Why Refrigerator Suction Line Frosts Up When Condenser Airflow Blocked

[RefDes]

Would someone explain step by step why the suction line frosts up when the condenser is blocked as explained below?

We are developing a small refrigerator slightly smaller than a previous model and using all the same components. It performs properly. However as a test when blocking 50% or more of the airflow across the condenser, the suction tube at the compressor begins to frost up quickly (with in a minute or so), even to the point that part of the compressor shell had some frost leading down in a short path below the suction port (almost as if liquid refrigerant (or cold gas) was pouring into the compressor shell) I did not get superheat and subcooling during this experiment.

Prior to partially blocking the condenser airflow the superheat on the evap was about 3 to 4 deg F, the subcooling at the condenser outlet was 3 to 4 deg F. This is a cap tube system with a short 1 1/2 foot long suction-cap tube heat exchanger. The condensing unit airflow is not finalized, eventually the condenser airflow will be directed past the compressor which will help with its cooling.

My understanding: reducing the condenser airflow will reduce subcooling, the refrigerant entering the cap tube will be lower quality/less subcooling, the cap tube flow refrigerant mass flow rate will be reduced, the evaporator will have less refrigerant to boil, so the refrigerant should boil off sooner in the evaporator and the low side pressure should drop. Due to the earlier boil off, I would expect super heat to be greater at the end of the evaporator and in the suction tube. I would expect the suction tube at the compressor to be warmer with the refrigerant boiling off at the begining of the evaporator and the heat load due to the air moving across the evaporator being the same. I would think with less mass of refrigerant and the same quantity of heat to remove that the refrigerant exiting the evaporator would have to be of higher quality (warmer). Instead the suction tube at the compressor is frosting up more.

Please help me understand. Thanks.

[2sac]

When properly charged, with a blocked coil you will have a higer subcooling temp, flashing, and a higher difference between condensing and evaporator temps which causes less efficiency.
If your unit is undercharged or under low ambient conditions, your metering device is starving. Block the condenser, raising the head pressure to "normal" your metering device is now feeding properly causing the frost.

[itsiceman]

If you have a undercharged condition blocking the condenser will result in very little change to return gas temp. A cap that is too small will show a much bigger change to return gas temp.

[bunny]

Blocking the condenser will result in a higher discharge/liquid pressure, which will increase the pressure drop across the cap tube.

Increased pressure drop = increased flow

[indy2000]

what happens to discharge pressure when you block condenser?

that will tell you what is going on.

[freeeze43]

Instead of liquid refrigerant entering cap tube you now have a saturated condition at cap tube which is less denser resulting in a higher pressure drop across cap tube, refrigerant will then change back into liquid towards the last rows in evap.

[jpsmith1cm]

Instead of liquid refrigerant entering cap tube you now have a saturated condition at cap tube which is less denser resulting in a higher pressure drop across cap tube, refrigerant will then change back into liquid towards the last rows in evap.

Please explain how this is possible.

In order for the refrigerant to condense, it must be giving heat off to it's surroundings. I don't see anything in your explanation that shows how that is occurring.

[barbar]

It is the pressure drop over the evaporator and capillary and the effect of heat transfer.
Think of it this way, the first part of the evaporator becomes part of the pressure drop process, so instead of all the evap being close to normal saturation temps, only part of the evap is now at normal saturation temps, so the refrigeration effect is moved to the exit of the evaporator. Hence you see frost on the suction.
The high liquid temp entering the cap, will continue to give energy to the surrounding, Sort of sub cooling matching the pressure drop. Therefore we are not getting flash gas which causes the greatest pressure drop (compared to that of liquid of the same mass flow), there fore the required pressure drop shifts further up the system. "first part of the evap"

For the commercial guys think of it similar to how your distributor and legs work. Normal saturation does not occur directly after the expansion valve but after the legs. Hence the requirement for external equalizied valves (I hope I have not confused the issue)

[MicahWes]

I think bunny has your, quite simple, answer.

[freeeze43]

required pressure drop shifts further up the system

. This theory makes sense to me.

[itsiceman]

From his measurements the evap is nearly full of refrigerant with some stacked in the cond. coil. He is not restricting severly so think of it in terms of the refrigerant needs to go somewhere. Buy blocking the coil it becomes smaller and less room for the charge moving it to the low side were he is now run out of room in the evap. coil. There will be a point where flow through the tube will slow but he is not at that point.
The problem is a cap that is too restrictive with a larger than required charge.

Quote RefDes
"My understanding: reducing the condenser airflow will reduce subcooling, the refrigerant entering the cap tube will be lower quality/less subcooling, the cap tube flow refrigerant mass flow rate will be reduced"
Blocking the coil will increase subcooling. I don't believe you measured this while you blocked the coil. Do it again and see if your understanding of this changes.

[MicahWes]

The refrigerant is always going to undergo its primary pressure drop at the expansion device, unless there is a drastic charge problem or restriction. This is simply a case of high head pressure causing a cap tube to overfeed.

Sent from my ADR6350 using Tapatalk 2

[itsiceman]

Higher head alone will not cause a cap to overfeed and also there is a difference between a blocked coil and warmer ambient.
If how much a cap tube feeds was directly linked to pressure it would not be considered a metering device.
There are papers somewhere in the pro section explaining this better than I ever could.