AC Axial-flow Fans
Common Information
M-47
Ot
h
e
rs
2. Recheck the Selected Fan
In the following diagram, the value for the system impedance
∆
P
1
is
unknown. It is assumed that a flow rate of Q
1
is required but mea-
surements of the cooling effect show a reduced flow rate of Q2, for
example. This result indicates that the system impedance was
∆
P
1
,
so the Fan one size larger is necessary to produce the flow rate Q
1
to
obtain the prescribed cooling effect.
3. Serial and Parallel Fan Operation
The characteristics of two identical Fans operated in series or paral-
lel are determined as shown in the following diagrams.
Serial Operation:
Parallel Operation:
Terminology
Nominal Value:
The average value of data based on actual measurements.
Nominal values cannot be treated as rated values. Enquire sepa-
rately for details on rated values.
Flow Rate: Q (m
3
/min.)
The volume of air discharged by the Fan in a unit of time.
Static Pressure: Ps (Pa)
The pressure difference across the front to the back of the Fan gen-
erated by the discharged air, which is unaffected by air flow speed.
Maximum Flow Rate: Q max (m
3
/min.)
The volume of air discharged by the Fan when the static pressure is
adjusted to zero (Pa) at the flow measurement unit.
Maximum Static Pressure: Ps max (Pa)
The pressure difference inside and outside the Unit when the flow
rate is adjusted to zero (0 m
3
/min.) at the flow measurement unit.
System Impedance:
The flow resistance inside a mounted Axial-flow Fan caused by the
density of parts and shape of the flow path.
Impedance Protection:
A method of preventing burning damage when the motor is restricted
from rotating by setting the motor winding impedance (AC resis-
tance) to a value giving a temperature rise in the windings below the
temperature at which burning occurs.
Thermal Protection:
A method of preventing burning damage when the motor is restricted
from rotating by setting a thermal element to interrupt (the restricting)
operation before the motor reaches a temperature at which burning
occurs.
Static pressure
Next larger size fan
Fan selected from
flow-rate calculations
Operating point
Theoretical operating point
Flow rate
Static pressure
2 units
1 unit
System
impedance
Pressure
increase
Flow rate
Static pressure
2 units
1 unit
System
impedance
Flow rate
Flow rate increase
P
P
Fan
Same as water level
P
P+Ps
Fan
Ps
1. The air pressure across the
front to the back of the fan does
not change when the fan is
stopped.
2. Static pressure (Ps) is
generated at the front of the
fan when it rotates.
P
P+
Ps max.
Fan
Maximum value of difference
Ps max.
