Humid enthalpy

Example 8 Determination of Air Properties For a barometric pressure of 25.92 inHg (Ap = —4), a dry-bulb temperature of 90 F, and a wet-bulb temperature of 70 F determine the following absolute humidity, enthalpy, dew point, relative humidity, and specific volume. 

Enthalpy data are given on the basis of kilojoules per kilogram of diy air. Entbalpy-at-saturation data are accurate only at the saturation temperature and humidity. Enthalpy deviation curves permit enthalpy corrections for humidities less than saturation and show how the wet-bulb-temperature hues do not precisely coincide with constant-enthalpy, adiabatic cooling hnes. 

Figure 13.5. Humidity-enthalpy diagram for air-water vapour system at atmospheric pressure...

Figure 13.7. Humidity-enthalpy diagram lor air-water system — rectangular axes...

The shape of the humidity-enthalpy line for saturated air is such that the proportion of the total area of the diagram representing saturated, as opposed to supersaturated, air is small when rectangular axes are used. In order to enable greater accuracy to be obtained in the use of the diagram, oblique axes are normally used, as in Figure 13.5, so... 

The gas formed by mixing two unsaturated gases may be either unsaturated, saturated, or supersaturated. The possibility of producing supersaturated gas arises because the 100 per cent humidity line on the humidity-enthalpy diagram is concave towards the humidity... 

Humid enthalpy H Heat content at a given temperature T of unit mass of dry air and the moisture it contains, relative to a datum temperature To, usually 0°C. As water is liquid at 0°C, the humid enthalpy also contains a term for the latent heat of water. If heat capacity is invariant with temperature, H = (Cpg 4-Cp Y)(T-To) + XqY, where Xo is the latent heat of water at 0°C, 2501 kj/kg (1075 Btu/lb). In practice, for accurate calculations, it is often easier to obtain the vapor enthalpy H from steam tables, when H = H +... 

Since the enthalpy is an extensive property, it could be expected that the enthalpy of a humid gas is the sum of the partial enthalpies of the constituents and a term to take into account the heat of mixing and other effects. The humid enthalpy Iq is defined as the enthalpy of a unit mass of dry gas and its associated moisture. With this definition of enthalpy. 

The calculations of absolute humidity, relative humidity, wet-bulb temperature, dew-point temperature, humid volume, humid heat, and humid enthalpy are based on information found in Pakowski and Mujumdar (2006) and Moyers and Baldwin (1997). 

The humidity enthalpy hp is marked on the side scale (point Qi for vaporous moisture and Q2 for liquid moisture, qualitatively). The line Pol P and Qj or Q2, respectively, gives the direction of the mixing line. Mixing points M, and M2 are found at the intesection of the parallels PQi or PQ2 through 1 with the lines X parallel to the ordinate according to Eq. (5-26). 

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