Filter-driers play a very important role in the operation of air conditioning and refrigeration systems. At the heart of the drier is the desiccant held in its cylindrical metal container. As important as the filter-drier is, many actually do not understand how it works. Here are some details.
The word desiccate means to dry out completely and a desiccant is a material or substance that accomplishes the moisture removal. Moisture in the refrigeration system is detrimental to the operation and life of the system. The filter-drier is an accessory that performs the functions of filtering out particles and removing and holding moisture to prevent it from circulating through the system.
It is extremely important that installation and service technicians prevent foreign materials, air, moisture, brazing flux, and carbon created during brazing from entering or remaining in a system. Good piping practice includes bleeding a small amount of dry nitrogen through the system while brazing. Pipe ends need to be sealed prior to sliding pipe insulation over the piping (this is done to keep the “powder” inside the insulation from getting into the piping). A good 500-micron evacuation should be reached to remove air and moisture before charging with refrigerant. And, the addition of a properly sized filter-drier is important on new systems. Keep in mind, that a lot of manufacturers install a filter-drier inside the condenser. The addition of a second drier could cause a pressure drop that could affect the system pressures. Even more important, anytime a system is opened for service, the filter-drier should be replaced.
The filter-drier is designed to remove any particulates that may circulate in the sealed system as well as collect and hold any moisture that may be present in the system. The use of a filter-drier containing a good desiccant has become even more important with the advent of R-410A systems, which utilizes the highly hygroscopic synthetic polyolester oils.
So, what is a desiccant material? The desiccant material is a porous solid consisting of a structure of pores and tiny chambers capable of collecting and holding moisture through capillary action. Adsorption is the physical trait of capillary action whereby moisture is drawn into small pores much like a sponge or paper towel collects liquid spills.
Capacity refers to the amount of moisture the desiccant in the filter-drier can hold. Capacity is measured in parts per million (ppm). One ppm is one part water per million parts of refrigerant. In practical terms, this would be approximately equal to one drop of water in a 125-pound drum of refrigerant. Desiccant capacities are rated at 75 and 125 degrees F. There has been used a number of different materials in filter-driers. The older desiccant, activated alumina, had a moisture holding capacity of 4 grams of moisture per 100 grams of desiccant. Silica gel had a moisture holding capacity of 3 grams of moisture per 100 grams of desiccant. Modern zeolite molecular sieve desiccants have a capacity of approximately 16 grams of moisture per 100 grams of desiccant.
The capacity of a desiccant is temperature dependent. The colder the desiccant, the more moisture it can hold. Therefore, locating a filter-drier in a cooler location is an advantage. Removing a brazed filter-drier with a torch flame causes moisture to be driven out of the desiccant and into the system. Generally, it is better to cut the filter-drier out with a tubing cutter.
The desiccant works better at removing and holding moisture when it is placed in a refrigerant line where the refrigerant is in the liquid state. The filter-drier is often called a “liquid line filter-drier” for this reason.
Suction Line Filter-Driers—The desiccant is still able to adsorb moisture when applied to the suction line but not quite as effectively. Special suction line filter-driers are made for cleaning up a system after a compressor burnout. A larger shell is used to minimize pressure drop on suction line driers. Suction line filter-driers marked as “HH” driers contain carbon filter material in addition to the desiccant. The carbon and desiccant are capable of capturing and holding acids as well as moisture. Suction line filter-driers used to clean up a system after a burnout should be replaced until the system is known to be clean and no longer tests positive for acids in the system.
A suction line filter-drier with an excessive pressure drop across it should not be left in a system. An excessive pressure drop in the suction line reduces the system operating capacity. Many suction line filter-driers have a pressure tap on the inlet end so the pressure on the inlet of the drier can be compared to the pressure at the suction service valve at the compressor. Still other suction line filter-driers have pressure taps on both the inlet and outlet.
Heat pump filter-driers – it is important to remember that when you are working on a heat pump that the filter-driers used in this application are usually bi-flow filter-driers. Depending where the filter-drier is located in the refrigerant system, since the refrigerant flow can be in either direction, you need to use the proper filter-drier or you could be putting contaminants back into the system.
Troubleshooting a filter-drier — A good “rule of thumb” for checking both suction line and liquid line filter-driers is, if there is a 2 degree or 2 PSIG drop across the filter-drier, it has reached its capacity and needs to be replaced.
Below, is a “QUICK REFERENCE CHART” for selecting filter-driers:
Liquid line filter-driers
Line size __ Tons of cooling CU. IN.
3/8 5 and under 16
1/2 12 and under 30
5/8 15 and under 30
Suction line filter-driers
Line size Tons of cooling CU. IN.
5/8 4 and under 35
3/4 5 and under 45
7/8 10 and under 45
1-1/8 13 and under 50
1-3/8 20 and under 75
Heat Pump bi-flow filter-drier
Line size Tons of cooling CU. IN.
3/8 5 and under 16
It Is Important To Confirm Refrigerant And Line Size For All Driers.
HEAT PUMPS, use the standard suction line filter-driers listed above for single flow direction.
Note !!!! Heat pumps that have discharge line filter-driers / mufflers must use OEM filter-drier only!!!!
Hopefully, all of the above information shows you the importance of filter-driers in a refrigerant system. Part of a good service call or when doing any maintenance on a refrigerant system is to check the filter-drier to make sure it is still effective and keeping the system “clean”.
(some of the information in this post is from and article written by Norm Christopherson – Sr. Trainer for JCI)