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Old 27-04-2010, 08:32   #1
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Problem with Isotherm 3700 ASU Fridge

I have an air cooled Isotherm unit with ASU (auto start up) charging a holding plate in cool box. The compressor is a single Danfoss BD35F.

Having spent $500 on servicing including replacing line couplings, condenser etc. I was disappointed that after only a month or so it failed again.

Now when I switch it on the compressor starts briefly but stops after between 10 seconds & 20 minutes. The lights on control panel indicate compressor overload. Checking refrigerant (R134a) pressure (low side) shows 25-30 psi with compressor off, dropping to 15 psi when it runs. It makes no difference if I try to run it in Manual or Auto (voltage sensing) mode.

Removing refrigerant (lowering pressure) allows compressor to run longer but it always shudders to a halt after a while suggesting a heavy load. The compressor gets quite warm but the high pressure line to condenser seems cool & no sign of cooling on cold plate at any time.

Trying different pressures (adding/removing gas) always gives similar result. If too high when running (> 18 psi) compressor stops after a few seconds & although it tries to restart this fails. If low (<= 15 psi) compressor will run for up to 30 minutes before stopping but it gets quite hot with no sign of cooling. Whilst running it never goes into high speed mode despite being on shore power & cycling battery charger into mass mode with approx 13.5 volts at control unit.

I am fairly sure there is no air in lines as they were vacuumed after last repair & I have been very careful purging re-charge lines before adding refrigerant. Also there are no signs of leaks. Have also checked & re-made all electrical connections.

Any help or suggestions would be appreciated.
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Old 27-04-2010, 10:48   #2
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If you have an Isotherm ASU 3700 air cooled unit with a BD35 compressor the suction pressure will never be over 6.5 psi when compressor is running. The pressure in system when compressor is not running will indicate nothing except there is pressure relative to ambient temperature surrounding unit.

Compressor stops do to overload (three LED flashes) indicates Poor condenser cooling or too much refrigerant in system or contaminated refrigerant.

Refrigerant normal suction pressure with compressor running is 3 to 6 psi depending on temperature of evaporator and compressor speed.

Recommendations; confirm condenser cooling is good, Reduce suction pressure to 6 psi when running with warm evaporator. If, with correct suction pressure and adequate condenser cooling the plate is not getting colder dehydrate system with vacuum pump and refill with 134a refrigerant.
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Old 27-04-2010, 12:25   #3
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Thanks for help Richard. Will try your suggestions but am guessing that there is a blockage as a result of recently fitted new condenser &/or re-connecting of high/low pressure lines after removing defective couplings.

Other possible guess is that refrigerant is contaminated, but this recent problem occurred before I worked on system myself & after being vacuumed and re-filled by professional leaving system running fine for several weeks.

The condenser at present doesn't even get remotely warm so don't think its cooling efficiency is relevant to problem.

BTW can I assume that the pressures you quote are those to be measured at the (only) service valve on compressor?
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Old 27-04-2010, 13:33   #4
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The suction pressure I gave was not correct for a warm holding plate it should be 8 to10 psi

First Post Revised

If you have an Isotherm ASU 3700 air cooled unit with a BD35 compressor the suction pressure will never be over 10 psi when compressor is running. The pressure in system when compressor is not running will indicate nothing except there is pressure relative to ambient temperature surrounding unit.

Compressor stops do to overload (three LED flashes) indicates Poor condenser cooling or too much refrigerant in system or contaminated refrigerant.

Refrigerant normal suction pressure with compressor running is 3 to 10 psi depending on temperature of evaporator and compressor speed.

Recommendations; confirm condenser cooling is good, Reduce suction pressure to 9 psi when running with warm evaporator. If, with correct suction pressure and adequate condenser cooling the plate is not getting colder dehydrate system with vacuum pump and refill with 134a refrigerant.

Responce to your last post

Most systems like yours only have a service fitting on the low pressure side of system. The electronic module that senses an overload and stops compressor is actually monitoring voltage and amperage. If compressor is overloaded because of high liquid pressure then amperage will be higher than 7 amps on this unit.

You reported If low (<= 15 psi) compressor will run for up to 30 minutes before stopping but it gets quite hot with no sign of cooling. This indicates too much refrigerant in system and not a restriction in high side of system. A restriction in high pressure side with the correct amount of refrigerant should result in low suction pressure. Unit ran 30 minutes also points to too much refrigerant.

A solder joint restriction between compressor and condenser can not be ruled out at this point but if there is a restriction after condenser there will be a cooling affect after a flow restriction pressure change.
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