Fridge evaporator plate has no frosting after coolant recharge

[RaymondR]

Originally Posted by Bellinghamster

Glad to hear it! Ideally, when things are stabilized (running for half hour or so, box reasonably cold) you want the "cold progression" (i.e. frost line) to cover the whole evaporator, but not down the suction line leading back to the compressor, indicating all the expansion is happening in the evaporator.
Frost down the suction line is wasted energy.

My understanding is that the majority of the refrigeration effect occurs because of the change of state of the refrigerant from liquid to vapour and there is no change in pressure whilst this is occurring. Consequently the frosting occurs at any place that liquid refrigerant is absorbing heat. However, as the evaporator temperature decreases the "boiling point" temperature will decrease and this will occur at a decreasing pressures.

In a TXV refrigerant flow controlled system the volume of liquid refrigerant in the evaporator is kept at optimum levels by the valve and provided there is sufficient total volume control is not critical. However in capillary tube flow control systems the optimum volume is achieved by tuning the system volume either by charging the system from empty with a specified weight of refrigerant or by observing the point on the return line where no change of state, as indicated by frosting, is occurring.

Bit pedantic I know but in my case attempting to analyse the workings of a refrigeration system using P/V relationships resulted in years long misunderstanding. Once I comprehended that flow control and state change were the dominant factors the workings became clear to me.

[Bowdrie]

Originally Posted by RaymondR

My understanding is that the majority of the refrigeration effect occurs because of the change of state of the refrigerant from liquid to vapour and there is no change in pressure whilst this is occurring. Consequently the frosting occurs at any place that liquid refrigerant is absorbing heat. However, as the evaporator temperature decreases the "boiling point" temperature will decrease and this will occur at a decreasing pressures.
In a TXV refrigerant flow controlled system the volume of liquid refrigerant in the evaporator is kept at optimum levels by the valve and provided there is sufficient total volume control is not critical. Once I comprehended that flow control and state change were the dominant factors the workings became clear to me.

^
Yeah, sometimes the idea of something "boiling" at -20F is strange.
Both TXV and cap-tube systems have their BTU rating in heat removal predicated not only on how much refrigerant they are capable of circulating but also condensing and evaporation temps.
Sometimes a TXV unit can throttle itself two early, (even with adjusting the superheat).
If the evaporator has too much pressure drop across its inlet/outlet it "fools" the sensing bulb into thinking that the outlet has now reached a temp where the valve can start throttling down the inlet flow.
That in turn causes too low of pressure on the suction side.
The temp/pressure curve "gets out of kilter" so to speak.
That's when an externally equalized valve is used, to compensate for pressure drop that's more than the valve wants to see.
It essentially tells the valve, "Hey, don't throttle down just yet, my pressure doesn't agree with what your temp sensing bulb is telling you".
As an aside, it's a bit intriguing how the Carnot cycle of a steam engine is kind of "the same but different" to the refrigeration cycle, in doing work thru a repeating process of compression/heating/expansion/cooling.

[MCOS]

Probably overcharged. You have two options:

1. Put a gauge on the vacuum side, larger line on the compressor. While running bleed off charge until vacuum gauge shows low side 30 to 40. If you have high side hooked up it should show 150 to 180 with the compressor running. If your system doesn't have fittings for attaching lines you can add them by installing line taps.

2. Preferable way. Drain system. Attach vacuum pump and evacuate system for about ten minutes. If pressure starts to rise very slowly after you shut off the vacuum pump, vacuum for another ten minutes. Might be some moisture in the system. If it does not hold a vacuum with pump off after two cycles, you have a leak.


After getting a good vacuum that holds, recharge. You should add pag oil / r134a first. This usually comes in a small can containing 2 oz oil and 1 oz refrigetant. Use one that has fluorescent dye which will help you locate any future leaks. You can calculate the systems required refrigerant and fill by weight. Should be two cans or less. Since you already have or should have at least a low side gauge hooked up, feed vapor only by keeping your canister up right, until the low side levels off between 30 and 40. Let the system run awhile and add a bit more if necessary. If you do have high side gauge, it should show 150 to 180. DanFoss 150 should draw about 3 amps running after start up. Over 4 you probably have too much refrigerant.

Good luck.

[Bowdrie]

Originally Posted by MCOS

1. Put a gauge on the vacuum side, larger line on the compressor. While running bleed off charge until vacuum gauge shows low side 30 to 40. If you have high side hooked up it should show 150 to 180 with the compressor running. If your system doesn't have fittings for attaching lines you can add them by installing line taps.

30>40 psig on the low side would barely chill your beer.
For any kind of a decent freezer with 134A you've got to get the low side down in the ~5>8 psig range at the end of the run cycle where the thermostat is just ready to shut off.
We don't like to see much, (if any,) negative low side pressure though, once the low side goes into a vacuum the oil return to the compressor slows and those little units don't have much flow rate of refrigerant to spare.
150>180 is higher than should be unless the air is unbearably hot.
Even with 100F condensing the high side should only be around 125 psig.
Better than "good enough" air flow thru the condenser and over the compressor is highly desirable on those 12V units.
That system most probably has a low side connection, but not necessarily a high side one.
Line taps are cheap and easy, but they far too often leak over time.
For longevity and peace of mind only a soldered/brazed in Schrader Tee should be used.

[RaymondR]

Originally Posted by claptrap

Our fridge had only partial frosting on the evaporator plate for a few weeks (so low on coolant).
I've recharged it and now, the evaporator plate does not show any frosting (even after one night of running) and sits at about 10C.

Did I overcharge?

The pressurized hose is warm to the touch, the return line from the evaporator cold, but not frosting, the compressor is warm and running.

Probably over charged.

When the liquid refrigerant draws in enough heat to vaporise the vapour has to have somewhere to go. If you put so much liquid refrigerant in that it fills everything back to the compressor it will either stall the compressor or vaporise in the hermetic housing (which might keep the compressor cool but the beers going to stay hot)

If the compressors running,Let a little bit out at a time and watch to see if the return line starts to frost, if so keep going until the defrosted bit reaches the evaporator outlet and you should have the fridge nicelly tuned.