• SkillCat Team

A Complete guide to Metering Devices: Fixed & Modulating

Updated: Jul 3

Refrigeration Equipment: Chapter 4


Topic: Metering Device


In this module, we will take a look at what the metering device component does and what different types there are. Skip to quiz!


Review: Metering Device Function


In the refrigeration cycle, refrigerant flows from the condenser to the metering device through the liquid line.


Remember that the previous component, the condenser, cools down the refrigerant until it turns into a liquid. So when refrigerant enters the metering device, it is in liquid state. This is why this line is called the liquid line, because the refrigerant is in liquid form.


Leaving the condenser, the refrigerant is a high pressure, low temperature liquid. We need to pass this refrigerant back to the evaporator so that it can continue absorbing heat to provide cooling.


But the refrigerant’s pressure is too high to pass back to the evaporator. If the pressure is too high, the refrigerant will not be able to absorb heat in the evaporator. This is where the metering device comes in.


The metering device’s job is to decrease the pressure of the refrigerant. Once the refrigerant’s pressure decreases, then we can send it back to the evaporator to absorb even more heat. This completes the refrigeration cycle.


We can think of the metering device as a garden nozzle. Water that comes out from a garden nozzle is like a spray. The spray that comes out is lower pressure than the water in the garden hose. This is what a metering device does.


After refrigerant leaves the metering device, it is now a low pressure liquid. Now, this low pressure liquid refrigerant can be passed back to the evaporator to continue absorbing heat.



Capillary Tube


The most simple form of a metering device is a capillary tube. A capillary tube is just a long tube with a fixed length and a small diameter. It meters, or controls the refrigerant flowing from the condenser to the evaporator.


Because the capillary tube is long and has a small diameter, there is more friction inside the tube. As refrigerant flows through the capillary tube, pressure decreases due to the friction inside.

Recall that there is friction is the resistance force that we experience with any type of motion.We can imagine this by thinking of bowling.


When we bowl, the bowling ball starts off fast at first. But then, it slows down as it reaches the end of the lane. This is because of friction. Friction slows down the movement of the bowling ball.


The longer the bowling lane is, the slower the ball will move by the time it reaches the end of the lane. This shows that the longer the distance, the more friction can slow down the motion over that distance. The same concept applies to the capillary tube.


The capillary tube, the more friction is acting on the refrigerant inside it. As more friction acts on the refrigerant, the refrigerant inside will decrease in pressure. This causes the refrigerant to move slower, just like the bowling ball.


To summarize, long capillary tubes will decrease the pressure of the refrigerant as the refrigerant flows through its inside.


The drawback to capillary tubes is that they will not stop the flow of refrigerant after the compressor has turned off. Normally, when the compressor turns off, other types of metering devices will stop the flow of refrigerant.


Because the capillary tube is just a tube, the remaining refrigerant will keep trickling through the system. It will keep moving until the high and low pressures equalize.


Other Metering Devices


As we said, the capillary tubes are a very simple device used to decrease refrigerant pressure. More complex categories of metering devices are:

  • Fixed Metering Devices, and

  • Modulating Metering Devices

The fixed metering devices are preset and designed for a specific set of operating conditions.

This means that they process a certain amount of refrigerant at a certain rate. This rate does not depend upon the cooling requirements or the outside temperature.


Fixed metering devices cannot be adjusted in any way. Outside weather can affect the temperature and pressure of our systems. This changes how our systems operate but fixed metering devices cannot respond to this.

Because of their limited applications, fixed metering devices are found in small devices or older systems. This includes: small domestic refrigerators, window ACs, and many old cooling and refrigeration appliances.


Modulating metering devices can respond to different weather conditions. This is the reason that modulating metering devices are used more widely.


Modulating metering devices can respond to different weather conditions. This is why they can be more efficient than fixed metering devices. For these reasons,modulating metering devices are used more often in a large range of applications.


The downside is the modulating metering devices are more expensive than fixed metering devices.


Both the fixed and modulating metering devices are types of metering devices. This means that there can be different types of fixed metering devices. There can also be different types of modulating metering devices. We will talk more about them in the next modules.


In this module, we reviewed the concept of metering devices and looked at how a capillary tube works. We also took a look at fixed metering devices and modulating metering devices.

Next, we will look at these two types of metering devices in more detail.



