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Herrmidifier Load Calculator User Manual

Page 13

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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°

+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°

+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.