
Forced Convection Heat Transfer: Internal Flows 275
P
P
D
D
N
N
2
1
2
1
5
2
1
3
2
1
=
ρ
ρ
,
(11.78)
SPLSPL
N
N
21
2
1
5
2
1
2
10=+
log.
ρ
ρ
(11.79)
The following observations can be made:
The fan ow rate is independent of density while its pressure rise is propor-•
tional to it. Air density decreases with altitude and therefore the performance
curve of a fan will be different at various altitudes. Figure 11.13 compares
performance curves of a typical fan at different altitudes. It must be noted
that this is true for constant speed fans such as the AC fans. For the constant
voltage DC fans, a reduction in air density may cause an increase