The Purpose of the Metering Device
The metering device forces the liquid refrigerant to drop its pressure from high to low. This, in turn, drops the temperature. An air conditioning system is designed so that the metering device drops the pressure/temperature of the refrigerant to a saturation temperature below that of the air entering the air conditioning system. The refrigerant is also in a saturated state where any addition of heat will cause a change of state.
There are three main types of metering devices:
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Capillary tube
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Fixed Metering
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Thermal Expansion Valve (TXV)
The Evaporator Circuit
The evaporator coil is placed in contact with the warm air of the conditioned space. An indoor blower assembly or furnace blower will circulate the warm air from the conditioned space, across the cold evaporator coil, and back out into the conditioned space.
The refrigerant that enters the evaporator coil is in a saturated state where any addition of heat will cause the refrigerant to evaporate or flash off into a vapor as it travels through the evaporative circuit. It is this time during the refrigeration cycle that massive amounts of heat are being transferred from the warm air into the cold refrigerant. This massive heat transfer is due to a large amount of heat required to change the state of the refrigerant from a liquid to a vapor.In this example, we have a suction pressure of 74.5 PSIG with R-22 refrigerant and 130PSIG with R-410A both correspond to 44F. This is the temperature of the refrigerant as it flashes from liquid to vapor.
With a properly charged and installed system all of the liquid will eventually boil off in the evaporator circuit and the only vapor will remain. At this point in the evaporative cooler, the change of state process ends.
by Ron Walker