The Metering Device


In residential air conditioning there are 2 devices that lower the pressure of the refrigerant before it enters the evaporator coil, the smaller copper line.  One is known as a Piston or Fixed metering device and the other is known as a TEV or TXV (Thermostatic Expansion Valve).  What the metering device does is lower the pressure of the refrigerant so we can boil it off at a lower temperature.  The Piston or Fixed metering device is just a hole that allows refrigerant to flow through.  The TXV is a more sophisticated metering device that can actually help make your system more efficient.  Click on TXV to see a picture of one and a video.  The TXV has a diaphragm inside that has a spring force pushing it closed as well as the outlet evaporator pressure.  There is a sensing bulb, that has refrigerant inside of it, that rests on the outlet of the evaporator suction line, the bigger copper line.  If the outlet of the evaporator suction line is warm then the bulb also becomes warm, which will force the TXV to open more allowing more refrigerant to flow through the evaporator coil.  As the evaporator suction line cools down the pressure from the bulb becomes lower which allows less refrigerant to flow through the evaporator coil.

Superheat is adding heat to the refrigerant to prevent liquid refrigerant from entering the compressor and is taken on the suction line (bigger copper pipe) at the condenser (outside unit).  The TXV should maintain about a 10-15 degree superheat.  The Piston’s superheat will change as the temperature changes.  If it is cooler outside then the superheat will be lower and if it is warmer the superheat will be higher.

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Published in: on April 25, 2010 at 12:58 pm  Leave a Comment  

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