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Psychrometry:, This branch of science deals with the study of properties of moist air and its behaviour under, different conditions. The properties of moist air include Dry-bulb Temperature (DBT), Wetbulb, Temperature (WBT), humidity. Relative Humidity (RH). etc., Dry Air:, The dry air includes all components of atmospheric air except water vapour (moisture). Thus, the dry air is, the mixture of oxygen, nitrogen, carbon dioxide, argon, etc., , Moist Air:, The term moist air is the mixture of the dry air and water vapour, and each component of the mixture, behaves as an ideal gas at states under considerations., Unsaturated air:, In atmospheric air, the moisture appears in the form of superheated vapour as invisible gas at, atmospheric pressure. The mixture of superheated vapour and dry air is called unsaturated air., Saturated Air:, The saturated air is moist air which contains maximum possible water vapour. The partial pressure of, water vapour in saturated air is equal to the saturated pressure of steam corresponding to the, temperature of moist air. Cooling of saturated air causes the separation of moisture and formation of fog., , Dry-Bulb Temperature (DBT), It is the temperature of air measured or recorded by a thermometer. It is denoted by T or Tdb or DBT., Wet-Bulb Temperature (WBT), It is the temperature of air recorded by a thermometer when its bulb is covered with wet wick or cloth, over which air is moving at a velocity at 2.5 to l0 m/s., Dew Point Temperature (DPT), It is the temperature of air recorded by a thermometer when the moisture present in it starts, condensing. It is denoted by Tdp or DPT., , Consider that a certain sample of unsaturated moist air shown by state A in given figure, is cooled at, constant pressure slowly by passing over the cooling coil. Its temperature goes on decreasing till it, reaches a temperature DPT, at which the first drop of dew will be formed. It means that the water, vapour in the air starts condensing. In the case of dehumidification of air, it is required to maintain, the temperature of cooling coil well below DPT. During the cooling process, the partial pressure of, water vapour and the specific humidity w remain constant until the vapour starts condensing.

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The DPT (saturated temperature) can be found from the steam table corresponding to the partial, pressure of water vapour pv, , Humidity Ratio (Specific Humidity), It is the ratio of mass of water vapour to the mass of dry air contained in the sample air. It is denoted, by V. It is normally expressed in g/kg of dry air., w = mass of water vapour in air/mass of dry air in air = mv/ma, Let pa, va T, ma and Ra be the pressure, specific volume, DBT in K, mass and gas constant, respectively., Let pv, vv T, mv and Rv be the pressure, specific volume, DBT in K, mass and gas constant, respectively., Using equation, pv = mRT, For dry air, Pa Va = maRaT, For water vapour, Pv Vv = mvRvT, As air and water vapour have the same volume and temperature, we get, ∗, ∗, , =, , ∗, ∗, , =, , ∗, ∗, , =, , ∗, ∗, , ∗, ∗, , Gas constant R = 287 J/kg-K, Rv = 461 J/kg-K, ∗287, , =, , =0.622 kg/kg dry air., ∗461, Where, pv and pa are partial pressure of water vapour and dry air respectively., According to Daltons law of partial pressure,, W=, , P=, , +, =, , W=0.622, , −, kg/kg dry air

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Q1. The dry-bulb temperature and dew point temperature of atmospheric air are 30°C and 14°C,, respectively. If the barometer reading is 758 mm of Hg, determine the humidity ratio., From steam tables , the partial pressure of water vapour corresponding to DP temperature 140C is, 0.015973 bar., Atmospheric pressure is 758 mm Hg (758 x 0.001332 = 1.0105656 bar)., , Absolute Humidity, It is the mass of water vapour present in one cubic metre of dry air. It is expressed in terms of grains, per cubic metre of dry air (g/m3 of dry air). Many a time it is expressed in terms of grains per m3 of, dry air. One kg of water vapour is equal to 15,430 grains., Pv V = mv RvT, Where pv = vapour pressure in air or saturation pressure at dew point, V= volume of air, which is also of water vapour, T = dry bulb temperature, Rv = gas constant of water vapour (462 kJ/kg-K), mv = mass of water vapour in kg, Vapour density = mv/V = pv/RvT, , Degree of saturation (m), It is the mass of water vapour in a sample of air to the mass of water vapour in the same air when it is, saturated at the same temperature. Mathematically,, , Where ps = partial pressure of water vapour when air is separated. It is obtained from steam tables, corresponding to DBT (Tdb)., Pv = partial pressure of water vapour in a moist air., p = total pressure of moist air.

