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I still see no mention of discharge pressure, therefore any diagnosing is guess work.

mass flow is reduced as flash gas is increased.

therefore raising discharge may increase or decrease mass flow depending on how much the liquid is subcooled entering the tube, again we are guessing until we know the discharge pressure change.

MicahWes I'm not sure we are reading the same thing or if you are giving another example why the OP's system is engineered incorrectly

We see a cap tube as a simple device, but what happens in a cap tube and the system as whole is far from simple.
What happens in a cap tube can not be based on a single state flow (liquid or vapour)
If this was the case, then the bigger the pressure differential, then the greater mass flow. How ever the flow is not single state, over the cap tube length, as the pressure drops vapour is formed, as more vapour is formed the greater the pressure drop will over the next section, and so on . So changes are happening over the overall length. The majority of the pressure drop occur within the last few % of the cap tube. A greater % of volume of flash gas. Note; By % of mass vapour will cause a bigger pressure drop, compared to that of liquid.

Um I'll have to read it again but I though the whole point was with a tube and charge that were correct. So when it was warmer or cooler at the condenser it would not matter.

That document says a few times about 3/4 of the way through that elevated head pressures will cause a cap tube to feed more and raise evap pressure. In these instances he is attempting to illustrate the problem of adding gas to a restricted cap tube system in order to raise the evap pressure (which you do in this instance by raising the head pressure with your overcharge).

http://www.supco.com/images/pdfs/Man...g%20Manual.pdf

If it does I would be too short the incorrect size.

A cap tube will most certainly feed more when head pressure is elevated!

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.

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.

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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.

required pressure drop shifts further up the system

. This theory makes sense to me.

I think bunny has your, quite simple, answer.

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)

Originally Posted by 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.

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.

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.

what happens to discharge pressure when you block condenser?

that will tell you what is going on.

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

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.

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.