# Thermal Engineering Questions and Answers – Psychrometers

This set of Thermal Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Psychrometers”.

1. Wet bulb depression, under saturated ambient air conditions _____________
a) Is always positive
b) Is always negative
c) Is always zero
d) May have value depending upon the dew point temperature

Explanation: The wet bulb depression, under saturated ambient air conditions is always zero. At RH=100% DBT=WBT=DPT. Hence depression is equal to zero.

2. What is the minimum temperature to which water can be cooled in a cooling tower?
a) The dew point temperature of air
b) The dew bulb temperature of air
c) They dry bulb temperature of air
d) The ambient air temperature

Explanation: At saturation condition the dry bulb, wet bulb and dew point temperature are same with 100% relative humidity. The minimum temperature to which water can be cooled in cooling tower is equal to the wet bulb temperature of the air.

3. When a stream of moist air is passed over a cold and dry cooling coil such that no condensation takes place, then the air stream will get along the line of __________
a) Constant wet bulb temperature
b) Constant dew point temperature
c) Constant relative humidity
d) Constant enthalpy

Explanation: When a stream of moist air is passed over a cold and dry cooling coil such that no condensation takes place, then the cooling is sensible cooling. It is along dew point temperature as it remains constant during sensible cooling.

4. During adiabatic saturation process of air, wet bulb temperature ___________
a) Increases and dry bulb temperature remains constant
b) Remains constant and dry bulb temperature increases
c) Remains constant and dry bulb temperature decreases
d) Decreases and dry bulb temperature remains constant

Explanation: During adiabatic saturation process, enthalpy and wet bulb temperature remains constant and dry bulb temperature decreases. In this the dew point temperature increases.

5. The performance of an evaporation condenser largely depends on ________
a) Dry bulb temperature of air
b) Wet bulb temperature of air
c) Hot water temperature
d) Air conditional room temperature

Explanation: The evaporative cooler performance is directly related to evaporate water at a given relative humidity. The temperature of the water does not have a great effect upon the cooling produced through evaporation.

6. What happens when warm saturated air is cooled?
a) Excess moisture condenses
b) Excess moisture condenses but relative humidity remains unchanged
c) Excess moisture condenses and specific humidity increases but relative humidity remains unchanged
d) Specific humidity increases and relative humidity increases

Explanation: When warm saturated air is cooled the excess of moisture condenses. The relative humidity remains constant but specific humidity decreases due to which the excess of moisture condenses.

7. Air at 35 and 25 degree Celsius dew point temperature passes through the water shower whose temperature is maintained at 20 degree Celsius. What is the process involved?
a) Cooling and humidification
b) Sensible cooling
c) Cooling and dehumidification
d) Heating and humidification

Explanation: The process involved is cooling and dehumidification. Since the temperature of spray water is less than the dew point temperature cooling and dehumidification will take place.

8. Which one of the following is correct according to temperature?
a) Effective temperature is the index which correlates the combined effects of air dry bulb temperature, air humidity and air movement upon human comfort
b) The value of effective temperature in winter and summer should be same for human comfort
c) Effective temperature and wet bulb temperature are same
d) The valve of the effective temperature should be higher in winter than in summer for comfort

Explanation: The valve of effective temperature is low in winter and high in summer. In winter it is 19 degrees Celsius and in summer 22 degrees Celsius. Therefore the effective temperature is the index which correlates the combined effects of air dry bulb temperature, air humidity and air movement upon human comfort.

9. What is the relative humidity when the wet bulb depression is zero?
a) 100%
b) 60%
c) 40%
d) Zero

Explanation: Wet bulb depression is the difference between the dry bulb and wet bulb temperature. Hence, when DBT = WBT implies that relative humidity = 100%. Therefore when the wet bulb depression is zero the relative humidity is equal to 100%.

10. The latent heat load in an auditorium is 25% of sensible heat load. What is the value of sensible heat fact?
a) 0.3
b) 0.5
c) 0.8
d) 1.0

Explanation: Sensible heat factor is the ratio of sensible heat and the total heat is equal to latent heat plus sensible heat
SHF = SH/ (SH+LH), but LH = 0.25SH
Hence, SHF = 0.8.

11. What is the saturation temperature at the partial pressure of water vapor in the air water vapor mixture called?
a) Dry bulb temperature
b) Wet bulb temperature
c) Dew point temperature
d) Saturation temperature

Explanation: At 100 RH, WBT = DBT = DPT. The temperature corresponding to the saturation temperature at the partial pressure of water vapor is called dew point temperature.

Sanfoundry Global Education & Learning Series – Thermal Engineering

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