## Refrigeration and Air-conditioning Miscellaneous

#### Refrigeration and Air-conditioning

1. A building has to be maintained at 21 °C (dry bulb) and 14.5°C(wet bulb). The dew point temperature under these conditions is 10.17°C. The outside temperature is –23°C (dry bulb) and the internal and external surface heat transfer coefficients are 8 W/m2K and 23 W/ m2K respectively. If the building wall has a thermal conductivity of 1.2 W/mK, the minimum thickness (in m) of the wall required to prevent condensation is

1. NA

##### Correct Option: B

NA

1. For a typical sample of ambient air (at 30°C, 75% relative humidity and standard atmospheric pressure), the amount of moisture in kg per kg of dry air will be approximately.

1. Given data:
Dry bulb temperature:
Tdb = 35°C
Relative humidity
φ = 75° = 0.75
At 35°C, Ps = .05628 bar

 φ = Pv ⇒ 0.75 = Pv Ps .05628

Pv = 0.75× .05628 =.04221 bar
Specific humidity,
 ω = .622Pv = .622 × .04221 P - Pv 1.0135 - .04221

= 0.0270 kg/kg of dry air.

##### Correct Option: B

Given data:
Dry bulb temperature:
Tdb = 35°C
Relative humidity
φ = 75° = 0.75
At 35°C, Ps = .05628 bar

 φ = Pv ⇒ 0.75 = Pv Ps .05628

Pv = 0.75× .05628 =.04221 bar
Specific humidity,
 ω = .622Pv = .622 × .04221 P - Pv 1.0135 - .04221

= 0.0270 kg/kg of dry air.

1. 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.

1.  w = 0.622 Pv P - Pv

 φ = Pv Ps

 ⇒ 0.55 = Pv Ps

⇒ Pv = 2235.3
 w = 0.622 × 2235.3 - 2235.3 (100 × 103)

w = 0.0142 kg/kg of dry air
w = 14.2 gm/kg of dry air

##### Correct Option: A

 w = 0.622 Pv P - Pv

 φ = Pv Ps

 ⇒ 0.55 = Pv Ps

⇒ Pv = 2235.3
 w = 0.622 × 2235.3 - 2235.3 (100 × 103)

w = 0.0142 kg/kg of dry air
w = 14.2 gm/kg of dry air

1. For air with a relative humidity of 80%

1. ##### Correct Option: B

1. For air at a given temperature, as the relative humidity is increased isothermally,

1. DBT = DBT2
DBT →
φ ↑, Twb ↓
Relative humidity is increased isothermally.

##### Correct Option: A

DBT = DBT2
DBT →
φ ↑, Twb ↓
Relative humidity is increased isothermally.