Capillary tube

November 1, 2021

Among the laboratory materials we have the capillary tube, a glass tube with a tiny internal diameter and a reduced length. It is used to demonstrate the different effects of capillarity.

What is it?

It is a small tube of small diameter and length used in chemistry laboratories to demonstrate the properties of capillarity. This small tube also has applications in refrigeration, since by having a tiny internal diameter – similar to the thickness of a hair – it allows controlling the pressure with which the flow of refrigerant moves.

In refrigeration, the capillary tube is made of copper and maintains the pressure with which the refrigerant passes from the condenser -high pressure zone- to the evaporator -low-pressure zone- transforming from the liquid state to the vapor state in order to equalize its temperature with that of saturation of the evaporator.

The term “capillary” comes from its internal diameter similar to the thickness of a hair (although in reality it is a little more), the diameter of a capillary tube ranges from 0.5 to 3 mm in diameter and in the case of applications for refrigeration, it can be 1 to 6 m long.

What is capillarity?

It is one of the many properties that fluids have and is related to the surface tension and cohesion of the fluid, these properties are what determine the power that the fluid will have to go up or down through the capillary tube, a property that is known as capillarity.

What is it for?

In the case of capillary tubes for laboratories, we have that you can use it to measure the temperature at which a substance completely melts, that is, its melting temperature.

The mass of product that you place in the tube will be directly proportional to the square of the diameter, this means that a narrow tube can suck the liquid to greater lengths than a wide tube and its capillary pressure will be greater.

In refrigeration it is used in expansion devices in which, due to its diameter, it causes the refrigerant to lose the pressure with which it leaves the compressor and expands towards the inlet of the evaporator. This loss will be determined based on the length of the capillary tube and the friction that causes resistance to the passage of the liquid refrigerant.

Advantage

Capillary tubes are preferred over other expansion equipment because of:

  • It is reliable because it has no moving parts.
  • It’s very cheap.
  • It is very easy to repair.
  • It does not need to contain liquid deposits.
  • Its simplicity.

Disadvantages

Among the disadvantages represented by the use of a capillary tube in refrigeration we have:

  • A filter drier must be installed in order to avoid clogging.
  • An adequate amount of refrigerant is essential.
  • It does not adapt to changes in flow rate.
  • To choose a capillary tube, the specifications of the air conditioning, refrigerator or refrigeration equipment must be evaluated.
  • They are easily clogged if dirt is present in the system.
  • They are very rigid, the refrigerant flow cannot be modified when there are variations in the ambient temperature or the thermal load of the equipment.
  • Its melting point is low because it has a small diameter.

Capillary tube obstruction

A capillary tube for cooling becomes clogged for different reasons:

  • Oil degradation.
  • Detachment of the filter drier molecules due to wear.
  • Moisture that freezes the pipe.
  • Malpractice on the part of the equipment maintenance or repair technician.

Characteristics and shapes of laboratory capillary tubes

Among its features we have:

  • This tube has a 1mm diameter.
  • It allows the fluid to be guided through a very narrow circular section.
  • It can be made of copper, metal alloys or glass.

How is the melting point determined with a capillary tube?

To determine the melting point of a substance, it is enough to crush a small quantity and pour it into a dry capillary tube up to 3 mm in height.

Then you must heat with the melting point determination equipment to a temperature about 5 or 10 ºC below the temperature you expect to obtain. Subsequently, you just have to adjust the equipment in such a way that the temperature rises in the tube at a rate of one degree per minute.

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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