The MATLAB program (**psychro.m**)
(shown below) is used to plot the Simplified Psychrometric Chart
shown above. Data is read from 2 datafiles containing water saturation
temperature/pressure data, file **t_pg**
over 1°C intervals for plotting the saturation and constant
relative humidity curves, and file **t_pg1**
over 5°C intervals for plotting the wet-bulb temperature and
enthalpy lines. The source of the data:
**NIST
Chemistry WebBook** - accessed Feb 2008

The following four equations (refer back to
**Section a** and
**Section b**) are evaluated in the program:

specific humidity: (sometimes known as: humidity ratio or absolute humidity):

:

Notice in the psychrometric chart that the oblique enthalpy axis is given by = T+5, for T varying between 0°C to 25°C. Substituting this value of in the enthalpy equation (above) leads to the intercept of the enthalpy line with the enthalpy axis as follows:

h = T + 2.5 (T + 5)

or finally: T = (h - 12.5)/3.5

Thus the enthalpy lines can be plotted parallel to, but independant of the wet bulb temperature lines.

The complete MATLAB program follows. Note that it is the convention in programming that all variable names begin with lower case letters. Thus t represents temperature [°C], p represents pressure [kPa], w (in place of ) represents specific humidity [grams / kg dry air] (aka: absolute humidity, humidity ratio), h represents enthalpy (kJ/kg dry air) and the suffix g represents the saturated vapor state (following the convention used in steam tables).

% Plotting the Simplified Psychrometric Chart% Izzi Urieli 2/27/2008tpg = dlmread('t_pg','\t'); % saturation temp/pressuret = tpg(:,1); % temperature (C)pg = tpg(:,2); % saturation vapor pressure (kPa)patm = 101.325; % standard atmosphere (kPa)rair = 0.287; % gas constant of air (kJ/kg.K)wg = 622*pg./(patm-pg); % saturation specific humidityplot(t,wg,'r-')holdgridfor phi = 0.1:0.1:0.4, % phi = relative humidity 10% - 40%w = 622*phi*pg./(patm-phi*pg);plot(t,w)endfor phi = 0.6:0.2:0.8, % phi = 60%, 80%w = 622*phi*pg./(patm-phi*pg);plot(t,w)end% specific volume and enthalpy/wet-bulb-temptpg1 = dlmread('t_pg1','\t');t1 = tpg1(:,1); % saturation temperature (C)pg1 = tpg1(:,2); % saturation pressure (kPa)wg1 = 622*pg1./(patm-pg1); % saturation specific humidity% specific volume of dry air (cubic m/kg dry air) (green)vol = rair.*(t1+273)./(patm-pg1) % specific vol at saturationtv0 = patm*vol/rair-273; % air temperature at zero humidityfor i = 1:7,plot([t1(i),tv0(i)],[wg1(i),0],'g-')end% wet bulb temperature (also enthalpy) lines (red)h = t1 + 2.5*wg1 % enthalpy (kJ/kg-dry-air) (displayed)t0 = h; % temperature at zero humidity for enthalpy hfor i = 1:6,plot([t1(i),t0(i)],[wg1(i),0],'r-')end% enthalpy axis and enthalpy lines (black)for h = 10:10:110, % enthalpy (kJ/kg-dry-air)t0 = h; % temperature at zero humidityt1 = (h - 12.5)/3.5; % temperature on the enthalpy axisw1 = t1 + 5; % specific humidity on the enthalpy axisplot([t0,t1],[0,w1],'k-')endplot([0,25],[5,30],'k-') % the oblique enthalpy axisaxis([0,50,0,30]) % limit the range of the charttitle('Simplified Psychrometric Chart')xlabel('Dry Bulb Temperature (deg C)')ylabel('Specific Humidity (gm vap/kg dry air)')

Notice in the program that both the specific volume (vol) and the enthalpy values (h) are displayed (no semicolon) thus they can be subsequently added to the plot together with the relevant saturation/wet bulb temperatures. relative humidity values.and enthalpy values on the enthalpy axis.

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