The purpose of evacuation is degassing and dehydration, which often leads technicians to think that they can simply pull any moisture from the dryer during the evacuation process without the need to change it—a potentially very costly mistake.
Let me start by saying we are in the service business, and there is nothing wrong with charging and getting paid for good service. Good service includes doing what is right and in the best interest of your customer, and changing filters dryers when a system is open for service is good practice. Changing a dryer could very well be the difference between a system lasting for 5 years and a system lasting for 20. Consider the filter dryer a small investment, no less important or costly than a throwaway filter when you consider the damage not changing it can cost.
Trying to evacuate a dryer to remove moisture is like trying to vacuum dirt from a furnace filter. A good technician would never do it. You will never get that filter clean to a like new condition, and it is a waste of time and money on what is considered a disposable item. Just like a vacuum cleaner would only remove the surface dirt from a furnace filter, evacuation also only removes a small portion of moisture from a filter dryer compared to its true moisture holding capacity. In addition, the evacuation does nothing to remove the solids and solubles that the dryer is holding—a real concern for trouble-free operation.
A dryer is required on systems containing POE oils because evacuation alone will not reduce the parts per million (PPM) of moisture to a level that is adequate for trouble free operation.
Let’s start with this: no matter how long you evacuate a system, and no matter how deep the vacuum, there is still moisture in the system. As can be seen on Fig. 1, at a vacuum of 500 microns the dewpoint is -12°F. This means that if a section of the system dropped below -12°F, moisture that is in the system would start to condense on the interior piping surfaces.
When many technicians think of the pitfalls of moisture, they get fixated on it freezing and creating a system restriction. Formation of ice and blockage of a metering device can and does happen, but in reality what a technician should be concerned with are the chemical reactions that are occurring due to moisture in the system.
In reality, the dryness of the system, according to Emerson Climate systems, should be less than 50 ppm or 3% relative humidity. In air, this means dew point temperatures as low as -60°F. As can be seen again from the chart—even approaching a perfect vacuum—these dew points are not achievable with a field vacuum pump and this is where the dryer plays a critical role in final dehydration.
Desiccants in the dryer trap moisture drying the system. POE oil has an affinity for moisture, 100 times more than that of mineral oil. POE oils do not readily release moisture in a vacuum, and the dryer is the safety net for the system. At moisture levels above 75 ppm, chemical reactions with moisture and oil start to occur. These chemical reactions can cause unwanted chemistries that cannot be undone: hydrolysis of the lubricants, corrosion of metals, copper plating, and a chemical change in the motor insulation of a hermetic compressor. A good filter dryer will hold that moisture and prevent chemical reactions that would occur in the hottest parts of the system.
Beyond the chemical reactions, there is also the “filter” portion of the filter dryer that must be considered. Keeping the system clean and free of foreign contaminants that can damage compressors and/or capillary tubes is the best way to assure long term, trouble-free operation. The contaminants can be solids such a copper burrs from installation, excessive flux, dust & dirt, copper oxide from brazing, or soluble contaminants formed from the chemical reactions, such as acid, water, resins and wax. Once the filter is used to its capacity, many of these contaminants will simply continue to circulate through the system.
The moral of the story is this. When a system is opened for service, the filter dryer should always be changed. In addition, it is a good idea to also install a moisture indicator like the Emerson HMI so you can determine the moisture level. As removal will take several days, at least 48 hours of continuous operation is required to determine if the system is dry. To avoid a return trip, you can easily have the customer visually inspect the indicator (if it is easily accessible) to determine if the dryer will need to be replaced again. Moisture will cause system failure, and it must be removed.
Changing the dryer does not need to be hard or complicated. Many times you simply need to pump down the system without the need for a recovery. Changing a dryer by EPA definition is considered a non-major repair. You can also isolate the condensing unit with the service valves and simply recover refrigerant in the line set and evaporator on systems that are not conducive to pumping down—like microchannel systems. This will significantly reduce the recovery and evacuation times, lowering the cost of repair.
For business owners, an important piece of advice. If you want technicians to change dryers and add moisture indicators, when a system is opened for service, keep them well stocked on your service trucks. It is a lot less likely that corners will be cut, and revenue lost, when the parts needed to make a proper repair are on the service vehicle. Keeping some simple acid test kits will also allow for simple testing and easy identification of systems that are in need of repair before a catastrophic failure occurs.
One last important note, always cut out the dryer with your tubing cutter, do not sweat it out with your torches. Moisture is driven from the desiccant with heat, the same process used during filter dryer manufacturing. Heating the dryer will drive the moisture out of the dryer and back into the system, undoing all the work that the dryer has done. If you are so inclined, a flare dryer can be used during cleanup for easy removal and installation. After installation, the system should be evacuated to 200-500 microns and put back into service. If you can achieve your target vacuum but, after isolation, the system continues to decay to the moisture region (1,500-2,000 microns), you know the system is tight but wet and may require several dryer changes. You might not be able to make any more headway with your vacuum pump! Don’t fret if the moisture indicator does not immediately indicate that the system is dry. Removing moisture takes time, and may only be removable with several filter dryer changes. Again, after dryer replacement, recheck the system after at least 48 hours of operation.
If it is true that a vacuum pump cannot succulently dehydrate a system with POE oil how do mini splits ever survive? Most mini split systems use POE oil, and none of them have any desiccant at all nor to any of the manufacturers recommend adding a dyer after the system has been opened.
If a mini-split is open to long to the atmosphere and a dryer not installed it likely will not survive. The reason that most mini-split manufacturers do not recommend the installation of a dryer is that it is often not installed in the correct location. The two lines headed to the evaporator are a saturated liquid line and a vapor line. The liquid line is between the condenser outlet and the metering device typically located inside the condenser on a mini-split system. If a dryer is installed in that location it will work as intended, and it is typically only installed if the system has been opened for service of the compressor changed. You will have to add a small amount of charge for the dryer, but the dryer manufacturer should be able to tell you what you need. If the system is commissioned properly in the first place and a vacuum properly is drawn, the odds of a system failure is very low, so in most cases, a dryer is never needed and would be impractical to add during installation. But it is 100% correct, moisture cannot be removed from POE with a vacuum pump alone.
The POE is in the condenser and has not been exposed to air. The line set is evacuated properly at the time of install so there is essentially no residual moisture to contend with. The system is sealed, so unless improper practices are done, no moisture should ever enter the system.
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