//The Effect of Capillary Tube Length on the Performance of Refrigeration System

The Effect of Capillary Tube Length on the Performance of Refrigeration System

How the capillary tube length effects the performance of the refrigeration system. Capillaries generally refer to slender copper tubes with an internal diameter of 0.4 ~ 2.0 mm. As the throttling mechanism of the water chiller system, the capillary tube is the simplest. It is widely used in small cooling system because of its low price and flexible selection. How to eliminate the phenomenon of “Ice Block” in capillary?

1. Influence on suction, exhaust temperature, and pressure

At the same charge, the shorter the capillary, the larger the refrigerant flow, so the suction temperature, exhaust temperature will fall; similarly, a certain capillary, the larger the charge, the larger the refrigerant flow, suction temperature, the exhaust temperature is also decreased.
But the increase in flow makes the suction pressure rise. For exhaust pressure, the shorter the capillary is, the smaller the charge is, and the larger the charge is, the higher the capillary length is.

 

2. Influence on condensation temperature and pressure

The shorter the capillary, the lower the condensation temperature and pressure. When the capillary length is constant, the higher the charge, the higher the condensation temperature and pressure.

 

3. Influence on subcooling degree and superheat

When the refrigerant filling amount is constant, the longer the capillary, the greater the degree of supercooling and the higher the superheat; When the capillary length is constant, the larger the filling amount, the greater the degree of supercooling and the smaller the superheat.

 

4. Influence on evaporation temperature and pressure

When the refrigerant filling amount is constant, the shorter the capillary, the greater the evaporation temperature and pressure; With a certain capillary length, the higher the filling amount, the higher the evaporation temperature and pressure.

 

5. Influence on cooling capacity, power consumption and performance coefficient EER

When the refrigerant filling amount is fixed, the longer the capillary length, the smaller the power consumption, but the refrigerant also becomes smaller, EER becomes smaller;When the filling amount increased to a certain extent, because the heat transfer temperature difference had a large impact, then the cooling amount became larger, and EER also became larger.

6. Design Essentials of Capillary System

1) on the high-pressure side, the liquid reservoir is generally not used. In fact, whether the liquid reservoir is used does not depend on what kind of throttling device it is, but depends on whether the whole system needs to operate, such as the heat pump system and the shutdown and pumping system. For refrigerators, air conditioners and other original system design rarely need a liquid reservoir.

2) in the suction pipe, it is better to use the gas-liquid separator. For the refrigerator, if the return pipe is longer, the return temperature can be guaranteed to be within the design requirements (the minimum recommended by the compressor company is 50c ). For the air conditioning, most air conditioning compressors will take a gas-liquid separator when leaving the factory.
Because the high and low -pressure side of the capillary system will balance when the machine stops, the evaporator will accumulate refrigerant liquid, and the gas-liquid separator can well prevent liquid shock and refrigerant migration.

3) the high-pressure side can hold all the refrigerant filled, which is to prevent the damage of the high-pressure pipeline system and compressor when the capillary is blocked.

4) under the condition of a high load of the evaporator, since the capillary system can feedback to the condenser side, the condenser should consider whether the condensing pressure is too high under this condition, so the condensing heat transfer area needs to be increased.

5) the pipeline between the condenser outlet and the capillary inlet should not store refrigerant liquid;

One theory is that when the compressor is shut down, this refrigerant liquid will evaporate because of the pressure drop, and then flow into the evaporator for condensation, thus bringing a part of the heat to the refrigerating space. This may have an impact on the closed space of the refrigerator. Another theory is that this will delay the time of high and low voltage side balance, which may cause problems when the low-torque compressor starts up again. This can be generally solved by increasing the delay of starting up in the control (in fact, it is also beneficial to reduce the impact of starting current on other electrical components or the power grid).

6) the capillary inlet must be added with filters to prevent blockage, especially the current use of HFC refrigerants, such as R134A, R404a, R410a is more sensitive to water, in the design requirements must be added with the dryer.

7) before the refrigerant enters the capillary, it is better to have a certain degree of supercooling. This can be achieved by adding a section of supercooling tube in the evaporator or generating heat exchange with the suction tube, so as to minimize the flash of gas in the capillary, thereby increasing the refrigerant and ensuring the refrigerant flow.

However, it should be noted that in the low temperature working condition, the subcooling degree may be too large because the suction pipe has a little return liquid, thus increasing the capillary flow, which in turn makes the supercooling degree is now increased, and may eventually cause return liquid.

 

 

 

How does capillary produce ice to block a phenomenon to should come to eliminate

“Ice blocking” is caused by poor vacuum treatment of the refrigeration system, the excessive water content in the system or excessive water content of the refrigerant itself. Most of the “ice blocking” occurs at the capillary outlet. When the liquid refrigerant evaporates from the capillary to the evaporator, the volume expands greatly and becomes a gas, absorbing a lot of heat. At this point, the evaporation temperature can reach about -5 degrees, the system of trace water with the refrigerant circulation to the capillary outlet will be frozen into ice. As the refrigerant continues to circulate, the volume of the frozen ice increases to the point where the capillary is completely blocked.

1. How to estimate the capillary “ice blocking”

A: when the power is switched on and the compressor starts running, the evaporator frosts and the condenser heats up. With the formation of “ice blocking”, the evaporator frost turns into light, and the compressor runs with a dull sound and no cool air is blown into the room. After the machine is stopped, wrap the capillary tube into the entrance of the evaporator with a hot towel for many times. Since the ice block is melted, the sound of unobstructed refrigerant flow can be heard. After starting the compressor, the evaporator starts frosting again.

B.

1) A three-way inspection valve is installed on the feeding process pipe of the compressor.
2) If the low pressure is maintained at 0PA after running for a period of time, the capillary tube may be in a state of semi-dirty plugging. If it is in the vacuum, it may be completely dirty plugging and should be further examined. At this point, the compressor operation has a dull sound.

3) Stop after the compressor, if the pressure balance is very slow, to 10 minutes or more than half an hour, that capillary dirty plug. Dirty plug location in the general drying filter and capillary junction. If the capillary and drying filter connection cut, the refrigerant spray, this can determine capillary dirty plug.

2. Elimination method of Capillary “Ice Blocking “

After determining the capillary “Ice blocking”, the refrigerant in the water chiller system is released and vacuum drying is carried out again. The evaporators and condensers, the main parts of the cooling system, are cleaned once. When reconnecting the water cooling system, it is advisable to replace it with a new drying filter. If you do not have a new drying filter, you can remove the drying filter, pour out the molecular sieve inside, wash the filter wall with gasoline or tetrachloromethane, and use after drying.

If the refrigerant itself due to excessive water content and the formation of “Ice Block”, the refrigerant cylinder can be added at the exit of a drying filter. This allows the refrigerant to be absorbed as it is filled.

 

By |2019-03-18T12:10:20+00:00March 18th, 2019|Updates|