Restricted Metering Device
A restricted metering device (partially clogged TXV, piston, or capillary tube) starves the evaporator of refrigerant. Symptoms include high superheat because the evaporator cannot get enough liquid to fully boil, low suction pressure because refrigerant flow is choked, and a high temperature difference across the metering device. The condenser may show normal or slightly high subcooling since liquid backs up on the high side. A frost or sweat line may appear at the point of restriction.
Restricted metering device: high superheat, low suction pressure, possible frost line at restriction point.
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