8. Psychrometric chart
8.1 Psychrometric chart
Psychrometric chart shows the characteristics of moist
air by a diagram, including i-x, t-x, and t-i diagram. The
i-x diagram is shown in the right side. A point express-
ing the state of air in the psychrometric chart is called
a state point, which is used for finding a dry-bulb tem-
perature,wet-bulb temperature, dew point temperature,
relative humidity, absolute humidity, enthalpy, etc. For
example, at dry-bulb temperature 26°C, and relative
humidity 50%, the enthalpy, absolute humidity, wet-
bulb temperature, dew point temperature and specific
volume is found from the psychrometric chart as fol-
lows.
Enthalpy I = 12.6 (Kcal/kg)
Absolute humidity x = 0.0105 (kg/kg)
Wet-bulb temperature t’ = 18.71 (°C)
Dew point temperature t” = 14.70 (°C)
Specific volume v = 0.862 (m
3
/kg)
8.2 Discomfort index (DI)
In summer, Discomfort Index is frequently used in the weather forecast on TV or newspapers. Man feels hot
or dry according as the relative humidity is high or low, but not the absolute humidity. Generally, now that we
feel discomfortable at high relative humidity, the state which is felt uncomfortable at both dry-bulb and wet-
bulb temperature may be expressed empirically as an equation, Discomfort Index (DI), which is used as an
emotional index for summer temperature and humidity condition.
* DI = (Wet-bulb tempe Dry-bulb temperature) x 0.72 + 40.6
* Degree of DI
Figure. Psychrometric chart
0.0105
V=0.862
t'=18.71
I=12.6
t"=14.70
=50%
26
[
°
C]
DI
Degree of DI
86
80
75
70
68
Intolerable discomfort (Unable to bear the sultry weather.)
Everybody feels uncomfortable. (Sweat at hot weather.)
Half or more people feel uncomfortable. (Feel the heat a little.)
Start to feel uncomfortable.
Comfortable
TCL Air Conditioner Service Manual
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