ACHRNEWS

Revisiting the Cap Tube

November 12, 2007

As energy costs and regulatory demands for system efficiency increase, thermostatic expansion valves (TXVs) are replacing capillary tubes, particularly in residential air conditioning and heat pumps.

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 CHARGE

To 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 HISTORY

When 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 SYSTEMS

To 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 HELP

The 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.)



Sidebar: Top Ten Tips

- TIP ONE -
Start with a cap tube length longer than your initial estimate. Shortening is easier than stretching.

- TIP TWO -
When cutting a cap tube, do not use a tubing cutter or pliers. File a groove around the tube OD (outside diameter) and break the tube as you would a coat hanger wire or a piece of laboratory glass tubing, thus avoiding burrs and end restrictions. In production batches, cap tubes should be cut with a fine-tooth cutter.

- TIP THREE -
A change in cap tube suppliers merits a check of flow rate. The surface finish of the tube bore can affect the capillary tube performance.

- TIP FOUR -
Be careful of the length of cap tube inserted into the filter drier and evaporator inlet. Too little insertion length may lead to blockage by brazing material. Too much insertion into a filter drier can lead to blockage of the cap tube by the drier screen or possibly penetration of the drier screen by the cap tube.

- TIP FIVE -
Provide enough heat exchange between cap tube and suction line so as to raise the suction gas temperature at least 10°F above evaporating temperature. This helps reduce liquid refrigerant carryover to the compressor and improves system performance.

- TIP SIX -
Filter drier outlet should be as far below its inlet as possible to ensure a liquid seal at the cap tube inlet.

- TIP SEVEN -
To avoid leaks, do not allow the cap tube to chafe against itself or any other surface.

- TIP EIGHT -
Favor a larger bore size to avoid cap tube blockage.

- TIP NINE -
To avoid cap tube blockage, choose a drier desiccant resistant to vibration.

- TIP TEN -
To avoid contamination before use, seal the ends of cap tubes.

Publication Date: 11/12/2007