Capillary tube: Advantages and disadvantages of capillary tube

Capillary tube

Capillary Tube and Its

1. A capillary tube is
a long, narrow tube of constant diameter.

2. The word “capillary” is a
misnomer since surface tension is not important in refrigeration

application of
capillary tubes.

3. Typical tube diameters of
refrigerant capillary tubes range from 0.5 mm to 3 mm and the

length ranges
from 1.0 m to 6 m.

Some of the
advantages of a capillary tube are

1. It is inexpensive.

2. It does not have any moving parts hence it does
not require maintenance.

3. Capillary tube provides an open connection
between condenser and the evaporator

hence during
off-cycle, pressure equalization occurs between condenser and evaporator.

This reduces
the starting torque requirement of the motor since the motor starts with

same pressure
on the two sides of the compressor. Hence, a motor with low starting torque

(squirrel cage
Induction motor) can be used.

4. Ideal for hermetic compressor based systems,
which are critically charged and factory


Some of the
disadvantages of the capillary tube are

1. It cannot adjust itself
to changing flow conditions in response to daily and seasonal

variation in ambient temperature and load. Hence, COP is
usually low under off design


2. It is susceptible to
clogging because of narrow bore of the tube; hence, utmost care is

required at the time of assembly. A filter-drier should be
used ahead of the capillary to

prevent entry of moisture or any solid particles. 

3. During off-cycle liquid
refrigerant flows to evaporator because of pressure difference

between condenser and evaporator. The evaporator may get
flooded and the liquid

refrigerant may flow to compressor and damage it when it
starts. Therefore critical charge

is used in capillary tube based systems. Further, it is used
only with hermetically sealed

compressors where refrigerant does not leak so that critical
charge can be used. Normally

an accumulator is provided after the evaporator to prevent
slugging of compressor.

Capillary tube
Capillary tube

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