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The cap tube is a fixed metering device that is the correct diameter and length for the load it is designed to handle. The outdoor temperature affecting the head pressure, to a small extent, affects the rate of flow. There needs to be a strainer or filter dryer just ahead of the cap tube as it is easily plugged.
A TXV is far superior for regulating superheat and subcooling of a variable load.
Nah, ya' don't need one. Just put a pinch-off tool om the liquiid line, and squeeze it down until the superheat is satisfactory. But it only works after installing a used run capacitor, about 50% over sized.
But there is a strong argument out there that says they are a modulating metering device, theoretically I suppose they are, they do modulate flow to the evaporator in response to how much subcooling they have in the capillary tube itself, the more subcooling they have the more refrigerant they will meter through the tube, the lower the subcooling and the less refrigerant passes, it tends to modulate , where as a fixed oriface has no ability to respond to the load.
The capillary tube does not have the ability to directly adjust to the heatload on the evaporator coil as the TXV does. Therefore, if "for any reason" the heatload drops way off which also lowers the coil's pressure compared to the highside resulting in increased flow into the evaporator coil. Therefore, if the outdoor ambient is very hot it will flood the evaporator with liquid and slug the compressor. Alternatively, the TXV will throttle the refrigerant flow down in a direct attempt to maintain enough superheat to help protect the compressor.
The cap tube does not stop the flow of refrigerant when the unit cycles off, which reduces pressure differentials and starting torque but also can reduce the life of the compressor due to some possible compressor slugging on startup.
Both the cap tube and the Fixed Orifice will respond to an increase in the evaporator load in respect to how much that increases the highside pressure and thus the flow through the restriction. However, I see very little difference between those two devices; the TXV is far superior to either of them. Eddy, I would like to see that strong argument that a cap tube can modulate subcooling or superheat in a proper way. - udarrell - Darrell
Not looking to argue it, its just that it is classified as modulating metering device due to the action and reaction that happens within the bubblee length of th etube,,, it makes it a modulating refrigerant device the same class as the TXV, however is no where near as efficceint as the TXV.
Its classified as a fixed meter device and a TXV is classified as modulating. I suppose you could say a cap tube is more of a pressure reducing device but I wouldnt try to stretch that to mean modulating.
I know where you are trying to go with this modulating bit but it doesnt really modulate open to closed or vise versa the way a TXV does.
The inlet pressure determines the flow rate and the modulation of the flow rate is dependant on the bubble length of the cap tube.
Thats not true, the flow rate is determined by the quantity of the bubbles in the bubble length. and thats what modulates, the amount of gas in the tube, because vapor travels at a different speed than liquid does.
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