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Engg Thermodynamics - Lecture - 14 - Unit - 5 - Gas Mixtures & Psychrometry

 PSYCHROMETRY

Psychrometric properties,

Psychrometric charts.

Property calculations of air vapour mixtures by using chart and expressions.

Psychrometric process –

├ Adiabatic saturation,

├ Sensible heating and cooling,

├ Humidification,

├ Dehumidification,

├ Evaporative cooling and

├ Adiabatic mixing.

Simple Applications

Psychrometry

Psychrometry is the science dealing with the physical laws of air – water vapour mixtures.

When designing an air conditioning system, the temperature and moisture content of the air to be conditioned, and the same properties of the air needed to produce the desired air conditioning effect.

In other words, we can say that Psychrometry is the study of MOIST AIR or mixture of dry air and water vapour.

PROPERTIES OF PSYCHROMETRY

Dry-bulb temperature

Wet bulb temperature

Wet bulb depression

Dew point temperature

Dew point depression

Relative humidity (ф)

Humidity ratio (or) Specific Humidity (or) Moisture content (ῳ)

Degree of saturation (or) Percentage of saturation(or) Saturation ratio (μ)

Total Enthalpy (h)


Dry-bulb temperature (td )

The dry-bulb temperature is the temperature indicated by a ordinary thermometer exposed to the air in a place sheltered from direct solar radiation.

The term dry-bulb is customarily added to temperature to distinguish it from wet-bulb and dew point temperature.

Wet bulb temperature(tw )

Wet bulb temperature is the temperature recorded by thermometer when the bulb is enveloped by cotton wick saturated with water.

The accuracy of a simple wet-bulb thermometer depends on how fast air passes over the bulb and how well the thermometer is shielded from the radiant temperature of its surroundings.

Wet bulb depression (WBD )

WBD = Dry-bulb temperature (td ) - Wet bulb temperature(tw )

Dew point temperature(tdP )

When the air is cooled, The temperature at which moisture starts to condense out of the air is known as DEW POINT.

Dew point is also known as saturation temperature.

Dew point temperature is determined by moving from a state point horizontally to the left along lines of constant humidity ratio until the upper, curved, saturation temperature boundary is reached.

Dew point depression(DPD )

DPD = Dry-bulb temperature - Dew point temperature(tdP )

Relative humidity (ф)

The ratio of the amount of moisture the air holds (mv) to the maximum amount of moisture the air can hold at the same temperature (ms).


Humidity ratio (or) Specific Humidity (or) Moisture content (ῳ)

The mass of water vapor present in a unit mass of dry air.




Degree of saturation (or) Percentage of saturation (or) Saturation ratio (μ)

The ratio of actual humidity ratio to the humidity ratio of saturated air at the same temperature  and total pressure.


Total Enthalpy of Moist Air (h)

H = Cptd + ῳ hg
Cp - Specific heat Capacity at constant
Pressure (1.005 KJ/Kg.K)
td – Dry bulb Temperature
ῳ - Specific Humidity
hg- Specific Enthalpy of air corresponding to the dry bulb temperature

Dalton’s Law of Partial Pressure
Pb = Pa + Pv
Pb = Barometric Pressure
Pa = Partial Pressure of Dry Air
Pv = Partial Pressure of water vapour



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