8.3 Dewing
The most damp condition of air is at the saturated air condition of relative humidity 100% which is unable to
contain moisture any longer. As a result, the water vapor (moisture) in the air is condensed and turned into
liquid as waterdrop which can be seen. Such phenomenon is called dewing or condensation, commonly
“sweating”.
Dew point temperature means the temperature of air when starting to form dew. For example, when taking
out a bottle of beer from refrigerator in summer, you have probably seen several times the surface of beer
bottle sweats, that is, waterdrop form. The phenomenon is because the moist air around the bottle of beer is
changed rapidly into a state beyond relative humidity 100% with rapid cooling below the dew point tempera-
ture, and the vapor in the air not contained forms dew. Maybe you have ever experienced damages that sum-
mer cooling in your house caused the bedclothes in wardrobe to be damp by de wing or the winter heating
caused to form dew on window and waterdrop to flow into wall or floor carpet.
❒
Dewing prevention
Such a fine particle (vapor in air) that its shadow or shape has never appeared is rapidly cooled when it
touches a cold substance below dew point temperature. That is, shocking or starting by rapid cooling
causes waterdrops to be emerged out; dewing.
It is necessary to prevent from dewing in room because it gives trouble. For example, in order to keep
room windows from dewing in heating, insulation effect should be increased by making room humidity
down or using a double window.
Americans frequently does not humidify when heating, and if humidifying, they lower room humidity con-
siderably.
Relative humidity in room to prevent inner area from dewing
Heat loss rate
[Kcal/m
2
h°C]
Outdoor temperature [°C]
-10
-7.5
-5
-2.5
0
2.5
5
22[%]
25[%]
29[%]
33[%]
38[%]
43[%]
48[%]
29
32
36
40
45
50
55
37
41
44
48
53
58
62
48
52
55
59
62
66
70
62
65
67
70
73
76
79
79
80
82
83
85
87
89
6
5
4
3
2
1
TCL Air Conditioner Service Manual
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