24
SELECTION PROCESS
Fan Performance Chart
Example 1 :
The following are the design requirements for A5DB250H2 unit:
Model:
A5DB250H2
Supply Air Quantity
=
9000
CFM
External Static Pressure
=
350
Pa
Step 1: From the fan curve (at 9000 CFM),
Standard operating system;
Total Static Pressure
=
410
Pa
Internal Static Pressure, ISP =
110
Pa
External Static Pressure, ESP =
300
Pa
External Static Pressure of 300 Pa did not fulfil the design requirements.
Step 2: Therefore at 9000 CFM and 350 Pa external static pressure,
Total Static Pressure
=
ISP + ESP
Pa
=
140 + 350
Pa
=
490
Pa
Step 3: From the fan curve, the design requirement calls for RPM about 810, whereas the unit can only deliver
RPM about 715 under the same CFM. Therefore, it is necessary to RPM about 810 under the same CFM.
Therefore, it is necessary to resize the pulley sizes.
From the table:
Motor pulley
=
90
mm
Blower pulley
=
200
mm
Motor RPM
=
1440
In order to obtain 810 RPM, we recalculate the new blower pulley as:
(while maintaining the motor pulley)
)
0
1
8
/
0
4
4
1
(
x
0
9
=
b
D
m
m
0
.
0
6
1
=
The nearest pulley size will be a diameter of 160mm
Recheck, with Db =
160mm
Blower pulley
=
1440 x (90/160)
=
810
We thus need to change the blower pulley from 200mm to 160mm in order to obtain the higher operating
static pressure.
Step 4:
When the pulley is changed, the V-belt length must be rechecked. We have for horizontal air throw
con guration:
V-belt length, L
=
2C + 1.57 (Db + Dm) + (Db - Dm)2 / 4C
9
9
.
1
+
)
0
9
+
0
6
1
(
7
5
.
1
+
)
5
1
6
x
2
(
=
m
m
9
4
.
4
2
6
1
=
We thus can use a belt with a length of
1637mm
where,
C
=
distance between the centres of the two pulleys
Db
=
diameter of blower pulley
Dm
=
diameter of motor pulley
Step 5:
From the fan curve, we can also notice that the motor power input has increased, which is now higher than
the standard 4.0 kW. At the new operating point, the power is approximately 4.2 kW.
Thus, the existing motor is not sufficient to drive the blower with the smaller 180mm pulley.
Summary:
i) Fan motor kW
=
5.5
kW
ii) Blower pulley diameter
=
160
mm
iii) V-belt size
=
1637
mm