The thermostatic expansion valve (TXV) is considered by some as a mysterious and complex device. As a result, many valves are replaced needlessly when the cause of the system malfunction is not immediately recognized.
Actually, the TXV performs only one very simple function — it keeps the evaporator supplied with enough refrigerant to satisfy ALL load conditions. It is NOT a temperature control, suction pressure control, or a humidity control.
The TXV is a refrigerant metering device that improves system performance and, in some cases efficiency. Unlike a fixed metering device, the TXV controls superheat with suction line temperature and pressure. As the system load changes, flow is regulated allowing complete refrigerant evaporation within the coil.
So, How Does it work? the valve uses 3 forces to control the amount of refrigerant released into the evaporator. (1) Sensing Bulb Pressure. The bulb is partially filled with refrigerant. The refrigerant pressure in the bulb increases and decreases in response to the suction line temperature. When the suction line temperature increases, the pressure in the bulb increases and pushes down on the diaphragm on top of the valve. This downward force open the valve seat allowing more refrigerant to flow into the coil and visa-versa. (2 & 3) Spring and Evaporator Pressure. These two forces work against the bulb pressure to close the valve. The spring pressure is constant under the valve head while the evaporator pressure varies under the diaphragm. All the valves we use residentially are non-adjustable so the spring pressure is constant. When the load on the system is constant, bulb pressure equals the spring pressure plus the evaporator pressure. However, during When the system is operating, the evaporator pressure plays a role in maintaining system performance. If the bulb allows too much refrigerant into the evaporator, the suction pressure increases. This extra pressure under the diaphragm throttles the valve head to close to reduce the refrigerant flow and avoid evaporator flooding with liquid refrigerant. If there is a lack of refrigerant in the coil, low pressure exists. BULB PRESSURE will then push the valve head open allowing more refrigerant to flow. The TXV reacts to existing thermal conditions and searches for a balanced refrigerant flow.
All of our valves have an equalizer line but are non-bleed TXV’s. What this means is that the TXV will maintain the refrigerant pressure during the off cycle. The purpose of using a non-bleed TXV is to increase efficiency since valve operation is almost immediate and refrigerant is available immediately to start cooling.. Since pressures do not equalize at the end of the cycle, the compressor will be trying to start against unequal pressures. On single phase equipment, “hard start or start assist” components may be required for the compressor to start and run. The equalizer line is there to “bleed off” pressure under the diaphragm that may build up due to “push rod” leakage internal of the valve, If the pressure cannot escape, the valve will remain closed. So, we let the pressure “bleed off” into the suction line from under the head and still maintain pressure in the liquid line to the coil.
Hopefully, this explains how TXV’s work in a refrigerant system. In my next article, I will go int how to diagnose different scenarios and troubleshooting of the TXV.