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N°7 – Control of air flow to electric heating coils (contd.) – HSC

For training at PB level, this section and the others will not be studied.
Monitoring the non-operating of fans with a suction pressure switch does not guarantee the control of the absence of air flow.

In fact, if a damper, located at the inlet of the casing, closes the suction at the inlet of the fan (and its Tdh) will be maintained (and will be even maximal for a centrifugal fan).

So, controlling a vacuum pressure (or a Tdh) which is too low will not enable the absence of air flow to be identified due to the closing of the damper.

In the air handling unit above, the air flow is zero and the vacuum pressure switch is inactive.
In this situation, to ensure the protection of the fan motor against the risk of the drive belt breaking, and protection against overheating of the electric coil, we have 3 solutions:
Solution n°1:
Carry out a control of the discharge pressure of the fan (not the suction at the inlet).

If the damper at the inlet of the AHU below closes accidently, we should expect that the pressure of discharge will cease. The pressure control switch monitoring the overpressure (compared to the atmospheric pressure) will register this closure and shutdown both the fan motor and the electric heating coil.

Note: this first solution requires that at a normal operating regime the discharge overpressure of the fan is not too low (which can be the case when the air blowing network is not very long).


How do we explain that when the damper at the inlet of the AHU closes, the discharge pressure of the fan drops to zero?

If the damper at the inlet of the AHU closes, the air flow ceases.
In this case if the discharge pressure did not drop to zero, this would be contradictory. In fact it would mean that the fan is continuing to “push” a flow of air in the air ducts, which is impossible because the air flow is zero.
Another formulation: zero air flow does not lead to head loss between the fan discharge and the premises. The pressure detected by the pressure switch is therefore that of the room or premises, i.e. zero.

If the air flow is zero, the pressure of the fan discharge can only be zero as well.
Thus we can understand that in this situation, a pressure switch controlling the discharge pressure of the fan is a solution.

A third solution consists of having the operating of the electric coil controlled by an air flow which can be diagnosed by measuring the dynamic pressure (Pitot tube flow meter), or by a paddle switch detector (see below).
It could be fitted in the outlet duct of the AHU if the fan is not a variable speed model.

Note : below a speed of 4 [m/s], the Pitot tube meter does not work well because the pressure difference which it measures is very low. Its use is therefore not recommended for controls carried out within the AHU.

We can also use a paddle switch (see below) which is much more sensitive to low air speeds.