liebert xdc160 short cycle alarm c1b

[john120]

liebert tech support feels there has been a load change in the room. i am in this building 4 or 5 days a month an to the best of my knowledge there hasn't been any change in load.
the c1b compressor cycles 5 times in 10 minutes and then alarms. with the
c1b contactor coil disconnected the cooling is running at 68%.
thanks for your time, john

[lafittejdubb]

John, there's a thread on here with someone who has had the same problem. attached is the link to it.

http://hvac-talk.com/vbb/showthread....rt+cycle+alarm

[Phisher]

What is the alarm? What are the pressures and temperratures. You need to update your profile.

[GoColts]

Do you have the suction and discharge pressures? Is your hot gas bypass open? On the roof have you checked the referigerant level in the leetemp barrel? At 68% cooling compressor 1a,1b and 2a should be running. Email me if you want and we can go into further detail. This is not the same type of system in that other thread.

[john120]

thanks for the reply,
70#/220# with 1 compressor c1a
70#/240# with 2 compressors c1a,c1b
hot gas valve about 20%
lee temp level, between the bottom and middle sight glasses. [there are 3 ] 70* oat
pump circuit level is between the middle and top sight glasses.
application:
2 xdc chillers
26 xdv's divided evenly 13 per chiller
the unit take about 10 minutes to go from 34%clg to 54%clg. At 54% the c1b compressor is started and the clg% drops from 54% to 34% in 8 seconds! the refrigerant temperature will also drop 4* to 6*
i have increased the min temp st pt from 70* to 72*
thanks for your time
john120

[Phisher]

Sounds like low load. Is it possible your getting some carryover from the supply air back to the return?

[GoColts]

The book will give you a level for your ambient tempature. Is the remote stat being affected in some way? Did they put a perf tile under it? Whats the dew point temp? And if so is it swinging wildly as well? The XDH's are they getting a good load from the hot rows? There are a lot of variables here for the Xtream Density Chiller systems. I would look at the sensors first make sure they are calibrated correctly and not being directly affected by anything. The XDH's like hot air, I have a 98 degree return and a nice 70 degree supply and my cooling % is always around 40 and I never have any short cycle problems.

[john120]

there are 2 refrigerant circuits that each contain 2 15 ton compressors. the short cycling is occuring between 30 and 45 tons of cooling or the second and third stages. because of that i am having a hard time convincing myself that this is related to load. i checked sensor calibration and pumped circuit pressures 45# net +/- 3#s. added delay on break time delay in c1b contactor circuit. no more alarms. i will be revisiting this situation during monthly p.m.'s
thanks, john

[d_lee1]

I worked on one that had this problem before. It was a liquid line solonoid valve leaking by on shut down

[Liebert_Tech]

Are all XDV's running? Sounds like low load. you have 320kw of cooling capability with 2 chillers. You need at least 40%(130kW) to run them properly. What is the load in the space? U may have to turn some XDV's off or parameter units to load it up. When you added delay on break relay, did cooling % drop on the controller and the compressor still run? Did you see the cooling % rise back up before the compressor dropped out? Is this wall box controller or iCOM?

[john120]

All xdv's are running. The chillers are not cycling off, they are cycling between 30 and 45 tons of cooling, rapidly. The chillers were in for 2 years and never had a short cycle alarm, until recently.I will try turning some of the xdv's off. Since adding the relay the cooling % will go up to 60% when the relay is open, once the relay makes c1b contactor coil is energized and takes 30 seconds to cycle off at 36% cooling demand. When the cooling % is dropping and gets to 44% it will hesitate and climb to 45% for a second and then continue to 36% and cycle off. Wall box controller. As far as sizing and load goes I must plead ignorance. There has not been any load changes in the room. Is it possible that the hot gas bypass is not reacting quickly enough? Is there a checkout procedure to verify proper operation of the hgbp?
Thanks for your time,
John120

[Liebert_Tech]

HGBP would only come into play if you were having some Low Suction issues and that would be from Low Load. If these units have proper load then HGBP should not be activated. From what your saying with the fluctuation in the cooling %, it sounds like this may be a setting in the controller issue. That info your reading is all output control telling you what the board wants to do from the set points that are set. Make sure no one has changed the dew point set point and temp set points or the sensativity for those settings. Make sure the sensativity settings are resonable and not to close. If they are this could cause the cooling command to jump around and do this. If this has not gone into alarm for 2 years and has been running fine, then something changed. Just make sure all your sensors are reading correct and not faulty.

[nick_verm]

I've come acrossed short cycle alarms on these before and it was caused by weak discharge valves. Time your compressors when in the off cycle. If the suction pressure continues to rise after multiple pump downs then you have a leak at either the LL solenoid or the discharge valves. I would tell u how to check it but I think they dont want stuff like that posted in a non-pro forum.