Fixed Metering Devices


In this module, we will take a look at what the different types of fixed metering devices are. Skip to quiz!


Review: Fixed Metering Device

Recall that a fixed metering device has fixed operating conditions. This means its operating conditions stay the same no matter what the external factors.

It processes a constant amount of refrigerant because its parts are not adjustable.


There are two types of fixed metering devices:

  • Capillary Tube, and

  • Fixed Orifice.

We will take a look at each of these types of fixed metering devices.


Capillary Tube


We went over the capillary tube in the previous module. As we said before, the capillary tube works because the refrigerant has to travel through a long tube. As it is traveling, friction on the inside of the capillary tube will decrease its pressure.


Capillary tubes also have a small diameter, which reduces the amount of refrigerant that can flow through at a time. This also decreases the pressure of the refrigerant. Let’s take a look at a video explaining this concept.


we saw that when water flows through a longer garden hose, it loses its pressure and you only get a slow stream of water at the end. This is how the capillary tube works.


Refrigerant in the tube starts off with a very high pressure. As it flows through this long thin copper tube, the pressure of the refrigerant drops.


Recall that if volume is fixed, pressure and temperature are directly related. This means that with a constant volume, as pressure drops, temperature drops as well.


This means that in the process of reducing the refrigerant’s pressure, we will also decrease the refrigerant’s temperature.


The temperature drop is good because we want the refrigerant to be nice and cold. This way, when the refrigerant heads back into the evaporator, it will absorb more heat. This provides more efficient cooling.



Fixed Orifice


Fixed orifice metering devices are also called piston valves. Let’s take a look at a video to see how they work.


A fixed orifice metering device has a piston or an orifice with a small hole in it. The high-pressure refrigerant liquid is forced through the small hole in the orifice device. As the high-pressure liquid passes through it, there is a drop in pressure.


To think about how this works, we can think of a leaking water pipe. If we think of a typical leaking pipe, there is water inside the pipe. This water moves inside the pipe and has pressure acting on it. But the leaking water drops move very slowly.


If we think about the water leaking outside the pipe, its speed is much slower than the water moving inside the pipe. The pressure of the water inside the pipe is contained by the pipe.

The leaked water’s pressure is much lower than the pressure of the water inside the pipe.


The same concept is used in a fixed orifice metering device. In the fixed orifice metering device, the high-pressure refrigerant liquid coming from the condenser is contained in the liquid line.


The fixed orifice metering device has a hole that the refrigerant can pass through. But the size of this hole is smaller than the size of the liquid line. This setup reduces the pressure of the liquid refrigerant that passes through it.


This is how the fixed orifice metering device lowers the pressure of the refrigerant.



Differences


Fixed orifice metering devices were developed as alternatives to capillary tube metering devices. They are a little more complex and give us more control over the pressure of the refrigerant.


Capillary tubes are more susceptible to clogging and damage. Repairing or replacing a capillary tube metering is a hard task that not anybody can do. Because the capillary tube opening is very small, there is a high chance that the hole gets clogged during the brazing of the capillary tube itself.


The orifice meters or piston valves only have a small piston. This piston part can be easily replaced. Just for fun, let’s take a look at how we would replace the piston part.


Fixed orifice metering devices can be easily replaced. This is because fixed orifice metering devices only have an orifice that can be changed easily by unscrewing the cap.


Capillary tubes are cheap and simple. They are still used in:

  • Small domestic refrigerators,

  • Window ACs, and

  • Old cooling and refrigeration appliances.


In this module, we looked at the two types of fixed metering devices. We looked what the similarities and differences between capillary and fixed orifice metering devices.





Modulating Metering Devices - Part 1


In this module, we will take a look at what the different types of modulating metering devices are. Skip to quiz!


Review: Modulating Metering Device


The word “modulating” basically means changing.


A modulating metering device can change how it operates based on different ambient conditions. This means that the device can change its operation based on outside temperatures detected. The modulating metering device controls the amount of refrigerant flow based on the outside weather.


Being able to change makes modulating metering devices suitable for a wide range of applications. This also makes them more expensive than fixed metering devices.


In this module, we will talk about three types of modulating metering devices:

  • Automatic Expansion Valves (AEV)

  • Thermostatic Expansion Valves (TEV), and

  • Electronic Expansion Valves (EEV)


Automatic Expansion Valves (AEV)


Automatic expansion valves, or AEVs, use the same principle as a fixed orifice device. AEVs have a hole that allows the refrigerant move through it. This hole controls the pressure of the refrigerant.


