Everyone who works with refrigeration systems knows that the quality of the refrigerants is important. Suppliers ensure that new refrigerants are free of contaminants before they ship them, and they carefully clean and check any recovered refrigerants before certifying them for reuse. The oils that are used in a refrigeration system also must be clean and contaminant-free to ensure that the equipment will operate properly. Using clean refrigerants and oils certainly is one requirement for having a clean refrigeration system.
Sometimes, however, the problem is not the quality of what is put into the system, but contaminants that have entered the system through other means. Refrigerants are good cleaners and tend to do a decent job of picking up loose particulate matter (rust, metal filings, dirt, welding smoke and slag, for example) and keeping it in suspension. Because refrigeration systems are closed systems, though, any “dirt” in the system will stay in the system, in constant motion with the refrigerant, unless it is removed.
A standard industry practice is to place strainers in front of solenoid valves and other points to help protect the valves and components from contaminants. Good maintenance practice calls for these strainers to be periodically removed and cleaned - a labor-intensive process under the best of circumstances. Even if this good maintenance practice is followed, these strainers typically can remove particles only down to 200 µm (0.0078"), leaving a large amount of particulate matter to continue circulating through the system. The contaminants can cause solenoid valves to stick open (or closed), float switches to jam, ports to become clogged and even compressor cylinder walls and pistons to be scored. In general, the fine particles can cause excessive wear on every component that comes in contact with the refrigerant. The bottom line is that dirt in a refrigeration system is an ongoing maintenance headache.
The Origins of Dirt in Industrial Piping
How does this dirt get into the refrigeration system in the first place? One source is the components used in the construction of the system. A refrigeration system consists of major equipment such as compressors, condensers, evaporators and vessels as well as minor components such as pipes, valves, controls and fittings. When these items are purchased (particularly the pipes and vessels), the system dirt often is purchased unknowingly along with the product.
Because they usually are stored outdoors and rarely are capped, pipes can be a big contributor to system contamination. Exposure to the elements can create internal rust. Pipes also might contain foreign objects such as mud or even small animals.
Vessels are fabricated and then pressure tested, usually with water, before being shipped. The fabrication process generates weld slag and smoke, and pressure testing with water can leave internal rust or moisture in the vessel.
Evaporator coils also can be a source of contamination. If the coils are tested with water, some moisture might remain on the coils after testing, and this moisture can contaminate the refrigeration system.
Another source of system dirt occurs during the construction of the refrigeration system. For example, in a steel pipe refrigeration system, weld slag and smoke can be introduced into the system through the process of welding the pipes, fittings and valves together. Cutting the pipes to length using chop saws or band saws can create fine metallic dust and particles inside the pipes, as well as components from the cutting blade.
After cutting, the pipe must be beveled for welding (figure 1). The industry standard is to use a power grinder held at an angle to grind the pipe to the proper bevel, but the grinding process also creates metallic chips and dust inside the pipe. Automatic pipe cutters that both cut and bevel the pipe in one operation can be used; however, these devices also tend to leave chips and metallic dust inside the pipe.
A Clean Solution for Industrial Refrigeration Piping
If these basic processes create most of the dirt inherent in a refrigeration system, it would seem that this problem, along with the associated maintenance challenges, is unavoidable. That is not so. Having a clean refrigeration system simply requires starting with a clean system and implementing technology designed to keep it clean.
A clean system can be built by purchasing only clean pipe, vessels, heat exchangers and other components; storing pipe indoors and in pipe racks; using caps and plugs to keep the pipe clean; and prohibiting the use of chop saws, band saws and other cutting tools that generate metallic debris. Pipe can be cut without introducing dirt into the system using an automatic bevel cutter that employs an electric motor to rotate a gas-cutting torch around the pipe. The torch is set at the proper angle not only to cut the pipe but also to bevel it. The set angle tends to blow dust, smoke and slag away from the end of the pipe, making it a clean cutting method (figure 2).
Good welds also are essential to a good refrigeration system. The system must be able to resist high pressures and must be tight enough to prevent refrigerant leaks during operation. In some jurisdictions, certified welding procedures are required. A 6010 root weld process (using an E-6010 electrode) is the industry standard, and it achieves the required weld strength when performed correctly; however, this process can introduce weld slag and smoke into the system. Using a 7018 low hydrogen root welding process (performed with an E-7018 electrode), as well as chill rings (also known as weld backing rings), can produce superior welds and also is much cleaner (figure 3). To further verify clean welds, only pipe welders who have successfully passed a rigid pipe-welding certification examination should be allowed to perform the welding operation, and all welds should be traced back to the welder for quality assurance.
Keeping a refrigeration system clean requires an efficient filtration system. Using fine-gauge filters (capable of capturing particles as small as 5 to 10 µm) instead of strainers on the major sources of all refrigerant liquids will ensure that any contaminants carried along by the refrigerants are captured as they flow through the system. These contaminants can be removed by replacing the filters as needed. Such filters can be used to prevent contaminant buildup in new refrigeration systems or to clean existing, contaminated systems.
Installing a clean refrigeration system requires a little extra effort up front to verify that all of the components and methods meet the required standards, but it can provide a significant payback in reduced maintenance requirements. For example, one large refrigeration facility - with miles of rooftop piping, hundreds of solenoid and control valves, and many large vessels - experienced this benefit firsthand after installing a clean system. The plant opted to use strainers instead of fine-gauge filters in the system design. At other plants owned by this company, cleaning strainers was an ongoing process that started right after the refrigeration plants began operation. Plants of this size typically require about 555 man-hours per year to clean all of the strainers. The facility with the clean refrigeration system has experienced no clogged strainers or sticking solenoid valves, which has saved a significant amount of labor on maintenance. Other plants that have installed clean refrigeration systems have had similar levels of success. Systems with fine-gauge filtration have experienced an overall increase in equipment efficiency and longevity.
When contamination occurs in a refrigeration system, it is easy to blame the refrigerants or oils. However, the dirt might have been in the system from the beginning. By installing a clean system with adequate filtration, plants can optimize their equipment life while minimizing maintenance problems.