1 hvacr312 - refrigeration metering devices. 2 control the flow of refrigerant to the evaporator...

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HVACR312 - Refrigeration

Metering Devices

2Metering 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.

3Metering Devices• Types

– Capillary Tube– Thermostatic Expansion Valve– Automatic Expansion Valve– Fixed Orifice (AC Only)– Electronic Expansion Valve

4Capillary Tube• Non-mechanical• Provides a constant flow (or feed)

of refrigerant. • Non-Adjustable• Typical size: .031” diameter (very

small) as in 1/32 inch.

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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.

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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.

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Capillary Tube

8Thermostatic 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.

9Thermostatic 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.

10Thermostatic 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.

11Thermostatic 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.

12Thermostatic 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.

13Thermostatic Expansion Valve

14Thermostatic Expansion Valve

15Thermostatic Expansion Valve

The TXV starts in an equalized setting with 10 degree superheat.

16Thermostatic 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.

17Thermostatic 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.

18Thermostatic 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.

19Thermostatic 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.

20Thermostatic 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.

21Thermostatic 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.

22Thermostatic 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.

23Thermostatic Expansion Valve

• The TXV is designed to work at equilibrium.

24Thermostatic 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.

25Thermostatic 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.

26Thermostatic 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.

27Thermostatic Expansion Valve

• The equalizer line will be connected onto the suction line to assist the evaporator pressure (upward force) for proper operation.

28Thermostatic 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.

29Thermostatic Expansion Valve

• Back seating the stem of the valve (turning it out) will flood the coil with additional refrigerant and will lower the superheat.

30Thermostatic 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.

31Thermostatic 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.

32Thermostatic 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.

33Automatic 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.

34Automatic 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.

35Automatic 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.

36Automatic 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.

37Automatic 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.

38Automatic Expansion Valve