Environmental Engineering Reference
In-Depth Information
v. Entrainment. In re-entry, exhaust gases, vapors, and particles may
become entrained in outdoor air intake flows, resulting in varying degrees
of indoor air contamination. Entrainment and subsequent indoor contami-
nation may also occur from contaminants generated from nearby sources.
They may move through outdoor air intakes, loading docks with open doors,
and leakage sites by infiltration. Entrainment of contaminants generated
external to buildings is a common occurrence. It is more notable when high-
emission sources are located close to outdoor air intakes and when buildings
or portions thereof are depressurized relative to the outdoor environment.
Commonly reported entrainment problems include motor vehicle exhaust,
vapors from asphalt sealers and asphalt or rubber compounds used in seal-
ing roofing materials, particulate and vapor-phase substances associated
with lawn or agricultural pesticide applications, and nearby commercial
(restaurants, print shops) and industrial operations.
Ventilation engineers have developed new
systems designed to improve the effectiveness of conventional ventilation
in controlling contaminants, and alternative systems that may be more effec-
tive than conventional general dilution ventilation systems. In the first case,
demand-controlled systems have been developed to quickly respond to
changing ventilation needs by increasing and decreasing volumetric outdoor
air flow rates in response to changing indoor contaminant concentrations.
In the second case, displacement ventilation systems have been designed to
more effectively move contaminants generated in occupied spaces to exhaust
outlets in ceilings.
e.
Ventilation innovations.
i. Demand-controlled ventilation. Demand-controlled ventilation
(DCV) systems differ from conventional ones in that outdoor air flow rates
are varied in response to sensor-measured contaminant concentrations
located in the return air stream of a room, ventilated area, floor, or whole
building. These systems have the advantage of decreasing outdoor air flows
to conserve energy when selected contaminants are below acceptable levels
and of increasing outdoor air flows when contaminant levels begin to rise
to unacceptable ranges.
A variety of sensors are available for use in DVC systems. These include
CO
is a relatively
poor indicator of air quality other than human bioeffluents, CO
, relative humidity, and mixed gas sensors. Though CO
2
2
sensors are
commonly used in DCV systems. This reflects their relatively low cost,
reliability, simplicity of use, and the historical role of using ventilation rates
based on CO
2
as the determinant of ventilation adequacy. The relationship
2
between CO
levels and outdoor air flow rates can be seen in
Figure 11.18
.
2
ii. Displacement ventilation. In conventional ventilation systems,
contaminant levels are reduced by mixing ventilation air with room air.
Maximum contaminant reduction will result with “perfect” or complete
mixing which, in many cases, does not occur.
