CH. 20 Metering Devices

If an AEV does not have a bypass, what kind of compressor will be needed for the system?

A compressor capable of starting under a load.

What is i a distributor?

A device that splits the flow of refrigerant into several paths to reduce pressure drop.

What is a mechanical pressure limiter?

A device within the TXV that prevents evaporator pressure from exceeding a safe operating limit.

A capillary tube that has a smaller inside diameter will have _________.

A greater the pressure drop

Where would a filter-drier be placed in reference to the TXV?

Ahead of it (before)

AEV's must be adjusted for _________ when buying an AEV, the Valve Capacity must equal?

Atmospheric pressure/ Evaporator capacity.

What is the most common cause of failure?

Bent, crimped or plugged tube.

What control does the stepper motor have with the valve?

Can open, close or modulate the valve.

What determines the length and diameter of capillary tube needed?

Capacity of compressor, type of refrigerant, desired target temperature, type of condenser used.

What are 5 types of Metering Devices?

Capillary Tube, Metering Device, TXV, AEV, EEV

How is a Sensing bulb mounted on a vertical line? What is an externally equalized TXV?

Capillary tube on bulv must be on top. Has a tube connected to the evaporator suction line output, connected to evaporator side of TXV diaphragm.

What happens when a capillary tube is in contact with the suction line?

Capillary tube subcools, suction tube superheats as heat transfer from hot to cold.

What is a Modulating metering device?

Changes orifice size to account for changes in cooling load.

What prevents refrigerant from flowing backwards through some TXV's?

Check Valve

What must be done properly when attaching a sensing bulb?

Clean bulb with steel wool for good thermal contact. Use copper straps and non-rusting screws and nuts to prevent corrosion with a dissimilar metal. Make sure bulb is tightly attached. Bulb must be wrapped in insulation so only suction line temperature affects bulb.

If a TXV is closing to slow refrigerant flow, what temperature is the sensing bulb sensing relative to the input of the evaporator?

Cold refrigerant

What happens to correct missed steps over a period of time?

Controller will fully close valve briefly to restablish zero position

What is the Primary function of the metering device?

Create a pressure difference between the low side and high side

If the TXV superheat adjuster is turned clockwise, will the spring tension loosen or increase and what affect will that have on the valve?

Decrease tension, lower superheat, flood evaporator.

How does a Stepper motor work?

Either directly or using a gear train, the motor drives a pin against/ or piston up and down that opens a port allowing liquid to flow.

What part of an EEV is equivalent to a TXV diaphragm?

Electric operator.

What controls a stepper motor?

Electric signals sent by a controller which causes the valve to rotate its gear train a small, precise, repeatable amount.

What does EEV stand for?

Electronic Expansion Valve

What does an equalizer do?

Equalizes pressure between valve and output of evaporator to allow valve to provide correct refrigerant flow.

What pressure difference closes a TXV?

Evaporator and superheat adjustment spring. pressure that is greater than the sensing bulb pressure.

Why is an AEV system sometimes called a dry system?

Evaporator is never filled with refrigerant but with a mist or fog.

Where are Capillary Tube Metering Devices Used vs. Fixed Metering Devices?

Extensively in refrigerators and window AC's vs. Heat pumps and split systems

What are some of the dangers of having an overcharged capillary tube system in the OFF cycle?

Extra refrigerant could fill the evaporator too full, the compressor will work harder on start-up, there is an increased risk of liquid slugging

When liquid in a capillary tube evaporates, what is this called?

Flash Gas

The longer the capillary tube is, the ________.

Greater the pressure drop

What do the variables (Tube length, inside diameter, number of turns) determine in regards to how the capillary tube is going to perform?

How much the pressure will drop between the condenser and evaporator.

Under what circumstances will a TXV not remain closed in the OFF cycle?

If valve is adjusted incorrectly.

What effect in the overall system does a capillary/ suction line heat exchange have?

Improves systetm efficiency

If the TXV superheat adjuster is turned clockwise. will the spring tension loosen or increase and what affect will that have on the valve?

Increase tension, higher superheat, starve evaporator

How do you properly charge a capillary tube system?

It is a critical charge so use a scale and weigh in refrigerant.

What does a capillary tube metering device consist of?

Length of seamless tubing with a small and precisely formed inside diameter.

Using a metering orifice what needs to be taken into account to determine right metering orifice size?

Length of the line set and extra refrigerant requirement?

What is an advantage to using larger capillary tubes?

Less likely to become plugged with dirt, wax, ice . . .

Which two charges use the same refrigerant that is in the system?

Liquid-charged, Gas-charged

What are the 5 types of sensing bulb charges?

