Using Refrigeration Gauges to Convert Pressure to Saturation Temperature
The only purpose for using your refrigeration gauges is to convert the refrigerant pressures to corresponding saturation temperatures. When you connect your refrigeration gauges to the liquid and suction line service ports, the pressures they read correspond to the saturation temperature of refrigerants in both the condenser on evaporative coils. In the condenser coil the refrigerant is condensing from a hot gas into a liquid and in the evaporative coil the cool liquid is evaporating into a cool vapor. The use of these two temperatures is critical to properly service and troubleshooting an air conditioning system.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:-
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 stated 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 only vapor will remain. At this point in the evaporative cool, the change of state process ends.
by Ron Walker