Accumulator frosting and dropping suction pressure Caterpilar Machine

Guys, im looking for some inspiration on this issue,
This is not un-common with certain Cat crawler tractors i have to admit, some you never look at and others you live with more than your wife, this is my issue detailed below and some advice would be welcomed.

The a/c system is a sanden rotary compressor , through a large tube & fin condenser with stacks of air flow, then through a small inline filter drier into a Orifice tube, through the tube direct into the evaporator ( tube & fin) out and return to the front of the machine through a accumulator and exits up and over the engine by rubber hose pipe work to the compressor.
That's the flow path, issue is this, system is correctly charged as stated by the manufacturer RED DOT, on running the system performs as expected normal high and low pressures, however once the machine has been working at full revs for some time the high begins to drop slightly from say 10 bar to 8 bar ( ambient 22C) and the low goes almost into vacuum thus tripping the LP switch at 3 psi.
Once this pattern of pressures start there is no turning back, the compressor kicks back in at around 10Psi suction pressure only to cut out again in less than a minute due to the LP switch.

I have changed orifice tubes, tried bigger and smaller bores, placed a rec drier in with a sight glass and we have liquid visible, i have blown the pipes through and removed any residue of oil build up ( cats are poorly laid out and the pipe work goes up and down with a round a 3 foot fall in places.)

Its as if there is a restriction, but some days the machine can work for hours with no problem, then for example once stopped to fill with fuel and start again the a/c blows hot and no compressor activity from the LP cut off due to suction pressure.

The only thing of note is the accumulator frosts up and the suction pressure drops, originally i was thinking it was poor routing of the discharge pipe at the back of the exhaust re-boiling the already condensed refrigerant so moved the pipe work but no change ( hence the inclusion of a filter with sight glass as opposed to the filter cat use with out.)

bigger or smaller orifice tubes makes no difference, reducing the charge by 500gr ( charge weight is 2300g) only results in poor evaporator temps, so i am at my wits end with this as i can not see what is the issue causing after a period of running the suction pressure to drop.

Its worth noting as the suction pressure drops the evap temps rise but liquid is still visible in the sight glass... what am i missing as i fear i have forgotten a basic principle and i am looking for a atom within a atom because it cant be simple???
Nothing is easy to get at, the heat generated around the engine area is immense so its not like a commercial fridge system were you can access all pipes etc, and as said not all similar machines suffer the same problem, maybe out of 20 i look after there are 4 that do this.

Any pointers will be greatly welcomed as this machine is making me look a twat TBH.

Regards

Pat

Sounds like it blocking,134a is famous for it,have you looked at the colour of the compressor oil?,maybe best to recover the charge flush the system and recharge ie the procedure you would undertake after a compressor siezure.

Possible electrical issue, Evap fan intermittent fault/loose connection and cycling on and off, when off will lower the suction pressure, but possibly not into vacuum?

Dirty or frosted evaporator?

Sounds like an evap issue as others have stated, although as Cad suggests pull the charge, i presume you've not been adding oil to it?

Check that the capillary tube (expansion) is away from any heat source or touching anything hot. The capillary tube must be full of liquid refrigerant only for it to flow, but if the capillary tube is touching any hot surface then the liquid inside the tube will vapourize causing gas bubbles which will cause a blockage in the capillary tube . The evaporator becomes starved and the suction pressure drops accordingly. The liquid in the accumulator will drop to the system suction pressure/temperature which will be a negative temperature and become covered in frost. Until the capillary tube cools down to allow the gas bubble to return to a liquid state then no refrigeration effect can take place.
If necessary put the capillary tube inside some insulation so that it is not touching any direct heat source. I have had instances where the capillary tube has been routed next to the compressor shell and once the compressor gets hot the capillary becomes vapour blocked, simply rerouting away from any high heat source should solve your problem. You can make a capillary tube act like a solenoid valve by simply putting a piece of heater tape on the capillary and energising the heater, this causes the liquid to turn to vapour thereby blocking the capillary tube, swithching the heater off allows the vapour bubbles to return to liquid and the refrigerant starts to flow again. NEVER allow the cap tube to touch any hot surface as it will cause problems

Interesting Glenn, very very interesting!

Quote:

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Originally Posted by Glenn Moore

Check that the capillary tube (expansion) is away from any heat source or touching anything hot. The capillary tube must be full of liquid refrigerant only for it to flow, but if the capillary tube is touching any hot surface then the liquid inside the tube will vapourize causing gas bubbles which will cause a blockage in the capillary tube . The evaporator becomes starved and the suction pressure drops accordingly. The liquid in the accumulator will drop to the system suction pressure/temperature which will be a negative temperature and become covered in frost. Until the capillary tube cools down to allow the gas bubble to return to a liquid state then no refrigeration effect can take place.
If necessary put the capillary tube inside some insulation so that it is not touching any direct heat source. I have had instances where the capillary tube has been routed next to the compressor shell and once the compressor gets hot the capillary becomes vapour blocked, simply rerouting away from any high heat source should solve your problem. You can make a capillary tube act like a solenoid valve by simply putting a piece of heater tape on the capillary and energising the heater, this causes the liquid to turn to vapour thereby blocking the capillary tube, swithching the heater off allows the vapour bubbles to return to liquid and the refrigerant starts to flow again. NEVER allow the cap tube to touch any hot surface as it will cause problems

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Hi Goober
I was called to a customer who had 15 ice bank machines where according to him the compressors were faulty and stopped pumping and made no ice . They stopped the production line until I attended site. They showed me a couple of sample units they had on test and sure enough they seemed to stop pumping after a short time of running. Then I noticed the capillary tube had been routed around the base of the compressor. By moving the cap tube and putting a strip of armaflex tape between the cap tube and the compressor shell everything burst into life. I’ve seen this several times in production areas where th engineers didn’t understand the implications of what they were doing . A little bit of internal
training to the production guys stopped any further issues but it’s a simple error to make . They had changed 6 compressors before they contacted us . A few red faces!!! but a great learning issue.