Environmental Engineering Reference
In-Depth Information
Figure 11.16
Cross-contamination in adjoining strip mall business places.
from the adjoining clothing store (1) through wall penetrations associated
with plumbing, electrical wiring, etc., because the restaurant is strongly neg-
ative relative to clothing store 1. Occupants of the restaurant may or may not
find clothing store odors objectionable. However, the negative pressure in
clothing store 1 and slight positive pressure in the print shop is likely to
result in the movement of solvent vapors from the print shop into clothing
store 1, where occupants are likely to find them objectionable. Since the east
wall of clothing store 2 is positive relative to the adjoining print shop, vapors
from the print shop are unlikely to move into clothing store 2 unless some
factor causes the indicated pressure differences to change. Such changes do,
in fact, take place when doors are opened and remain open for several
minutes or more. In
Figure 11.16
,
air can flow from clothing store 2 to the
business supply store because the latter is negative relative to the former.
The degree of cross-contamination and its objectionableness depends on the
nature of contaminants and extent of their migration from one shop to
another. Contaminant migration depends on the magnitude of pressure dif-
ferences and the relative “leakiness” of wall structures separating each unit.
Assuming that the building (
Figure 11.16
)
is under single ownership, it
would be desirable to modify and operate ventilation systems in the print
shop so that both east and west walls are negatively pressurized relative to
adjoining spaces. The print shop should be equipped with an exhaust ven-
tilation system to remove solvent vapors and maintain the space under
negative pressure.
iv. Re-entry. Re-entry of exhausted contaminants is also a common
problem in buildings. This is the case when exhaust outlets and outdoor
intakes are located relatively close together, with intakes downwind of
exhaust outlets. Re-entry also occurs by infiltration when imbalances
between intake air and exhaust air flows occur. When exhaust air flow
exceeds air flow through intakes, the building or parts of it become depres-
surized (under strong negative pressure). Depressurized indoor spaces may
