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N°5 – Other methods for fan cycle control – HSC

For training at PB level, this section and the others will not be studied.
Fan cycle control can also be carried out by monitoring the Tdh value or the overpressure at discharge, but in the latter case the excess pressure to be measured is often less than the suction at the inlet and therefore more difficult to control.
Note : on the subject of Tdh values of fans, we can consult the paragraph « Tdh » characteristics of a fan


Explain why the suction at the inlet of the fan n°1 below is certainly greater than the overpressure on discharge.

The suction at the inlet n°1 below is much greater than the excess pressure on discharge because the pressure losses of the air handling unit are greater than those of the network of discharge air blowing from the fan.


Propose for the fan below a setting value for regulating the pressure switch, depending on whether it controls:
– The suction at the inlet
– Discharge overpressure
– The Tdh of the fan.
Setting to monitor the suction at the inlet: – 600/2 = -300 [Pa]
Setting to monitor discharge overpressure: + 200/2 = +100 [Pa]
Setting to monitor the Tdh of the fan: 800/2 = 400 [Pa]