Enthalpy-humidity chart

In the calcuiation of enthalpies it is necessary to define some standard reference state at which the t.-nthalpy s taken as zero. It is most convenient to take the melting fKfint of the malef ial constituting the vapour as the reference temperature, and the liquid state of the uajerial as its standard state, 

11 H is the enthalpy of the humid gas per unit mass of dry gas, Ha the enthalpy of the lirv gas per unit mass, the enthalpy of the vapour per unit mass, Ca the specific heat oj she gas at constant pressure, Cw the specific heat of the vapour at constant pressure, othe temperature of the humid gas, 9q the reference temperature, k the latent heat of. aporisation of the liquid at Oq and the humidity of the gas. 

If the gas contains more liquid or vapour than is required to saturate it at the tem( rature in question, either the gas will be supersaturated or the excess material will be present in the form of liquid or solid according to whether the temperature is gretrter or less than the reference temperature 0o- The supersaturated condition is unstable and will not be considered further. 

If the temperature 9 is greater than 9q and if the humidity is greater than the huimdity jfo. of saturated gas, the enthalpy H per unit mass of dry gas is given by  

If the temperature 9 is less than 9q, the corresponding enthalpy H is given by  

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Two cases may be considered to illustrate the use of enthalpy-humidity charts. These are the mixing of two streams of humid gas and the addition of liquid or vapour to a gas. 

FIG. 12-43 Enthalpy humidity chart used to generate the results in Table 12-13 plots humidity (abscissa) against enthalpy (lines sloping diagonally from top left to bottom right). 

Aspen Technology http //www.aspentech.com. PSYCHIC, one of the Process Tools, generates customized psychrometric charts. Mollier and Bowen enthalpy-humidity charts are produced in addition to Grosvenor. Any gas-vapor system can be handled as well as air-water data supplied for common organic solvents. Can draw operating lines and spot points, as shown in Fig. 12-7. 

Another alternative is the graphic representation of conditions of constant relative saturation on a vapor pressure-temperature chart (Figure 1.3). The curve for 50% relative saturation shows a partial pressure equal to one-half of the equilibrium vapor pressure at any temperature. A common method of portraying humidity charts is by using the enthalpy-humidity chart indicated earlier [10]. 

Construction of psychrometric charts by computer methods is common. Three types of charts are most popular Grosvenor chart, Grosvenor (1907) (or the psychrometric chart), Mollier chart, Mollier (1923) (or enthalpy-humidity chart), and Salin chart (or deformed enthalpy-humidity chart) these are shown schematically in Figure 3.3. 

Pakowski Z., 1986, Computer drawing of enthalpy-humidity charts. Hung. J. Ind. Chem., 14, 225. 

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