Why Refrigerator Suction Line Frosts Up When Condenser Airflow Blocked

[MicahWes]

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

[itsiceman]

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

[itsiceman]

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

[MicahWes]

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).

[itsiceman]

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.

[barbar]

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.

[itsiceman]

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

[indy2000]

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.