In automatic expansion valves, the opening hole is adjustable. The hole can be increased or decreased in size depending on the cooling demand.


In AEVs, if we need more cooling, the hole will be larger to let more refrigerant through. That way, we can provide more refrigerant to do more cooling in the evaporator. If we need less cooling, the hole will be smaller to let in less refrigerant.

In automatic expansion valves, the hole size is controlled by a diaphragm and a spring. Let’s take a closer look how the diaphragm and spring combination works.


The diaphragm is flexible piece of metal that can be bent. Because the diaphragm is made of metal, it won’t break or snap easily. The diaphragm works by partially covering the opening of the metering device.


The diaphragm is like the piston of a fixed orifice. It blocks a part of the opening, which controls the pressure of the refrigerant that moves through the hole.


The spring is a coil-like device that can compress or extend. We see springs in everyday things like door springs or mouse traps. In automatic expansion valves, the spring is attached to the diaphragm we just talked about.


The spring is attached to the diaphragm. So when we pull the spring back, the diaphragm opens up the orifice hole making the opening bigger. A bigger opening means more refrigerant can pass through through it and more cooling can be provided.


When we press on the spring, this makes the diaphragm close up part of the opening. This makes the opening size smaller. As a result, less refrigerant will flow through the opening.


So in AEVs, the combination of the spring and the diaphragm allow us to control the hole size.

Depending on the hole size, more or less refrigerant can flow through the metering device. This changes the refrigerant’s pressure.


So in AEVs, the combination of the spring and the diaphragm allow us to change and control the hole size. Depending on the hole size, more or less refrigerant can flow through the metering device. This changes the refrigerant’s pressure.


AEVs automatically change the diaphragm and spring position so that we can adjust to the amount of cooling required. The orifice opening size controls the pressure of the refrigerant and the pressure of the refrigerant controls cooling in the evaporator.


AEVs are most commonly found in industrial refrigerated air dryers. These are used to provide cold dry air in factory settings. Automatic expansion valves are metering devices that work by maintaining constant pressure output.


In this module, we looked at the working of automatic expansion valves. These are one of the most basic types of modulating metering devices. We rarely see them in the HVAC appliances that are seen in our daily life.



Modulating Metering Devices - Part 2


In this module, we will take a look at what the different types of modulating metering devices are. We already looked at the working of AEVs. We will now discuss about the TXVs and EEVs. Skip to quiz!


Review: Modulating Metering Device


The word “modulating” basically means changing.


A modulating metering device can change how it operates based on different ambient conditions. This means that the device can change its operation based on outside temperatures detected. The modulating metering device controls the amount of refrigerant flow based on the outside weather.


Being able to change makes modulating metering devices suitable for a wide range of applications. This also makes them more expensive than fixed metering devices.


In this module, we will talk about three types of modulating metering devices:

  • Automatic Expansion Valves (AEV)

  • Thermostatic Expansion Valves (TEV), and

  • Electronic Expansion Valves (EEV)

Thermostatic Expansion Valves (TEV)


Thermostatic expansion valves are also called TEVs or TXVs.

(The X in TXV also refers to the expansion part). These are the most commonly used metering devices. Let’s get started by looking at a video explaining how they work.


TXVs or TEVs are used in many different types of appliances, as we saw from the video. They are used in residential single split units as well as industrial chillers. Much smaller systems like household refrigerators would not use a TXV, they would use a fixed orifice device.


In thermal expansion valves, the sensing bulb is the key component. As we saw from the video, the sensing bulb is attached to the surface of the evaporator coils. This way, it can monitor the temperature of the refrigerant in the evaporator.


Since the sensing bulb sits on top of the evaporator coils, heat can transfer from the evaporator coils to the sensing bulb. This means that if the refrigerant inside the evaporator coils gets too hot, the sensing bulb also gets hot since they are touching each other.


The sensing bulb is also connected to the metering device through a capillary tube. This capillary tube is attached to the metering TXV metering device’s piston.


The sensing bulb is filled with a liquid that has similar properties to the refrigerant.


If the refrigerant in the evaporator is too hot, the heat will transfer from the evaporator to the liquid in the sensing bulb. This sensing bulb liquid will boil into a vapor from the heat transferring off the evaporating coils.


