Refrigeration and Air-conditioning Miscellaneous Difficult Questions and Answers | Page - 1

Refrigeration and Air-conditioning Miscellaneous

If a mass of moist air contained in a closed metallic vessel is heated, then its

1. relative humidity decreases
1. relative humidity increases
1. specific humidity increases
1. specific humidity decreases

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Correct Option: A

Air (at atmospheric pressure) at a dry bulb temperature of 40°C and wet bulb temperature of 20°C is humidified in an air washer operating with continuous water recirculation. The wet bulb depression, the difference between the dry and wet bulb temperatures) at the exit is 25% of that at the inlet. The dry bulb temperature at the exit of the air washer is closest to

1. 10°C
1. 20°C
1. 25°C
1. 30°C

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NA

Correct Option: C

NA

Atmospheric air at a flow rate of 3 kg/s (on dry basis) enters a cooling and dehumidifying coil with an enthalpy of 85 kJ/kg of dry air and a humidity ratio of 19 grams/kg of dry air. The air leaves the coil with an enthalpy of 43 kJ/kg of dry air and a humidity ratio of 8 grams/kg of dry air. If the condensate water leaves the coil with an enthalpy of 67 kJ/kg, the required cooling capacity of the coil in kW is

1. 75.0
1. 123.8
1. 128.2
1. 159.0

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Mass flow rate of condensate,
ṁ_w = ṁ_a (w₁ - w₂) 3 × (.019 - .008) = .033kg/s
By applying energy balance equation we get
ṁ_a h₁ = ṁ_a h₂ + ṁ_a h_fg + Q
3 × 85 = 3 × 43 + 0.033 × 67 + Q
Q = 123.8 kW

Correct Option: B

Mass flow rate of condensate,
ṁ_w = ṁ_a (w₁ - w₂) 3 × (.019 - .008) = .033kg/s
By applying energy balance equation we get
ṁ_a h₁ = ṁ_a h₂ + ṁ_a h_fg + Q
3 × 85 = 3 × 43 + 0.033 × 67 + Q
Q = 123.8 kW

Dew point temperature is the temperature at which condensation begins when the air is cooled at constant

1. volume
1. entropy
1. pressure
1. enthalpy

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Air is cooled at constant pressure to make unsaturated air to saturated one.

Correct Option: C

Air is cooled at constant pressure to make unsaturated air to saturated one.

Various psychrometric processes are shown in the figure below.

process in Figure	Name of the Process
P. 0-1	(i) Chemical dehumidification
Q. 0-2	(ii) Sensible heating
R. 0-3	(iii) Cooling and dehumidification
S. 0-4	(iv) Humidification with steam injection
T. 0-5	(v) Humidification with water injection

The matching pair are

1. P-i, Q-ii, R-iii, S-iv, T-v
1. P-ii, Q-i, R-iii, S-v, T-iv
1. P-ii, Q-i, R-iii, S-iv, T-v
1. P-iii, Q-iv, R-v, S-i, T-ii

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Process 0 - 1 : DBT ↑ ,w C; sensibleheating
Process 0 - 2 : DBT ↑ ,w ↓ ;heating & dehumidification
Process 0 – 3; Cooling and dehumidification
Process 0 – 4 ; Humidification with water injection (DBT↓, w ↑)
Process 0 – 5; DBT ↑, w ↑ : Humidification with steam injection.

Correct Option: B

Process 0 - 1 : DBT ↑ ,w C; sensibleheating
Process 0 - 2 : DBT ↑ ,w ↓ ;heating & dehumidification
Process 0 – 3; Cooling and dehumidification
Process 0 – 4 ; Humidification with water injection (DBT↓, w ↑)
Process 0 – 5; DBT ↑, w ↑ : Humidification with steam injection.