With the hygroscopic properties of the POE oils in use today we have found that replacing the cap tube with an AEV (that is Automatic Expansion Valve - NOT - a Thermostatic Expansion Valve) valve eliminates 90% of the call backs due to plugged cap tubes. You need to run a 1/4" line from the new drier- usually a 032s or 052s instead of the original bullet drier- to the evaporator and weld in the AEV as close as you can to the evaporator inlet. We have yet to find a sweat in AEV- so you will need to find an adapter to thread into the valve- and sweat your tubing to. Add an ounce extra of refrigerant and once the box has cooled, set your suction pressure so that you have the same suction pressure/temperature relationship. Check for cooling to the compressor. The AEV is a fixed metering device just the same as a cap tube- but does allow you the option of setting the proper suction pressure. We have been doing this for at least 6 or 7 years and have stopped our call backs and compressor failures - on the smaller refrigeration systems.
Note- I am not talking about a Thermostatic Expansion Valve (with the bulb) those have to be used on systems with a receiver and the compressor has to be able to start against a load( the TXV shuts off when the compressor stops). The AEV valves have been used for decades in Ice makers and Ice Cream machines and proabably would be used by True, Delfield and the others, except for the huge difference in cost. As you all know the problem is, once you open a POE system the oil turns "waxy" and bits are floating all around- the small drier becomes saturated-and then the cap tube plugs. This is a HUGE problem in freezers especially.
My question is- has any tried using multiple AEV's on a HVAC unit that has the multi port metering device feeding cap tubes to the evaporator? Seems to me that as long as you monitored your super heat and checked each evaporator circuit for similar temps the idea would work. Of course as is with any cap tube system- the freon charge is critical. The AEV does not open and close like a TXV so it does not need a receiver. We put on the larger drier due to the moisture and increase the charge due to the larger liquid line and drier. Anyone have pros or cons?