This sensing bulb vapor flows up through the thin capillary tube. The vapor fills the capillary tube and pushes down on the piston in the TXV or TEV. Pushing down on the piston increases the size of the opening that refrigerant can flow through. We can think of this piston as a button that opens up the refrigerant gates.


Because the opening in the metering device is larger, more refrigerant flows through. More refrigerant = more cooling.


The opposite can also happen. If the refrigerant is too cold, the substance in the sensing bulb will condense into a liquid. Then, there will be nothing pressing on the metering device piston. So less refrigerant will be let through, and less cooling will happen.



Electronic Expansion Valves (EEV)


Electronic expansion valves are also called EEVs or EXVs. Both mean the same thing. Electronic Expansion Valves (EEV) control the flow of refrigerants electronically. Let’s get started by looking at a video explaining how they work.


Electronic expansion valves control the amount of refrigerant going into the evaporator. EEVs receive information from temperature and pressure sensors in the evaporator. It changes the amount of refrigerant it lets through based on these sensor readings.


To put it simply: If the sensor reading says pressure is too high, the EEV will send less refrigerant through to decrease the refrigerant pressure. And if the sensor reading says that pressure is too low, the EEV will send more refrigerant through to increase the refrigerant pressure.


EEVs are more sophisticated than TXVs. Compared to TXVs, EEVs

  • Have more precise flow control,

  • Better performance, and

  • Higher efficiency.

Because EEVs are extremely precise and offer great control, they are used in larger and more high end applications. EEVs are used in:

  • Multisplits,

  • Air handling units, and

  • Chillers.

EEVs are generally not used for residential applications.


In this module, we looked at the two types of modulating metering devices. We looked at the thermostatic expansion valves, and electronic expansion valves. Of these, EEVs are the most precise and energy efficient metering devices.






Question #1: After refrigerant leaves the condenser, the refrigerant is a liquid that is:

  1. High temperature and high pressure

  2. High temperature and low pressure

  3. Low temperature and low pressure

  4. Low temperature and high pressure

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Answer: Low temperature and high pressure

Both of these are true. It the condenser’s job to throw heat out of the system’s refrigerant. The condenser also provides the necessary space for air to flow over the refrigerant coils.


Question #2: The metering device’s job is to

  1. Meter the space

  2. Increase pressure

  3. Decrease pressure

  4. All of these

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Answer: Decrease pressure

The metering device’s job is to decrease pressure.


Question #3: A capillary tube is too simple to be called a metering device.

  1. True

  2. False

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Answer: False

This is false. A capillary tube is a very simple device. But it is still considered a metering device because it is used to lower the pressure of a refrigerant or a liquid.


Question #4: Friction is the resistance to motion. Friction inside long capillary tubes ___ the pressure of the refrigerant inside.

  1. Equals

  2. Increase

  3. Decrease

  4. None of these

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Answer: Decrease

Friction decreases the pressure of the refrigerant inside capillary tubes.


Question #5: Which quality of the capillary tube is most responsible for decreasing refrigerant pressure?

  1. Its length

  2. Its length and material

  3. Its attachment to the condenser

  4. Its timing

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Answer: Its length

The length of the capillary tube is the primary reason why the refrigerant’s pressure decreases. We can think about our bowling lane example, the longer the lane, the slower the bowling ball will be moving at the end of the lane. Remember that friction is resistance to motion. Long capillary tubes mean the refrigerant has to move farther, which means more friction will act on the refrigerant to decrease its pressure.



Question #6: Capillary tubes automatically stop letting refrigerant travel through when the system stops running.

  1. True

  2. False

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Answer: False

This is false. When the system turns off, the refrigerant will continue to flow through capillary tubes until the pressures equalize.


Question #7: Fixed metering devices means that they stay in one place. Other metering devices can be moved around.

  1. True

  2. False

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Answer: False

Fixed metering devices means that they have one set of operating conditions. They can only process a preset amount of refrigerant at a time.


Question #8: Fixed metering devices cannot change based on weather conditions. They can only process a certain amount of refrigerant in a certain way. This process does not change.

  1. True

  2. False

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Answer: True

This is true. Fixed metering devices are preset and designed for a specific set of operating conditions. Outside weather conditions might affect the system, but these devices are unable to adjust to it.


