Unstable Static Pressure in Exhaust Duct

[Haotranphotomask]

Hello, I have a system where two exhaust fans are running simultaneously. However, they both discharge to the same outlet. The suction pressure seems to be quite stable, but the discharge pressure fluctuates between 2" wc to 4" wc frequently. Below is a rough setup of my system. The rectangles are the dampers that we have on the discharge side. The ones with the red circles are dampers with a linkage and secured. The other ones are backdraft dampers and are not staying in place. I believe this happens due to both fans discharging to the same header, which causes the airflow to fluctuate the backdraft dampers.

My questions are:

• Does this setup cause a reduced efficiency of the fans?
• Would it be possible to lock open the backdraft dampers to reduce the static pressure fluctuations? I cannot find any codes referring to not locking the backdraft dampers. I understand the risk of fan shutting down and having air circle back into the rooms. If I accept that risk, would it be a possible solution short term?

 

[Lnewqban]

I would replace both back-draft dampers with only one at the outlet of the common duct, if possible.

If not possible, modifying the ductwork layout and connection in order to reduce the flow from each blower from hitting the opposite damper head on, would help.

Reducing the airflow of one blower via VFD or discharge volume damper may improve the current situation as well.

Pressure pulses from each blower may be deforming the metal ducts walls and swinging the dampers, inducing vibrations, and the fluctuations in static pressure and airstream velocity.
Even resonance may be possible.

 

[Haotranphotomask]

I would replace both back-draft dampers with only one at the outlet of the common duct, if possible.

I was more worried if one fan fails, air blown from the running fan would travel through the failed fans into the rooms.

I have plans to redesign the entire duct work. I am trying to find short-term solutions. I may try to adjust the VFDs as suggested.

 

[Lnewqban]

I was more worried if one fan fails, air blown from the running fan would travel through the failed fans into the rooms.

I have plans to redesign the entire duct work. I am trying to find short-term solutions. I may try to adjust the VFDs as suggested.

The diagram suggests that both are working in parallel and extracting from a common point.
If changing velocities is not sufficient, replacing that tee with a Y connector may be an effective minimum change to the existing ductwork.

 

[jrmichler]

The ones with the red circles are dampers with a linkage and secured.

VFD's and dampers on the same fans make for a bad design. One or the other, but not both. Are those dampers secured in the full open position? If not, they should be.

the discharge pressure fluctuates between 2" wc to 4" wc frequently.

How frequently is frequently? If it is one to several times per second, it is probably a duct resonance. If it is once every several seconds, it might be the VFD's fighting each other. Is the blower speed under feedback control, or does an operator set the speed manually?

If that pressure is measured at the blower discharge, there is very high resistance in the discharge ductwork. What is the differential pressure across the blowers? What is the fan curve for the blowers? Please post the fan curve, and show the operating point(s).