Liebert stops in it's tracks....

[D'Laine]

Today I was running some Testo 523 logging on a 2002 model of a 20 ton Liebert. Just as I was wrapping up the hoses, the unit pumped down for one minute and then started up again.
Now the set point was 6 degrees below the return air temp. so it should have not shut off till it was satisfied. No alarms were generated, and the fan kept running so it shouldn't have been an external shut-down.

I have no idea why this would happen, can any one suggest something?

I know that once in a while the unit's controller equalizes run time for the compressors, but it shouldn't shut down when both are running.

Very strange.

Dave

[The Doctor]

what is your control type?

[Airmechanical]

what is your control type?

hopefully its set on proportional!



.

[madhat]

Did the fan shut off or stop-start too? If so its most likely in the EPO (Emergency Power Out) circuit. Check the wiring connections on the terminal strip on the right piller in side the compressor compartment,(Since you were in this area when it happened) and back to the control board. It maybe an external problem to the EPO, and/or fire panel.

[atmosphere]

Sometimes when you are buttoning up the unit you get a bit of that action.

Not to worry.

[D'Laine]

Yeah, the fan kept running, it wasn't the external EPO shut down circuit.

But now I found another unit in the room did stop for a minute as well. I've had the controls set to Liebert's 'intelligent' control, that didn't work at low loads in the room so I set it to proportional and the temps were better.
Now that the room is heavily loaded, I found the intelligent control gave better operation.

I'll have to check the terminals for tightness next, but I'm thinking it's something in the logic control. I mean 1 minute exactly, I had a recorder running and the room was too warm to cycle any compressors.

I'll give Liebert tech support a call next week and see what they have to say.

[madhat]

Another thing I'd check is the connections on the High Pressure switch, when the high pressure trips, the board closes the Liquid Line Solenoid valve so the refrigerant doesn't end up in the Evap. I believe the newer Lieberts will go thru a cycle on a broken signal at the HP switch. PS back in the early 90s, I made a suggest at Liebert to close the LLS when a unit had a high pressure fault. About a year later the units had that feature.

[The Doctor]

Yeah, the fan kept running, it wasn't the external EPO shut down circuit.

But now I found another unit in the room did stop for a minute as well. I've had the controls set to Liebert's 'intelligent' control, that didn't work at low loads in the room so I set it to proportional and the temps were better.
Now that the room is heavily loaded, I found the intelligent control gave better operation.

Can you be more specific? You can't go wrong with proportional control IMO. I've seen rooms with almost nothing in them, and rooms packed to the doors with heat, and "Liebert's" proportional is THE way to go. Why reinvent the wheel? I've seen maintenance men on campuses try intelligent, and call us when the room went sometimes 8* off course. It's not a science with intelligent, but it is with proportional.

I'll have to check the terminals for tightness next, but I'm thinking it's something in the logic control. I mean 1 minute exactly, I had a recorder running and the room was too warm to cycle any compressors.

I'll give Liebert tech support a call next week and see what they have to say.

Smart money's on tech support saying "go with proportional control".

[D'Laine]

I didn't really think that proportional or 'intelligent' control was the problem.
I was too far above setpoint.

I called Liebert's tech support today and they said that it's built into to logic, with units that run for long periods of time, will pump down for a minute now and then. It was meant for tertiary refrigerants to allow them to ... now here's the part that I don't understand and should be another topic, to allow them to mix together.

It's not going to overheat my rooms so I'll just leave it as is.

But for those of you working with R-507, or is it R407, could you explain Liebert's logic to the rest of us?

[madhat]

Wished you had said it had R407 in it. We have a Lab walk in box at work, that will keep the temperature within a half a degree, by modulating between an external and internal evap. During the summer it will run continously, and the R134a gets left behind in the internal evaporator. (this takes weeks)Then it will cool erractically. a couple of pump downs and all is back to normal. First couple of times it did this, we were pulling our hair out trying to figure out what was wrong. We recovered the refrigerant, goofed on weighing it. leak checked, changed the driers, evac'ed, recharged. Two weeks later same thing.

[The Doctor]

It sounds as though the heat load in the room is at 104% +- of your cooling potential. Did you say the temp is up to 8* away from setpoint at times?
How much 407 is out there? I talked with a friend the day before Independence Day and specifically asked him how much of that particular animal he works on and he said none. So let me qualify. He works for a Liebert USA contractor(or whatever they call it now) in the Capitol area. THey work on everything, as did I while working there,.....everything Liebert AND some, and he hasn't worked with it yet? Just asking.

[jim bergmann]

If the system is a pump down, could the low pressure control differential be set to low? Do you have a leaking liquid line solenoid?

[D'Laine]

I forgot to say that I'm using R-22 in these units. It seems that Liebert applied this occasional 'pump-down for a minute' logic to all their boards, regardless of refrigerant type.

Ten 20 ton machines in the room and only 7 are needed to keep it comfortable. Yes, the return air temp gets up to 77 or 78 in places, but walking around the room, the spaces vary from 69 to 74 degrees.

So from what I hear from madhat, tertiary gases can separate under continued operation, to the point where they don't work so well. Bummer!

Thanks for a the feedback guys,

D.