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PSYCHROMETRICS 485

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10. The relative humidity, during heating and humidification,
    (a) increases (b) decreases
    (c) may increase or decrease (d) remains constant.


11. The relative humidity, during cooling and dehumidification of moist air
    (a) increases (b) decreases
    (c) can increase or decrease (d) remains constant.
    ANSWERS


12. (b) 2. (a) 3. (b) 4. (a) 5. (a) 6. (b) 7. (d)


13. (b) 9. (d) 10. (a) 11. (c).

Theoretical Questions

1. Define the following terms :
   (i) Saturated air (ii) Dry bulb temperature
   (iii) Dew point temperature (iv) Relative humidity
   (v) Specific humidity.


2. State ‘Dalton’s law of partial pressure’.


3. Derive the following relations :

(i) Specific humidity, W = 0.622

p

pp

v

tv−

.(ii) Degree of saturation, μ =

p

p

p

p

p

p

v

s

s

t

v

t

1

1

F −

HG

I

KJ

F −

HG

I

KJ

L

N

M

M

M

M

M

O

Q

P

P

P

P

P

4. Explain briefly with a neat sketch a ‘sling psychrometer’.


5. Describe briefly any two of the following processes :
   (i) Sensible heating (ii) Cooling and dehumidification
   (iii) Heating and humidification (iv) Heating and dehumidification.


6. Write a short note on ‘by-pass factor’.

Unsolved Examples

1. The atmospheric conditions are 30ºC and specific humidity of 0.0215 kg/kg of air. Determine :
   (i) Partial pressure of air (ii) Relative humidity
   (iii) Dew point temperature.
   Atmospheric pressure = 756 mm Hg. [Ans. (i) 14.89 mm of Hg, (ii) 46.8%, (iii) 17ºC]


2. A mixture of air and water vapour at 1 bar and 25ºC has a dew point temperature of 15ºC. Determine the
   relative humidity and specific humidity. [Ans. 53.8%, 0.01078 kg/kg of dry air]


3. An air-water vapour mixture at 1.24 bar has a temperature of 38ºC and relative humidity of 60%. Calculate
   the kg-mass of water vapour per kg of air and per kg of mixture. Also find the dew point.
   [Ans. 0.0203 kg/kg of dry air, 0.0199 kg/kg of mixture, 28.5ºC]


4. In a house, the temperature of the windows on a day in winter is 5ºC. When the temperature in the room
   is 23ºC, and the barometric pressure is 748.8 mm Hg, what would be maximum relative humidity that could
   be maintained in the room without condensation on the window panes? Under these conditions, find the
   partial pressure of the water vapour and air, the specific humidity, and the density of the mixture.
   [Ans. 30.7%, 0.00872 bar, 0.989 bar, 0.00548 kg/kg of dry air, 1.164 kg/m^3 ]


5. Atmospheric air enters a heater at 4.5ºC and 60% relative humidity and leaves at a temperature of 21ºC.
   Calculate :