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measurement of humidity is Relative Humidity
(rH). It is the proportion of existing humidity
in the air to the capacity of humidity the
air can hold without saturation. If the
existing humidity is greater than such capacity,
the excess humidity condenses and saturates
into a liquid form. The higher the temperature
of the air, the more quantity of humidity
the air can hold and more vapor pressure
of the air takes place. Thus, the change
in temperature directly affects such capacity
and determines the dew point of any given
humidity in the air. Under a certain temperature,
the dew point of high %rH will be higher
than that of low %rH. If the temperature
accidentally drops, the condensation of
air with high %rH will occur before that
of air with low %rH. The below table displays
vapor pressure and water content of a given
temperature. |
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|
Dew
Point
(degree c) |
Vapor
Pressure (PSIG) |
Water
Content (g/cu.m.) |
|
0 |
0.0886 |
4.8452 |
|
10 |
0.1780 |
9.3902 |
|
15 |
0.2472 |
12.8122 |
|
20 |
0.3390 |
17.2724 |
|
25 |
0.6516 |
23.0111 |
|
30 |
0.8586 |
30.3212 |
|
35 |
0.8156 |
39.5383 |
|
40 |
1.0700 |
51.0509 |
|
45 |
1.3902 |
65.2722 |
|
50 |
1.7897 |
82.7282 |
|
55 |
2.2838 |
103.9594 |
|
60 |
2.8900 |
129.5802 |
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| From the
table, at 35C, the vapor pressure is 0.8156
(measured in PSIG unit) and the air can
hold water vapor not exceeding 39.5383 grams
per cubic meter. If the existing rH is 85%,
the vapor pressure is then 0.6933 and the
water vapor is 33.6076 grams per cubic meter.
Should the temperature drops to 30C that
the air can not hold more than 30.3212 grams
per cubic meter, the excess 3.2864 grams
of water per cubic meter will condense into
liquid. By knowing the %rH of the air, we
can estimate the level of moisture vapor
and calculate the desiccant to adsorb the
undesired humidity and to prevent the condensation. |
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