BENG3 THERMAL & FLUIDS ENGINEERING (THER305)
TUTORIAL EXAMPLES in PSYCHROMETRY
Solve questions 1 to 5 from first principles (do not use a
psychrometric chart)
1.
Air at 32°C is saturated with water vapour
and the barometric pressure is 101.3 kPa. Determine the partial pressure of water vapour and of
the dry air. What volume of the mixture would contain 1 kg of vapour? Calculate
also the mass of air associated with this amount of vapour, the moisture
content and the relative humidity of the mixture.
[4.75 kPa; 96.55 kPa; 29.6 m3; 32.5
kg; 0.0306; 100%]
2.
The pressure of water vapour in an
atmosphere which is at 32°C and 101.3 kPa is 2.062 kPa.
By how much is the water vapour superheated ? What are the specific and
relative humidities of the air? To what temperature would the air have to be
cooled for it to be just saturated with water vapour? If the air is cooled to
10°C from its original condition, how much condensate is formed per kg of dry
air?
[14 K; 0.0129; 42.2%, 18°C; 0.0053 kg]
3.
An air and water vapour mixture at 1 bar
and 26.7°C has a moisture content of 0.0085. Determine the percentage
saturation.
[37.8 %]
4.
A mixture of air and water vapour at
101.3 kPa and 16°C has a dew point of 6°C. Determine the moisture content and
relative humidity. [0.00578 kg/kg dry air, 51%]
5.
Atmospheric air at SSL pressure has a
temperature of 32°C and a percentage saturation of 52%. Calculate:
(a) The partial pressures of the vapour and the dry air,
(b) The moisture content,
(c) The dew point,
(d) The density of the mixture
[2.527 kPa; 98.798 kPa; 0.015875; 21°C;
1.147kg/m3]
6.
The readings taken in a room from a sling
psychrometer gave a dry and a wet bulb temperature of 25°C and 20°C
respectively. Using a psychrometric chart (pair =101.325 kPa)
determine:
(a) The moisture content,
(b) The percentage saturation,
(c) The dew point,
(d) The specific volume of the mixture,
(e) The enthalpy per kg of dry air.
Chart [0.0126 kg/kg;
62.5%; 17.5°C; 0.861 m3/kg; 57 kJ/kg]
7.
If the atmosphere of Q.6 is cooled to 5°C
and then heated until the dry bulb temperature is 17°C, both processes
being at constant pressure, determine,
from the chart, assuming that the air leaving the cooler is saturated:
(a) The
final percentage saturation,
(b) The
final moisture content,
(c) The
final wet bulb temperature,
(d) The
amount of condensate collected at the cooler per kg of dry air.
(e) The
heat removed in the cooling process and that supplied in the heating process
per kg of dry air.
Chart [45%; 0.0054; 10.8°C;
0.0072 kg/kg; 38.0 & 12.7 kJ/kg d.a.]