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Application of Control Output Lower Limit in both MAU and Exhaust sys.

Application of Control Output Lower Limit in both MAU and Exhaust sys.

Application of Control Output Lower Limit in both MAU and Exhaust sys.

(OP)
Application of Control Output Lower Limit in both MAU and Exhaust system are different.

IN MAU (Make-up air unit) system, the PIC control logic  modulates the damper 0 to 100% by the cleanroom pressure.

FMS has programmed to limit both the upper & lower control output controlling the damper in open/close postion.

The control outputUPPER limit allows the damper to open 100%, whereas the control output LOWER limit allows the damper to close 0%.

This has advantage of limiting the damper from closing to 0% position.

When the MAU is shutdown, the damper is software interlocked to fully close 0%.

Should the input of control output LOWER limit still be 0%?

2. In Exhaust system,the PIC control logic modulates the VSD speed by the air flow.

FMS has programmed to limit both the upper & lower control output controlling the VSD speed to ramp from 0 to 100%.

The control output UPPER limit allows the VSD to ramp maximum 100%, whereas the control output LOWER limit allows the VSD to stay at 0%.

This has advantage of limiting the VSD from falling to 0% speed.

Should the input of control output LOWER limit be 40% (20Hz-similar to the minimum VSD setting)?

RE: Application of Control Output Lower Limit in both MAU and Exhaust sys.

It sounds like you're asking if the MAU damper should coincide with the exhaust fan VFD positions.  There is nothing that says their relationship is linear or proportional.  Intervening ductwork and fan efficiencies dictate that a linear, equivalent relationship between the two is least likely.

You haven't detailed all of the system components, but closing a damper to 100% on an open air system is normal and prudent.  If the MAU is not heating, there is the potential that freezing air could be brought into the space, destroying the MAU coils, the sprinkler piping within the space, or worse.

Try again if you were asking about different things.

RE: Application of Control Output Lower Limit in both MAU and Exhaust sys.

FWIW the reason the low limit of the fan is 20% is that below that, if the fan is belt drive....it stops - due to bearing/belt friction.  VFD's on belt drive equipment should ALWAYS be checked at low speeds to make sure the fan is actually turning when at the low speed.

As far as OA is concerned, you need to check local codes for min OA damper settings.  Obviously when the fan is off...damper closes.  When fan is on usually the min could be 20% of volume at all speeds/volumes.  (depends on space usage etc.)  So that governs OA requirements, then depending on space use/ positive pressure etc, that will govern the EXH damper position.

Not sure if this helps.
Good Luck
Ed

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