Thermodynamics ME 2105 Credit: 4.0 Psychrometry Presented By
Md. Shariful Islam Lecturer Department of Mechanical Engineering Khulna University of Engineering & Technology The psychrometry is that branch of engineering science, which deals with the study of moist air i. e. dry air mixed with water vapour. It also includes the study of behaviour of dry air and Psychrometry water vapour mixture under various sets of conditions. Though the earth's atmosphere is a mixture of gases including nitrogen (N2), oxygen (O2), argon (Ar)
and carbon dioxide (CO2), yet for the purpose of psychrometry, it is considered to be a mixture of dry air and water vapour only.
3/5/2020 Md. Shariful Islam, Lecturer, Department of Mechanical Engineering, KUET 2 A hygroscopic substance is the substance that readily attracts water from its surroundings, either by absorption or by adsorption. Most hygroscopic materials are salts, but many other materials display the property Hygroscopic Examples: Honey, glycerin, ethanol, methanol, concentrated H SO , concentrated NaOH Substance 2 4
3/5/2020 Md. Shariful Islam, Lecturer, Department of Mechanical Engineering, KUET 3 Dry Air: Dry air is a mechanical mixture of the gases: oxygen, nitrogen, carbon dioxide, hydrogen, argon, neon, krypton, helium, ozone, and xenon. However, oxygen and nitrogen make up the major part of the combination. Dry air is considered to consist of 21% oxygen and Properties of 79%nitrogen by volume, and 23% oxygen and 77% nitrogen by mass. Atmospheric Moist Air: It is a mixture of dry air and water vapour. Air The amount of water vapour, present in the air, depends upon the absolute pressure and temperature of the mixture. Saturated Air: It is a mixture of dry air and water vapour, when the air has diffused the maximum amount of water vapour into it.
3/5/2020 Md. Shariful Islam, Lecturer, Department of Mechanical Engineering, KUET 4 Completely dry air does not exist in nature. Water vapour in varying amounts is diffused through it. If pa and pw are the partial pressures of dry air and water vapour respectively, then by Dalton’s law of partial pressures
pa + pw= p Properties of where p is the atmospheric pressure, Atmospheric So, mole-fraction of dry air, xa p = a = p ( as p = 1 atm) Air a
and mole reaction of water vapour, xw p = = p w
Since pw is very small, the saturation temperature of water vapour at pw is less than atmospheric temperature, tatm
3/5/2020 Md. Shariful Islam, Lecturer, Department of Mechanical Engineering, KUET 5 Relative humidity: It is the ratio of actual mass of water vapour in a given volume of moist air to the mass of water vapour in the same volume of saturated air at Properties of the same temperature and pressure. Atmospheric Relative humidity (RH ϕ ) can also defined as the ratio of partial pressure of water vapour, pw, in a mixture to the Air saturation pressure, ps, of pure water, at the same temperature of the mixture. ϕ =
3/5/2020 Md. Shariful Islam, Lecturer, Department of Mechanical Engineering, KUET 6 If water is injected into unsaturated air in a container, water will evaporate, which will increase the moisture content of the air, and pw will increase. This will continue till air becomes saturated at that temperature, and there will be no more evaporation of water. For saturated air, the relative humidity is 100%. Assuming water vapour as an ideal gas