Liquid-charged, Gas-charged, Liquid cross-charged, Gas cross-charged, Adsorption gas cross-charged.

A sensing bulb sensing cold will respond with what type of pressure?

Low pressure

What inputs does a controller need in order to determine whether the stepper motor needs to open or close the valve?

Low-side pressure and temperature at the evaporator outlet.

What does MSS stand for?

Minimum Stable Signal

Which is more efficient, Fixed metering or modulating metering devices? why?

Modulating M.D's are more efficient because they can vary flow based on load.

How is a sensing bulb mounted on horizontal section of suction line on a small system?

Mounted on top

What is main advantage of a capillary tube from a maintenance point of view?

No moving parts

Are check valves used to prevent refrigerant from flowing the wrong direction in capillary tubes?

No, Refrigerant is free to flow either direction which means pressures equalize in the off cycle.

Does the AEV use a sensing bulb to control superheat?

No.

Do pressures always balance in the OFF cycle with a TXV? A sensing bulb sensing heat will respond with what type of pressure?

No/ High pressure

How does an AEV operate?

Opens and closes based on low-side pressure.

When the Pressure in the evaporator drops, how does the AEV react?

Opens to increase refrigerant flow.

Where is a sensing bulb mounted?

Outlet of the evaporator.

What component is used to determine pressure?

Pressure transducer

What does PWM stand for?

Pulse Width Modulation

What does RPB stand for and what is it?

Rapid Pressure Balancing - Secondary port opens during compressor OFF cycle allowing pressures to equalize allowing a lower torque compressor to be used.

How does the controller work?

Receives the temperature and pressure signals and uses them to calculate the proper valve response using an algorithm and P/T tables for proper refrigerant, then sends commands to the stepper motor.

What is a danger or a faulty valve seat?

Refrigerant leaks in off cycle. If it gets to the compressor it will cause severe knocking and possible danger.

What changes the pressure inside a sensing bulb?

Refrigerant temperature in Suction Line.

Upon replacement of capillary tube what else should be done?

Replace filter-drier.

Explain what pressures exist in a TXV when the compressor is OFF?

Sensing bulb and evaporator pressures pushing against each other in the diaphragm cancel each other out which leaves spring pressure to close and hold the valve closed.

What pressure difference opens a TXV?

Sensing bulb pressure that is greater than the combined pressure of the evaporator and superheat adjustment spring.

What is a visual clue that a system might be overcharged?

Severe frosting of suction line.

What is a danger that could happen with a hunting TXV?

Slugging compressor

What is the most common electric operator?

Stepper motor.

What is a visual sign of an oversized valve that is hunting?

Suction line sweating or frost

When a compressor is running, what is the delta T between the input and output of the evaporator called?

Superheat

What practice do manufacturers use to help installers have the proper metering device?

Supply multiple orifice sizes.

If a low torque compressor was in a unit, what type of TXV would we need?

TXV with a bleed port so hi and low side pressures can equalize in the OFF cycle.

What does hunting mean in reference to a TXV?

TXV's response opens valve too much then overcorrects and closes valve too much.

What is MSS?

The Best Superheat setting for an evaporator that results in the smallest variation in sensing bulb temperature change.

Where is the Component from the above question installed?

The Metering Device is installed between the Liquid line and the Evaporator

What type of valve receives a pressure signal from a sensing bulb and modulates refrigerant flow based on varying loads?

Thermostatic Expansion Valve

What do a TXV and EEV have in common when it comes to why the components function the way they do?

They both restrict superheat to a desired setting.

What variables does the performance of a capillary tube depend on?

Tube length, inside diameter, number of turns

How does a PWM solenoid metering device work?

Uses a pulse width modulation signal to control rapidly opening and closing a solenoid at different speeds in order to vary refrigerant flow.

How does the TXV respond when all liquid vaporizes too late in an evaporator?

Valve closes to provide a lower flow rate

AEV pressure does not balance in the off cycle unless:

Valve is equipped with a bypass or bleeder.

How does the TXV respond when all liquid vaporizes too early in the evaporator?

Valve opens to provide a higher flow rate

If a TXV is opening to allow higher refrigerant flow, what temperature is the sensing bulb sensing relative to the input of the evaporator?

Warmer refrigerant

What is an overdriving routine?

When controller is first powered up, it sends more steps to the motor than it can use to drive the valve fully closed/zero position.

Under what circumstances would an equalizer be used?

When evaporator pressure between valve and output of evaporator to allow valve to provide correct refrigerant flow.

After initialization what are subsequent signals to the stepper motor based on ?

Zero position

How far should you turn the adjustment screw at a time?

¼ turn