EXV Hunting

[Dan H]

I am having troubles with the EXV (electronic expansion valve) on circuit A of a Carrier 30GT090 chiller.
The EXV continuously cycles, the measured superheat at the compressor suction sensor ranges between 5 deg F and 40 deg. F. The Service manual list total superheat at 30degF, (10deg after the chiller plus 15-20degF from the motor winding.)
I've tested and inspected the EXV and all works ok. The temperature sensors all check ok and the sight glass is clear so why is the valve continuosly overshooting?

The EXV on circuit B is working fine, maintaning right around 30degF superheat.

Any suggestions will be apprecieated.
Dan

Below are compressor stats when I was troubleshooting this morning. Outside air temp was in mid 50s.

Compressor A Running Conditions:
suct pressure: 40 psi approx
Disch Pessure: 140 psi approx
Chilled water entering temp: 29 degf
Chilled water leaving temp: 27degf
Saturated suction temp:13
Compressor return gas temp:varies 5 to 40F
Condensing temp:68F

Compressor B Running Conditions:
suct pressure: ? not recorded
Disch Pessure: 155 psi approx
Chilled water entering temp: 29 degf
Chilled water leaving temp: 27degf
Saturated suction temp:17degf
Compressor return gas temp:47degf
Condensing temp:69degf

[chemi-cool]

Hi Dan.

Although I have never played with one of these, It seems to me that from what you explaining, that the problem is in the controls

To completely eliminate the doubt of the EXV, change the wires with the other one only for a minute and just to make sure that it operates OK.

Check all the connections of all the wires to make sure that no loose connection is giving you these results.

If none of this will help, then the problem is in the control.

Is it powered with ComfortLink Controls?

Chemi

[botrous]

Was this EXV replaced before ? or it's the original one ?
I suggest that you check the stepper motor wires for faults . . .
Please let us know what happens next .

[Dan H]

Botrous;
Both EXV valves have been replaced, but not by our company. I don't have any records to show why. The stepper motor checks out ok.

Chemi;
This chiller has a Flotronic Control System.
I checked all the wiring and the valve operation per the service manual and found no problems.

I like your idea of swapping wires to the other EXV. The controller is contiuously overshooting, which makes it seem like the valve is moving to slow.

I will be working on the unit again tommorrow, I'll let you know what happens.

Dan

[botrous]

ok Dan I am looking forward to hear what will happen . Hope that you will find the problem and fix it.

[chemi-cool]

One more thing Dan,

Most of these step motors, work on 24 acv, the command is 1 to 10 volts.
It is not hard to disconnect it from the controller and check it to see that it operates OK.
If it works OK, then no doubt, the problem in the controller.

If it doesn't work smoothly, replace it.

Having two units work from one controller, will make it easier on you to eliminate the parts that are in fine working order.

Chemi

[rbartlett]

I think hunting should be banned...

cheers

richard

[Peter_1]

The Carrier guys can connect very easily a portable PC on the unit and adapt the PID algorithm for the EXV.
We had the same problem some years ago.

To measure an EXV: just check the 4 windings of the stepper motor which has to be 4 times the same value (mostly mentioned on the technical papers delivered with the unit)

Is evaporator correct loaded, pumps running, water filters not clogged

Another strange thing on Carrier Chiller with multiple compressors, controlled by that bloody Prodialog electronics (how easy was in those days without all that fancy electronics): the first compressor is the master, the other the slaves. The Prodialog monitors the resistance of each crankcase heaters If the heater of the master fails, the whole unit is shut down!!. If this chiller is in a production line, then you're serious in trouble. Just switch the heaters temporarily so that the unit can run again.

[Jase]

Make sure the compressor is pumping correctly as I have come across problems with exv's on this type of carrier chiller.
Is the chiller actuating any fault codes?

Jase

[Peter_1]

Make sure the compressor is pumping correctly...

How do you do this or change it in case it isn't?

[Dan H]

Thanks to all for your comments.

It turns out that Jase was correct....the compressor disharge valves were damaged. I installed new valve plates today and the unit is working fine.

I took me four days of testing and troubleshooting before I began suspecting the compressor to be the problem.

I ran all of the factory service manual tests on the processor, I tested and swaped all sensors with known good ones...nothing had an effect on the EXV.

I noticed compressor motor amps were inconsistent.. going from 30 amps to 65 amps. (FLA =57)

I spoke with a compressor rebuilder, he said bad discharge valves were probably the cause of the high motor amps.

The Flotronics Processor measures the superheat temerature entering cylindar head "A". As discharge pressure is forced back down the suction port due to the bad valve, the temperature reading fluxuates...causing the EXV to compensate for the temperature change.

Anyhow..I am glad to have this one working. I am a controls specialist first, with ok mechanic abilities. I just couldn't see how the compressor could be at fault, but it makes sense now.

Dan

[maddymoo2]

I had one do something like that .It was showing large superheat due to gas shortage but only faulted once!! on low gas charge since the gas charge pressure sensor was on the high side!! and after the system filled with air it read a good head pressure again.kept showing erratic or eventually high superheats though I think you can swap tx valvbes by swapping 2 plugs around pretty easy to try I think??