High pressure trip w/ low charge

[KB Cool]

How is this possible on a 30RAP106. The IOM says it could!

This is what it says:

T126 Alert Circuit A High
Head Pressure
Compressor operation outside
of operating envelope. Circuit shut down
Automatic, only after
first 3 daily occurrences.
Manual reset thereafter.
Reading from OAT
sensor must drop 5 F
(2.8 C) before restart


And these are the possible causes:

Faulty transducer, low/
restricted condenser
airflow, low refrigerant charge,
faulty EXV

[GT Jets]

How is this possible on a 30RAP106. The IOM says it could!

This is what it says:

T126 Alert Circuit A High
Head Pressure
Compressor operation outside
of operating envelope. Circuit shut down
Automatic, only after
first 3 daily occurrences.
Manual reset thereafter.
Reading from OAT
sensor must drop 5 F
(2.8 C) before restart


And these are the possible causes:

Faulty transducer, low/
restricted condenser
airflow, low refrigerant charge,
faulty EXV

Think about it for a minute.....

Makes perfect sense to me. What would happen if you had just enough refrigerant to keep the system from dropping out the low pressure switch. No condensing, no cooling, no cooling the pressure just goes higher and higher to to heat build up.

GT

[KB Cool]

Think about it for a minute.....

Makes perfect sense to me. What would happen if you had just enough refrigerant to keep the system from dropping out the low pressure switch. No condensing, no cooling, no cooling the pressure just goes higher and higher to to heat build up.

GT

Have you seen it?

[Tech Rob]

I have seen it on an RTAA; not the scenario GT described, but a high pressure trip as a result of low-charge and a hot chilled water loop. The condenser fans on the RTAA are staged based on system delta T converted to delta P. Low charge = low differential pressure = no condenser fan action = high pressure trip

[KB Cool]

On this particular machine! Would you expect to see a temp. drop across the Liquid line dryer, if undercharged? I got a 12 degree drop across it and sight glass is flashing. Could the temp. drop be because the dryer is so close to the EXV. And again the T126 lockout boggled me. I'm thinking dryers plugged. Now i'm not sure?

[Tech Rob]

I have never seen this chiller, but I would say that any liquid line drier that is dropping 12 degrees is a problem. How close are we talking? Close enough to the EXV where the condensation from the valve body is affecting your temperature readings? I would think that 8-12" would be enough distance to get a reliable reading.

I would expect to see a temperature drop across a plugged liquid line drier only get worse as the charge is decreased. When you have a low charge, the liquid is closer to the saturation point and will easily flash into a vapor with resistance to flow.

[KB Cool]

I have never seen this chiller, but I would say that any liquid line drier that is dropping 12 degrees is a problem. How close are we talking? Close enough to the EXV where the condensation from the valve body is affecting your temperature readings? I would think that 8-12" would be enough distance to get a reliable reading.

I would expect to see a temperature drop across a plugged liquid line drier only get worse as the charge is decreased. When you have a low charge, the liquid is closer to the saturation point and will easily flash into a vapor with resistance to flow.

The EXV is probally a good 2 feet away!!

[GT Jets]

Have you seen it?

Yes I have, it was a Carrier Aquasnap and it had a low charge and kept tripping the high pressure cut out.

Virtually ANY liquid line drier with the refrigerant at saturation at a high rate of flow (read velocity) due to a high evaporator load and low refrigerant charge will read a higher than expected TD, not sure about 12*F but way more than 2* I think the highest I have seen was 7*F and the drier was fine. Once the system was topped off after a leak repair was made it dropped right back down to less than 1*F

Liquid line driers are designed to filter liquid, not gas, so if the refrigerant is flashing off in the drier, you will see a large temp. drop.

GT

[KB Cool]

Yes I have, it was a Carrier Aquasnap and it had a low charge and kept tripping the high pressure cut out.

Virtually ANY liquid line drier with the refrigerant at saturation at a high rate of flow (read velocity) due to a high evaporator load and low refrigerant charge will read a higher than expected TD, not sure about 12*F but way more than 2* I think the highest I have seen was 7*F and the drier was fine. Once the system was topped off after a leak repair was made it dropped right back down to less than 1*F

Liquid line driers are designed to filter liquid, not gas, so if the refrigerant is flashing off in the drier, you will see a large temp. drop.

GT

That makes sense!! Thanks for the info!

[KB Cool]

Well guys, pulled the charge and cut out of the dryer and i could barely blow through it!!! I'm still baffled by the head trip.

[chilbrig]

If you have Micro Channel Coils, any restriction will cause a high pressure trip. Cut your drier open and see if a magnet will pick up the fiber filter material. If it does, get ready to change out your compressor.

[KB Cool]

If you have Micro Channel Coils, any restriction will cause a high pressure trip. Cut your drier open and see if a magnet will pick up the fiber filter material. If it does, get ready to change out your compressor.

Yeah, it is a Micro channel. But, when i first got to the job. I think my head psi was no where near the tripping point!! But, anyways this what my boss had me do. What a PITA!!! I know the charge amount on the data plate is no longer correct!!

[Carrene #2]

Well, I had this alarm. I called for explaination.

The official reason that all those symptons will flag a High Head alarm, "..we're computer programers...picked the five conditions and chose what we thought would be the most common....."......"essentially the compressor is running outside it's design envelope".
Hope that helps. Hahaha