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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
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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 • Automatic Expansion Valve • Fixed Orifice (AC Only) • Electronic Expansion Valve
Capillary Tube Non-mechanical Provides a constant flow (or feed) of refrigerant. Non-Adjustable Typical size: .031” diameter (very small) as in 1/32 inch.
Capillary Tube 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.
Capillary 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.
Thermostatic Expansion Valve 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.
Thermostatic Expansion Valve 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.
Thermostatic Expansion Valve 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.
Thermostatic Expansion Valve 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.
Thermostatic Expansion Valve • The TXV is adjustable. • Turning the adjustment counter clockwise sends more liquid into the coil, which reduces the superheat. • Turning the adjustment clockwise chokes off the flow of refrigerant and increases the superheat. • Make any adjustments VERY slowly and give the system time to respond.
Thermostatic Expansion Valve The TXV starts in an equalized setting with 10 degree superheat.
Thermostatic Expansion Valve • As load conditions change and heat is added to the conditioned space: • The sensing bulb starts warming up. • The valve opens. • Allows more liquid into the evaporator. • Lowers superheat.
Thermostatic Expansion Valve The load on the evaporator goes up as the load increases and the valve opens increasing the flow of refrigerant into the coil.
Thermostatic Expansion Valve • As the demand for cooling decreases: • Cools off the bulb. • Takes pressure off of the diaphragm. • Closes the valve. • Decreases the flow of refrigerant. • Raises the superheat.
Thermostatic Expansion Valve The load requirement drops and the evaporator cools down; the valve starts to close and decreases the flow of refrigerant to the coil.
Thermostatic Expansion Valve 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.
Thermostatic Expansion Valve Distributors are the “octopus” looking things following the expansion valve on larger multiple pass evaporators. The distributors distribute the refrigerant through the multiple passes.
Thermostatic Expansion Valve • The rate of flow of liquid through the TXV is directly proportional to the load conditions. • The forces that control a TXV are: • Sensing bulb – This is the downward force that will open the valve. • Evaporator pressure – Creates an upward force that will close the valve along with the spring pressure.
Thermostatic Expansion Valve The TXV is designed to work at equilibrium.
Thermostatic Expansion Valve • Adjustments • By adjusting the spring pressure, the superheat can be changed. • TXV’s can be internally or externally equalized. • Internal has two lines, one is the liquid inlet and the other is the evaporator port outlet.
Thermostatic Expansion Valve • External has three lines, the liquid line, the evaporator outlet line, and the equalizer line. • With externally equalized TXVs, the bulb must be mounted between the evaporator coil outlet and the equalizer line.
Thermostatic Expansion Valve 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.
Thermostatic Expansion Valve The equalizer line will be connected onto the suction line to assist the evaporator pressure (upward force) for proper operation.
Thermostatic Expansion Valve • Superheat Adjustments • TXV’s are adjusted at the factory. • When an improper superheat is suspected, first check the manufacturer’s recommendations. • Front setting the valve (turning it in) will starve the coil or increase the superheat. By front seating we are turning clockwise.
Thermostatic Expansion Valve Back seating the stem of the valve (turning it out) will flood the coil with additional refrigerant and will lower the superheat.
Thermostatic Expansion Valve • Superheat Measurement • The best place to get the temperature reading is at the sensing bulb of the TXV. • If you can not access this point and the compressor has a long run to it, add 2 psi to your gauge reading. • Convert the compound (low side) gauge to temperature.
Thermostatic Expansion Valve • Subtract the saturation temperature (boiling point temperature from your gauges) from the suction line temperature (near sensing bulb). • This is the superheat. • It is VERY important to realize that it takes a few minutes for superheats to change.
Thermostatic Expansion Valve • Sensing bulb location • When mounting the bulb, make sure the suction line area is clean for good heat transfer. If it is not, sand it. • Should be secured tightly (by at least two straps). • Should be insulated. • Should not be mounted under the pipe, as liquid refrigerant and oil can sit on it and cause incorrect readings.
Automatic Expansion Valve 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.
Automatic Expansion Valve 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.
Automatic Expansion Valve 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.
Automatic Expansion 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.
Automatic Expansion Valve 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.
