Liebert CRAC unit - short cycle problem

[josephpatrickallan]

The new Crac units have a setting in the Service Menu that allows you to use PI, Proporational or Intelligent. I found that Intlligent setting will cause the unit to overshoot at times but will not allow a short cycle.
The last proporational band I changed had 10,000 starts and a total of 71 hrs of work. Very hard on the unit causing a PDF problem.

[lafittejdubb]

I've run across this a couple times on Liebert units.

There's a good chance your LLSV is leaking-by causing the unloader to pump the compressor down. The problem is after 5 restarts within an hour, it actuates the short-cycle alarm. You can test this by gauging-up to your suction service valve and monitoring the psi's. If the psi starts to rise after the unloader has pumped the compressor-down to the LPCO more than once (sometimes there's enough gas to raise it causing one more pump down), then your LLSV is leaking-by.

If that is your problem you will probably need to replace the LLSV unless you just want to disable that alarm. You can disable the alarm by going into the alarm menu on the control panel. If you don't know the password, you can disable the password protection by turning microswitch #6 to the off position.

Last but not least, increase your deadbands also! They're set at 3 degrees by default (which is about as high as you can go), however they might be set to less.

Hope this helps!

[mark_ballard]

I agree with Lafittejdubb on the leak through. Have seen it a few times myself. Also, did they load this end of the server room more so than the rest? Have seen that a few times as well.
Also, guys, he is in the AOP section as a owner, so why not help instead of dogging him? Just saying.

[Gib's Son]

CRAC=Computer Room Air Conditioner

ACU Air Conditioning Unit

I think he was pointing out the irony of his request not to abbreviate.

[GoColts]

How about a model number? Does it have a semi hermetic compressor? If so have you checked the valve plates by testing to see if they hold pressure? Let the system pump down, front seat the suction valve, does your pressure in your compressor body rise? If so you are leaking through the discharge valves. If not try front seating the discharge valve with the suction valve open after pump down, if pressure rises you have a bad solenoid valve. The bad discharge valves has been the cause for 90% of my short cycle alarms in the past because most of my CRAC's see constant loads. I have seen one solenoid valve bad and it had something stuck in it not allowing it to close off properly. A burn out that was not properly cleaned up, clogged the TXV as well.

[mixsit]

How about a model number? Does it have a semi hermetic compressor? If so have you checked the valve plates by testing to see if they hold pressure? Let the system pump down, front seat the suction valve, does your pressure in your compressor body rise? If so you are leaking through the discharge valves. If not try front seating the discharge valve with the suction valve open after pump down, if pressure rises you have a bad solenoid valve. The bad discharge valves has been the cause for 90% of my short cycle alarms in the past because most of my CRAC's see constant loads. I have seen one solenoid valve bad and it had something stuck in it not allowing it to close off properly. A burn out that was not properly cleaned up, clogged the TXV as well.

Ding. Two months to get to this

Liebert (ie Carlyle) back about then apparently had some 'new improved valves that tend to leak. (Liebert support indicated to me they set them (carlyle) straight once the warranty stuff started coming in I guess.

Hey there Sac'..

[mixsit]

just to add.. If that is the problem some of it can possibly be mitigated if you can shift more load (setpoint on that one vs others nearby?). Every minute the thing is running is time not cycling.
Also the ones I'm seeing it on won't let you select lead compressor = problem child' ..which would have been nice.

[GoColts]

We had seven DH380's that were installed in late 2007. There were serious compressor issues, some would shake and vibrate so badly that we were breaking the cap tubes on the low pressure switches and a few of them just broke. We finally got Liebert to admit that they had gotten a bad batch of caryle compressors. We ended up replacing 5 compressors and all of the valve plates even the ones on the new replacement compressors that Liebert had sent us. Liebert really steped up on this one, and they took it in the shorts and covered all of the parts and labor which was very cool on their part.