Herrmidifier Load Calculator User Manual
Page 13
L o a d C a l c u l a t i o n G u i d e
L o a d C a l c u l a t i o n G u i d e
13
w w w. h e r r m i d i f i e r- h v a c . c o m
So, the MAXIMUM HEATING DEMAND LOAD for the wood-
working plant in the example would be 860 lbs. of water
needed to be added per hour when the outside air is at O°F.
Traditional heating systems are rather easy to calculate hu-
midification demand loads for, however, with the advent of
economizer cycle heating and cooling systems this calcula-
tion gets a bit more complicated as we shall see.
Heating Load: Economizer Cycle
In air handling systems using the economizer cycle, outside
air is added to return air in varying quantities to provide a set
mixed air temperature reaching the heating/cooling section
of the system. The amount of outside air added is based on
the outside air temperature and the desired mixed air tem-
perature to be maintained. This volume of outside air is add-
ed automatically in varying amounts as the outside air tem-
perature changes. THE MAXIMUM DEMAND LOAD FOR
HUMIDIFICATION is based on the desired indoor conditions
the moisture content of the outside air added and the volume
of outside air being introduced.
Weather Data and Design Conditions (from the ASHRAE
Handbook) do NOT give sufficient information to determine
the moisture content of outside air at various temperatures.
It is necessary to seek assistance from the U.S. Weather
Bureau, Aviation Weather or the like in the area in which the
plant is located. You will need to know the MINIMUM level
of relative humidity experienced at the outside design tem-
perature in the area and for 1O°F increments up to the tem-
perature at which I00% outside air is admitted. With this data
and the characteristics of the system, you can then calculate
the demand load for each outside temperature. The highest
load calculated becomes the maximum demand load for hu-
midification. This maximum demand load will occur at some
outside air temperature OTHER THAN design temperature
when the Economizer Cycle is used. Following, in Tables 2-B
and 2-C, are typical characteristics of two economizer cycle
systems.
TABLE 2-B: ECONOMIZER WITH 70 F RETURN AIR
Mixed Air Temp. °F
% OUTSIDE AIR REQUIRED AT TEMP. SHOWN °F
-10°
0°
+10°
+20°
+30°
+40°
+50°
+55°
+60°
+65°
50°
25
29
33
40
50
67
100
55°
19
21
25
30
38
50
75
100
60°
12
14
17
20
25
33
50
67
100
65°
6
7
8
10
13
16
25
33
50
100
TABLE 2-C: ECONOMIZER WITH 75 F RETURN AIR
Mixed Air Temp. °F
% OUTSIDE AIR REQUIRED AT TEMP. SHOWN °F
-10°
0°
+10°
+20°
+30°
+40°
+50°
+55°
+60°
+65°
50°
30
33
38
45
56
71
100
55°
23
26
31
36
44
57
80
100
60°
18
20
23
27
33
43
60
75
100
65°
12
13
15
18
22
29
40
50
67
100
The above information must be provided for the system being used. Tables 2-B and 2-C are typical only. Therefore the HUMIDIFICATION DE-
MAND LOAD for an economizer system such as that shown in Table 2-B could be calculated as follows:
EXAMPLE:
Indoor Design Conditions: 70°F 40% R.H. (8.10x.40=3.24 gr/CF)
Mixed Fair Temperature: 55°F
Outside Design Temperature: -10°F
System CFM: 30,000
Outside Temp. °F % R.H. Minimum (1) Outside gr/CF (2)
% Air(3) (Outside) CFM Air (Outside)
gr to Add*
Load lb/hr
-10
70
.203
19
5,700
3.037
148
0
65
.312
21
6,300
2.928
158
+10
60
.468
25
7,500
2.772
178
+20
55
.682
30
9,000
2.558
197
+30
50
.975
38
11,400
2.265
221
+40
45
1.260
50
15,000
1.980
254
+50
40
1.644
75
22,500
1.596
308
+55
35
1.712
100
30,000
1.528
393
(1)From Weather Bureau; (2)From Table 2-A; (3)From Table 2-B above; *7,000 gr/lb.
Information in (1) above is the key to the solution. The maximum demand load for Humidification for systems using the Economizer Cycle is almost always at some outside temperature other than
the design temperature for the heating system.