UNIT III
HUMIDIFICATION
1. What is humid heat?
Humid heat is the heat required to raise the temperature of unit mass of gas and its
accompanying vapor one degree at constant pressure.
2. What is natural draft?
It ensures more positive air movement even in calm weather by depending upon the
displacement of warm air inside the tower by the cooler outside the air.
3. Define Humidity
It is the mass of vapor carried by a unit mass of vapor free gas. It is denoted by H or y
4. What is saturated gas?
It is a gas in which vapor is equilibrium with the liquid at gas temperature.
5. Define Relative humidity
It is defined as the ratio of the partial pressure of vapor to the vapor pressure of the liquid at gas
temperature.
6. What is Percentage humidity?
It is the ratio of actual humidity to the saturation humidity.
7. What is Humidification?
In the processing of materials the operation of increasing the amount of vapor present in a gas
stream is known as humidification.
8. What is dry bulb temperature?
This is the temperature of a vapor free gas mixture as ordinarily determined by immersion of
thermometer in a mixture.
9. What is wet bulb temperature?
It is the steady state temperature reached by a small amount of liquid evaporating in to a large
amount of unsaturated vapor gas mixture.
10. What is bound water?
If the equilibrium moisture content of a given material is continued to its intersection with 100%
humidity line the moisture is called bound water
11. What is unbound water?
If a material contains more water than indicated by intersection with 100%humidity time. It
exerts only a vapor pressure as high as ordinary water at same temperature.
12. What are the types of cooling towers?
Natural draft towers, Induced draft towers and forced draft towers
13. What are Induced draft towers?
In this case the fan is placed at the top avoids the dis advantages of previous and permits more
uniform internal distribution of air.
14. What are Tray towers?
These are very effective not commonly used in humidify, dehumidify or gas cooling operation
for reasons of cost relatively high pressure drop.
15. What are Spray chambers?
These are horizontal spray towers. These are used for adiabatic humidification cooling
operations with recirculating liquid.
16. What are the methods of determination of humidity?
by chemical methods, by dew point, By WBT, By piezo electric hygrometer.
17. What are the methods of increasing humidity?
Live steam addition
Water spray
The gas mixed with a gas of higher humidity.
18. What is humid volume?
Humid volume is defined as volume occupied by unit mass of dry gas and its associated
vapor.
19. What are the characteristics of good refrigerants?
It should be nontoxic and nonflammable, Cost must be less, Must have low viscosities.
20. What is Air conditioning?
It is the simultaneous control of temperature humidity motion and purity of the atmosphere in
confined space.
PART B:
1. (a) Describe the methods available for estimating humidity of a sample of air.
(b) What are the different types of cooling towers used in industries? Briefly explain them.
2. The temperature of air in a room is 40.2OC/50 OC and the total pressure is 101.3 kPa abs.
The air contains water vapor with a partial pressure PA = 3.74 kPa. Calculate:
(i) the humidity
(ii) the saturation humidity and percentage humidity
(iii) the percentage relative humidity.
3. Explain the theory of adiabatic saturation curves and wet bulb temperature theory
4. Water is to be cooled in a cooling tower counter currently from 55OC to 30OC, using air
counter- currently. The entering air is at 20OC and an absolute humidity of 0.007 kg water / kg
dry air. The value of Kya is 0.3 kg/m3.sec and the liquid flow rate is 1500 kg/hr.m2. Estimate
the minimum air requirement and the height of the cooling tower, if 1.3 times the minimum air
flow rate is used
Temperature, to OC 20 30 40 50 55
Enthalpy, H' kJ/kg 60 102 166 279 355
5. a) Explain the principle and theory of a cooling tower.
b) With a neat sketch, explain the operation of a counter-current induced draft tower.
6. Explain how the height of a cooling tower can be determined.
7. A mixture of oxygen-acetone vapor at a total pressure of 1050 mm. Hg. at 25OC has a
percentage humidity of 70%. The saturation vapor pressure of acetone at 25OC is 290 mm. Hg.
The specific heats of oxygen and acetone vapor are 0.25 and 0.352 Kcal/Kg OC respectively.
Calculate its molal humidity, absolute humidity, relative humidity, molal humid volume and
molal humid heat.
8. Fresh air at 21.2°C, in which the partial pressure of water vapor is 0.0118 atm. is blown at
the rate of 214 m3/hr., first through a preheater and then adiabatically saturated in a spray
chamber to 100% saturation and then again reheated. The reheated air has a humidity of 0.024
kg water vapor/kg. Dry air. It is assumed that the fresh air and air leaving the preheater have
the same % humidity. Determine:
a) The temp. of the preheater, spray chamber and reheater.
b) Heat requirement for preheating and reheating.
9. a) Write short notes on absolute humidity, relative humidity and dew point.
b) Describe Interphase mass transfer
C) Write a note on induced and forced draft cooling tower
10. a) Discuss briefly about spray chamber with a neat sketch
b) Write a note on two industrial cooling towers with a neat sketch
C) Write briefly about adiabatic saturator
