Capillary Tube:

Rather than an orifice, a length of a little diameter tube may offer the similar restrictive effect. Small diameter tubing is known as 'capillary tube', meaning 'hair-like'. The inside diameter of the capillary utilized in refrigeration is in general approximate 0.5 to 2.28 mm (0.020 to 0.090'). The longer the capillary tube and/or the get smaller the inside diameter of the tube, higher is the pressure drop it may create in the refrigerant flow; or in other terms, greater shall be the pressure difference required among the high side & low side to establish a given flow rate of the refrigerant.

The length of the capillary tube of a specific diameter needed for an application is first find out roughly by empirical calculations. This is further then correctly established by experiments. The capillary tube is not self-adjusting. If the conditions modify, such like an increase in the discharge/condenser pressure because of a go up in the ambient temperature, decrease in evaporator pressure, etc. the refrigerant flow-rate shall also change. So a capillary tube, chosen for a specific set of conditions & load shall operate somewhat less efficiently at other conditions. But if correctly selected, the capillary tube may work satisfactorily on a reasonable range of conditions.

As soon as the plants end, the low and high sides equalize via the capillary tube. For this cause, the refrigerant charge in a capillary tube system is critical and therefore no receiver is utilized. If the refrigerant charge is more than the minimum required for the system, the discharge pressure shall go up while in operation. It can even lead to the overloading of the compressor motor. Furthermore, throughout the off- cycle of the unit, the excess amount shall enter the cooling coil & it can cause liquid flood back to the compressor at the time of beginning. So, the refrigerant charge of the capillary tube system is decisive. For this cause, a refrigerant liquid attains cannot be utilized. The charge should be precisely the quantity as showed by the manufacturer of the refrigeration unit.

As the capillary tube equalizes the high side along with the low side throughout the off- cycle, the idle pressures at the discharge & suction of the compressor shall be equal. So at the time of beginning, the compressor motor has not overcome the stress of the dissimilarity of pressure in the suction & the discharge sides. In other terms the compressor is said to begin unloaded. It is a great advantage since a low starting torque motor is adequate for driving the compressor.

The capillary tube is fairly a simple device and is also not expensive. Its pressure equalization property permits the use of a low beginning torque motor. The liquid receiver is also removed in a capillary tube system due to the need to limit the refrigerant charge. All of these factors help to decrease the cost of manufacture of the systems employing a capillary tube like the throttling device.

The capillary tube is utilized in small hermetic units, such like domestic refrigerators, freezers & room air conditioners.