Question #9: Modulating metering devices

  1. Can change operating conditions

  2. Can only be used in chillers

  3. Are cheaper than fixed metering devices

  4. All of these

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Answer: Can change operating conditions

Modulating devices can change operating conditions. The other options are incorrect.


Question #10: Modulating metering devices can be used in a wide range of applications, but they are more expensive than fixed metering devices.

  1. True

  2. False

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Answer: True

This is true. Modulating metering devices can adjust the refrigerant flow as per the needs of the system. So they are more expensive.


Question #11: The capillary tube is only one type of fixed metering device.

  1. True

  2. False

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Answer: False

There are different types of fixed metering devices. We will talk about these in the next module. The capillary tube is one type of fixed metering device. But there are others.


Question #12: Fixed metering devices can process refrigerant in only one set of conditions.

  1. True

  2. False

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Answer: True

This is true. Fixed metering devices are preset and designed for a specific set of operating conditions. Outside weather conditions might affect the system, but these devices are unable to adjust to it.


Question #13: A fixed orifice metering device works by passing refrigerant through a smaller opening. This decreases the speed the refrigerant travels at, which decreases the pressure.

  1. True

  2. False

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Answer: True

This is true. Passing the high-pressure refrigerant through a small hole decreases its pressure.


Question #14: Which of the following factors help to decrease the pressure of the refrigerant in capillary tubes?

  1. Long length

  2. Small diameter

  3. Both of these

  4. None of these

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Answer: Both of these

Both the long length and small diameter of the capillary tubes are factors that increase the friction acting on the refrigerant. Remember that friction is resistance to motion. So increased friction means the refrigerant will move slower, and be lower pressure.


Question #15: As refrigerant moves through a capillary tube, what decreases?

  1. The length of the tube

  2. Temperature

  3. Pressure

  4. Temperature and pressure

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Answer: Temperature and pressure

As the refrigerant moves through the capillary tube, both its temperature and pressure decrease. We talked about how the capillary tube decreases the refrigerant’s pressure. In this process, the volume stays the same. So the relationship between pressure and temperature tells us that as pressure decreases, temperature also decreases.


Question #16: In a fixed orifice metering device, which part is considered to be “fixed” or constant?

  1. The hole or opening size

  2. The pressure

  3. Both of these

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Answer: The hole or opening size

Of these answer choices, only the hole size is fixed in a fixed orifice metering device. Pressure changes, which is the whole point of a metering device.


Question #17: In fixed orifice metering devices, the piston can be replaced easily.

  1. True

  2. False

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Answer: True

This is true. The piston in a fixed orifice metering device is easy to replace. We only need to unscrew a nut to expose the piston. This can be easily removed.


Question #18: Fixed orifice metering devices are more complex than capillary tubes.

  1. True

  2. False

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Answer: True

This is true. Fixed orifice meters are a bit more complex compared to capillary tubes.

Capillary tubes are one of the simplest metering devices found in appliances.


Question #19: Modulating metering devices

  1. Have multiple operating settings

  2. Can change based on their environment

  3. Can adjust the amount of cooling needed

  4. All of these

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Answer: All of these

All of these are true. Moduating metering devices have different operating settings. These settings are based on the cooling demand and the temperature of the surroundings. By adjusting the refrigerant flow in different conditions, the cooling provided by the system can be changed.


Question #20: Modulating metering device can be used in a lot of different systems. This is because they can provide different amounts of cooling based on operating conditions. They are also more expensive.

  1. True

  2. False

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Answer: True

This is true. A variety of systems use modulating metering devices. They are also expensive compared to fixed metering devices.


Question #22: In an AEV, the diaphragm controls the opening size of the orifice. The position of the diaphragm controls the amount of refrigerant that can flow through to the evaporator.

  1. True

  2. False

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Answer: True

This is true. In an AEV, the diaphragm controls the opening size of the orifice.The position of the diaphragm controls the amount of refrigerant that can flow through to the evaporator.


Question #23: In an AEV, the spring is not attached to the diaphragm and acts independently of the diaphragm.

  1. True

  2. False

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Answer: False

This is false. The spring is attached to the diaphragm in an AEV so the spring and diaphragm move together. The spring can push or pull on the diaphragm to adjust the opening size. This controls the amount of refrigerant that can flow through the AEV.