SCHOOL of ENGINEERING
BENG3 THERMAL & FLUIDS ENGINEERING (THER305)
TUTORIAL EXAMPLES in AIR CONDITIONING & COOLING TOWERS
Use the psychrometric chart for pair = 101.325 kPa
1.
In an air conditioning system air at 32°C
is drawn into a building with a percentage saturation of 66.5 %. It is required
to maintain the air in the building at 25°C and 35% percentage saturation.
Calculate the temperature to which the inlet air should be cooled at the
cooling coil, the refrigerating load, and the heat input to the heater. The
system requires 5 m3 of incoming air per second.
[8.7°C, -325 kW, 92.5 kW]
2.
Air is required to be delivered to a computer
room at a dry bulb temperature of 16°C and with a relative humidity of 50%. The
plant to be specified will include a cooler, a condensate extraction system and
a heater in series. A fan is to be located downstream of the heater. Assuming
an inlet air flow of 0.75 m3/s with a fan consumption of 1.5 kW.
Calculate:
(1)
The condition of the air when it leaves
the heater. [tdry = 14.1ºC, %sat = 56%]
(2)
The temperature required at the
cooler. [5.8 °C]
(3)
The heat input to the heater.
[7.6 kW]
If the supply of air to the cooler is made up of recirculated air and fresh air
in volumetric proportion of 2.5 to 1, and the conditions of the two streams are
(a)
recirculated air: 27°C, % sat 50%
(b)
fresh air: 21°C, % sat 75%, then assuming
adiabatic mixing, determine:
(4) The condition of
the mixed air stream.
[25.4 ºC, 56%]
(5) The heat to be
removed in the cooler. [-31.3 kW]
3.
In an air-conditioning unit water is
injected into a stream of dry air at the rate of 1 kg of water
per 100 kg of dry air. The temperature of the mixture is subsequently found to
be 25°C. The mixture is then cooled under constant pressure steady flow
conditions and the temperature reaches 15°C.
(a) Show that the mixture is initially unsaturated and determine the partial
pressures of the air and water vapour.
(b) To what temperature would the mixture have to be cooled at constant
pressure for it to become saturated?
(c) What are the values of the mean relative molecular mass and mean specific
heat capacity at constant pressure for the mixture?
(d) How much heat is rejected per unit mass of mixture during cooling to 15°C?
Assume: Mair = 29
kg/kmol, Cpair =
1.005 kJ/kgK,
Mwater vapour
=18 kg/kmol, Cpwater vapour
= 1.87 kJ/kgK
[99.715
kPa, 1.61 kPa; 14°C; 28.826, 1.0136 kJ/kgK; -8.5 kJ/kg]
4.
A cooling tower with an induced draft fan
(located at the exit) is required to cool 5.2 kg/s of water from 45°C. The fan
produces a flow of 10 m3/s of air through the tower. At entry, the
air has the condition 18°C and % sat = 60%. At exit, the air is saturated at
27°C.
Calculate:
(1) The final water temperature
[20.4°C]
(2) The cooling water make-up flowrate.
[0.17 kg/s]