advantages and disadvantages
of adding the internal heat exchanger to the air-conditioning
system.
_______________________________________________________________________
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)______________________
