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6 Fans and Flow Control Devices
amount of static pressure is transformed into velocity pressure. In addition, both give
the airflow the desired exit direction.
Moreover, it is also common to transform a part of the dynamic pressure at the
fan outlet into static pressure by means of a gradual expansion. This is achieved with
devices called diffusers in the case of blower fans, which also help to couple the fan
to the duct and evasés in exhaust fans, which discharge directly into the atmosphere.
Question 6.1 Pressure measurements have been made on the exhausting system
shown in the figure.
a
b
c
20 m
3 s
-1
They provide the following results:
(a) 3.2 cmwc,
(b) 4.6 cmwc, and
(c) 1.4 cmwc.
You are asked to determine the:
(a) Static, dynamic and total pressure SI units and with the corresponding sign with
regard to the atmospheric pressure; and
(b) Fan power.
Answer
(a) With the equivalence 1 Pa = 0.010197 cmwc; 1 cmwc = 98.068 Pa
• Dynamic pressure (a): 3.2 · 98.068 Pa = 313.8 Pa
• Static pressure (b): −4.6 · 98.068 Pa = −451.1 Pa
• Total pressure (c): −1.4 · 98.068 Pa = −137.3 Pa
(b) For an exhausting system, the pressure grade lines are similar to those in the
figure.
6 Fans and Flow Control Devices
amount of static pressure is transformed into velocity pressure. In addition, both give
the airflow the desired exit direction.
Moreover, it is also common to transform a part of the dynamic pressure at the
fan outlet into static pressure by means of a gradual expansion. This is achieved with
devices called diffusers in the case of blower fans, which also help to couple the fan
to the duct and evasés in exhaust fans, which discharge directly into the atmosphere.
Question 6.1 Pressure measurements have been made on the exhausting system
shown in the figure.
a
b
c
20 m
3 s
-1
They provide the following results:
(a) 3.2 cmwc,
(b) 4.6 cmwc, and
(c) 1.4 cmwc.
You are asked to determine the:
(a) Static, dynamic and total pressure SI units and with the corresponding sign with
regard to the atmospheric pressure; and
(b) Fan power.
Answer
(a) With the equivalence 1 Pa = 0.010197 cmwc; 1 cmwc = 98.068 Pa
• Dynamic pressure (a): 3.2 · 98.068 Pa = 313.8 Pa
• Static pressure (b): −4.6 · 98.068 Pa = −451.1 Pa
• Total pressure (c): −1.4 · 98.068 Pa = −137.3 Pa
(b) For an exhausting system, the pressure grade lines are similar to those in the
figure.