12V 4WD Freezer temperature question

Hi everybody.

Thanks for letting me join. A family member dropped off a camping fridge/freezer for me to have a look at. It is not a commercial device, apparently a local firm made a run of them some years ago for a specific purpose. It is very nicely made, fully stainless steel with a Danfoss BD-3F compressor and controller, dual thermostats, one for fridge one for freezer, is capilliary and has a replaceable filter/dryer fitted. The condenser is a coil inside a vertical tube with a fan at the top.

Now I am an electronics tech, so I know enough to be dangerous about refrigeration and in Australia I can't legally tinker with the refrigeration side of things, so I just need to determine whether it needs to go to a fridge mechanic or not. It's 12 volt, so no dangerous voltages to worry about.

If there was a sticker indicating the refrigerant and charge, it is long gone. Google suggests that the compressor can use R134a or R22.

So my task was to clean up the engine bay and run some tests as he said it was constantly drawing current.

It was full of good ol Aussie red dust and the fan wiring was bad, so I fixed that and ran her up. One thermostat is for fridge mode and the other freezer. In fridge mode it seems to work fine after a clean and adjusting the thermostat.

In freezer mode it just keep running the compressor and settles around -11 deg C. but doesn't seem to go any further. The thermostat hysterisis (cutin-cutout range) seems to be around 10-11 degrees, so I can't get it to stay below zero and cutoff properly.

So my question I guess is should it go lower than -11 degrees Celsius? Ambient is around 21 C currently. Or is the thermostat dodgy and its control range should be tighter?

Thanks,
Glenn.

Been a long time since Ive looked at one so I cant remember how low they go.
They can take a stupidly long time to cool down properly though.
Those Danfoss compressors normally come with a electonic control unit attached to the side of them, I'm sure there was info online where it lets you check a few things, and theres possibly a terminal you stick a resistor onto to change the speed I think.
If you dont have fridge gauges, then I would probably run it and see what temperatures you get at:
- Before and after the filter.
- Compressor head temp.
- Discharge pipe temp.
- Suction pipe temp.
And keep an eye on the frosting pattern of the evap plate (they normally build up over time), as well as where along the capillary tube the frosting starts.

If you do put a gauge on it, mind the gas charge is very small. Small losses and small additions (air as well as refrigerant) can have a noticable change.

Thanks for replying.

So with what gear I have and where I can measure, temps are:
Ambient 17
Fan cooled Condenser 19/22.6
Compressor suction line -8
Filter/dryer 22.2/24.5
Cooling plates -11.5 where lines are.

It appears to have condensation around all sides along the looping lines behind, but I don't know where the capillary starts and stops.
I did find the compressor is marked R134a.

Cheers,
Glenn.

Got some more accurate readings:
29 where the tube from the condenser enters the fridge compartment.
26 where it exits and goes to the dryer.
24 where it goes from the dryer back into the fridge compartment.
-8 where it goes back to the compressor.

Regards,
Glenn.

Edit: leaving it on for some time, the coldest place on the cooling plate is -12.6 using a temperature gun on stainless steel. Oh, and this controller doesn't have that resistor terminal. P/N 102N3030.

Are you able to measure what temperature the top of the compressor is at, and also the compressor outlet pipe temp? (about 100-200mm out from the comp)

Ah sorry, the capillary might be inside the return/suction pipe.

If theres lot of condensation on the pipework or the box, you might need to think about the insulation.

Once the units ran for awhile is the frost on the cooling plate reasonable even and covering nearly all of the plate?

Is the thermostat one of these thats fixed to the cooling plate? Or one thats measuring air temperature?

Hi there. I'll try tomorrow.
Only a normal amount of condensation because we are in a humid area and I keep opening it up to check. The insulation seems to be very good, no frost on the outside or any feeling of cool.
The frost is inside along what appears to be the loops of the pipes behind the cooling plate. It seems to be even over the whole length of the "pipe" in behind. The thermostat capilliary tube disappears inside somewhere along the pie to the cooling plate, so I assume it is in intimate contact with the plate at some point.