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2008-10-01
(b) Formula 7
Assume that during the summer the HVAC system is in the cooling mode. The
air leaving the cooling coil is at 55°F and 90% rh. In order to maintain the
desired 47% rh in the space, moisture must be added using the following
formula:
Desired space 80°F 47% rh= 5.19 gr/ft
3
(Formula 1)
55°F 90% = 4.89 x 0.90 = 4.40 gr/ft
3
________________________ __________
Grains to add = 0.79 gr/ft
3
15,000 x 0.79 x 60 min/hr
_________________________
= 101.57 lbs/hr
7,000 grains/lb
(8) In considering Process and Environment, in Tables 5 through 7 you will find conditions
and processes that may affect your calculation and should be addressed.
(a) Table 5 shows the outdoor temperature at which the rh would cause condensation
on the windows to the outside. Should your outdoor conditions make this a
possibility, an outdoor setback sensor may be a solution.
(b) Table 6 describes the moisture gain of various material and if your process or
environment includes a great amount of these materials that are constantly
introduced to the area, its affects must be considered.
(c) Table 7 identifies many recommended indoor conditions for various locations and
processes, these can be of use when deciding what conditions would be most
beneficial in your application.
(d) In Figure 2, you will find the ASHRAE physchrometric chart describing the
enthalpy of dry air and the effects to and from rh in the air.
Table 5. Inside Relative Humidities At Which Moisture Will Condense On Windows
Outside
Temperature
Inside Temperature (°F/°C)
Outside
Temperature
Inside Temperature (°F/°C)
°F °C
60/
15.5
65/
18.3
70/
21
75/
23.8
80/
26.6
°F °C
60/
15.5
65/
18.3
70/
21
75/
23.8
80/
26.6
Single Windows, Still Air Single Windows, Wind
-50-4599988-50-4543332
-40-401212111010-40-4054443
-30-341614141313-30-3476665
-20 -29 20 18 18 17 16 -20 -29 10 9 9 8 7
-10 -23 25 23 22 21 20 -10 -23 14 13 12 11 10
0 -18 32 29 27 25 24 0 -18 20 18 16 15 13
10-12393633312910-122824222018
20 -7 47 43 40 37 35 20 -7 36 32 30 26 24
30 -1 57 52 50 45 42 30 -1 48 41 38 34 30
40 4 70 63 60 53 50 40 4 62 54 49 43 40
NOTE: Bold areas indicate that moisture will be in the form of frost on windows.