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The Drying Pie
by
Ernie Storrer
Injectidry Systems, Inc.
The
cleaning industry has long talked about the concept of looking at the sum of
factors involved in carpet cleaning as a “cleaning pie.” These factors have
been:
·
time
·
temperature
·
chemical
·
agitation.
If
one ingredient, factor or activity is increased, one or more of the other
components can be decreased to compensate and still achieve “clean.”
The concept of structural drying can be looked at in the same manner. The sum total of factors (that we can influence or change) involved in structural drying could be looked at as the “drying pie.” The factors involved include:
·
temperature
·
air
·
materials
·
relative
humidity of air
·
air
movement
·
time.
If
one ingredient, factor or activity is increased or decreased, one or more of
the other components can be changed to compensate and still achieve structural
drying.
In
order to properly accomplish structural drying with desiccant dehumidifiers, a
contractor has to have an understanding of the principles of psychometrics.
Additionally, he must also grasp the concept of “air exchanges.” A desiccant
will remove a given quantity of moisture per hour from the air, given certain
relative humidity and temperature situations. Each desiccant will process back
into the room or structure, a quantity of cubic feet of air per hour. In order
to provide adequate moisture removal, the air must be exchanged, usually on the
order of one to three times per hour.
If
we consider each stud and ceiling cavity as a separate area, we can apply these
factors and consider what happens. When structural materials such as drywall
and studding are subjected to water, the air trapped within these areas rapidly
becomes saturated. When the air becomes saturated, absent significant air
movement within the wet assembly, the only drying that will occur will happen
through moisture migration. The cellular structure of the wood and drywall will
transmit moisture by capillary action. With significant quantities of water,
this will be a slow process. If there were no paint on the surface of drywall
this could happen quickly enough in some cases. With various wallboard
coverings, however, the capillary action is virtually stopped.
In
order for proper drying to occur, we need to achieve in the wet structure,
similar results to what a home dryer does with clothing. A home dryer moves air
through the tumbler, air that has the ability to carry moisture. This air has
been heated, increasing its ability to hold moisture. The air moving through
the cylinder past the wet clothing provides the carrying capacity for the
grains of moisture. The wet air is then moved out of the cylinder until the
clothing loses the excess moisture.
In
structural cavities, we should attempt to move as much dry air as practical
past the wet material. This can be accomplished by accessing the area by
drilling small holes in the wall. Then we can move air into the interstitial
cavity. We must be certain that air is going into the wet area. Just drilling
holes and moving air at or near the surface is no guarantee that there is an
exchange of air.
Now
back to our pie. I trust there will be no rhubarb (pun) here over the concept
that the drier the air (relative humidity) and the warmer the air, the more
rapid the transfer will be. This is the same principle that applies when you
use a hair drier. Moisture will always go to lower vapor pressure also. The
most important concept, however, is that of air movement in the form of air
exchanges.
Let’s
assume for a moment that we have a home where the room air temperature is 70°f and the relative humidity 60%. Can we
remove the excess moisture from the carpet without altering or lowering the
relative humidity? The answer is “yes,” as can achieve this through rapid air
movement. Remember, the air is not totally saturated and can hold 40% more in
terms of actual grains of moisture. Obviously, it is usually better not to have
60% humidity during the drying process, but drying could be accomplished.
If
you have the goal of drying a floor, where you could, the airmover would be set
on high speed. The more air moved by the wet material the faster the transfer.
The more air you move through the dryer, the faster your clothes will dry. The
higher setting on a hair dryer will dry your hair faster.
A
pie chart can perhaps explain better what we need. With all factors in balance,
the chart could look like this:
If
temperature is increased, the chart could look like this:
Definitions:
Permeability describes the quality or state of having pores or openings that permit liquids or gases to pass through.
Vapor pressure is the pressure that is
exerted by a vapor that is in equilibrium with its solid or liquid form
Vapor diffusion is the spreading or
movement of moisture in the form of vapor (migration)
Relative humidity is the ratio of the amount
of water vapor actually present in the air to the greatest amount possible at
the same pressure.
Water in the air is always trying to seek a condition of equilibrium. Moisture will move to an area where pressure is low from areas of high pressures. In the same manner, air in our atmosphere is constantly moving toward low-pressure areas sometimes creating tornadoes or at the wind or measurable air movement. The greater the pressure differential, the faster the air will move. The same is true with the moisture. In fact, moisture will move against air movement to achieve equalization.