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Relative humidity, RH = pv/ps = 0 when moist air is totally dry, i.e. which does not contain water, vapour., If the moist air is saturated, then pv =ps then RH = 1 and µ = 1. It shows that the degree of saturation, varies between 0 and 1., Pressure, In air conditioning terms, air means a mixture of water vapour and remaining gases., So by Dalton's law of partial pressure, p = pa + pv, where p = total pressure of air, , Relative humidity (RH), It is the ratio of mass of water vapour in a given volume of air at any temperature and pressure to the, maximum amount of mass of water vapour which the same volume of air can hold at the same, temperature conditions. The air contains maximum amount of water vapour at the saturation, conditions., Let vv and vs be the specific volumes of water vapour in the actual and moist saturated air at, temperature T and in a volume V.

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Relative humidity is therefore defined as the ratio of vapour pressure in a sample of air to vapour, pressure of saturated air at the same temperature, i.e., RH = vapour pressure of water vapour/vapour pressure of saturated, air at the same temperature, Relative humidity is measured in percentage. It has great influence on evaporation of water in the air, and therefore on the comfort of human beings., Enthalpy of air (h), , Air is a homogeneous mixture of dry air and water vapour. Therefore, enthalpy of air is found, taking the sum of enthalpy of dry air and enthalpy of water vapour in the moist air., Enthalpy of air/kg of dry air = Enthalpy of dry air + enthalpy of w kg of water vapour, , = ha + whv, Considering the change in enthalpy of perfect gas as a function of temperature only, the enthalpy of, dry air part, above a datum of 0°C, can be found as, , ha = cpaTdb = 1.005Tdb kJ/kg, Assuming enthalpy of saturated liquid at 0°C as zero, the enthalpy of water vapour at point A in, given figure, is expressed as, , hv = cpwTdp + (hfg)dp + cpv(Tdb – Tdp), where ,cpw = specific heat of water vapour (kJ/kg-K), Tdb = dry-bulb temperature

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Tdp = dew point temperature, (hfg)dp = latent heat of vaporization at dew point temperature, cpv = specific heat of water vapour (kJ/kg-K)., , Specific volume, It is the volume of air per unit mass of dry air. It is measured in m3/kg of dry air. Air flow is, measured by anemometer as volume rate of flow and the heat added or cooling requires mass, flow rate. So specific volume is essential to relate the two.

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Q2. For a dry-bulb temperature of 25°C and a relative humidity of 50%, calculate the following, for air when the barometric pressure is 740 mm Hg. Find without using psychrometric chart:, (a) Partial pressure of water vapour and dry air, (b) Dew point temperature, (c) Specific humidity, (d) Specific volume, (e) Enthalpy, Dry-bulb temperature, Tdb = 25°C, Relative humidity, RH = 0.50, Atmospheric (barometric) pressure, pb = 740 mm Hg, = 740 x 133 Pa = 98,420 N/m2 = 98.420 kPa

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PSYCHROMETRIC CHART, A psychrometric chart is a graphical representation of the thermodynamic properties of moist air., These properties of moist air vary with atmospheric pressure and altitude. One such chart for, atmospheric pressure of 1.01325 bar at sea level is shown in following figure, , The variables shown on a complete psychrometric chart are: DBT, WBT, relative humidity, total, heat; vapour pressure and the actual moisture content of the air., As shown in this figure the dry-bulb temperature is taken as the x-axis and the mass of water vapour, per kg of dry air as the ordinate., The following illustrations will help in locating the different lines and scales on the chart., DBT lines, These dry-bulb temperature lines extend vertically upwards and there is one line for each degree of, temperature., WBT lines, The wet-bulb temperature scale is found along the 'in-step' of the chart extending from the toe to the, top. These lines extend diagonally downwards to the right. There is one line for each degree of, temperature.

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RH lines, On the psychrometric chart, the relative humidity lines are the only curved lines on it. The various, relative humidifies are indicated on the lines themselves. The 100% RH line or saturation curve, becomes the boundary of the chart on the left side. The region beyond this line is the supersaturated, zone or fog zone., , Specific humidity lines, The scale for specific humidity is a vertical scale on the right side of psychrometric chart. The scale, is in grams of moisture per kilogram dry air., , DPT lines, The scale for dew point temperature is identical to the scale of WBT lines. The DPT lines run, horizontal to the right.

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Specific volume lines, The specific volume lines are drawn along the sole chart and they are equally-spaced diagonal lines., , Specific enthalpy lines, The specific enthalpy scale is located along the 'in-step' of the chart. These lines are similar to WBT, lines. Specific enthalpy lines indicate the total heat content.