Environmental Engineering Reference
In-Depth Information
for dilution, contaminant concentrations would be expected to increase in
an inverse manner to the dilution-induced decreases described above.
In the real world, the level of contaminant reduction associated with
general dilution ventilation is often less than that described above. Contam-
inant concentrations are determined by how they are generated (episodic,
constant, increasing), ventilation capacity (the HVAC system's ability to
mechanically deliver outside air to dilute contaminants to acceptable levels),
and the operation and maintenance of HVAC system equipment. Elevated
contaminant levels may result from high source generation rates, inadequate
design capacity, reduced outdoor air flows associated with energy-manage-
ment practices, and poor operation and maintenance of HVAC systems.
Because of the large air volumes required to reduce elevated contaminant
levels, general dilution ventilation is not the most efficient control measure.
However, when contaminant sources are diffuse and cannot be easily con-
trolled in other ways, it is the control method of choice. Such conditions are
common in many large nonindustrial, nonresidential public access buildings.
1.
HVAC systems
The ventilation function is incorporated into systems that provide climate
control, such as heating and cooling (and sometimes humidification), in most
buildings where general dilution ventilation is used to reduce contaminant
levels and provide a comfortable and healthful indoor environment. These
HVAC systems include one or more air-handling units (AHUs), supply
ducts, diffusers, return air grilles, return air plenums, dampers, exhaust fans
and exhaust outlets, intake grilles, mixing boxes, etc. A relatively simple
HVAC system design that provides conditioned air to a single space is
illustrated in
Figure 11.7
.
In this case, the AHU is a small box suspended
above ceiling level. It consists of a filter, thermal sensors, and heating and
cooling coils. Air that enters the AHU can be 100% outdoor air, nearly 100%
recirculated air, or various mixtures of recirculated (from the conditioned
space) and outdoor air.
Outdoor and recirculated air percentages are varied to meet building
operating needs. In middle latitudes in North America, such systems are
operated with 100% outdoor air during limited periods of spring and autumn
months when temperatures are favorable for using outdoor air for free-
cooling. That is, outdoor air is used to cool building spaces in lieu of acti-
vating energy-consuming cooling units. During cold weather, HVAC sys-
tems may be operated on nearly 100% recirculated air to prevent freezing
of system coils. In many instances, it is still common (particularly in school
buildings) for facilities managers to operate HVAC systems on or near 100%
recirculated air as an energy management strategy. In the latter case, HVAC
systems are not being operated as designed, and building air quality is
unlikely to be acceptable. Good operating practice would require that at a
minimum 15 to 20% outdoor air ventilation be provided during normal
building occupancy hours to maintain acceptable air quality. In older AHUs,
dampers that regulate the percentage of recirculated and outdoor air have
