Automatic Expansion Valve (AEV)
- The automatic expansion valve (AXV or AEV) is an expansion device that meters the refrigerant to the evaporator by using a pressure-sensing device.
- The AXV maintains a constant pressure in the evaporator. Notice that superheat plays no part in this valve.
- When the evaporator pressure drops for any reason, the valve begins to open and feeds more refrigerant into the evaporator.
- The adjustment of this valve is normally at the top of the valve.
- This adjustment changes the spring tension that parallels the atmosphere in pushing down on the diaphragm.
- When the tension is increased, the valve will feed more refrigerant and increase the suction pressure.
- The AXV responds differently than the TXV to load changes. It actually acts in reverse.
- When a load is added to the coil, the suction pressure starts to rise.
- The AXV will start to throttle the refrigerant by closing enough to maintain the suction pressure at the set point.
- This has the effect of starving the coil slightly.
- A large increase in load will cause more starving.
- When the load is decreased and the suction pressure starts to fall, the AXV will start to open and feed more refrigerant into the coil.
- If the load reduces too much, liquid could actually leave the evaporator and proceed down the suction line.
- The best application for this valve is where there is a fairly constant load.
- One of its best features is that it can hold a constant pressure.
- When the AXV is used with a thermostat, the thermostat should be adjusted to shut the compressor off before flooding of the entire evaporator occurs.
- The AXV should be adjusted to maintain an evaporator pressure that corresponds to the lowest evaporator temperature desired through the entire running cycle of the compressor.
- AXVs are usually found on smaller equipment with somewhat constant evaporator loads, including ice cream freezers. ice makers and drinking fountains.
Special Considerations for the TXV and AEV
- The TXV and the AEV both are expansion devices that allow more or less refrigerant flow, depending on the load.
- Both need a storage device (receiver) for refrigerant when it is not needed.
- The receiver is a small tank located between the condenser and the expansion device.
- Normally the condenser is close to the receiver.
- It has a king valve that functions as a service valve.
- This valve stops the refrigerant from leaving the receiver when the low side of the system is serviced.
- This receiver can serve both as a storage tank for different load conditions and as a tank into which the refrigerant can be pumped when servicing the system.