Trane chiller tripping on "LOW EVAPORATOR REFRIGERANT TEMP"

Hi

I have a ~8yr old Trane ERTHA300 (rotary screw chiller) used for mall air-conditioning in South East Asia. (ambient: 30C, 90%RH)

It has been running fine for at least the past 3 years, but recently, it is always tripping after about 15-30 minutes of running:

starts with status A74 (normal running),
15 minutes later status changes to A77 "Run: evaporator limit(3)"
3 minutes later it trips on status bFb "Low evaporator refrigerant temp"

Water temperatures in and out of the evaporator and condenser seem normal.
Evap water in/out: 11-13C / 7-8C
Cond water in/out: 27-30C / 31-34C

Any ideas?

Some possibilities I've thought of:

• dirty evaporator heat exchange tubes causing large approach
• bad thermistor under-reporting evap refrigerant temp

Hi Kaon
Haven't worked on Trane chillers but if they are similar to Carrier chillers and have an electronic TX valve I would look at both the temperature and pressure transducers that control the TXV. If these are good it could be the motor in the TX valve. We have had this problem with some of the Carrier chillers we installed, we changed both transducers and ended up replacing the TX valve which solved the problem.

Paul

Hi Kaon,

I have just repaired the same fault on a trane unit, it was a faulty suction return sensor, which was causing it to fail on low evap ref temp, maybe worth checking yours sensor.?

Regards
GSXR1340

Is there a water strainer? and if so is it clean?

Another thing is to check the screw is unloading as the load drops off. If the slide valve (i assume it has one) does not unload then the system will try to pump too much refrigerant around for the current conditions, thus potentially flooding liquid through the evap and dropping the temp.

May be worth a look, who knows?:)

kaon,

whats the cooler approach?
whats the suction super heat?

possibly you have under charge unit or fouled cooler tubes.

Quote:

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Originally Posted by joel

kaon,
whats the cooler approach?
whats the suction super heat?
possibly you have under charge unit or fouled cooler tubes.

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I'm pretty sure, this unit does not display those data.

I was mistaken, the chiller is more like 15 years old...
It doesn't display details like approaches.

The forum doesn't allow me to post photos because of my low post count.
so i have to use the attachment feature.

Quote:

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Originally Posted by slingblade

Is there a water strainer? and if so is it clean?

Another thing is to check the screw is unloading as the load drops off. If the slide valve (i assume it has one) does not unload then the system will try to pump too much refrigerant around for the current conditions, thus potentially flooding liquid through the evap and dropping the temp.

May be worth a look, who knows?:)

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Think the strainers are clean, because we opened a different one recently (serving another chiller on the same header), and it was very clean.
I doubt it is a lack of chilled water flow causing this trip, since the output water is normal temperature... about 7-8C.
Also, this chiller is not expected to need to unload. There's always plenty of heat load to pump out of this retail building.

Quote:

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Originally Posted by gsxr1340

Hi Kaon,
I have just repaired the same fault on a trane unit, it was a faulty suction return sensor, which was causing it to fail on low evap ref temp, maybe worth checking yours sensor.?
Regards
GSXR1340

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Thanks, GSXR, PaulZ and everyone for the ideas.
Will post back here with the results.

Hello Kaon
the most obvious cause would be loss of refrigerant charge, this would cause evap limit which in turn would cause the compressor to unload if the suction pressure continued to fall then it would trip on low evap refrigerant temp. What are the refrigerant pressures?

evap approach...
suction press converted to temp - chilled water out temp.

check your super heat.will tell you if the unit is under charged.

Quote:

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Originally Posted by joel

evap approach...
suction press converted to temp - chilled water out temp.

check your super heat.will tell you if the unit is under charged.

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Anyone know how to get those readings from this ERTHA300?

Quote:

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Originally Posted by kaon

Anyone know how to get those readings from this ERTHA300?

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Yes me.
Suction pressure can be read from a gauge connected to any section of pipework between the evap outlet and the compressor inlet. this can then be converted ito a relevant temp by using a comparitor.

Chilled water outlet can be measured with a thermometer on the evap water outlet pipe.

Superheat can be measured by subtracting the difference between suction pipe temperature and comparated suction temp on a guage.

but of course you allready know this as an engineer.

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Also, this chiller is not expected to need to unload. There's always plenty of heat load to pump out of this retail building.

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not convinced.

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Water temperatures in and out of the Evaporator and Condenser seem normal.
Evap water in/out: 11-13C / 7-8C
Cond water in/out: 27-30C / 31-34C

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That is not under constant load, or is it?
make your mind up.
11-13 OK 7-8, shorter td less load and lower temp.

have a think.

sling.

measure the temperature drop over the filter, but be ware there's an orifice before it!

more then 5 K of difference? then the filter is saturated and needs to be replaced!

Lost interest or is it working now?

Quote:

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Originally Posted by gsxr1340

Hi Kaon,
I have just repaired the same fault on a trane unit, it was a faulty suction return sensor, which was causing it to fail on low evap ref temp, maybe worth checking yours sensor.?
Regards
GSXR1340

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My problem was the same as yours.
Problem went away after change of sensor.
Thanks everyone.

Quote:

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Originally Posted by kaon

My problem was the same as yours.
Problem went away after change of sensor.
Thanks everyone.

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That is quick fix!:D:D