Revisiting the Cap Tube
But in many applications, particularly in domestic refrigerators and window units, the use of capillary tubes continues to dominate.
Because it regulates flow of refrigerant into a system’s evaporator, refreshing our knowledge of capillary tube function and design is probably a good idea.
A capillary tube, typically copper, is similar in construction to other interconnecting tubing in a refrigeration circuit, except that the diameter of copper capillary tubing is small compared to that used for suction, discharge, and liquid lines. A capillary tube ranges from 0.026 to 0.070 inches ID (inside diameter) and from 0.71 to 0.125 inches OD (outside diameter).
RELATION TO THE SYSTEM CHARGETo function properly, the capillary tube in a refrigeration circuit must be associated with a refrigerant charge suitable for the system.
For a given system, too much charge results in liquid refrigerant backing up into the condenser, causing excessively high condensing temperatures and excessive power consumption. Too much charge also may cause flood back of liquid refrigerant to the compressor and danger of compressor damage.
Too little refrigerant in a given system will starve the supply of liquid refrigerant to the evaporator, leading to abnormally low suction pressure. The result will be poor system performance with extended run time, and again, higher than normal power consumption. In extreme cases, too little refrigerant in the system can lead to motor overheating leading to compressor failure.
A LITTLE HISTORYWhen household refrigerators first came on the market, a high-side float regulated the liquid refrigerant supply to the evaporator. It functioned in a manner similar to the float in an automotive carburetor. (Remember the days of automobiles before fuel injection?)
Called a high-side float because it was at the outlet of the high-pressure side (condenser), the float and a needle valve ensured that enough liquid remained in the condenser outlet so that only liquid refrigerant, not uncondensed gas, entered the evaporator.
There were problems with high-side floats such as leaks in the float ball, pivot wear, and erosion of the needle and orifice. These shortcomings led to the development of the capillary that is found in small refrigeration systems today. To control refrigerant moisture level, and to protect the capillary tube from being plugged, a strainer or filter drier is placed at the inlet of the capillary tube.
TROUBLESHOOTING TODAY'S SYSTEMSTo function properly the capillary tube inlet should see only liquid. Sight glasses are not normally provided in cap tube systems, so to determine that a liquid seal is present at the capillary tube inlet, measure the temperature at the outlet of the filter drier and compare that temperature to the temperature of a condenser tube about halfway through the condenser.
The temperature at the capillary tube inlet should be from 2 to 4°F lower than the condenser midpoint temperature. Measurement of the condenser tube temperature too close to the condenser inlet can easily lead to error; the refrigerant may still be superheated at that point. The refrigerant gas entering the condenser left the compressor in a superheated state. Until the superheat lost is rejected in the condenser, no liquid refrigerant is generated.
Once the superheat is lost in the initial run of condenser tubing, the temperature of the gas drops rapidly as it moves through the condenser until the refrigerant begins to liquefy. Between that point and another further downstream where all of the refrigerant has condensed to a liquid, the temperature of the refrigerant remains constant. After the refrigerant is fully liquefied, the temperature will drop as additional heat is removed. This further reduction in refrigerant temperature below its condensing temperature is referred to as subcooling.
It is important to position the filter drier with its outlet well below its inlet to ensure that a liquid seal is maintained.
The most accurate way to determine the level of subcooling at the inlet to the capillary tube is to measure the pressure at the outlet of the condenser, find the saturation temperature for the refrigerant from a refrigerant properties table, and compare that pressure determined temperature to the temperature measured directly at the inlet of the capillary tube. This presumes that you have an accurate pressure gauge.
SOFTWARE HELPThe industry has tube selection software such as that offered on the Website http://compressors.danfoss.com. (At that site, you can look for “Selection Software” in the left-side menu and download the 4MB self-extracting file “dancapv1.exe”4.)
Such a program can provide a starting point for further refinement. Final cap tube selection and refrigerant charge should be based on tests in a controlled ambient with careful control of charge amount.
To use the tool mentioned you would need to choose a refrigerant and estimate the system’s evaporator cooling capacity, the system’s evaporating temperature and its expected condensing temperature.
You can choose to work with Btu/h cooling capacity and degrees Fahrenheit or cooling capacity in Watts and degrees Celsius. Evaluate system performance under normal operating conditions as well as the extreme conditions expected in the application. (In the case of the Danfoss program, authors have presumed that the capillary tube and suction lines are soldered in the manner found in domestic refrigerators.)
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Publication Date: 11/12/2007