refrigerant discharge piping noise

[hvacrayg]

Hello fellow mechanics.....i have recently replaced a 80hp refcomp compressor with a 90hp Handbell screw compressor.Reason for the replacement was refcomp is no longer in the United States and i needed parts/gaskets etc.Long story short i have a high pitch noise throughout the discharge line all the way to the evap condensor tower.I installed vibration eliminators on both the discharge and liquid line at the chiller.Still cant get rid of the noise in the discharge line.Im installing a vfd Friday to finish the quoted job and hoping this will help the hi noise pitch.It sounds like hi pitch sand in the discharge line....any ideas.:

[Screwit]

The source of the high pitched noise may be due to the discharge pressure pulse gasfrequency at the particular load matching the natural frequency of you discharge line, the interaction of the matching frequencies cause the high pitched noise

Does the pitch of the noise alter with the compressor loading?

Previously when I have encountered this phenomonon have had some success with the use of weights to change the natural frequency of the discharge line. Quick test is, wearing welding gloves for protection, grab hold of the discharge line at various places and see if it alters the effect of the noise. If so you can experiment with adding weights to the line to change the frquency.

If this is not the cause could be the check valve or similar in the compressor outlet attributing to the problem.

[Screwit]

Or add a muffler - involve handbell for recomendations

[flange]

higher velocities due to pumping more cfm with larger compressor? ? is it noise or vibration?

[jayguy]

higher velocities due to pumping more cfm with larger compressor?...

this was my first thought too. if this is the case (assuming that the compressor itself isn't the issue), the noise will get better with unloading and worse with loading. if it is a natural resonance (like screwit mentioned) then the issue will come and go at certain loading spots.

[CCSPIERCE]

I aagree with scewit. Need to get a properly selected muffler.

[flange]

80 horse screwy should have a king valve at discharge. slowly close it to restirct discharge, and see if noise waves goodbye. if not, muffler will likely be in order, but I would try the weight thingy and reducing velocity first to be sure.

[hvacrayg]

Great answers......i did try grabbing on to the discharge piping at diff locations (150 feet w/ hydrozorbs every 10 feet) with not much success,noise was still super annoying.Well i ended up installing a VFD on the the Handbell screw machine.I had to pull the gas out to reconfigure the unloader/loading set up for the drive.With no gas in compressor i inspected the discharge check valve...it seemed to be in working order.Long story short being that the other compressor did not have a check valve i removed this check valve for testing.The noise is all but gone......only problem is i dont know if it was the check valve or the vfd itself removing the noise problem.Another issue is as you guys would probably know....when the machine turns off there is now a longer delay(30 seconds)before the pressures equalize and the screw stops spinning due to no check valve.Is there any possible problems with machine spinning backwards even though machine is off.Pressures equalizing to 80 to 100 psig during the daywhen machine off.

[r404a]

saw something like a weight or clamp on the discharge line of an RTAC that looked factory, was that a noise reduction option?

[jayguy]

saw something like a weight or clamp on the discharge line of an RTAC that looked factory, was that a noise reduction option?

it is for vibration, not noise.

[r404a]

thanks jay

[klove]

Great answers......i did try grabbing on to the discharge piping at diff locations (150 feet w/ hydrozorbs every 10 feet) with not much success,noise was still super annoying.Well i ended up installing a VFD on the the Handbell screw machine.I had to pull the gas out to reconfigure the unloader/loading set up for the drive.With no gas in compressor i inspected the discharge check valve...it seemed to be in working order.Long story short being that the other compressor did not have a check valve i removed this check valve for testing.The noise is all but gone......only problem is i dont know if it was the check valve or the vfd itself removing the noise problem.Another issue is as you guys would probably know....when the machine turns off there is now a longer delay(30 seconds)before the pressures equalize and the screw stops spinning due to no check valve.Is there any possible problems with machine spinning backwards even though machine is off.Pressures equalizing to 80 to 100 psig during the daywhen machine off.

You'll backspin at shutdown unless you set up the controls to let it coast to a stop on the drive, but you won't backspin with it off (hope I interpreted the question correctly). If the sytem were on pumpdown, you'll have lost that ability. Refrigerant migration to the evaporator during the off cycle will be very bad. Might want to look at manipulating the controls for a slow ramp loading or pumpdown on restart to avoid carrying a lot of liquid into the oil separator and possibly washing it out.

[hvacrayg]

Well i put the discharge check valve back in compressor and the high pitch noise is pretty much gone still.So installing the vfd by itself really took care of the high pitch problem.I am getting a low oil level trip every now and then which im assuming is somehow tied into the drive install(wasnt get that trip before).I have MCS controls looking into this since they configured/downloaded all of the parameters for the new drive and exv remotely.Also by reinstalling the disch. check valve the helical rotors come to a stop right away when machine shuts down.Beginning to see the light at the end of the tunnel at this time....tx for the responses.