Psychrometer

Any instrument capable of measuring the psychrometric state of air is called a psychrometer. As mentioned before, in order to measure the psychrometric state of air, it is required to measure three independent parameters. Generally two of these are the barometric pressure and air dry-bulb temperature as they can be measured easily and with good…

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Wet-Bulb Thermometer

In practice, it is not convenient to measure the wet-bulb temperature using an adiabatic saturator. In stead, a thermometer with a wetted wick is used to measure the wet bulb temperature as shown in Fig. It can be observed that since the area of the wet bulb is finite, the state of air at the…

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Psychrometric chart

A Psychrometric chart graphically represents the thermodynamic properties of moist air. Standard psychrometric charts are bounded by the dry-bulb temperature line (abscissa) and the vapour pressure or humidity ratio (ordinate). The Left Hand Side of the psychrometric chart is bounded by the saturation line. Figure below shows the schematic of a psychrometric chart. Psychrometric charts…

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Humid specific heat

From the equation for enthalpy of moist air, the humid specific heat of moist air can be written as: where cpm = humid specific heat, kJ/kg.K cp = specific heat of dry air, kJ/kg.K cpw = specific heat of water vapor, kJ/kg W = humidity ratio, kg of water vapor/kg of dry air. Since the second term in…

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Enthalpy

Enthalpy is the measure of heat energy in the air due to sensible heat or latent heat. Sensible heat is the heat (energy) in the air due to the temperature of the air and the latent heat is the heat (energy) in the air due to the moisture of the air. The sum of the…

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Important psychrometric properties

Dry bulb temperature (DBT) is the temperature of the moist air as measured by a standard thermometer or other temperature measuring instruments. Saturated vapour pressure (psat) is the saturated partial pressure of water vapour at the dry bulb temperature. This is readily available in thermodynamic tables and charts. ASHRAE suggests the following regression equation for saturated vapour…

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MASS AND MOLE FRACTION

If we want to deal with the problems of gas mixture, two laws should always be taken into consideration. One is the conservation of mass. And the other is the conservation of moles for a nonreacting mixture. Assuming that a gas mixture consists of components According to the conservation of mass, we have: According to…

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Second Law of Thermodynamics

The application of first law of thermodynamics process led to the establishment of a new property named internal energy (U), the application of the second law to a process leads to the establishment of another new property names as entropy (S). Entropy is defined as follows For a reversible thermodynamic process between two states, from state 1…

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