The capillary tube — the simplest of refrigerant controls — is the most popular method for metering refrigerant flow from the condenser to the evaporator in compression-type refrigeration systems. Capillary tubes are used instead of thermal expansion valves.

The length and diameter of the capillary tube is chosen to match the flow capacity of the tube to the pumping capacity of the compressor. These seamless tubes are made of copper and act as a throttling valve to meter the amount of refrigerant entering the evaporator. There is only one set of conditions at which a capillary tube will operate at maximum efficiency.

The inside diameter of capillary tubes is as small as 0.02 in. (50 mm). Dealing with such small diameters can lead to blockage if large particles reach the tube.

If the tube does become blocked, then the evaporator becomes starved. The suction pressure drops, less refrigerating effect takes place, and the compressor is forced to work harder, resulting in increased head pressures with accompanying increases in temperature.

The cause of capillary tube blockage is the accumulation of particles large enough to significantly block refrigerant flow. In refrigeration systems, an insoluble chemical or particles may be introduced during the system assembly process or during the system installation process. Blockages may also form during system operation.

It has been documented by former employees of Spauschus Associates Inc. (SAI) in an ASHRAE paper (“Corrosion of Metals in Contact with New Refrigerant/Lubricants at Various Moisture and Organic Acid Levels,” by Jay E. Field, Ph.D., and David R. Henderson, P.E.) that polyol ester (POE) refrigeration oils in the presence of iron and elevated temperatures can form iron calboxylates, which are insoluble in the POE lubricants. The referenced work was performed in sealed glass tubes prepared per ASHRAE Standard 97 and aged at various temperatures for various time intervals in a laboratory environment.

In an operating refrigeration system in the field, the following steps can result in one possible scenario for the development of capillary tube blockage:

1. Metal-to-metal wear occurs where the POE lubricant is not forming a film at the bearing load, which results in poor boundary lubrication.

2. Iron metal scrapings are formed, possibly due to excessive wrist pin wear and higher than normal temperatures are caused by the friction from metal-to-metal wear.

3. The higher temperatures exceed the thermal stability of the POE.

4. The POE is degraded to the polyalcohol and the organic acid building blocks from which the POE was originally made.

5. The organic acid reacts with the iron scrapings to form iron carboxylates.

6. The iron carboxylates are insoluble in the POE oil.

7. The iron carboxylates leave the compressor, pass through the tubing, and through the condenser where the tubing diameter is very large.

8. The iron carboxylates are too large to pass through the small diameter capillary tubes. They accumulate there, increasing in size with time. This blocks the flow of refrigerant to the evaporator.

Inadequate Boundary Lubrication

The causes of inadequate boundary lubrication leading to excessive metal wear are varied. They could derive from improper compressor engineering, inadequate construction of the compressor at the factory, use of the wrong lubricant, the presence of contaminants (such as processing chemicals or metal slag in the system), or some other reason.

Concerning the use of the wrong lubricant or the presence of processing chemicals which are incompatible with the refrigeration lubricant, tests at SAI have demonstrated that incompatible chemicals will often result in the formation of residues. These residues can plug the tubes.

For many years, CFCs and mineral oils that are both non-polar were in use in the majority of refrigeration systems. At that time, the processing fluids chemically formulated over many years for use in the manufacture of system parts — such as evaporator coils, condenser coils, and compressors — were also non-polar chemicals. They were soluble in non-polar CFC refrigerants and non-polar refrigeration oils. If the processing chemicals were not totally rinsed out of the component before being incorporated as part of the refrigeration system, there likely would be no harm because the non-polar processing oil was soluble in the non-polar refrigeration oil

With the introduction of polyol esters and HFCs, chemical compatibility problems occurred. This is not a criticism of polyol ester refrigeration oils. It is only a fact concerning the difference in chemical polarity between POEs and mineral oils. This difference in polarity may cause chemical insolubility.

New lubricants that were polar in chemical nature had to be substituted for the non-polar mineral oils in order that they could be used with HFC refrigerants. The chemically polar POE is soluble in the new HFC refrigerants, but the non-polar mineral oil is not and, therefore, immiscibility occurs.

Extremes in temperature also affect the solubility of chemicals. The cold temperatures in the evaporator — the lowest temperature component of the refrigeration system — provide an ideal place for incompatible chemicals to precipitate. These precipitates manifest themselves as residues. If these precipitates leave the evaporator and are circulated with the refrigerant, the point of constriction in the system will be at the entrance to the cap tube. Particles will accumulate here and restrict flow.

Processing oils (such as wire- cutting lubricants, thread-cutting lubricants, cleaners in soak baths, etc.) need to be formulated to be chemically polar and soluble with the polar POEs and HFCs. Or, better yet, they should be completely removed from the system by a thorough rinsing prior to the addition of the POE refrigeration oil to the compressor.

The amount of capillary tube blockage can be quantified to determine the degree of blockage of a capillary tube. Dry nitrogen is flowed through a length of new capillary tubing (control) at a known rate. Next the capillary tube removed from the system is measured for nitrogen flow through the tube at the same initial pressure as for the control tube. The lengths of tubing are identical, as are the diameters of the capillary tubes. The degree of blockage is determined simply by dividing the flow rate through the system cap tube by the control tube flow rate. An ASHRAE test method describes the procedure.

Miscibility Testing

To avoid possible capillary tube blockage, miscibility testing can be performed to determine the compatibility of refrigeration oil mixtures, refrigeration oil/processing oil mixtures, or oil/refrigerant mixtures. These tests should be run prior to the processing chemical or lubricant being offered for sale to the refrigeration industry.

In the event of capillary tube blockage, techniques have been developed to aid in the identification of processing chemicals present in the refrigeration oils. These are useful for identifying the possible culprit involved in an immiscibility problem. Most oems are aware of these problems and expend large amounts of time and money testing their products before sending them to market. Good compressor lubrication and chemical compatibility of all chemicals involved will prevent most cap tube blockage.

For more information, contact Spauschus Associates Inc., 300 Corporate Center Court, Eagle’s Landing, Stockbridge, GA 30281; 770-507-8849; or (e-mail).

Publication date: 04/02/2001