Psychrometric Chart - Printable Version

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Psychrometric Chart - Pankaj Garg - 06-02-2019

Psychrometric Chart
History :
The Psychrometric chart – 1904
Willis H. Carrier’s historic paper “Rational Psychrometric formulae”, 1911


A Psychrometric chart is a graphical representation of physical and thermodynamic properties of environment such as Dry bulb temperature, Wet bulb temperature, Relative Humidity, enthalpy, and air density.
Psychrometric Chart – Dry-bulb temperature (DBT)

The dry-bulb temperature (DBT) is the temperature of air measured by a thermometer freely exposed to the air but shielded from radiation and moisture.
DBT is the temperature that is usually thought of as air temperature, and it is the true thermodynamic temperature. It indicates the amount of heat in the air and is directly proportional to the mean kinetic energy of the air molecules.
Psychrometric Chart – Wet-bulb temperature (WBT)
The Wet Bulb temperature is the adiabatic saturation temperature.
Wet Bulb temperature can be measured by using a thermometer with the bulb wrapped in wet muslin. The adiabatic evaporation of water from the thermometer bulb and the cooling effect is indicated by a “wet bulb temperature” lower than the “dry bulb temperature” in the air.
The rate of evaporation from the wet bandage on the bulb, and the temperature difference between the dry bulb and wet bulb, depends on the humidity of the air.

Psychrometric Chart -Relative Humidity

Relative Humidity is the amount of water vapour present in air expressed as a percentage of the amount needed for saturation at the same temperature.
When the air is saturated, dry-bulb, wet-bulb and dew point temperatures are all equal, and the relative humidity is 100 %
Psychrometric Chart -Dew point (DP) temperature
This is the temperature of the air at which a moist air sample reaches water vapor saturation. It is equivalent to a wet bulb temperature at 100% relative humidity. At this combination of temperature and humidity, further removal of heat results in water vapor condensing into liquid.

A practical view of the dew point is the temperature to which air must be cooled before condensation will begin. An example is when you take a bottle out of the refrigerator. Water condenses on the outside of the bottle only if the original temperature of the bottle was below the dew point. The instantaneous temperature of the water is the dew point temperature.
Psychrometric Chart -Moisture content

This is also known as the humidity ratio. It is the proportion of the mass of water vapor per unit mass of dry air.
Humidity ratio is dimensionless, it may be expressed as grams of water per kilogram of dry air or as a percentage.
The moisture content is the vertical axis of the chart.
Psychrometric Chart – Enthalpy (total heat)
nEnthalpy (h) is the total amount of heat energy of the moist air and therefore includes the amount of heat of the dry air and the water vapor in the air. nIn the approximation of ideal gases, lines of constant enthalpy are parallel to lines of constant WB temperature. Thus the enthalpy is indicated by diagonal lines on the chart.


To read more.. http://frontdesk.co.in/building-services/hvac-heating-ventilation-and-air-conditioning/

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