RTAA- Low evap refr temp

[mtnwrench]

This problem started near the end of last fall but would never duplicate while there. On a cold start, I can get it to happen this spring.

Circuit starts, runs normally for a few minutes and will switch to "running, evaporator limit." While this happens, suction pressure drops between 20 and 40 psi causing evap refr temps to drop as well. At times it will self correct, suction raises to normal and the machine is good for the day. At others, it crashes on the alarm.

What could be causing this? Electric expansion valve? Slide Valve?

Problem happens with warm water and I can almost get to duplicate when evap water temps are near setpoint by shutting machine off and restarting. After a few minutes, suction will drop but rise back up to normal and machine will run normal.

Where to start?

[allstar08]

flow!

[toptwo]

Yes... water flow need to check there first.

[MrSlim]

(qoute from RTAA 70-125 IOM)
Low Evap Rfgt Temp - Ckt 1 or 2 CMR
a. The Saturated Evap Rfgt Temp - Circuit 1 or 2 dropped below
the Low Rfgt Temp Cutout Setpoint while the circuit was
running for 30 deg F seconds.
b. See the low ambient ignore time on startup.

If it isn't water related, check your fan staging. Start the system and watch your refrigerant pressures as the fans stage on. Which RTAA do you have and is it equiped with fan inverters?

[chillerguy81]

This problem started near the end of last fall but would never duplicate while there. On a cold start, I can get it to happen this spring.

Circuit starts, runs normally for a few minutes and will switch to "running, evaporator limit." While this happens, suction pressure drops between 20 and 40 psi causing evap refr temps to drop as well. At times it will self correct, suction raises to normal and the machine is good for the day. At others, it crashes on the alarm.

What could be causing this? Electric expansion valve? Slide Valve?

Problem happens with warm water and I can almost get to duplicate when evap water temps are near setpoint by shutting machine off and restarting. After a few minutes, suction will drop but rise back up to normal and machine will run normal.

Where to start?

Just FYI, there is currently a thread on this same subject located in the chiller section.

[mtnwrench]

RTAA080, installed in 2000. No inverters on the fans,just staged.

Problem stays with the same circuit, never on the other circuit. Makes me think not flow but will check superheats, subcools and approaches today.

[servicetrane]

MTN:Low Evap Rfgt Temp - Ckt 1 CMR a. The Saturated Evap Rfgt Temp - Circuit 1 dropped below
the Low Rfgt Temp Cutout Setpoint while the circuit was
running for 30 deg F seconds.
b. See the low ambient ignore time on startup.

[mtnwrench]

This thing isnt running on real low ambient temps. Starts at 50 degrees. The problem stays with circuit 2. Circuit one will start up and run normal.

On the first start of the day, the compressor starts, runs for a few minutes with normal pressures and begins to fall down to around 50psi. This will build back up to 65-70 and a within another minute, pressures will fall to 20-40 causing running, evap limit,sometimes followed by low evap refr temps. Other times, it recovers and runs normal.

I performed the exv test and circuit 1 matches the book step for step. Circuit 2 has very short stoke timings (5 seconds) with resistance values per book. Manual states stuck valve.

Guessing I will try these timings once more in the morning to verify. If this was a flow problem, I should be seeing the same problems with circuit 1, shouldnt I?

I will check this low ambient ignore time in the morning as well but circuit 1 doesnt have these goofy suction fluctuations.

[MrSlim]

The nice thing about the RTAA chillers is they have 2 circuits. You can check the programs/settings against each other and confirm they are the same. When I log my RTAA chillers, I'll lock one circuit out and load up the other, log it and then do the same with the other side. If they don't match, I know where to start.

http://www.trane.com/Commercial/Uplo...-svd03a-en.pdf

[MrSlim]

RTAA 130 to 400 T-shoot manual

[servicetrane]

resistance checks temperature sensors TSE AND TSS

[jdcoolj]

low refrigerant charge will also give you that diagnostic. I would check subcooling if the oat is warm enough. You may even want to pull the charge and confirm. This takes the guess work out of the picture. also check flow and percentage of glycol if any. Then you could confirm your low evap setpoints are set correctly based on the fluid you have and your leaving fluid temp setting.