HVACR312 - Refrigeration. Metering Devices. Metering Devices. Control the flow of refrigerant to the evaporator coil. Maintain the correct superheat. Create the flash gas at the start of the evaporator coil. Metering Devices. Types Capillary Tube Thermostatic Expansion Valve
Control the flow of refrigerant to the evaporator coil.
Maintain the correct superheat.
Create the flash gas at the start of the evaporator coil.
Provides a constant flow (or feed) of refrigerant.
Typical size: .031” diameter (very small) as in 1/32 inch.
Sometimes used to form a heat exchanger by attaching it (by solder) to the suction line, or by wrapping it around the suction line.
The best way to cut a capillary tube is to gently notch it with a file and then snap it at that point. Do not crush the tube.
Installing a cap tube is done by crimp connections because of its diameter.
When replacing a cap tube, cut the same length of new tubing as the original one in the system. Be sure it is the same diameter.
Known as TXV’s
TXV usually has a marking on the top designating the refrigerant type they can be used with.
They are most common on commercial refrigeration systems.
The TXV is a temperature actuated metering device.
The valves mechanisms respond to load variations.
The purpose is to keep the temperature of the evaporator coil constant.
The bulb is attached to the suction line after the evaporator coil. The bulb must be insulated and mounted on a horizontal section of line.
The bulb CANNOT be installed at the bottom of the line.
The sensing bulb senses the temperature in the suction line and the force created by the gas in the bulb will open or close the valve.
If the valve is not responding the first thing you should do is to check the strainer.
The TXV starts in an equalized setting with 10 degree superheat.
The load on the evaporator goes up as the load increases and the valve opens increasing the flow of refrigerant into the coil.
The load requirement drops and the evaporator cools down; the valve starts to close and decreases the flow of refrigerant to the coil.
With newer evaporators there is a pressure drop from the metering device to the suction line.
If the pressure drop exceeds 2.5 psi, a TXV with an external equalizer line should be used.
The external equalizer is used to compensate for the pressure drop from the inlet to the outlet of the evaporator.
Distributors are the “octopus” looking things following the expansion valve on larger multiple pass evaporators.
The distributors distribute the refrigerant through the multiple passes.
The TXV is designed to work at equilibrium.
The equalizer line must be as close to the compressor side as possible to ensure that 100% vapor is entering the ¼” line. Any liquid will cause improper TXV operation.
External equalizers are used on large evaporator coils where there is a pressure drop.
The equalizer line will be connected onto the suction line to assist the evaporator pressure (upward force) for proper operation.
Back seating the stem of the valve (turning it out) will flood the coil with additional refrigerant and will lower the superheat.
Also known as an AEV or a constant pressure valve.
The AEV does the same thing as a capillary tube – it acts like a water valve.
It is not seen as often as the TXV.
The force that operates an AEV is the evaporator pressure. This is the upward force on the bottom of the diaphragm that tends to close the valve.
When you front seat the valve on the AEV (clockwise), you are opening the valve, which puts more liquid into the coil and lowers the superheat.
When you backseat the valve on the AEV (counter clockwise), you are starving the coil, which raises the superheat.
Atmospheric and adjustable spring pressure exert a downward force that will open the valve.
AEVs are designed to maintain a constant evaporator pressure.
When checking AEVs, you rarely have a pressure port right next to the evaporator, and need to add 2 psi to your readings to account for pressure drop.
Systems with AEVs and most systems with TXVs should have a receiver to ensure a proper refrigerant flow to the valve.
The systems with a capillary tube will rarely (or never) have a receiver.
The receiver is a type of storage tank to hold extra refrigerant.