Trouble Free Expansion Valves at Start-Up
PROBLEM: I just installed an expansion valve and I can’t get it to feed correctly. The superheat is too high and I’m starving my evaporator coil. It’s the right tonnage, the correct refrigerant and proper charge application. Did I just get a bad TXV out of the box?
All things considered, 99 percent of the time expansion valves are not bad out of the box. My aim in this article is to tie together good installation and piping practices for proper expansion valve operation at start-up.
Your expansion valve has one simple job! The evaporator it feeds must be fed enough refrigerant to satisfy the load conditions. We measure the performance of expansion valves by how well it controls the superheat to the evaporator. Too low or no superheat the valve is flooding or too high of a superheat the valve is starving the coil.
A high percentage of valves returned for warranty exchange have no manufacturing defect. The most common problem lies with system contaminants causing the valve to malfunction. When brazing an expansion valve or any brazing of pipe in the system flowing nitrogen to purge air from the system is a must. If the lines have air in them during the brazing process the oxides burn up and will cause soot or dirty scale formation in the lines. The following picture is of an expansion valve internal push rod that has this soot scale deposited on the upper end nearest the inlet port.
This system had multiple evaporators and upon system startup had exhibited high superheat and inability to adjust both the valves open. The TXV’s upon inspection had the same soot deposits on the push rods as seen in the picture.
Some expansion valve designs have inlet screens that are placed into the inlet of the valve prior to installation. Make sure the screens are utilized to prevent contaminants from reaching the internal valve workings. A preferable screen is one that is part of the valve and externally removable without un-brazing the entire TXV inlet connection.
A proper evacuation is essential as well. A system should be evacuated to 500 microns and be able to hold the vacuum for at least one minute. Water or a mixture of water and oil frozen in the port of the TXV is a common source of trouble. This can freeze the valve open, closed, or about any position in between. Importance of sight glass and drier installation can’t be over stated to keep your TXV install free of trouble.
See your cfm SuperStore parts professional with any of your refrigeration expansion valve needs or questions today.
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