Influence of capillary tube length on refrigeration system parameters

Capillary tube usually refers to the inner diameter of 0.4 to 2.0 mm between the thin copper tube, as a refrigeration system in a simple throttling mechanism, it is widely used in small refrigeration equipment with its low cost and flexible selection advantages. Next, we will discuss the influence of the length of the capillary tube on the parameters of the refrigeration system, and introduce how to eliminate the phenomenon of capillary tube “ice blockage”.

The influence of capillary tube length on the parameters of the refrigeration system

The length of the capillary tube has a direct impact on the performance of the refrigeration system. First of all, under the same refrigerant charge, the shorter capillary tube will lead to a larger refrigerant flow, which will lead to the compressor suction temperature and discharge temperature decrease. At the same time, as the flow rate increases, the suction pressure rises; however, when the charge is fixed, the shorter the capillary tube, the lower the discharge pressure.

In the case of condensers, shortening the capillary tube reduces the condensing temperature and pressure when the refrigerant charge is constant; conversely, if the capillary tube length is fixed, increasing the charge causes the condensing temperature and pressure to rise. The evaporator follows a similar pattern: at a fixed charge, shortening the capillary tube increases the evaporating temperature and pressure; while at a fixed capillary tube length, an increase in charge will also cause an increase in evaporating temperature and pressure.

Regarding the degree of subcooling and superheating, the longer the length of the capillary tube, the degree of subcooling and superheating will increase; while in the case of a fixed length of the capillary tube, increasing the charge volume will lead to an increase in the degree of subcooling, but the degree of superheating will be reduced.

In addition, the capillary tube length also affects the cooling efficiency (EER) of the whole system. At the same charge, extending the capillary length reduces power consumption but sacrifices cooling capacity and EER; however, once the charge reaches a critical point, both cooling capacity and EER may improve due to the enhanced heat transfer temperature difference.

Design points of capillary tube system

In order to ensure stable operation of the refrigeration system, the following points need to be noted when designing:

• Liquid reservoirs are generally not used on the high pressure side, unless required for specific needs such as heat pump systems or shutdown evacuation systems.
• The suction pipeline should be equipped with a gas-liquid separator to prevent the liquid back to the compressor to cause damage.

• The high pressure section of the system must have sufficient capacity to hold all the refrigerant charge to prevent damage to other components in case of capillary tube blockage.

• Considering the possible rise in condensing pressure under high load conditions, it is necessary to enlarge the condenser area appropriately.

• The line between the condenser outlet and the capillary tube inlet should not hold refrigerant liquid to avoid unnecessary heat loss or delayed system balance.

• A filter is installed at the capillary tube inlet to avoid impurities from entering and to ensure that the refrigerant has a proper degree of subcooling, thus reducing gas flashing and optimizing the cooling effect. 

Methods for troubleshooting capillary tube ice blockage

When encountering a capillary tube “ice blockage” problem, the first step is to confirm the existence of the problem through the methods described above. Once the problem is identified as an ice blockage, the refrigerant in the system needs to be discharged and vacuum dried thoroughly. Also clean key components such as the evaporator and condenser, and consider replacing the filter drier with a new one or cleaning the existing one.

If the problem is found to be due to high refrigerant water content, additional filter driers can be installed at the outlet of the refrigerant cylinder to ensure that the excess water is absorbed during recharging.

Properly designed capillary tube lengths and their associated configurations are essential to ensure efficient and reliable operation of the refrigeration system. At the same time, timely and effective treatment of such common failures as “ice blockage” is also the key to maintaining the normal operation of refrigeration equipment.

Shanghai KUB Refrigeration Equipment Co., Ltd.

Free Call Video Chat