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Sizing and Selection

Step 5: What type of humidiﬁ er to use. In this example, in-

tegral fan units are preferable to steam jet units installed in

conjunction with unit heaters. Since the unit heater fans are

on or off to control temperature, it follows that the humidi-

stat may call for steam when the nearest unit heater is nor

running. With the low ceiling, the discharge from a steam

jet humidiﬁ er might rise to the ceiling and produce conden-

sation. Therefore, the integral fan type should be used.

Step 6: Location of humidiﬁ ers. Several patterns are pos-

sible, and actual location can usually conform with the ex-

isting steam supply and return lines to make an economical

installation with a minimum of new piping.

In our problem of a 400' x 160' x 10' room, there would

likely be steam lines along both sides of the room, and

humidiﬁ ers can be located as shown in Fig. 5-1. If the sup-

ply lines run down the center of the room, the center line

pattern would be practical. Runouts to integral fan units

in a 160' wide room would be about 20' long. If the room

were only 60 or 80 ft. wide, runouts need be no longer than

required for actual hook-up.

Step 7: Location of humidistat. This should be form 20 to

30 feet away from the humidiﬁ er and slightly to one side of

the air stream from the unit. The humidistat should "see" its

humidiﬁ er. Do not hide it behind a post or in the channel of

an H-beam. It must get a good sample of the air, to control

the humidity.

Figure 5-1.

Where practical, locate humidifiers to minimize

piping. Locations shown where steam supply

lines are along outer walls; and where supply

is in center of room.

Table 5-1

70°F. Humidiﬁ cation. Pounds of Steam Per Hour, Per Air Chcange for Each 1000 cu. FT. of Space to Secure Desired Indoor Relative

Humidity at 70°F. with Various Outdoor Temperature (Outside Air 75% Saturated).

Outdoor

Temp.

25%

30%

.079

.136

.154

.212

.204

.262

.237

.294

-10

.257

.314

-20

.270

.327

70°F. - RELATIVE HUMIDITY DESIRED INDOORS-70°F

35%

40%

45%

.194

.251

.309

.269

.327

.385

.319

.377

.434

.352

.409

.467

.372

.430

.487

.385

.442

.500

Sizing and location with forced ventilation.

Typical jobs: Mill and sanding rooms in furniture factories.

Here, the problem of selecting and installing humidiﬁ ers is

much the same as previously described except for:

Determining the number of air changes.

Location of humidiﬁ ers and humidistat.

Air Changes: These can be determined from exhaust fan

capacity where known. The cu. ft. per hour capacity of the

fans divided by the cubic feet of space to be humidiﬁ ed will

give the number of air changes.

Where the capacity of fan or fans is not known, air changes

can be measured with velometer readings at all open

doors, elevator shafts, etc., leading to the room and with

fans operating at full capacity. Your Armstrong Representa-

tive can determine air changes for you.

Humidiﬁ er Location: Bear in mind that humidiﬁ ers will

have to control the humidity 24 hours a day, seven days a

week during the heating season. Exhaust fans may operate

only 40 hours or 80 hours per week. Thus the humidiﬁ ers

and humidistats must be located for good distribution of

humidity during the fan-off periods as well as when the fans

are operating. Figure 5-2 outlines a typical requirement.

Figure 5-2.

Schematic layout of humidifiers in woodworking plant

where exhaust fans are used. Arrows indicate air flow

induced by fans. Humidifiers are sized for load conditions

imposed by fan. Humidifiers are located to give uniform

distribution of humidity when fans are off or when fans

are running.

50%

55%

60%

.367

.424

.482

.441

.499

.557

.492

.549

.607

.524

.582

.639

.545

.602

.660

.557

.615

.672

65%

70%

75%

.539

.597

.654

.615

.672

.730

.665

.722

.780

.697

.754

.812

.717

.775

.832

.730

.787

.844