Application.
For a humidification application we are basically interested only
in how much dry air is entering the space to be humidified. If we
have a room in which we wanted to maintain 70^{o}F and 50%
relative humidity, and the room was air tight with a vapor barrier,
we would only have to introduce the proper amount of water into the
air once. If we maintain a constant 70^{o}F we would also
maintain a constant 50% RH. This would be due to the fact that no
dry air could get in to mix with our conditioned air and no
moisture could get out. But, even our most modern buildings are not
that tight. Outside air enters through open doors, cracks,
ventilation, makeup air or exhaust systems. This leakage flow is
called infiltration.
Ground
Rules for Estimating.
In estimating a humidification
application we must find : 1. Indoor design condition: The desired temperature and relative
humidity. For example, 70^{o}F and 50% RH. The psychrometric chart
gives the amount of moisture in the air at these conditions as 55
gr/lb.
2. Outdoor design condition: The given winter temperature and
relative humidity for the location. It is the temperature for which
heating systems are designed. For example, it may be
10^{o}F and 40% RH (moisture = 2gr/lb.) in the North, or
35^{o}F and 60% RH (moisture = 17 gr/lb.) in the South.
3. Volume of outside air entering the space to be humidified.
Calculations
In a residence, outside air enters by natural infiltration, which
in turn, depends on tightness of construction. Typically this
varies from 1/4 to 1 air volume exchange per hour and may be more
with a fireplaces or fresh air exchange devices. In a factory,
warehouse or other buildings without air ducts, infiltration,
exhaust fans or loading docks are the major sources of fresh air.
Infiltration is difficult to calculate and is usually an
“engineering estimate” based on a percentage of total
volume.
Example: A building with 100,000 cubic feet of space. There is no
mechanical ventilation or makeup air system. Assume 1 air change
per hour. The outdoor heating design temperature is 0^{o}F
and we require 50% RH at 70^{o}F. The formula for H
(lbs/hr) is:
H =
Volume X Air Changes X Grains of Moisture
Required
Specific
Volume X 7000
Grains of Moisture Required From psychrometric chart = 56 grains of
moisture per pound of air at 70^{o}F and 50% RH, minus 9
grains of moisture per pound already in the air (56  9 = 47).
Specific Volume From psychrometric chart = 13.5 cu. ft./lb. of air
at 70^{o}F, 50% RH and 7,000 = Number of grains per pound
of water, a conversion constant.
Calculation – Buildings With Ductwork.
In offices and other buildings with air duct systems, the fresh air
is deliberately brought in through outdoor air dampers in measured
amounts. This makes calculations easier by simply adding the total
volume of all fresh air intakes. Using the above example , with
identical conditions, but now with ducts bringing in 4000 cfm of
outside air, we multiply the 4000 cfm by 60 minutes to get 240,000
cu. ft. per hour. Use this number for volume in the formula
below.
H = Volume Of Air Changes
Per Hr. X Grains of Moisture Required
Specific
Volume X 7000
H =
240,000 cu. ft. X 1 per hr.
X 47 =
119 lbs. per hr.
13.5
cu. ft. per lb. X 7,000 grains/lb
The above examples are for general principles only. Every
application is different. Please contact our technical support at
any time for specific advice and data sheets about a particular
application. For detailed information, you may email Humidity Source , or contact us at
9739161001.
