Capillary Tube, what is it and how to choose the best one?

November 2, 2021

The capillary tube is the simplest of all refrigerant flow controls, with no moving parts. They normally consist only of a copper pipe, diameter 0.5 to 1.5 mm and length 1.5 to 6 m. The expansion function is simply caused by the pressure drop induced by the long, narrow tube. The mass flow through the tube depends on the pressure difference between the condensing and evaporating sides.

Capillary can be found in small, high-volume commercial systems, such as home refrigerators, but can also be used for larger systems if operating conditions are relatively stable. In general, the capillary tube is vulnerable to clogging , so a filter drier and filter are typically mounted prior to inlet. The low pressure side of a refrigerant system with a capillary expansion device must be able to hold the entire refrigerant charge. When the compressor stops, the refrigerant will migrate to the low pressure cold side.

Often the low pressure side is equipped with a liquid separator, which acts as a receiver, just before the compressor. Refrigerant charge must also be carefully considered for capillary tube systems. An overloaded system will back up the condensate in the condenser.

This will eventually totally flood the capacitor if the overload is large enough or if there is a large change in operating conditions. Low load, on the other hand, will starve the evaporator, with hunting as a result.

Next, we will talk in detail about the capillary tube and capillary action, which is what it is generally used for. This one of the many laboratory tubes, such as the Pitot Tube . Join us!

What is a capillary tube?

Capillaries or capillary tubes are very thin tubes made of a rigid material , such as plastic or glass, in which a liquid flows into the tubes against gravity in a process called capillary action. Capillarity is the movement of a liquid in a small cross-sectional capillary tube, such as openings in porous materials. Capillary action of a fluid can occur in both vertical and horizontal directions. In capillarity or capillary action, the fluid moves upward against the force of gravity.

In the capillary tube, capillarity occurs due to intermolecular forces between the liquid and the surrounding solid surfaces of the tube. It is caused by the pressure of cohesion and adhesion, which makes the liquid move even against gravity.

The effect of capillary action can be seen in:

  • Drawing liquids between the hairs of a brush.
  • Thin tubes
  • Porous materials such as paper.
  • Some non-porous materials, such as liquefied carbon fiber
  • Cells

Capillary action has many important applications, such as thin-layer chromatography, in which a solvent moves vertically up a plate through capillary action. In brazing, capillary action causes a filler metal to draw into the space between the workpieces.

What is the capillary tube made of?

The capillary tube is generally made of stainless steel , since mercury amalgamates with other metals. Changes in temperature affect the capillary and the mercury it contains and therefore the temperature reading. But if the capillary has a very small capacity, the error due to changes in ambient temperature will be negligible.

Capillary tube properties

When using a capillary tube of appreciable length, it is necessary to compensate for the effects caused by changes in temperature in the vicinity of the tube. This can be done in several ways. To achieve complete temperature compensation, two thermal systems are used that are identical in all respects except that one has a light bulb and the other does not.

The capillary tubes run side by side, and the Bourdon tubes are very close within the same case. If the pointer is arranged to indicate the difference in movement between the free ends of the two Bourdon tubes, it will indicate an effect that is due only to the change in temperature in the bulb.

If only housing temperature compensation is required, the capillary tube is omitted in the compensation system, but in this case the length of the capillary tube used in the uncompensated system should not exceed 8 meters.

What is capillary action?

Even if you have never heard of hair action, it is still important in your life. Capillary action is important to move water and all the things that dissolve in it. It is defined as the movement of water within the spaces of a porous material due to the forces of adhesion, cohesion and surface tension.

Capillary action occurs because water is sticky , thanks to cohesion forces, water molecules like to stay together, and to adhesion, water molecules are attracted and adhere to other substances. Adhesion of water to the walls of a container will cause an upward force on the liquid at the edges and will result in an upturned meniscus.

Surface tension acts to keep the surface intact. Capillary action occurs when adhesion to the walls is stronger than the cohesive forces between liquid molecules. The height to which the capillary action will carry water in a uniform circular tube is limited by surface tension and, of course, gravity.

Not only does water tend to stick to a drop, it sticks to glass , fabric, organics, soil, and luckily to the fibers in a paper towel. Dip a paper towel in a glass of water and the water will “creep” over the paper towel. In fact, it will keep pulling the towel up until the force of gravity is too much to overcome.

Capillary action applications

The practical use of capillary action is evident in all forms of our daily life. It makes doing our jobs efficient and effective. Some applications of this unique property include:

  • The fundamental properties are used to absorb water through the use of paper towels. The cohesive and adhesive properties draw the fluid towards the paper towel. The liquid flows onto the paper towel at a certain speed.
  • A technique called thin-layer chromatography uses capillary action in which a layer of liquid is used to separate mixtures of substances.
  • Capillary action helps us naturally by pumping tear fluid into the eye. This process cleanses the eye and clears all the dust and particles that are around the eye passages.
  • To generate energy : One possible use for capillary action is as a renewable energy source. By allowing the water to rise through the capillaries, evaporate once it reaches the top, the condensate, and falls back to the bottom by spinning a turbine on its way to create the energy, the capillary action can generate electricity! ! Although this idea is still in the works, it shows the potential that capillary action has and how important it is.

Capillary action and capillary tube

The rise or fall of liquids in a capillary tube depends on the interactions between the tube and the liquid. The meniscus is the curve caused by surface tension on the upper surface of a liquid. It can be convex or concave . A convex meniscus occurs when molecules have a stronger attraction to each other (cohesion) than to the container material (adhesion), causing the surface of the liquid to sink downward.

This can be seen between mercury and glass in barometers and thermometers. Rather, a concave meniscus is produced when molecules in the liquid are attracted to those in the container, causing the surface of the liquid to sink upward. This can be seen in a glass of water.

Forces in capillary action

The cohesive forces are the forces of attraction which, as its name suggests, cause cohesion or bind the molecules of the liquid. It is the cohesion forces that drive surface tension, the property of a liquid to resist penetration. They exist between similar atoms, like the branched hydrogen atoms that make up water.

The bond strengths are not that different. These are attractive forces too, but the attraction they promote is not between similar atoms, but different atoms. As their name implies, they cause adhesion or stick a family of molecules, in this case, the liquid, to another family of molecules, here, the capillary surface.

A liquid will rise only if the adhesion forces between its molecules and the molecules in the tube are greater than the cohesion forces between its own molecules. A tube, like the thin stem of a plant that carries water and nutrients, attracts and “pulls” the liquid further and further along its sticky surface.

Dr. Loony Davis5
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Born and raised in Brussels in an English family, I have always lived in a multicultural environment. After several work experiences in marketing and communication, I came to Smart Water Magazine, which I describe as the most exciting challenge of my career.
I am a person with great restlessness and curiosity to learn, discover what I do not know, as well as reinvent myself daily, someone who is curious about life and wants to know. I enjoy sharing knowledge.
This is my personal project but I also collaborate in other blogs, it is the case, the most important web on water currently exists in the US, if you are interested you can read my articles here.

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