Chapter hvac engineering Fundamentals: Part 1 Introduction

96
Chapter Five
The fan laws expressed in the following equations relate only to the
effect of varying speed, assuming that fan size and air density remain
constant.
RPM
2
CFM
⫽
CFM
(5.1)
2
1
RPM
1
2
RPM
2
SP
⫽
SP
(5.2)
冉 冊
2
1
RPM
1
2
RPM
2
TP
⫽
TP
(5.3)
冉 冊
2
1
RPM
1
3
RPM
2
BHP
⫽
BHP
(5.4)
冉 冊
2
1
RPM
1
3
where CFM
⫽
airflow rate, ft / min
SP
⫽
static pressure
TP
⫽
total pressure
BHP
⫽
brake horsepower, bhp
Expressed in simple language, the fan laws say that when fan size
and air density are unchanged, the airflow rate varies directly as the
change in speed, the pressure developed by the fan varies as the
square of the change in speed, and the power required to drive the
fan varies as the cube of the change in speed.
The complete fan laws also include terms for changes in fan size
and air density. The laws are valid only when fans of different sizes
(diameters) are geometrically similar.
3
CFM
RPM
D
2
2
2
⫽
⫻
(5.5)
冉 冊
CFM
RPM
D
1
1
1
2
2
TP
SP
VP
RPM
D
d
2
2
2
2
2
2
⫽
⫽
⫽
(5.6)
冉 冊 冉 冊
TP
SP
VP
RPM
D
d
1
1
1
1
1
1
3
5
BHP
RPM
D
d
2
2
2
2
⫽
䡠
(5.7)
冉 冊 冉 冊
BHP
RPM
D
d
1
1
1
1
where
D
⫽
fan diameter and
d
⫽
air density.
For further variations, see ASHRAE Handbook,
2000 HVAC Systems
and Equipment,
Chap. 18, Table 2, p. 18.4.
Design Procedures: Part 3
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