capillary tube

What is a Capillary Tube Used For?

Did you know that capillary tubes actually control the flow of refrigerant within an HVAC system? Overall, they have several important functions, such as removing air bubbles from fluids and transferring heat from one area to another. We’re going to take a closer look at the purpose of capillary tubes and why they are needed within HVAC systems.

We will also discuss how to select the right tube for your needs and what some of the most common causes of capillary tube failure are. So what is a capillary tube used for? Keep reading to find out.

What Is a Capillary Tube Exactly?

So, what is a capillary tube? They are fixed-length tubes that have a small diameter and you’ll find them between the evaporator and condenser of your unit. It’s used for metering any refrigerant from those two pieces it sits between.

Capillary tubes are most commonly used in air conditioning and refrigeration systems. However, they are also used in a variety of other industries and applications, including:

  • Automotive industry
  • Medical industry
  • Food and beverage industry

These tubes are made from a variety of materials, such as copper, aluminum, brass, and stainless steel. The material you choose will depend on the specific application.

For example, capillary tubes made from copper are often used in automotive applications because they can withstand higher temperatures. Aluminum capillary tubes are often used in air conditioning systems because they are lightweight and easy to work with.

The Purpose of a Capillary Tube: Why Is It Needed?

The purpose of using a capillary tube within an HVAC system is to transfer heat from one area to another area within a building or structure. Heat can get transferred from areas that are too hot (such as near furnaces) and carried away by fluids such as water or refrigerant, which travel through pipes throughout an HVAC system before being transferred to areas that are too cold (such as near air conditioning units).

The capillary tube is used in conjunction with a thermostat, which is what measures the temperature of the air and sends signals to other parts of the HVAC system. The thermostat will send signals as well.

So, if the thermostat senses that an area is too hot, it will send a signal that essentially allows more fluid to travel through the tube. This allows for heat from an area that is too hot to be transferred into other areas to even out the temperature. Overall, the tube is a component that acts as a maintainer or a regulator in a sense.

So, How Do They Work?

Capillary tubes work by using a small diameter to create a large surface area. The small diameter allows it to be flexible and move with the fluid that it is transferring. This allows for a greater amount of heat to be moved around.

Capillary tubes are often used in HVAC systems to remove air bubbles from fluids as well. So, to better understand its application, the tube itself is placed in a larger pipe or tubing that carries the fluid through the system. The capillary tube has a very small opening at one end and a larger opening at the other end.

This allows for fluid to enter the tube from one direction and exit out of the other. The fluid enters the capillary tube through an opening called an inlet port, which is located on top of the capillary tube near its smaller opening.

The fluid then travels through the length of the tube until it reaches its larger opening, where it exits the tube and continues on its way through other pipes or tubing within the HVAC system. When you’re selecting a capillary tube for a project, it is important to consider the following factors:


The diameter of the tube will determine the amount of fluid that can flow through it.


The length of the tube is a factor that will drive the amount of time it takes for the fluid to travel.


The material of the tube will determine its durability and overall resistance.

What Are the Benefits of Using a Capillary Tube in HVAC Applications?

The capillary tube is a very useful component that makes up a portion of your HVAC system. It helps the transfer of heat as we already know. That helps the temperature of an area to be better maintained and at a comfortable level.

It is also very easy to manufacture, easy to install, and simple to use. Overall what it’s doing is providing solution control. It limits the amount of refrigerant that’s charged to a system.

In addition, working with capillary tubes provides more benefits that could make different projects more easy to manage. Some of those are:

  • The efficiency can be set to maximum
  • The component has a high level of accuracy
  • High chance of reliability
  • Accessible
  • Pressure reduction

This piece responds to the load on the evaporator of your HVAC system and this is what helps liquid refrigerants to vaporize at desired rates of pressure. The component itself is easy to work with and to have on hand due to the ease of manufacturing it.

Are There Disadvantages of Using a Capillary Tube in HVAC Applications?

