PSYCHROMETRICS 457dharm
\M-therm\Th10-1.pm5- Constant relative humidity lines in per cent are indicated by marking off vertical dis-
tances between the saturation line or the upper curved line and the base of the chart. The relative
humidity curve depicts quantity (kg) of moisture actually present in the air as a percentage of the
total amount possible at various dry bulb temperatures and masses of vapour. - Enthalpy or total heat at saturation temperature in kJ/kg of dry air is shown by a
diagonal system of co-ordinates. The scale on the diagonal line is separate from the body of the
chart and is indicated above the saturation line. - Wet bulb temperatures are shown on the diagonal co-ordinates coinciding with heat co-
ordinates. The scale of wet bulb temperatures is shown on the saturation curve. The diagonals run
downwards to the right at an angle of 30º to the horizontal.
Dry bulb temperature Cº Enthalpy (h) k cal/kg dry air(^01020304050)
–90
–80
–70
–60
–50
–40
–30
–1.0
–20 –10 (^0102030)
1.0
80
70
60
40
Enthalpy (h) k cal/ kg dr y air
10
15
20
25
30
10% Relative humidity
Volume m /kg dry Air
3
90%80%
60%50% 40%
30%
20%
70%
0.78 0.79
0.81 0.82
0.830.84
0.86
0.87
0.88
0.89
0.90
0.91
0.92
0.93
0.94
0.80
0.85
.028
0.95
.004
.008^90
.012
.016
.020
.024
50
Humidity ratio (W) kg/kg dry air
Saturation temp°.C
Fig. 10.5. Carrier chart.
- The volume of air vapour mixture per kg of dry air (specific volume) is also indicated by
a set of diagonal co-ordinates but at an angle of 60º with the horizontal.
The other properties of air vapour mixtures can be determined by using formulae (already
discussed).
In relation to the psychrometric chart, these terms can quickly indicate many things about
the condition of air, for example : - If dry bulb and wet bulb temperatures are known, the relative humidity can be read from
the chart. - If the dry bulb and relative humidity are known, the wet bulb temperature can be deter-
mined. - If wet bulb temperature and relative humidity are known, the dry bulb temperature can
be found.