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Thermodynamics ME 2105 Credit: 4.0 Psychrometry Presented By 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 = = ṜT and = = ṜT Properties of where V is the volume and T the temperature of air, the subscripts w and s indicating unsaturated and saturated states of air respectively. Atmospheric ϕ= = Air = ℎ ℎ = = 3/5/2020 Md. Shariful Islam, Lecturer, Department of Mechanical Engineering, KUET 7 Specific Humidity: or humidity ratio, W , is defined as the mass of water vapour (or moisture) per unit mass of dry air in a mixture of air and water vapour. Properties of If G = mass of dry air Atmospheric m = mass of water vapour W = Air Specific humidity is the maximum when air is saturated at temperature T, or = = 3/5/2020 Md. Shariful Islam, Lecturer, Department of Mechanical Engineering, KUET 8 If dry air and water vapour behave as ideal gases = = Now specific humidity W = = ./. Properties of = ./ Atmospheric = 0.622 − Air where p is the atmospheric pressure, pw and pa are the partial pressure of water vapor and dry air. If pw is constant, W remains constant. If air is saturated at temperature T = = 0.622 − where ps is the saturation pressure of water vapour at temperature T. 3/5/2020 Md. Shariful Islam, Lecturer, Department of Mechanical Engineering, KUET 9 Degree of Saturation: The degree of saturation, μ, can be defined as the ratio of the actual specific humidity and the saturated specific humidity, both at the same temperature T. μ = .∗ Properties of = .∗ Atmospheric = ∗ Air If ϕ= = 0, = 0, = 0, = 0, . μ=0 If ϕ=100%, = , = , μ=1 Therefore, μ varies between 0 and 1. 3/5/2020 Md. Shariful Islam, Lecturer, Department of Mechanical Engineering, KUET 10 It is known that, the degree of saturation, μ, can be defined as the ratio of the actual specific humidity and the saturated specific humidity, both at the same temperature T. 0.622 ∗ − μ = = 0.622 ∗ − − Properties of = ∗ Atmospheric − (1 − ) = ∗ Air (1 − ) ( ) ( ) = ∗ = ϕ∗ ( ) ( ∗ ) ϕ Upon simplification……(see lecture notes) ϕ= () 3/5/2020 Md. Shariful Islam, Lecturer, Department of Mechanical Engineering, KUET 11 Dry Bulb Temperature: Dry bulb temperature (dbt) is the temperature recorded by the thermometer with a dry bulb. Wet Bulb Temperature: Wet bulb temperature (wbt) is Properties of the temperature recorded by a thermometer when the Atmospheric bulb is enveloped by a cotton wick saturated with water. As the air stream flows past it, some water evaporates, taking Air the latent heat from the water-soaked wick, thus decreasing its temperature. Energy is then transferred to the wick from the air. When equilibrium condition is reached, there is a balance between energy removed from the water film by evaporation and energy supplied to the wick by heat transfer, and the temperature recorded is the wet bulb temperature. 3/5/2020 Md. Shariful Islam, Lecturer, Department of Mechanical Engineering, KUET 12 Dew Point Temperature: It is the temperature of air recorded by a thermometer, when the moisture (water vapour) present in it begins to condense. In other words, Properties of the dew point temperature is the saturation temperature (tsat) corresponding to the partial pressure of water Atmospheric vapour (pw). Air Note: For saturated air, the dry bulb temperature, wet bulb temperature and dew point temperature is same. Dew Point Depression: It is the difference between the dry bulb temperature and dew point temperature of air. 3/5/2020 Md. Shariful Islam, Lecturer, Department of Mechanical Engineering, KUET 13 Psychrometer: A psychrometer is an instrument which measures both the dry bulb and the wet bulb temperatures of air. And ultimately humidity and other properties. Properties of Atmospheric Air Adiabatic Saturation Temperature: Adiabatic saturation temperature refers to a temperature at which air can be brought to saturation state, adiabatically, by the evaporation of water into flowing air. The device used for this type of process is known as adiabatic saturator. The adiabatic saturation temperature and the wet bulb temperature are taken to be equal for all practical purposes. 3/5/2020 Md. Shariful Islam, Lecturer, Department of Mechanical Engineering, KUET 14 When unsaturated air flows over a long sheet of water in an insulated chamber, the water evaporates, and the specific humidity of the air increases. Both the air and water are cooled as evaporation takes places. The process continues until the energy transferred from the air Adiabatic to the water is equal to the energy required to vaporize the Saturation water. Process 3/5/2020 Md. Shariful Islam, Lecturer, Department of Mechanical Engineering, KUET 15 Since the system is insulated and no work is done, the first law yields ℎ + ℎ + − ℎ = ℎ + ℎ where (m2 - m1) is the mass of water added, hf2 is the Adiabatic enthalpy of the liquid water at t2(= twb2), ha is the specific enthalpy of dry air, and hw is the specific enthalpy of Saturation water vapour in air. Dividing by G, and since hw2 = hg2 ℎ + ℎ + − ℎ = ℎ + ℎ Process Solving for W1 (ℎ − ℎ) + (ℎ − ℎ ) = ℎ − ℎ 3/5/2020 Md. Shariful Islam, Lecturer, Department of Mechanical Engineering, KUET 16 − + ℎ = ℎ − ℎ Adiabatic Where W = = = 0.622 Saturation The enthalpy of the air-vapour mixture is given by Process ℎ = ℎ + ℎ where h is the enthalpy of the mixture per kg of dry air (it is not the specific enthalpy of the mixture) ℎ = ℎ + ℎ 3/5/2020 Md. Shariful Islam, Lecturer, Department of Mechanical Engineering, KUET 17 Again, Let us consider the energy balance for the adiabatic saturation process ℎ + ℎ + − ℎ = ℎ + ℎ Since ha + Whw = h kJ/kg dry air ℎ − ℎ = ℎ − ℎ Adiabatic where subscript 2 refers to the saturation state, and subscript 1 Saturation denotes any state along the adiabatic saturation path. Therefore, Process ℎ − ℎ = Since Whf2 is small compared to h (of the order of 1 or 2%) ℎ = indicating that the enthalpy of the mixture remains constant during an adiabatic saturation process. 3/5/2020 Md. Shariful Islam, Lecturer, Department of Mechanical Engineering, KUET 18 It is a graphical representation of the various thermodynamic properties of moist air. The psychrometric chart is very useful for finding out the properties of air (which are required in the field of air conditioning) and eliminating lot of calculations. There is a slight variation in the charts prepared by different air- conditioning manufactures but basically they are all alike.
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