Working principle and process of capillary tube in refrigeration system

- Jul 19, 2019-

1. Working principle of capillary tube:

Capillary tube is a common throttling device in refrigeration system. Capillary tube generally refers to slender copper tube with inner diameter of 0.4~2.0mm.

As refrigeration system of the throttle body, capillary is one of the most simple, because of its cheap, flexible, so it is widely used in small refrigeration device, in the larger refrigerating capacity of the unit has recently adopted, such as 10 ark adopts the horse, even in the larger single cooler system is also useful to a company of 40 kw &water-cooled packaged units have adopted.


The specifications of capillary tubes commonly used by the company are:

1.24mm, 1.37mm, 1.63mm.

Custom capillary specifications are:

1.8mm, 2.1mm, 2.4mm

There are 6mm, 8mm copper pipe can also do larger system throttling.


Throttling device mainly controls the refrigerant flow rate needed by evaporator through the pressure drop of refrigerant flow in the device.

Capillary (though called capillary, but actually does not have a capillary action) is really just a relatively small diameter of copper tube, it is a head of connection export condenser (generally in the capillary entry will add a filter, in case of capillary jam), on the other end connection evaporator inlet, this structure because of no heat, called adiabatic capillary tubes.

In some refrigerator refrigeration systems, the capillary and return tube are welded together, and there is heat exchange between them, called exothermic capillary.

High pressure refrigerant liquid flows in the tube. Because the inner diameter of capillary tube is relatively small, small and medium-sized refrigeration devices are usually 0.4-2.5mm, so the pressure drop is relatively large. Therefore, the pressure drop of refrigerant flow can be changed by changing the size of capillary tube, so as to achieve the purpose of controlling the flow rate.

Ii. Working process of capillary tube:

The following graph is commonly used to describe the flow state of refrigerant in the capillary tube along the length direction:


1. 0-2 process:

In this section, because the refrigerant is still a completely supercooled liquid, the flow rate does not change much, so the pressure drop of the refrigerant in the capillary tube remains constant and changes linearly. The pressure of the refrigerant keeps decreasing and approaches to the saturation pressure. Because it is an adiabatic process, the temperature remains unchanged.

2. "2" process:

It is saturation state of liquid refrigerant, refrigerant can be started in this heat evaporating out of gas, so general called flash point, but according to Lietal. (1990) and Mikol (1963) research, points out that although the refrigerant reach saturation point "2", but still not evaporate, the refrigerant pressure must continue to drop down the short length of the capillary to point "2" evaporated, 2, 2 'this process is called the steady state or overheating liquid state.

3. 2-3 process:

The refrigerant liquid starts to evaporate from the point "2". Because of the large volume of the gas, the velocity is accelerated, and the pressure drop of the refrigerant increases. Therefore, this section is a two-phase section with a large pressure drop.

At this time the capillary flow of the plug, that is, to reach the maximum flow. In a real refrigeration system most systems will have the refrigerant down to this pressure, so the capillary outlet pressure is usually at point 3.

4. "4" process points:

Point "4" is the evaporator inlet pressure, which is related to the size of evaporator, refrigerant flow and compressor exhaust volume. Here, it represents the situation when the critical pressure is less than, for most refrigeration systems, it is around point "4".

From above can clear understanding of refrigerant in capillary pressure and temperature changes, now let's look at the refrigerant flow, when the evaporator inlet pressure is greater than the critical pressure of the capillary, it and capillary entry pressure (" 0 ") and capillary outlet pressure (also called export back pressure), namely the evaporator inlet pressure "4" (point), increases with the increase of capillary entry pressure, with the decrease of the evaporator inlet pressure increases. When the inlet pressure of evaporator is less than the critical capillary pressure, it only increases with the increase of capillary inlet pressure.

Because the resistance increases with the increase of refrigerant gas in the capillary tube, the flow of refrigerant increases with the increase of supercooling degree.