Some often wonder if there are any downfalls to using a capillary tube. In short, there are not many disadvantages associated with using a capillary tube in an HVAC system because it is needed. However, it can be difficult at times to find the right size capillary tube that will fit into your system.

If you are unable to find the right size, you may have difficulty installing your system how it should be. We’ll get more into sizing later in the article.
Since there aren’t any notable disadvantages to using this type of tube it’s easy to focus on more of the benefits of the product.

Think about it like this, a capillary tube doesn’t have any moving parts. This means you don’t have to worry about wear and tear as too much of a factor.

Also, the tube doesn’t maintain superheat or pressure given off by the evaporator. These are factors you won’t have to worry about.

Selecting the Right Capillary Tube for Your Needs

There are a variety of different factors that you need to consider when selecting a capillary tube for your system. The first thing you need to think about is the size of your HVAC unit. The size of your unit will determine the size of the capillary tube you need.

When looking for the proper size, you usually want to decide on the tube based on the condenser system. That system is between the inner diameter of your tube and the evaporator of the HVAC system.

There are times you’ll stick between 0.76 and 2.16 mm for the pipe but it will depend on your unit. One important thing to keep in mind is that the shorter the tube is, the lower the pressure and temperature the condensation will be. In comparison though, the more constant the length is, the higher the charge will be.

Another thing to consider is the type of refrigerant you are using. Some refrigerants are more corrosive than others and this can affect the type of capillary tube you need as well.

The temperature of the refrigerant is another factor to consider. If you are using a refrigerant that has a high operating temperature, you will need to use a capillary tube that can withstand high temperatures.

What Are Some Common Applications for Capillary Tubes?

There are several uses for capillary tubes. They are often used in residential and commercial heating and cooling systems as well as industrial heating and cooling systems. This means that they are often applied to multiple industry applications if it calls for it.

Overall, they’re used as throttling devices. These devices are also used in water coolers and deep freezers. In addition, you will see them used in domestic refrigerators and air conditioners. There main need is the sample collection of fluids that flow against gravity.

Process of Installation

To better understand how the components are needed within an HVAC system, let’s take a look at how the installation process will generally go. The process is quite simple and after you turn off the power to your HVAC unit and shut off the power at the breaker box you will need to locate the suction line of your unit. It will be the larger of the two refrigerant lines and will be connected to the compressor of your unit.

To continue, you’ll often cut into the suction line using a tubing cutter. Keep in mind that you will need to cut into the line about 3 inches from where it connects to the compressor. Then you can remove any burrs from the end of the suction line if there are any.

To complete the project you’ll measure the length of the tube you will need. The length of the tube will depend on the size of your HVAC unit as discussed earlier.

Next, you will install the capillary tube into the suction line and use a soldering torch if needed and that’s it. Once you’re done the power can come back on for you to test the connection.

If you do not use a capillary tube in an HVAC system, the refrigerant will not be able to flow well and the system will be less efficient than it should be. This can lead to several problems, such as your unit not cooling properly which can lead to unplanned spikes in electric bills.

It can also lead to your unit freezing up or overheating. If you run into this issue, connecting with other professionals can help to make the installation process easier to navigate as well.

Usual Causes of Capillary Tube Failure

The blockage of the capillary tube is often caused by the accumulation of particles that are big enough to cause a disruption to the flow of refrigerant. This can be a significant problem if the tubes aren’t cleaned as well. For example, foreign materials that become stuck over time can also start to freeze within the tubing.

Kinks in the tube itself can also make this worse. Anything from dirt, refrigerant oil, or steel wool can cause blockages as well.

Another common cause of failure is due to vibration. If the tube is not installed correctly, it can rub against other components in your HVAC system and this will cause it to vibrate.

The vibration can cause the tubing to wear down over time and eventually fail. Since it is one piece, this wear process will take longer than it would for other parts. Another potential cause of failure is due to pressure.

This can happen if the capillary tube is not installed in the correct way or if the tube’s damaged before going into it. Air pressure is another effect that you can consider.

However, because of air backpressure, the smaller the diameter of the tube, the longer the length will be to fill the tube regardless of the viscosity of the liquid.

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