Filter-Driers And HFC Refrigerants
The following overview describes how engineers have met the challenges of working with R-410A.
MOISTURE AND FILTRATIONThe basic job of a liquid-line filter-drier, especially in R-410A systems, is to remove water and solid contaminants from a refrigeration system.
Water can get into systems from various sources. Some of these are leaks, improper cleaning and evacuation, POE lubricants, and windings in electric motors. This water has to be removed to prevent freeze-ups, corrosion, and possible hydrolysis of the ester lubricant. Solid contaminants, like copper oxide, carbon, braze particles, etc., can clog metering devices or cause problems in the compressor. They are removed by filter media such as fiberglass pads, screens, and cores.
New refrigerants like R-410A and lubricants like POEs are excellent solvents. As a result, contaminants that R-22 and mineral oil left in place are now removed by HFC refrigerants such as R-410A and R-407C and the POE lubricants used with them. Some of these common contaminants are residual detergents from cleaning, drawing compounds, rust preventatives, plasticizers, and lubricant additives. Contaminants such as braze material, flux, and moisture can also be introduced during installation. These contaminants can deposit in capillary tubes, restrictor orifices, or TXV seats.
POEs are made by reacting organic acids with alcohol to form the ester chains that make up the lubricant and water. This reaction is reversible, so under the right conditions, the lubricant can react with excess water to reverse the ester reaction and decompose into the original organic acid and alcohol. This was not an issue with the earlier HFC/CFC/mineral oil systems’ chemistry. In addition, POEs are hygroscopic, meaning they can adsorb a large amount of water. Studies have shown that activated alumina in the liquid line can play a role in the hydrolysis of the polyolester.
Refrigerant blends that have R-32 (a component of R-410A) pose another issue for drier manufacturers. R-32 is very close to water in terms of molecular size. This means that some desiccants can adsorb R-32, which lowers the water capacity of the desiccant and causes a premature pressure drop. The more immediate effect is that the composition of the blend can change as R-32 is adsorbed.
A new desiccant (drying agent) had to be employed to adsorb water and exclude R-32. One new desiccant is UOP’s XH11. Specifically, the pore openings are designed to adsorb moisture while excluding the R-32, lubricant, and additives. This ensures maximum water capacity in the drier and allows the water to be physically separated from the POE lubricant to prevent hydrolysis. Conversely, activated alumina has pore openings that will adsorb not only the water but also the R-32, and POE lubricant, and its additives, which lowers its drying capabilities and removes essential additives from the lubricant. It also gives the water and the lubricant a surface for the hydrolysis reaction, since they can coexist in the alumina’s openings. The water is not physically separated from the lubricant.
RESEARCHIn research done by Parker Hannifin Corp., molecular sieves had approximately three to four times higher water capacity than activated alumina and silica gel.
Another study looked at the water capacity for different molecular sieve desiccants in R-410A. Research resulted in high water capacity at low water levels, meaning that filter-driers are now available to dry refrigerants to low levels of water.
In another study, after exposure of the desiccants to the POE and HFC refrigerant in a sealed glass tube for 14 days at 180 degrees F, the activated alumina showed organic acid levels almost 40 times higher than that of molecular sieve. This is an indication of the breakdown of the lubricant. The fluoride levels were low, indicating compatibility with the refrigerant.
CONCLUSIONFiltration capacity in this newest generation of liquid-line driers has been designed to ensure that the drier is able to handle three times the amount of contaminant of typical core and other types of driers.
Valued because of its versatility and environmentally conscious properties, R-410A is increasingly becoming the refrigerant of choice in new residential and light commercial air conditioning equipment. But components with such equipment, including filter-driers, must be designed to work with the refrigerant. Contractors should consult with manufacturers and wholesalers for guidance when working with R-410A.
The material was drawn from Engineering Bulletin RAC-410A from Parker Hannifin Corp., Climate and Industrial Controls Group, Cleveland, OH.
Publication date: 03/04/2002