or moist air is able to flow backward to the drive in any case, even during off-time or
while servicing the drive or the ventilation system.
Calculating the required static pressure difference
The required static pressure difference between the exit air duct and the drive
installation room can be calculated as follows:
Δ
p
s
= (1.5…2) ·
p
d
where
p
d
= 0.5 ·
ρ · v
m
2
v
m
=
q
/
A
c
Dynamic pressure
p
d
Air density (kg/m
3
)ρ
Average air velocity in the exit duct(s) (m/s)
v
m
Rated air flow of the drive (m
3
/s)
q
Cross-sectional area of the exit duct(s) (m
2
)
A
c
Example
The cabinet has 3 exit openings of 315 mm diameter. The rated air flow of the cabinet
is 4650 m
3
/h =1.3 m
3
/s.
A
c
= 3 · 0.315
2
· π / 4 = 0.234 m
2
v
m
= 1.3 / 0.234 = 5.5 m/s
p
d
= 0.5 ·
ρ · v
m
2
= 0.5 · 1.1 · 5.5
2
= 17 Pa
The required pressure in the exit air duct is then 1.5…2 · 17 Pa = 26…34 Pa below the
pressure in the room.
Mechanical installation 25
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