Civil Engineering Reference
In-Depth Information
range can be expanded on either side by a few percentages for less critical appli-
cations. Therefore designers should employ means of air humidifi cation or dehu-
midifi cation to keep the total indoor air relative humidity within these limits
throughout the year.
To achieve this goal the obvious fi rst and most important step is to limit entrance
of excessive amount of outdoor unwanted moisture to the building through cracks,
building joints and openings. This goal simply can be achieved by employing high
quality workmanship in construction phase of the building and also by installing
suitable vapor barriers on the building envelope exterior surfaces. Proper architec-
tural designs such as utilizing revolving doors or vestibules can also contribute to
the elimination of the building interior environment contact with the outdoor
through direct openings. It should be noted that cracks and openings can be a source
of infi ltration of pollutants and even bacteria into the building as well.
Three major contributing sources of entrance of unwanted moisture (and also
pollutant) into the building via infi ltration through the cracks and joints are known
to be wind pressure, stack effects, and improper pressure balance in the building.
One of the most effective methods of preventing the outdoor air moisture and
pollutant to enter the building is designing a mechanical pressure system utilizing a
fan that continuously monitor the outdoor air pressure and keeps the building under
positive air pressure relative to the building outdoor condition. The only negative
outcome of this method is its need for extra indoor air quantities over what it is
really needed for people comfort. Therefore additional conditioned air, which can
be translated to additional energy consumption, should be generated and delivered
to the space.
A poor air handling unit design, installation and maintenance or lack of proper
draining of the condensate water generated at the cooling coils is another path for
allowing unwanted moisture into the ducts and building. When the velocity of the
air that is moving over the cooling coil reaches or moves above a proper level
(usually above fi ve hundred foot per minute (500 fpm)), it creates an opportunity
for some of the moisture in the air stream to pass over the coil without proper con-
tact with the coil surface and therefore leave the coil section without draining all of
its moisture content in the drain pan. The moisture left in air stream then will be
carried on to the ductwork or inside the building and will become the source of
unwanted conditions such as uncomfortable environment or even creation of mold.
The most common way to prevent this condition is to design a large enough contact
surface for the cooling coil to minimize the maximum velocity of air over the coil
to not more than 500 fpm.
After all means of preventing entrance of unwanted moisture to the building is
provided an easier follow-up task then would be to control the humidity that is
generated inside the building. This humidity can be generated from the daily build-
ing functions and occupant activities. A well designed cooling and dehumidifi cation
system can provide proper humidity level inside the building during the summer.
In winter time however a well-designed heating and humidifi cation system can
maintain the indoor humidity above a minimum proper level.
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