This set of Air-Conditioning Multiple Choice Questions & Answers (MCQs) focuses on “Psychrometry – Humidification and Dehumidification”.
1. Which of the following denotes Humidification process on the psychrometric chart?
a) A horizontal line with an arrow towards right
b) Vertical line with arrow upwards
c) Horizontal line with an arrow towards right
d) Vertical line with arrow downwards
View Answer
Explanation: Psychrometric chart has DBT on the x-axis and sp. Humidity on the y-axis. So, if a process is going up on the chart, then there is an increase in relative as well as specific humidity. So, the vertical line with an arrow upwards denotes humidification process.
2. Which of the following denotes dehumidification process on the psychrometric chart?
a) A horizontal line with an arrow towards the right
b) Vertical line with arrow upwards
c) A horizontal line with an arrow towards the right
d) Vertical line with arrow downwards
View Answer
Explanation: Psychrometric chart has DBT on the x-axis and sp. Humidity on the y-axis. So, if a process is going down on the chart, then there is a decrease in relative as well as specific humidity. So, the vertical line with an arrow downwards denotes dehumidification process.
3. Which of the following is denoted by LH = (h2 – h1) = hfg (W2 – W1)?
a) Sensible cooling
b) Sensible heating
c) Humidification
d) Dehumidification
View Answer
Explanation: Latent heat transfer i.e., LH = (h2 – h1) = hfg (W2 – W1) denotes Humidification. As in the humidification process, there is an increase in specific humidity, so W2 > W1 and also increase in enthalpy, h2 > h1.
4. Which of the following is denoted by LH = (h1 – h2) = hfg (W1 – W2)?
a) Sensible cooling
b) Sensible heating
c) Humidification
d) Dehumidification
View Answer
Explanation: Latent heat transfer i.e., LH = (h1 – h2) = hfg (W1 – W2) denotes dehumidification. As in the dehumidification process, there is a decrease in specific humidity, so W1 > W2 and also decrease in enthalpy, h1 > h2.
5. What is the relation between DBT and WBT if the relative humidity is 100%?
a) DBT = WBT
b) DBT > WBT
c) DBT ≫ WBT
d) DBT < WBT
View Answer
Explanation: When the dry bulb temperature is equal to the wet-bulb temperature, then the relative humidity tends to be 100%.
6. The minimum temperature to which moist air can be cooled under ideal conditions in a spray washer is ___________
a) DPT of inlet air
b) Water inlet temperature
c) WBT of inlet air
d) Water outlet temperature
View Answer
Explanation: Wet-bulb temperature is the minimum temperature to which moist air can be cooled under ideal conditions in a spray washer. It is an integral part of the process.
7. Which of the following process is used in winter air conditioning?
a) Humidification
b) Dehumidification
c) Heating and Humidification
d) Cooling and Dehumidification
View Answer
Explanation: In Winter, the weather is dry and cold, so the process of heating and humidification is done to get the desired comfort conditions.
8. Which of the following process is used in summer air conditioning?
a) Humidification
b) Dehumidification
c) Heating and Humidification
d) Cooling and Dehumidification
View Answer
Explanation: In Summer, the weather is humid and hot, so the process cooling and dehumidification is done to get the desired comfort conditions.
9. What is the value of Latent heat transfer in the process of humidification having enthalpies as 20 and 40 kJ / kg of dry air respectively?
a) 60
b) 40
c) -20
d) 20
View Answer
Explanation: Latent heat transfer i.e., LH = (h2 – h1) = hfg (W2 – W1) denotes Humidification. As in the humidification process, there is an increase in specific humidity, so W2 > W1 and also increase in enthalpy, h2 > h1. So, as per given data, h1 = 40 and h2 = 20 kJ / kg of dry air.
So, latent heat transfer = h2 – h1 = 40 – 20 = 20 kJ/ kg of dry air.
10. What is the value of Latent heat transfer in the process of dehumidification having enthalpies as 19.45 and 13.67 kJ/ kg of dry air respectively?
a) 5.78
b) -5.78
c) 33.12
d) -33.12
View Answer
Explanation: Latent heat transfer i.e., LH = (h1 – h2) = hfg (W1 – W2) denotes dehumidification. As in the dehumidification process, there is a decrease in specific humidity, so W1 > W2 and also decrease in enthalpy, h1 > h2. So, as per given data, h1 = 19.45 and h2 = 13.67 kJ / kg of dry air.
So, latent heat transfer = h1 – h2 = 19.45 – 13.67 = 20 kJ / kg of dry air.
11. What is the value of Latent heat transfer in a process of humidification having hfg = 10 kJ / kg of dry air, W1 = 0.0071 kJ / kg of dry air and W2 = 0.0017 kJ / kg of dry air?
a) 0.054
b) 0.0054
c) 0.54
d) 5.4
View Answer
Explanation: Latent heat transfer i.e., LH = (h2 – h1) = hfg (W2 – W1) denotes Humidification. As, in the humidification process there is an increase in specific humidity so W2 > W1 and also increase in enthalpy, h2 > h1.
So, latent heat transfer = hfg (W2 – W1) = 10 (0.0071 – 0.0017) = 0.054 kJ / kg of dry air.
12. During humidification process the dry bulb temperature ___________
a) increases
b) decreases
c) tends to zero
d) remains the same
View Answer
Explanation: During the humidification process, DBT remains the same as the increase is in specific humidity and relative humidity. As the process is vertical on chart keeping DBT constant.
13. Which of the following is not an indirect method of air-washer humidification?
a) Using vaporization after cooling
b) Using heated spray water
c) Using re-circulated spray water without prior heating of air
d) By pre-heating the air and then washing it with re-circulated air
View Answer
Explanation: There are direct and indirect methods of obtaining humidification and dehumidification. There are three types of Indirect methods which are as follows:
i.Using heated spray water
ii.Using re-circulated spray water without prior heating of air
iii.By pre-heating the air and then washing it with re-circulated air.
14. During the sensible cooling process, specific humidity decreases.
a) True
b) False
View Answer
Explanation: Sensible cooling process is the one where dry bulb temperature decreases. It is a horizontal process on the psychrometric chart. Hence, the specific humidity neither increases nor decreases, but there might be some change in relative humidity.
15. In order to cool and dehumidify a stream of moist air, it must be passed over the coil at a temperature between DBT and WBT.
a) True
b) False
View Answer
Explanation: In order to cool and dehumidify a stream of moist air, it must be passed over the coil at a temperature which is lower than the dew point temperature of the incoming stream to get desire state of the air.
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