Regarding the design of capillaries
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Dear marine engineers, when working with low-temperature systems, one can choose either a throttle valve or a capillary tube for throttling. Generally, it is possible to calculate a Cv value based on the operating conditions, and this Cv value is then used to determine the size of the valve element. When using a capillary tube, experience is usually relied upon in such cases. I would like to ask whether there is any calculation method that allows one to determine the corresponding Cv value based on the inner diameter and length of the capillary tube
Calculation method for capillary throttling: In refrigeration systems, steel tubes with an inner diameter of around 0.5 to 2 mm and a length of 1 to 4 meters are generally referred to as capillaries. Once the inner diameter and length are determined, the flow rate of the capillary is primarily influenced by the pressure difference between the inlet and outlet sides, that is, between the high and low pressures. It is also affected by factors such as the degree of subcooling of the liquid flowing in, the amount of flash gas present, as well as the degree of bending of the tube and the number of turns it makes. Therefore, when the unit system is fixed, the operating conditions cannot be changed arbitrarily, nor can capillaries of any other specification be used. According to relevant experiments, under the same operating conditions and flow rates, the length of a capillary is approximately proportional to the 4.6th power of its inner diameter; that is, L1/L2 = (d1/d2)4.6. When the ambient temperature rises or too much refrigerant is added, the pressure in the condenser increases, and an increase in capillary flow leads to an increase in the pressure and evaporation temperature in the evaporator. Conversely, when the ambient temperature drops or the amount of refrigerant is insufficient, the condenser pressure decreases. The reduced flow rate in the capillary tube causes the evaporator pressure and evaporation temperature to drop as well, resulting in a decrease in cooling capacity, and in some cases the desired temperature cannot be achieved. Therefore, in refrigeration equipment that uses capillaries, it is necessary to strictly control the amount of refrigerant charged according to the design requirements. For example, for a refrigerator with a capacity of around 200 liters, the amount of R12 required is about 150 grams, with a deviation of no more than 5 grams. The initial filling volume M for a typical system can be approximated by the following formula: M = 20 + 0.6V (grams), where V is the volume inside the evaporation coil in cm3. Typical refrigerators use copper capillaries with an inner diameter of 0.5 mm and a length of 3 m. Typically, air conditioners use copper capillaries with an inner diameter of 1.42 mm and a length of 450 mm. (Adjust as appropriate)