The metering device is located after the condenser coil. There are two kinds of metering devices, thermal expansion valves (TXV) and capillary tubes. Either one does the same thing; they lower the high-pressure liquid pressure by forcing it through a small hole or nozzle. The metering device acts like your garden hose nozzle, turning a steady stream of liquid into sprayed droplets. Lowering the pressure of the refrigerant and spaying it into droplets lets the refrigerant boil at a much lower temperature. As refrigerant saturation point is relevant to pressure, the lower the pressure the lower the saturation point. Refrigerant droplets are more easily boiled than a steady stream. That is the purpose of the metering device.
The main purpose of the TXV is to maintain superheat. It does this by delivering refrigerant to the evaporator in an amount that is necessary for efficient heat absorption. The txv is always fighting to maintain balance. The pressure from inside the evaporator is constantly pushing the needle and seat inside the valve closed. While the pressure inside of the bulb and capillary tube, are pushing open the TVX. Always feeding the right amount of flow. If the temperature in the cooled space goes up, the refrigerant in the evaporator, absorb more heat. Now the pressure in the system has gone up, so will the pressure in txv bulb. Causing the TXV to close until the temperature of the refrigerant gets back to normal. Therefore, the txv will adjust itself to always maintain the right superheat.
Air Conditioning capillary tubes do the same as the txv except for one major difference. A cap tube cannot adjust for superheat. A capillary tube is a very small tube or usually multiple tubes that refrigerant is forced through. As the refrigerant moves from a large tube and into a very narrow, small tube it obviously has a major drop in pressure. The length and size of the cap tube are vital. All capillary tube air conditioners are critically charged systems. What that means is the cap tube is designed to deliver a certain amount of refrigerant to the evaporator, by its designed size and length, to maintain proper superheat. If we change the charge at all then we no longer will have correct superheat and the ac system will fail premature and need repair. If ever you have a clogged cap tube, it is acceptable to cut off 1 inch from the front of the tube. Anymore and you need to replace entire tube, with new tube designed to deliver same superheat. If we do not have the right size tube, we can use a different size as long as we make the proper length adjustment. If we use a larger diameter tube, then we should lengthen the tube also. As we can restrict the flow by either lengthening the tube or using a smaller diameter. Anytime you have a critically charged system try to weigh in your charge.