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Psychrometry

Persons who want to control temperature and humidity of the air in the air conditioning system should have an understanding of psychrometry. They make use of psychrometric charts to determine various parameters of the air at any condition. For example, by knowing two parameters, you can look up the chart to find the other values. The values found in a typical psychrometric chart are:

Dry Bulb Temperature

The dry-bulb temperature of air is measured by a thermometer which is freely exposed to the air but is shielded from radiation and moisture
Wet Bulb Temperature
The Wet-bulb Temperature of air is measured by a thermometer whose bulb is covered by a muslin sleeve which is kept moist with distilled and clean water, freely exposed to the air and free from radiation
 
Dewpoint Temperature
Dew point temperature is the temperature at which moist air becomes saturated (100% relative humidity) with water vapour when cooled at constant pressure, i.e. temperature at which condensation of moisture begins when the moist air is cooled.
 
Relative Humidity
Humidity ratio or relative humidity is the ratio of the actual mass of water vapor in the air to the mass of water vapor that would saturate the air at the same temperature
 

Enthalpy

The enthalpy of moist air is defined as the sum of its internal energy and the product of its pressure and volume. Specific enthalpy h (kJ/kg) of moist air is defined as the total enthalpy of the dry air and water vapour mixture per kg of moist air.
 
Moisture Content
Specific humidity or moisture content of air is the ratio of the mass of water to the mass of dry air in a given volume of moist air
 
Psychrometric processes, i.e. any changes in the condition of the atmosphere, can be represented by the movement of the state point on the psychrometric chart. Common processes include:

Sensible cooling / sensible heating

When an air-vapor mixture is heated or cooled, without the addition or removal of moisture, the resulting "process" may be plotted as a horizontal line on the chart:

 
Humidification / dehumidification
Humidification can be made by spraying water or steam to air.If humidification made by water, air will move through wetbulb temperature line and dry bulb temperature will decrease.If humidifaction made with steam air will move through drybulb temp line and there will be no significant change in drybulb temprature
 
Cooling and dehumidification / heating and humidification
 
 
Evaporative cooling / chemical dehydration
 
Sample Psyhrometric Systems

 

 

 
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