Environmental Engineering Reference
In-Depth Information
the NPP, indoor and outdoor pressures are equal. In Figure 11.2 , the NPP is
located at approximately mid-level, suggesting that leakage areas are uni-
formly distributed over the building face. The location of the NPP depends
on the distribution of leakage sites. In older single-family dwellings, the NPP
is often above mid-height, and in the case of houses with flue exhaust of
combustion by-products, the NPP may be above the ceiling level during
exhaust operation. The construction of residential dwellings has included a
number of energy-conserving measures, most notably tighter building enve-
lopes. Since leakage areas have been reduced, pressure characteristics have
changed, resulting in reduced infiltration and exfiltration. In such houses,
the NPP would be expected to occur at mid-height (absent the active oper-
ation of combustion exhaust systems).
In tall buildings, the NPP may vary from 30 to 70% of the building height.
The inflow and outflow of air discussed above is called the stack effect
because airflows are similar to those which occur in a smokestack. The
magnitude of the stack effect increases significantly with building height.
The change in pressure with height in a large building has been reported to
be approximately 0.001” H
O (0.25 pascals) per story. Stack effect flows
upward are particularly noticeable in elevator and other service shafts and
in open stairwells. Each story, if constructed in an airtight way, can behave
more or less independently, i.e., have its own stack effect.
The influence of stack effect on building air exchange rates is, for the
most part, proportional to
2
T, the difference between indoor and outdoor
temperatures. This relationship can be seen in model predictions graphed
in Figure 11.3 for
T increases, air exchange increases,
with maximum values on cold days. Minimum air exchange occurs when
T = 0°F and 40°F. As
Building air exchange associated with different stack effect and wind
speed conditions.
Figure 11.3
 
 
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