Herrmidifier Load Calculator - Manual (Page 5)

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
5 w w w. h e r r m i d i f i e r- h v a c . c o m
The Psychrometric Chart
To further illustrate the moisture holding ability of air at vari-
ous temperatures, following is a chart showing the grains of
water per cubic foot of saturated air, at various temperatures.
This chart was abstracted from the ASHRAE Handbook.
Table 1-A
GRAINS OF WATER PER CUBIC FOOT (AIR)
Saturated Air, Various Temperatures
°F Grains °F Grains
0 .475 60 5.795
5 .609 65 6.845
10 .776 70 8.055
15 .984 75 9.448
20 1.242 80 11.04
25 1.558 85 12.87
30 1.946 90 14.94
35 2.376 95 17.28
40 2.863 100 19.95
45 3.436 105 22.95
50 4.106 110 26.34
55 4.889 115 30.13
As you can see, from the above Table 1 -A, as the tempera-
ture of the air increases, so does its ability to hold moisture.
Determinations of MAXIMUM HUMIDIFICATION DEMAND
LOAD are made from this chart. To determine a heating hu-
midification load, simply take the maximum indoor and mini-
mum outdoor temperatures, refer to the chart above, and
write down the corresponding grains of moisture. Then multi-
ply these times the percent of relative humidity desired. Use
the lowest design R. H. for the outdoor figure. Now subtract
the lower temperature figure from higher temperature figure
and insert the result into the following formula for Grains/
Cu. Ft., also inserting the cubic foot of air to be humidified
per hour:
LOAD = Grains/Cu. Ft. X Cu. Ft./Hr.
(Lbs. Water/Hr.) 7000/Grains/lb.
Other factors influencing the maximum humidification de-
mand load are:
• Natural Air Infiltration
• Exhaust Equipment
• Make-up Air
• Construction of Building
• Air Conditioning or Refrigeration
• Vacuum Equipment
• Vapor Barriers
• Window Area
• Hygroscopic Material
Because of all the factors involved, humidification can be
simple or rather complicated. It is best to consult a humidifi-
cation expert such as Herrmidifier Co., Inc. and allow them to
conduct a humidification survey. (Free help can be obtained
by filling out Herrmidifier’s form DI-5 and mailing it to us. This
form contains all necessary information for us to help you
design a project).
Equilibrium Moisture Content
Dry air pulls moisture from everything it contacts- HYGRO-
SCOPIC materials (able to absorb or emit moisture), liv-
ing tissues, etc. Conversely, moist air gives up moisture to
dry hygroscopic materials. An equilibrium will eventually be
reached when the moisture gain of a material equal its mois-
ture loss. This is the EQUILIBRIUM MOISTURE CONTENT
OR “EMC.” This “EMC” changes with temperature, R.H. and
from material to material. The reaching of this “EMC” can
sometimes be detrimental as we shall see.
“Regain” Of Hygroscopic Materials
All hygroscopic materials-and a long list is so classified-take
on or give off moisture when the EMC is disturbed. At a given
level of R.H. a given hygroscopic material will hold a certain
amount of moisture. The weight of this moisture so help com-
pared with the dry weight of the material is called “Regain”
and is expressed as a percentage. Regain varies with R.H.
for a given material. For example: EMC values for average
wood are 5.9% Regain with 30% R.H. air; 9.3% Regain with
50% R.H. air; 14% Regain with 70% R,.H. air; etc. A stan-
dard Regain is often specified for hygroscopic items sold by
weight. If the standard Regain is not attained, economic loss
to the seller results.
Variations in Regain cause hygroscopic materials to change
in dimensions, weight, quality, workability, etc.; therefore, it
is best to stabilize the Regain at the most desirable level for
manufacturing, processing, testing, storage, use, consump-
tion, etc. To do this, the R.H. of the air in contact with the ma-
terial must be stabilized at the proper level, as we shall see.
“Dry Air” And Comfort
Dry air pulls moisture from anything it contacts, including
the human body. Dry air will cause moisture to evaporate
more readily from the surface of the skin, causing a feeling
of chilliness-even at temperatures as high as 75°F! Dry air
also pulls moisture from living membranes, such as the nasal
passages, throat, eyes and ears, leading to the uncomfort-
able parched throat and sore eyes of winter. Proper control
of relative humidity can help to eliminate these problems.