Refrigeration and Air-Conditioning

Part #2

Psychrometry 

➢ Psychrometry is the study of the properties of mixtures of air and water vapour.

➢ Atmospheric air is a mixture of many gases and water vapour.

➢ The amount of water vapour vary from place to place.

➢ This mixture is known as moist air.

➢ The molecular weight of dry air is found to be 28.966 and th gas constant R is 287.035 J/kgK.

➢ The molecular weight of water vapour is taken as 18.015 and its gas constant R is 461.52 J/kgK

Specific Humidity or Humidity Ratio

➢ The humidity ratio (or specific humidity) w is the mass of water associated with each kilogram of dry air.

➢ The humidity ratio is given by:



➢ Now assuming water vapour and dry air as ideal gas.

➢ By putting values we get

Relative Humidity

➢ Relative humidity (φ) is defined as the ratio of the mole fraction of water vapour in moist air to mole fraction of water vapour in saturated air at the same temperature and pressure.

Dry Bulb Temperature

➢ It is the temperature of the moist air measured by ordinary thermometer.

Wet Bulb Temperature

➢ It is the temperature shown by thermometer whose bulb is covered with wet cloth.

Wet Bulb Depression

➢ It is the different between DBT and WBT

Dew Point Temperature

➢ If unsaturated moist air is cooled at constant pressure, then the temperature at which the moisture in the air begins to condense is known as dew-point temperature (DPT) of air

For Unsaturated Air

➢ DBT > WBT > DPT

➢ Relative Humidity is less then 100 %

➢ Wet Bulb Depression is greater then zero.

For Saturated Air

➢ DBT = WBT = DPT 

➢ Relative Humidity is 100 %

➢ Wet Bulb Depression is zero.

Degree of Saturation or Percentage Humidity

➢ Degree of Saturation is the ratio of the specific humidity of moist air - to the specific humidity of saturated moist air at the same temperature and pressure.

Enthalpy of Moist Air

➢ Enthalpy of Moist Air is given by :

➢ Here t = dry bulb temperature.

Psychrometry Chart 

➢ Psychrometric chart include physical and thermodynamic properties such as

✓ Dry bulb temperature.

✓ Wet bulb temperature.

✓ Specific humidity

✓ Relative humidity

✓ Dew point Temperature

✓ Enthalpy.

Dry Bulb Temperature Lines

➢ The dry bulb temperature lines, on the psychrometric chart are vertical and uniformly spaced.

Specific humidity lines and Dew Point temperature lines

➢ On psychrometric chart, wet bulb temperature lines are inclined with uniformly spaced.Wet bulb temperature lines and Specific Enthalpy lines.

➢ Specific Enthalpy lines are usually similar to wet bulb temperature lines but they are non uniformly spaced.


Wet bulb temperature lines and Specific Enthalpy lines

➢ On psychrometric chart, wet bulb temperature lines are inclined with uniformly spaced. Wet bulb temperature lines and Specific Enthalpy lines

➢ Specific Enthalpy lines are usually similar to wet bulb temperature lines but they are non uniformly spaced.

Relative humidity Curve

➢ The RH lines are the curves extending from the lower left to the upper right part of the psychrometric chart.

➢ The different curves of RH indicate different values of humidity measured in percentage.

Sensible Heating/Cooling

➢ Dry bulb temperature is increased/decreased but specific humidity remains constant

Isothermal Humidification/ Dehumidification

➢ Specific humidity is increased/decreased but Dry bulb temperature remains constant.

Adiabatic (Chemical) Humidification/Dehumidification

➢ Specific humidity is increased/decreased as well as Dry bulb temperature is increased/decreased but specific enthalpy remains constant.


SOME QUESTION FROM THESE TOPICS

1.Isothermal Dehumidification is the process of removing moisture from air with dry bulb temperature.

(A) Increasing

(B) Decreasing

(C) Remaining same

(D) Changing in any direction

2.If the specific humidity of moist air remains the same but its dry bulb temperature increases, its dew point temperature

(a) Remains the same

(b) Increases

(c) Decreases

(d) May increase or decrease depending on its relative humidity

3.The wet bulb depression is zero, when relative humidity is equal to

(a) 100%

(b) 60%

(c) 40%

(d) Zero

4.Dew point is _______ .

(A) The temperature at which condensation of steam in saturated air will start

(B) The lowest attainable temperature for a mixture of air and steam

(C) Dependent on pressure of air

(D) Used in connection with air conditioning

5.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) (b) 55%

(c) 83%

(d) 71%

 

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