strange suction pressure and floodback dilemma

[jakabus]

I observed work being done on a crappy system. 12v R134a mid range system. condenser fan was changed (thats when i started on the job) and a 30a circut breaker and a 25a fuse (for the 3rd time) After that he charged the system by the sight glass 210/60 psi. The LP dropped immediately to 0 psi as soon as he turned the bottle off. sight glass was clear. (thats my first query) There was a strange grinding noise comming from the reciprical compressor. He added a small canaster of pag oil. it was quiet although i don't agree with it.

I went around a put my hand on the discharge line entering the condenser and liquid line exiting the condenser. dis was HOT (i couldnt hold it it was that hot) and the liquid line was not freezing but cold. but the hp was only 210psi. high head pressure or non-condesibles? fan was not running at the time. I believe he should of checked his supperheat and subcooled liquid temps. But he does not believe in the idea, its time wasting.

I asked him how would he know about floodback to the compressor before he had t change the compressor... he said the low side gauge will go up and down erratically. does any 1 ells agree with this?

im not a trady, im only an apprentice trying to learn

[mikeref]

What is this 12 volt system exactly? A portable fridge/freezer?.. Mike.

[Tradewinds]

If it is R134a then those pressures are pretty high.

210psi = 57'c (should be <43'c)
60psi = 17'C (should be about +5'C for AC)

"The LP dropped immediately to 0 psi as soon as he turned the bottle off. sight glass was clear. (thats my first query)" might have had a liquid line solenoid valve that was closed whilst charging.


For AC the discharge pipe should be about 60-70'C. So you should be able to touch for a few seconds.

Why wasn't the fan running?

If the compressor was changed did he vac the air out of the system before charging?

To test for non condensibles in the system, put your gauges on the system but leave it switched off for about 20 minutes. This should be enough time to allow the refrigerant in the system to get down to ambient temperature. You can force the condenser fan on to speed up the process. Then get you PT chart out. Measure the ambient temperature and then find that temp for R134a on your comparator. The pressure value you get on the card, should match what you get on your gauges. If the system pressure is higher then you have non-condensibles in the system and will need new refrigerant.

Floodback - If you are getting liquid floodback into the compressor, you will get the following:
Low/Zero superheat
Cold compressor housing
White/frothy oil in compressor
Fluctuating compressor current
Cold discharge pipe temp
Risk of pumping oil out of compressor leading to compressor failure

Tell your mate he is probably waisting more time when he has to go back (or someone else does) to fix it again. And he's not exactly teaching you good habits either.

ALWAYS measure your subcooling and superheat. Even if I weigh the correct charge in I will still measure these values.

Try taking these readings on every job:
Refrigerant type:
Ambient temp
Discharge Pressure
Saturated Condensing Temperature
Suction Pressure
Saturated Suction Temperature
Liquid line temperature
TD across condenser
TD across evaporator
Subcooling
Superheat
Sight Glass
Oil Level

Have a small notebook in your pocket that you record the values in. Then you can refer back to them on the next job.

All this will take you about ten minutes when you get up to speed.

BTW - Well done for questioning your mentor and following up with questions in here.

[moggy66]

is this a transport unit? if so what type of unit is it?

[jakabus]

I do not know what type of system it was, all i seen was the compressor to the condenser and thats it. I wish i follow what you said tradwinds but my work wont allow me to do that. But thanks for your support

[Gary]

I observed work being done on a crappy system. 12v R134a mid range system. condenser fan was changed (thats when i started on the job) and a 30a circut breaker and a 25a fuse (for the 3rd time) After that he charged the system by the sight glass 210/60 psi. The LP dropped immediately to 0 psi as soon as he turned the bottle off. sight glass was clear. (thats my first query) There was a strange grinding noise comming from the reciprical compressor. He added a small canaster of pag oil. it was quiet although i don't agree with it.

I went around a put my hand on the discharge line entering the condenser and liquid line exiting the condenser. dis was HOT (i couldnt hold it it was that hot) and the liquid line was not freezing but cold. but the hp was only 210psi. high head pressure or non-condesibles? fan was not running at the time.

Airflow (through both coils) has an enormous effect upon pressures and temperatures throughout the system. Unless/until you have proper airflow through both coils, everything else is meaningless. Always start with airflow.

[young gun]

i see you are from ipswich and it is May, average temperature around there this time of year im guessing would be around 20-24 degrees during the day? and you said mid range system, im guessing you mean medium temp which is usually approx a 0degree coldroom? they are very high pressures on both sides at any time of year, especially in may. You mentioned he changed the condenser fans, has he changed the fans to the orignal correct motor and blade that is specifically designed for that system, or to a fan with the same capacity. Alot of 12v-24v transport, companies will replace original fans with cheaper fans, i see a million thermoking trucks getting around with car radiator fans as condenser fans.

The high pressures could explain partly the loud compressor, with out any cooling the compressor is going to get very hot very quick, and being a 12v system very possible there is no thermal overload.

And you said suction pressure dropped immediately 0psi as soon as he turned the bottle off and the sight glass was full. Was there actually any refrigeration happening, did it come down to temp? Is it possible the sight glass was actually 100% empty?