Can You Pull Moisture from a Filter Dryer with a Vacuum Pump?

Written by Jim Bergmann

May 10, 2017

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.

Fig. 1: From the Engineering Tool Box

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.


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  1. Jim
    Spot on, as our learning continues. Your chart showing that -12 and 500 microns, in addition to your frequent comments that require an understanding of Thermodynamics and Chemistry relative to the damage that occurs from residual moisture, is so relevant that all the Schools need to use this simple blog post. I remember Alco used to build in a “bypass” into their driers, not allowing a complete restriction in supermarkets. I saw these FROST up. I became a huge fan of Sporlan at that point in my career as when they plug, they plug and refrigeration ceases.

  2. I am a huge fan of filter-dryers, but not so much so that I want to do multiple replacements. I would suggest that the system should be ‘dried out’ more thoroughly.

    Anytime the vacuum gauge stalls you know that you are beginning to boil some liquid. I suggest that the system should be blotted with nitrogen. With a safe nitrogen pressure, you can run the compressor a few seconds and get volatile fluids out of the oil and into circulation. Blow the nitrogen charge and begin the evacuation again. Repeats of this process are less time consuming than changing liquid line dryers, and I think more effective.

    If there is a lot of liquid left in the system, as in condenser or chilled water systems, then meter nitrogen through the system at say, 1200 microns, while applying heat.

    I just don’t like opening a system for anything any more than I have to.

    1. That is good advice, especially if moisture is trapped under the compressor oil. One resulting issue that I have seen happen though is if the system pressure is raised to high with nitrogen, moisture that is vapor can fall out as a liquid and take a considerable time to remove with a vacuum pump.

      1. Good point Jim. I have caused this problem with large capacity vacuum pumps on small systems and actually frozen the liquid in place. Actually, you don’t need a lot of nitrogen pressure so it shouldn’t be a problem under 150#, keeping in mind evaporator test pressures.

        If you keep a heat gun pointed at places where ambient air condenses you can really speed up the process. A really speedy evacuation is possible if you utilize good technique.

        Joe Kokinda is correct in his statement that “an understanding of Thermodynamics and Chemistry relative to the damage that occurs from residual moisture.” I have a lot of stories to tell about copper plating and catastrophic failures due to moisture, air, and nitrogen left in a system by technicians(?).

      2. Jim

        Moisture or entrained refrigerant in compressors has become the bane of the evacuation process. I have a compressor rebuilder that continued to send us compressors for racks that customers wished to be changed out with either wet internals( the boiling out process of compressor bodies) or bad oil. We for years have used a flush provided by Supply Houses to get residual oil out of circuit components in addition to pulling tin cans and changing the oil. It is imperative that we all understand that with POE and refrigerants that glide and fractionate will leave contamination throughout any system. Built up systems have a need for drier changes so they continue to do what they are designed for. Many systems have conditions of operation that exceed their maximum tolerances relative to cycle rates, discharge temperatures, low mass flows and many more that all cause degradation of internal motor windings or lubricity of the oil such that the drier is the most under appreciated system component out there. Especially in AC applications. As your post states and taught by us to our Clients, the driers are there to do work. Neglecting them will lead to trouble. If a system is opened, the drier must be changed.

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