6.2 Fans
227
There are guide vanes that are already integrated with the fan before purchase,
while others are coupled later on. In the first case, the effect of the vane is frequently
included in the fan characteristic curves supplied by the manufacturer; in the second
case, the user must calculate their effect.
Variable-pitch control is possible for axial fans with adjustable propeller blades.
In such cases, the airflow rate can be controlled by varying the angle of the impeller
blades so that if the angle of attack increases, the flow rate increases. Varying the
angle of attack can be performed with the fan at standstill or in motion. Presentday variable-pitch adjustment is obtained using a hydraulic actuator axially stationary
relative to the main shaft. When the fan is in operation the system permits instant
matching to the demand. Fan duty control employing variable-pitch blades provides
an infinite range of airflow rates, has relatively high efficiency and can move air
backward employing negative pitch angles. The greatest drawbacks of this system
are its high cost and very high risk of stall if the pitch angle adjustment is not correct.
The last options for flow control would be either to expel the excess airflow rate
to the atmosphere or to feedback this excess current to the suction side (bypass
control). Bypass control acts by lowering down the system impedance curve while
maintaining the fan curve. The system is only used in fans with steep fan curves,
such as centrifugal backward-curved and axial fans. The method is not often used as
it creates high losses.
Question 6.2 Analyse, using a graph showing pressures and flows, the effect that the
increase in the total length of the ventilation circuit has on the airflow rate supplied
by a fan. Make a synoptic table indicating the main differences between discharge
damper control and vane control with regard to the system characteristic and fan
characteristic.
Answer
Increasing the length of a circuit means increasing its aerodynamic resistance. Therefore, since the basic relationship P = R Q
2 is met, the increase in resistance results
in an increase in the slope of the curve, with the same considerations as those made
for Fig. 6.26.
The main differences between discharge damper control and inlet vane control
are:
Control system
System characteristic
Fan characteristic
Discharge damper
Altered
Unaltered
Inlet vane control
Unaltered
Altered
It is key to remember that due to the pre-whirl in the direction of fan rotation
created by the inlet vanes, the airflow, pressure and power are reduced, thus altering
the fan performance curve.
227
There are guide vanes that are already integrated with the fan before purchase,
while others are coupled later on. In the first case, the effect of the vane is frequently
included in the fan characteristic curves supplied by the manufacturer; in the second
case, the user must calculate their effect.
Variable-pitch control is possible for axial fans with adjustable propeller blades.
In such cases, the airflow rate can be controlled by varying the angle of the impeller
blades so that if the angle of attack increases, the flow rate increases. Varying the
angle of attack can be performed with the fan at standstill or in motion. Presentday variable-pitch adjustment is obtained using a hydraulic actuator axially stationary
relative to the main shaft. When the fan is in operation the system permits instant
matching to the demand. Fan duty control employing variable-pitch blades provides
an infinite range of airflow rates, has relatively high efficiency and can move air
backward employing negative pitch angles. The greatest drawbacks of this system
are its high cost and very high risk of stall if the pitch angle adjustment is not correct.
The last options for flow control would be either to expel the excess airflow rate
to the atmosphere or to feedback this excess current to the suction side (bypass
control). Bypass control acts by lowering down the system impedance curve while
maintaining the fan curve. The system is only used in fans with steep fan curves,
such as centrifugal backward-curved and axial fans. The method is not often used as
it creates high losses.
Question 6.2 Analyse, using a graph showing pressures and flows, the effect that the
increase in the total length of the ventilation circuit has on the airflow rate supplied
by a fan. Make a synoptic table indicating the main differences between discharge
damper control and vane control with regard to the system characteristic and fan
characteristic.
Answer
Increasing the length of a circuit means increasing its aerodynamic resistance. Therefore, since the basic relationship P = R Q
2 is met, the increase in resistance results
in an increase in the slope of the curve, with the same considerations as those made
for Fig. 6.26.
The main differences between discharge damper control and inlet vane control
are:
Control system
System characteristic
Fan characteristic
Discharge damper
Altered
Unaltered
Inlet vane control
Unaltered
Altered
It is key to remember that due to the pre-whirl in the direction of fan rotation
created by the inlet vanes, the airflow, pressure and power are reduced, thus altering
the fan performance curve.