Air Con Past Exam Questions

School of Mechanical and Chemical Engineering
University of Western Australia
Thermofluids 3 MECH3401

Past exam questions (from TF3, TF306/312/311)
2006
An air-handling unit (AHU) for a cinema receives outside air at 30
o
C Φ=60% at a total
mass flow rate of 0.8kg/s, and return air from within the cinema at 27
o
C Φ=75% at a total
mass flow rate of 1.2 kg/s. The unit supplies air at 14
o
C Φ=100%. Pressure throughout
the system is 109kPa. How much water is condensed per hour, and how much heat
leaves the AHU?

2005
1. A cooling tower receives 20 l/s of warm process water at 44
o
C. Air enters the tower at
34
o
C, with a relative humidity of 25%, and leaves saturated at 40
o
C. Air pressure is
101kPa throughout.
(i) If the cooling tower removes 1575kW from the process water, and neglecting the
proportion lost to evaporation, show that the cooled process water leaves the tower at ≈
25
o
C.
(ii) Calculate the air mass flowrate through the tower, and the percentage of the process
water lost to evaporation.
Show all your working clearly, and state any assumptions that you make.



2004
A refrigeration plant using Ammonia as the refrigerant produces chilled water at 5
o
C.
The evaporator operates 10 degrees below the chilled water temperature. The
compressor (isentropic efficiency 85%) is driven by a nominal 25kW electric motor,
which is 90% efficient. Refrigerant enters the compressor superheated by 10
o
C. The
condenser, which operates at 2 MPa, is cooled by a water circuit which uses a cooling
tower to reject heat to the atmosphere. Air enters the cooling tower at 32
o
C, with a wet-
bulb temperature of 17
o
C, and leaves the cooling tower at 41
o
C, with a relative humidity
of 40%. The mass flowrate of dry air through the cooling tower is 2.33 kg/s. Cooling
water from the condenser arrives at the cooling tower at 60
o
C, 1.2 kg/s. Pressure
differences in the cooling tower are negligible; atmospheric pressure is 101.3 kPa. Cp for
air is 1.005 kJ/kgK
(i) Calculate the temperature of the water leaving the cooling tower, and hence
the heat being removed from the cooling water (and therefore the condenser).
[7]
(ii) Using the electric motor power and compressor efficiency information,
calculate the refrigerant flowrate.
[5]
(iii) Use the heat lost to the condenser cooling water to calculate the temperature
and enthalpy of the refrigerant leaving the condenser.
[5]
(iv) Calculate the CoP of the refrigeration cycle.
[3]


2003.
(a) Explain briefly why air entering an air conditioning system is often cooled below
the delivery temperature required, then reheated. (4 marks)

An air conditioning system consists of a refrigerated heat exchanger to cool the
incoming air, a mixing chamber in which the cooled fresh air is mixed with recycled
air, and a heater to raise the temperature of the resulting mix to the required value.
Fresh air enters the unit with a relative humidity of 60%, and the cooling coil
condenses out moisture at a rate of 0.003 kg/s, before the air is delivered to the
mixing chamber at a volume flow rate of 20 m
3
/minute.
In the mixing chamber, the cooled fresh air is mixed with recycled air, which has a
temperature of 30
o
C and partial water vapour pressure of 2.12 kPa. The mixed air
flow is delivered to the heater at a temperature of 17
o
C and a specific humidity of
0.0098 kg/kg dry air, and leaves the system at 25
o
C. Pressure change through the
system is negligible.

(b) Show the processes involved on a psychrometric chart, clearly labelling each
point. (8 marks)
(c) Calculate the ratio of fresh to recycled (dry) air mass flow rates entering the
mixing chamber. (6 marks)
(d) Determine the dry bulb and wet bulb temperatures of the fresh air entering the
air conditioner. (6 marks)
(e) What is the air’s relative humidity as it leaves the system, and how much heat is
input in the heater? (6 marks)


