05-12-2012, 12:25 PM
Psychrometry and Air Processes
Psychrometry.ppt (Size: 2.78 MB / Downloads: 60)
What is meant by psychrometry?
Psychrometry is the science of studying the thermodynamic properties of moist air and the use of these properties to analyze conditions and processes involving moist air.
Moist air is a mixture of dry air and water vapour. In atmospheric air, water vapour content varies from 0 to 3% by mass.
Psychrometrics or psychrometry are terms used to describe the field of engineering concerned with the determination of physical and thermodynamic properties of gas-vapor mixtures.
The thermal parameters of moist air
Atmospheric air
Atmospheric air is not completely dry but a mixture of dry air and water vapor.
In atmospheric air, the content water vapor varies from 0 to 3% by mass.
The processes of air-conditioning and food refrigeration often involve removing water from the air (dehumidifying), and adding water to the air (humidifying).
Dry air
The pure dry air is a mechanical mixture of oxygen, nitrogen, carbon-dioxide, hydrogen argon, neon, helium etc.
The oxygen and nitrogen have the major portion of the combination. Neglecting the minor components.
Considered to be a mixture of oxygen 21% by volume together with nitrogen 79% by volume.
Moist air
It is a mixture of dry air and water vapour. The amount of water vapour present in the air, depends upon the absolute pressure and temperature of the mixture
Measurement of wet bulb temperature
A simple wet bulb thermometer used to measure the wet bulb temperature is based on the principle of the adiabatic saturation process.
It is an ordinary thermometer being wrapt with a cloth sleeve of wool or flannel, around its bulb.
The cloth sleeve should be clean and free of oil and thoroughly wet with clean fresh water.
The water in the cloth sleeve evaporates as the air flows at a high velocity (≥2.5m/sec).
The evaporation, which takes the heat from the thermometer bulb, lowers the temperature of the bulb.
The thermometer indicates approximately the wet bulb temperature tWB.