Water Vapor Saturation Data (Tsat, Pg) and the Psychrometric Chart

The following Water Vapor Saturation Data is required in order to plot the Saturation Curve (Relative Humidity Φ=100%) of the Psychrometric Chart.

Temp Pressure   Temp Pressure Tsat (°C) Pg (kPa)   Tsat (°C) Pg (kPa) 0.01 0.6117   25 3.1699 1 0.6571   26 3.3639 2 0.7060   27 3.5681 3 0.7581   28 3.7831 4 0.8136   29 4.0092 5 0.8726   30 4.2470 6 0.9354   31 4.4969 7 1.0021   32 4.7596 8 1.0730   33 5.0354 9 1.1483   34 5.3251 10 1.2282   35 5.6290 11 1.3130   36 5.9479 12 1.4028   37 6.2823 13 1.4981   38 6.6328 14 1.5990   39 7.0002 15 1.7058   40 7.3849 16 1.8188   41 7.7878 17 1.9384   42 8.2096 18 2.0647   43 8.6508 19 2.1983   44 9.1124 20 2.3393   45 9.5950 21 2.4882   46 10.099 22 2.6453   47 10.627 23 2.8111   48 11.177 24 2.9858   49 11.752 25 3.1699   50 12.352

The basic information used to construct a psychrometric chart is the water vapor saturation data (Tsat, Pg) which is obtained from steam tables over the range from Tsat = 0.01°C through 50°C. The specific humidity ω is then evaluated using the relative humidity Φ as a parameter to produce the various relative humidity curves (blue lines) as follows: where P is the standard atmospheric pressure 101.325 [kPa]. The saturation curve (100% relative humidity) is also known as the dew point curve is drawn as a red line. Notice that on the saturation curve the wet and dry bulb temperatures have the same values. For more information refer to Chapter 10b on the Psychrometric Chart ____________________________________________________________ Source of the (Tsat, Pg) data: NIST Chemistry WebBook - accessed Feb 2008

The major simplifying assumption in the construction of the chart is that the enthalpy of the mixture is assumed to be constant throughout the adiabatic saturation process (described in Chapter 10a). This implies that the evaporating liquid added does not significantly affect the enthalpy of the air-vapor mixture, leading to the constant slope wet bulb temperature / enthalpy lines defined by:

Note that this simplyfied linear equation results in an error of less than 1% over the temperature range 0.01°C to 50°C.

Finally the specific volume curves which complete the chart are given by:

Refer to the complete MATLAB program for Plotting the Psychrometric Chart using these equations.

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Engineering Thermodynamics by Israel Urieli is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License