2002.
The city of La Paz in Bolivia lies at an altitude of 3650m above sea level. Average
atmospheric pressure is 62.3 kPa, average daytime temperature is 8.8
o
C, and average
relative humidity is 55%. The power station at La Paz airport uses a Baltimore Aircoil
3501A cooling tower; water coming from the condenser cooling circuit enters the top of
the tower at 144 litres/s at 36
o
C. Moist air at average ambient conditions enters the
bottom of the tower and exits at the top at 12
o
C with a relative humidity of 100%. The
change in air pressure between inlet and outlet is negligible.
(a) If the evaporative cooling process in the tower loses 4% of the incoming water
stream, what is the temperature of the cold water leaving the bottom of the tower?
(15 marks)
(b) Under these conditions, what is the cooling capacity of the tower in tons? The
rated capacity of the tower at normal ambient conditions (i.e. sea level, 21
o
C wet-
bulb) is 500 tons: why is it operating so far from this point in La Paz?
(8 marks)
(c) What is the volume flow rate of dry air leaving the tower? (2 marks)
[(a) ≈ 8.5
o
C (b) 4750 tons, because the incoming air is very cold and has a very low ω
(c) 719.1 kg/s]

2001
An air-conditioning system draws in 100 m
3
/min of ambient air at 101kPa, 38
o
C and 30%
relative humidity. Half of the incoming air is diverted into a refrigerated section where it
is cooled to 5
o
C. Once cooled, it is mixed adiabatically with the other half of the ambient
air and introduced into a room. Assuming that pressure changes throughout are
negligible, determine:
(i) how much water is condensed in the cooled section per minute; (7 marks)
(ii) how much heat is removed from the cooled section; (8 marks)
(iii) the temperature and relative humidity of the air entering the room. (6 marks)
(iv) If all of the ambient air had been cooled to the room entry condition by direct
refrigeration, use the psychrometric chart to estimate the amount of heat that
would have had to be removed. (4 marks)
[(i) 0.405 kg/min (ii) 48 kW from chart, 40 kW if you do it longhand (iii) 21.5
o
C, 55% (iv)
48kW]


2000
1. An air conditioning system receives 1.3 m
3
/s of air at 38
o
C and 100 kPa with a
specific humidity of 0.014 kg/kg dry air, and passes it through an evaporative cooler.
The saturated air leaving the evaporative cooler is then passed through a refrigerated
section during which moisture is removed. The air is then heated to give an outlet
condition of 25
o
C, Φ = 50%.
(a) Show the path followed by the air on a sketch of the psychrometric chart. (4
marks)
(b) What is the minimum temperature (to the nearest 0.5
o
C) reached in the
evaporative cooler, and what is the minimum temperature that the air is cooled to
throughout the whole system? (4 marks)
(c) Assuming that the air exiting the evaporative cooling section is saturated, and that
any water condensed leaves the system at the minimum temperature, calculate the
amount of heat removed in the refrigerated section. Comment on the accuracy of any
values that you have used from the psychrometric chart. (12 marks.)
[(b) 24.5
o
C, 14
o
C (c) 50.2 kW or 50.8kW depending on your method]


1999
In part of an air conditioning system, two air streams are mixed in an adiabatic mixing
chamber before being passed into an evaporative cooler. The conditions of the two flows
being mixed are: 50 m
3
/min, 20
o
C, Φ = 20%, and 30 m
3
/min, 40
o
C,
ω = 0.0148 kg/kg. The air leaves the evaporative cooler at 22
o
C and 55% saturation.
(a) Using the psychrometric chart, determine the temperature, relative humidity and
specific humidity of the mixed air stream as it enters the evaporative cooler. (10
marks)
(b) Determine the rate at which water must be supplied to the evaporative cooler to give
the outlet conditions specified above.(4 marks)
(c) If a conventional heat exchanger were used to cool the air to 22
o
C instead of the
evaporative cooler, what would be the required cooling power? (4 marks)
(d) What is the lowest temperature that the mixture could be cooled to in an evaporative
cooler? (2 marks)
[(a) 27
o
C; φ = 32%; ω = 0.0072 kg/kg (b) 0.186 kg/min (c) 7.96 kW (d) 16.4
o
C]

1998
An air conditioning system is used to cool a laboratory. Air at a rate of 800 m
3
/min at
32
o
C, 101 kPa and Φ = 80% is first chilled to 10
o
C to remove moisture, and then heated
until Φ = 40%
Find:
(a) the heat removed in the cooling section per minute;
(b) the heat added in the heating section per minute;
(c) the rate of condensate removal.

In order to save energy, it was later decided to recirculate some of the return air from the
laboratory and mix it with atmospheric air before entering the chiller. If the mass flows
of recirculating air and atmospheric air are equal, recalculate (a) and (c) above assuming
that the air leaves the laboratory at 25
o
C and Φ = 50% and that the total mass flow of air
entering the chiller is the same as before. (The psychrometric chart may be used if
desired.)

acrt March 2011