Relative humidity (RH) is the actual amount of water vapor in the air as
a percentage of the maximum amount of water vapor which the air
could hold at a given temperature. When air is warmed, its ability
to hold water vapor is increased, and when cooled, it can hold less
water vapor. If the amount of water vapor in the air were held
constant as the temperature was increased, this would cause the
relative humidity to fall, because the warm air would now be able
to hold more water vapor then when it was cool. In the winter, as
cold, moist outdoor air is brought indoors and heated, it becomes
warm, dry air just by being heated. Air at 20oF & 70%
RH, when heated to 72oF, will have a relative humidity
of just 8%. Humidifiers bring the relative humidity back to
the normal levels we need for comfort, safety & protection from
1 1/2 Ounces
of water in each glass.
Glass on left is 80% full,
glass on right is 15% full.
two glasses representing the same pound of air at different
temperatures are a good visual example of relative humidity. The
small glass contains 1 1/2 ounces of water and is 80% full. This
could represent a pound of air at 30oF and 80% relative
humidity. If we pour the 1 1/2 ounces of water from the small glass
into the large glass, we now have only 15% of the glass full. This
larger glass would represent our same pound of air heated to
70oF, but with the same pound of air and the same water
vapor content we now have only 15% relative humidity.
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 70oF 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 70oF 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, make-up air or exhaust systems. This leakage flow is
Rules for Estimating.
In estimating a humidification
application we must find :
1. Indoor design condition: The desired temperature and relative
humidity. For example, 70oF and 50% RH. The psychrometric chart
gives the amount of moisture in the air at these conditions as 55
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
-10oF and 40% RH (moisture = 2gr/lb.) in the North, or
35oF and 60% RH (moisture = 17 gr/lb.) in the South.
3. Volume of outside air entering the space to be humidified.
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
Example: A building with 100,000 cubic feet of space. There is no
mechanical ventilation or make-up air system. Assume 1 air change
per hour. The outdoor heating design temperature is 0oF
and we require 50% RH at 70oF. The formula for H
Volume X Air Changes X Grains of Moisture
Volume X 7000
Grains of Moisture Required From psychrometric chart = 56 grains of
moisture per pound of air at 70oF 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 70oF, 50% RH and 7,000 = Number of grains per pound
of water, a conversion constant.