MOIR compact liquid chillers can be used to achieve micro refrigeration tasks. For example, industrial lasers, medical devices, race driver body cooling, and other small liquid cooled systems. We have focused on micro refrigeration for decades and offer a full lineup of products, including mini dc compressors, miniature liquid chiller modules, dc condensing units, and other refrigeration units.
As the throttling mechanism of liquid chillers, the capillary is the simplest one. Because of its low price and flexible selection, it is widely used in small refrigeration devices. Recently, it is also used in large refrigeration capacity units.
What’s Capillary?
Basically, the miniature chiller consists of a mini compressor, condenser, throttling device, and evaporator.
Capillary is one of the most commonly used throttling devices in refrigeration and air conditioning systems. A capillary is a piece of copper tube with a small inner diameter. It’s long and coiled several times, so it takes up less space. Internal diameters of capillaries for refrigeration and air conditioning applications range from 0.5 to 2.28 mm (0.020 to 0.09 in). Capillaries are used as throttling devices in domestic chillers, deep coolers, water coolers, and air conditioners.
How does Capillary Work?
The capillary works by creating a pressure difference between the high-pressure and low-pressure sides of the compressor. The refrigerant is compressed by the compressor, which increases its temperature and pressure. The high-pressure refrigerant is then directed to the capillary tube, located in the chiller’s evaporator section. As the refrigerant passes through the narrow diameter of the capillary tube, the pressure drops, causing a decrease in temperature.
As the refrigerant leaves the condenser and enters the capillary, its pressure drops suddenly because of its small diameter. In a capillary, the refrigerant pressure drops not because of the orifice but because of the small opening in the capillary.
The decrease in refrigerant pressure through the capillary depends on the diameter and length of the capillary. The smaller the diameter and longer the length of the capillary, the greater the pressure drop as the refrigerant passes through it.
Under normal operating conditions of refrigeration equipment, the refrigerant pressure on both sides of the capillary decreases, but when the equipment is stopped, the refrigerant pressure on both sides of the capillary is balanced. For this reason, when the compressor is restarted, there is not much load. Also, for this reason, one could not overfill the refrigeration system with refrigerants, and receivers were not used.
Capillaries are non-adjustable devices, which means that the flow of refrigerant through them cannot be controlled as in automatic throttles. Thus, the flow of refrigerant through the capillary varies as the surrounding conditions change. For example, the flow of refrigerant through the capillary changes due to increased pressure in the condenser due to high atmospheric pressure and reduced pressure in the evaporator due to a small refrigeration load. Therefore, capillaries are designed for certain environmental conditions. However, it can work well in a wide range of conditions if chosen properly.
The length of a particular diameter capillary required for refrigeration applications cannot be found by fixed formulas, but by empirical calculations. Find some approximate lengths required for some applications and then correct them by experiment.
Capillary Advantages
Here are some advantages of using capillaries as throttling devices in refrigeration and air conditioning systems:
- Capillaries are very simple devices that are easy to make and inexpensive.
- Because no receiver is required in these systems, capillaries limit the maximum amount of refrigerant that can be charged into the refrigeration system.
- When the miniature liquid chiller is stopped, the pressure at both ends of the capillary becomes the same and remains constant throughout the refrigeration cycle. This means that when the equipment is stopped, the pressure on the suction and discharge sides of the compressor is the same. Therefore, when the compressor restarts, it does not have much load because it does not have to overcome very high pressure. Therefore, a compressor motor with less torque can be selected to drive the compressor and reduce the cost of the compressor. This, together with the above two advantages, helps to reduce the overall cost of refrigeration and air conditioning systems.
Capillary Size Calculation Method
After the inner diameter and length have been determined, the flow rate of the capillary is mainly affected by the pressure difference between the inlet and outlet sides, namely the high and low-pressure ends, and is also related to the degree of refrigerant liquid undercooling, the amount of flashing gas contained, the degree of tube bending and the number of coils. Therefore, when the unit system is fixed, the working condition can not be changed arbitrarily or the capillary of any specification can be replaced.
According to relevant experiments, under the same working condition and flow condition, the length of the capillary is approximately proportional to the 4.6 power of its inner diameter, namely:
F1/F2= (D1/D2) ^ 4.6
L1/L2 =( D1/D2) ^ 4.6
Nowadays, ID: 0.4-2.5mm is mainly used for the capillary diameter. In some large systems, the ID6 copper pipe will also be used as the distribution pipe. Although the pressure drop of refrigerant in the pipeline should be considered, the throttling of such systems is mainly on the expansion valve, and the distribution pipe can only be considered as the pressure loss.
When the ambient temperature rises or the refrigerant is charged too much, the condenser pressure becomes higher, and the capillary flow increases, resulting in the evaporator pressure and evaporation temperature rising. On the contrary, when the ambient temperature decreases or the amount of refrigerant is insufficient, the pressure of the condenser becomes lower, and the decrease of the capillary flow will reduce the pressure of the evaporator and the evaporation temperature, leading to the decrease of the refrigeration capacity, or even less than the required temperature. Therefore, the use of capillary refrigeration equipment must be strictly controlled the refrigerant refilled volume according to the design requirements.
Conclusion
In summary, the capillary is an essential component of a mini liquid chiller that is used to cool liquids in various applications. It regulates the flow of the refrigerant from the high-pressure side of the compressor to the low-pressure side, causing a pressure drop that results in a decrease in temperature. The capillary is designed to have a specific length and diameter that is matched the cooling requirements of the liquid in the chiller. It is a simple and reliable component that is widely used in small liquid chillers.
