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Professor: Grant C, Baker Psychrometric Charts General Mathematical relationships can be used to derive the needed air properties for design, However, it is much easier for these calculations to be done once and placed on a graph for reference, Thus, Psychrometric Charts were created to provide this convenience. Psychrometric Charts are available at 1 atm as well as other pressures commonly used for design work. For discussion of Psychrometric Charts, some important notes are: Water vapor is treated as an ideal gas. Dry air is air (oxygen and nitrogen) containing no water vapor (HyOvapor). The subscript "a" refers to dry air. The subscript "v" refers to water vapor in air. The subscript "g" refers to saturated water vapor. hg =Napy air “ep T=C.005 ki/kgeo CHT where Tis in °C, Eq.1a Ab, = Abary air CpAT=(1.005 kI/kge? C)AT where T is in °C, Eq.1b or hy =hyy air =c,T=(0.24 Btu/lbm-° F)T where T is in °F Eq.2a Ab, = Ahary air CpAT=(0.240 Btw/lbm-° F)AT where T is in °F Eq.2b NOTE: The reference temperature for h, on a Psychrometric Chart in English units is different than for ST units. For the Psychrometric Chart in English units, the reference temperature is 0°F. For the Psychrometric Chart in SL units, the reference temperature is 0°C, Remember, 0°F # 0°C. In other words, since h depends on h, , the h values determined from SI Psychrometric Charts are not comparable to h values determined from the English Psychrometric Chart for the same air conditions. For example, at T= 70°F and $= 60%, we get hg «27.3 Btu/lbm dry air. On a SI Psychrometric Chart for the same air conditions (i.e. T=70°F (=21.11°C) and o = 60% ) we get hg « 45 kJ/kg dry air. Using the conversion factor 2.326 kJ/kg=1 Btu/lbm we get 1 Btu/lom 2.326 kI/kg Therefore, h values from the SI Psychrometric Charts are not comparable to h values from the English Psychrometric Charts and vice-versa. However, the difference in reference temperatures can be corrected (i.e. hg must have the h, contribution corrected from 0°F to 32°F. So, the correction factor (conversion) would be hg #45 ki/kg dry ar | Btu/Ibm dry air # hy hig, = hy —0.240(32° F) = hy, 7.68 Btu/ Ibm dryait p=p, +p, (ie. total pressure is equal to sum of partial pressures) Eq.3a Py = OP cat + OPg Eq.3b For Eqs. 4 to 6, ~10
