Civil Engineering Reference
In-Depth Information
A common method of dehumidifi cation of the delivered air to the space through
air conditioning system is cooling and then reheating the air before introduction to
the space. This helps to remove the excess humidity out of the air stream that is
cooled down close to saturated level, but due to the fact that simultaneously cooling
and heating the same stream of air is not energy effi cient, its usage have been
limited by the governing standards. Another more accepted way is reducing the
humidity in the outdoor air stream by using energy recovery units. In these units a
wheel equipped with some kind of moisture absorbent is used to not only remove
the excess moisture from the air stream before delivering it to air handling unit, but
also to transfer the heat that is usually wasted via exhaust air streams to the outside
air and therefore economically preheat this air, where it is applicable.
6.3
Contamination Control
The outdoor environment can be source of a wide verity of contaminants such as
nitrogen dioxide (NO
2
), sulfur dioxide (SO
2
), carbon monoxide (CO), airborne dust,
airborne volatile organic compounds (VOCs), and airborne odors. When we are
designing a building, the existence of these contaminants in outdoor air shall be
recognized and their contamination levels shall be closely monitored and controlled.
Contamination can be reduced with proper contamination reduction methods before
the outdoor air is introduced to the air system of the building via intake air louvers,
hoods, or any other means of introduction of fresh air to the building. Environment
Protection Agency (EPA) has developed the National Ambient Air Quality Standards
(NAAQS) in which, the level of pollution of different areas in the USA in regard to
different pollution agents are specifi ed. These guides specify the areas in the country
that the building indoor has to be protected against specifi c regional outdoor pollu-
tions. For example if NAAQS specifi es an area as nonattainment for sulfur dioxide
(SO
2
) pollution, then the HVAC design team in addition to the other general build-
ing ventilation requirements, such as minimum required outdoor air regulated under
relevant standards, shall specify specifi c provisions such as gas-phase air cleaner as
part of their design in order to resolve this specifi c region related contamination
problem as well. It should be noted that the EPA provides both primary (to protect
human health) and secondary (to protect environment) attainment requirements to
be complied with.
As it was said earlier, similar to moisture, contamination can enter the building
from different sources. For example in locations where the outdoor air contamina-
tion level is high and a proper air pressure relation between the contaminant area
and the building is not maintained, contaminants can fi nd their way into the building
through cracks or open windows. Also lack of proper design or operation of the air
handling unit air fi ltration could be a resource of entrance of contamination to the
building. That implies that the fi rst step in controlling level of contamination inside
the building obviously is selecting the proper geographic location and climatic con-
dition (wind) orientation for the building. By selecting locations that the outdoor
