# Psychrometry | RAC | GATE MCQs (solved)

## GATE ME solved Questions on Psychrometry (RAC)

Question #1

Ambient air is at a pressure of 100 kPa, dry bulb temperature of 30 C and 60% relative humidity. The saturation pressure of water at 30 C is 4.24 kPa. The specific humidity of air (in g/kg of dry air) is ________ (correct to two decimal places)

[GATE 2018 Set 2, 2 marks]

Question #2

Moist air is treated as an ideal gas mixture of water vapor and dry air (molecular weight of air = 28.84 and molecular weight of water = 18). At a location, the total pressure is 100 kPa, the temperature is 30°C and the relative humidity is 55%. Given that the saturation pressure of water at 30°C is 4246 Pa, the mass of water vapor per kg of dry air is _____________ grams

[GATE 2017 Set 1, 2 marks]

Question #3

If a mass of moist air contained in a closed metallic vessel is heated, then its
(A) relative humidity decreases
(B) relative humidity increases
(C) specific humidity increases
(D) specific humidity decreases

[GATE 2017 Set 2, 1 mark]

Question #4

The partial pressure of water vapour in a moist air sample of relative humidity 70% is 1.6 kPa, the total pressure being 101.325 kPa. Moist air may be treated as an ideal gas mixture of water vapour and dry air. The relation between saturation temperature (Ts in K) and saturation pressure (ps in kPa) for water is given by ln(Ps/Po)= 14.317 − 5304/Ts, where po = 101.325 kPa. The dry bulb temperature of the moist air sample (in oC) is __________

[GATE 2016 Set 2, 2 marks]

Question #5

In a mixture of dry air and water vapor at a total pressure of 750 mm of Hg, the partial pressure of water vapor is 20 mm of Hg. The humidity ratio of the air in grams of water vapor per kg of dry air (gw/kgda) is __________

[GATE 2016 Set 3, 2 marks]

Question #6

A stream of moist air (mass flow rate = 10.1 kg/s) with humidity ratio of 0.01 kg/ kg dry air mixes with a second stream of superheated water vapour flowing at 0.1 kg/s. Assuming proper and uniform mixing with no condensation, the humidity ratio of the final stream (in kg kg dry air) is _____________

[GATE 2015 Set 1, 1 mark]

Question #7

Air in a room is at 35°C and 60% relative humidity (RH). The pressure in the room is 0.1 MPa. The saturation pressure of water at 35°C is 5.63 kPa. The humidity ratio of the air (in gram/kg of dry air) is ________

[GATE 2015 Set 3, 2 marks]

Question #8

A sample of moist air at a total pressure of 85 kPa has a dry bulb temperature of 30°C (saturation vapour pressure of water = 4.24 kPa). If the air sample has a relative humidity of 65%, the absolute humidity (in gram) of water vapour per kg of dry air is _______

[GATE 2014 Set 3, 1 mark]

Question #9

Moist air at 35°C and 100% relative humidity is entering a psychrometric device and leaving at 25°C and 100% relative humidity. The name of the device is

(A) Humidifier

(B) Dehumidifier

(C) Sensible heater

(D) Sensible cooler

[GATE 2014 Set 4, 2 marks]

Question #10

The pressure, dry bulb temperature and relative humidity of air in a room are 1 bar, 30°C and 70%,
respectively. If the saturated steam pressure at 30°C is 4.25 kPa, the specific humidity of the room
air in kg water vapour/kg dry air is

(A) 0.0083

(B) 0.0101

(C) 0.0191

(D) 0.0232

[GATE 2013, 1 mark]

Question #11

A room contains 35 kg of dry air and 0.5 kg of water vapor. The total pressure and temperature of air in the room are 100 kPa and 25°C respectively. Given that the saturation pressure for water at 25°C is 3.17 kPa, the relative humidity of the air in the room is

(A) 67%

(B) 55%

(C) 83%

(D) 71%

[GATE 2012, 2 ,marks]

Question #12

If a mass of moist air in an airtight vessel is heated to a higher temerature, then

(A) Specific humidity of the air increases

(B) Specific humidity of the air decreases

(C) Relative humidity of the air increases

(D) Relative  humidity of the air decreases

[GATE 2011, 1 mark]

Practice other topics  from Thermodynamics  :

Basics of  Thermodynamics

Heat and work

First Law of Thermodynamics

Second Law of thermodynamics

Pure substances