HVAC Static Pressure Issue

[superdumpy]

Hello there,

Let me give a brief history of the issue:

Our HVAC runs at 900 pascals supply static pressure set-point and our supply fan maintains this just fine. At irregular intervals however we have a sudden dip in pressure of roughly 50 pa and the fan will ramp up to counteract and it will take a few oscilations (four minutes) to steady out again. This all sounds ok but the more this happens the more the pulley belts are loaded and damaged (due to the ramping up and down) until a point where the fan throws the belts and we have a failure.

We have changed the static pressure transmitter, changed the motor, alligned the pulleys and went through many many belts.

Has anybody dealt with or witnessed a similar problem with HVAC systems. We are running out of ideas!!

Kind and warm regards,
Thomas

 

[MintJulep]

Perhaps you are operating the fan in the unstable region of its performance curve.

 

[GMcD]

What's controlling the fan speed? If it's a variable frequency drive, can't you just program the response time to smooth out the ramp-up/ramp-down time response so it changes the fan speed a bit slower. Basically tune the PID control feedback to the speed control.

 

[KenRad]

GMcD is right. If your static control response is so fast that it's jerking belts apart, then a PID parameter adjustment is in order.

A 50 pa deviation from setpoint is probably not significant to the operation of this system anyway. I would start by slowing down the control response.

---KenRad

 

[gepman]

superdumpy
I agree with the previous responses but I think that you should also check you decel rate parameter in your VFD. Your fan could have a high inertial load and be slowing down at a slower rate than the motor. This would cause the belt tension to be reversed and possible cause the belts to come off.

 

[superdumpy]

The fan is controlled by a VFD. I had one of our automation guys take a look at the issue and he is happy with the set-up. It does take some time to smooth out the control but he reckons this 50pa drop in the first place is where the problem lies and slowing down the PID response may be just masking a mechanical problem.

Thanks for the many replies so far.
Thomas

 

[marcoh]

900Pa setpoint seems high and you may need to check the location of the pressure sensor. How close to the fan is the pressure sensor and how far from outlets etc?