View Full Version : Air handler vibration / pulsation
I have a large McQuay air handler (LSL141DH) that vibrates / pulsates when the VFD ramps up (>50 Hz). If I open the door at the blower side (downstream of the coil) the vibration / pulsation stops. It “appears” that the vibration is the greatest at the discharge vent of the air handler. The walls in the offices adjacent to the machinery space also vibrate. No vibration is felt at the concrete pad.
Question: If an air handler is not getting a sufficient return air flow, can it result in vibration / pulsation of the air handler or discharge ventilation? If so, what would be the theory behind it?
02-13-2007, 10:18 AM
I would bet that your having a harmonics problem with your VFD and it needs to be adjusted, call the rep and have them make the proper adjustments. Whats happening is called cogging and usually takews about 1/2 hour for the adjustments. I learned this the hard way on an ABB drive, changed motor, had a balancer check it and then talked to the right tech at the ABB rep who came out and fixed it in 30 min.
02-13-2007, 05:33 PM
if the vibrtion/pulsation is downstream, and you relieve the pressure downstream and the vibration goes away...you most likely have surge. you may have heard a chiller surge before. it is the same thing. the chiller is designed to be able to "lift" the refrigerant from one low pressure area to a high pressure area. if the low side gets too low or the high side gets too high (in pressure) the chiller can not create enough lift to keep the refrigerant moving from the low side to the high side. you probably have the same thing. a fan wheel does almost the exact same thing to air. your air can not get from the low side to the high side and the sound you hear is the air blowing backwards through the fan to relieve the pressure. find the low end of the vibration/pulsation vfd setting and watch your amp draw as it pulsates. your amp draw will raise briefly and then drop dramatically just as the surge goes away. it will then repeat.
find out why you need to have the fan run so hard...are you not able to make discharge duct setpoint? is the setpoint too high? is the sensor accurate? is there a blockage between the fan outlet and the duct pressure sensor? check your amp draw and voltage to the motor with a "True RMS" meter and compare to the motor nameplate (keep in mind that you have a vfd). find out the engineering specs and compare the setpoints and pressure differentials to what you are experiencing. the fan curve chart for your fan will tell you a lot. in your submittal data (if you have it), you will find the chart with the selection criteria that was picked for your application.
02-13-2007, 05:56 PM
Sounds like it could be either problem.
PBCD, you will need to follow up on this thread so we know the outcome.
I'll try to get back to that job soon and check it out.
02-14-2007, 03:54 PM
I have had this same issue and found it to be a damper issue. Return is up high and OSA down low. The plenum is not mixing the way it should and some times on start up the outside damper will close down. As one person said, slow ramp up pulls that air across the element before it can mix. I also have an issue with the freeze stat liking to trip on this same unit. The plenum holds the heat and during start up the discharge sensor tells the heating valve to close and with an electric Belimo it takes a while longer then air , so by the time it understands the need to open again TRIP . Unit is running nice again but that was my story. Hope it helps.
02-14-2007, 06:09 PM
Is this AHU a pull thru design? If so then removing the blower access door would be relieving a return air issue. Also most harmonic issues I have experienced with a VFD have been at mid range speeds- not toward the full speed of the drive.
Yes, it is a pull thru design.
Thanks for all your comments.
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