Handbook of air conditioning and refrigeration / Shan K

20.8 CHAPTER TWENTY
(a)
Height
ft
Neutral pressure
level (NPL)
�p between
space air
and air in
exhaust duct
(exhaust fan
is shut off)
Exhaust duct
for restrooms
Elevator
door
Elevator
shaft
�p el �p o
where p so, p si � outdoor and indoor absolute static pressure, in. WG (Pa)
� o, � i � outdoor and indoor air density, lb/ft 3 (kg/m 3 )
H NPL � vertical distance between NPL and reference level, ft (m)
T Ro,T Ri � outdoor and indoor absolute temperature, °R
For a high-rise building with two openings on the external wall, N NPL, in ft (m), measured from
lower opening can be calculated as
where Ho � vertical distance between two openings, ft (m)
A1, A2 � area of lower and higher openings, ft2 (m2 Ho HNPL �
1 � (A1/A2) )
TRi, TRo � absolute temperature of indoor and outdoor air, °R (K)
2 (TRi / TRo) (b)
Building
Outside
door
Outside
�p in WC
Outside
Elevator
shaft
Building
(20.5)
In Eq. (20.5), T Ri � T Ro. If the indoor air is cooler, that is, T Ro � T Ri, then T Ri/T Ro should be
changed to T Ro/T Ri. For conditioned spaces where T Ri � T Ro, the chimney and exhaust systems raise
the NPL level and the cold outdoor air supply system lowers the NPL level.
In Fig. 20.1b, it can be seen that the absolute static pressure of outdoor air p so is greater at the
low level than at the high level. This is because the air column is taller at the lower level.
In an ideal single-cell building with no inside partitions, with a vertical distance of 100 ft (30 m)
between the lower air inlet and upper air outlets and an indoor and outdoor air temperature difference
of 50°F (10°C), the static pressure difference �p st is about 0.15 in. WC (38 Pa).
Most buildings have inside partitions; i.e., they are multicell buildings. They have doors at the
lower entering inlet, windows at the upper discharge outlets, and doors to the stairwells. Usually
(c)
(d)
Elevator
shaft
Outside
Building
FIGURE 20.1 Stack effect in a high-rise building (a) schematic diagram of building airflow; (b) pressure profiles; (c) pressure profiles
for tight building shell; (d ) pressure profiles for leaky building shell.