Dehumidification load.

TM 38-260

b. Dehumidification load. The rate at which water vapor must be removed from the building to maintain the desired

level of relative humidity is known as the dehumidification load. Once the moisture in excess of the residual has been

removed, the daily dehumidification load in a warehouse is based upon maintaining an inside relative humidity of 50

percent. Since the outside relative humidity varies greatly, ranging from 10 to 100 percent, depending upon weather

factors, the quantity of moisture gained from the outside air never remains fixed. Estimates based upon weather data

from previous years may be used. Sources of humidity are:

(1) Open doors. During the period that an external cargo door is open, outside air has free access to the

warehouse, the rate being dependent upon the wind velocity and direction at the time. Actual observations of air flow

through an open cargo door have shown that a 5 mile per hour wind will produce a mean velocity through a doorway of

approximately 70 cubic feet per minute resulting in the potential for a drastic increase in relative humidity.

(2) Infiltration. Infiltration of air into the warehouse is due to wind forcing outside air through cracks and other

apertures on the windward side of the warehouse, while a like quantity of dehumidified air leaves through similar openings

of the-lee side. The quantity of infiltrated air is a function of wind velocity and the number, size, and character of

openings through which it can enter.

(3) Floors. The vapor permeability of concrete has a wide range and may be a critical factor in the transmission

of moisture through a floor slab. The resistance of concrete to the penetration of moisture depends to a considerable

degree on the curing time and the amount of water used in the mix. While there can be considerable transmission of

moisture through the floor, the cost of treating floors to reduce this transmission may be greater than potential savings.

However, where unusually damp conditions prevail, savings can be made.

(4) Breathing. The moisture gained from breathing is caused by the daily temperature changes in the building

and the changes in barometric pressure. An increase in temperature in the building causes the air to expand and forces

some dehumidified air out of the building. Shrinking of the air in the structure as the inside temperature falls causes a

like quantity of humid air to be drawn into it. The difference in quantity of water in the indrawn air and that in the dry air

which it replaced constitutes the dehumidification load due to breathing.

(5) Transmission. Calculated transmission loads through the roof, walls, and floor are all based upon empirical

data obtained in laboratories and filed tests. Some leakage through the concrete floor is undoubtedly due to capillary

transmission. Where the underlying rock strata and the hydraulic conditions in the area combined to deliver water under

pressure to the lower side of the concrete floor, excessive quantities of water may be forced through the concrete to add

to the dehumidification load.

(6) Receipts of equipment. Moisture in excess of the residual moisture brought in with equipment adds to the

dehumidification load. The equipment may have been shipped from any portion of the country, and may have been

exposed to snow or rain en route. Wooden skids will retain moisture, temporarily adding to the dehumidification loads.

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