Wheeler On HVACR
With all the renewed concern about keeping moisture and air out of air conditioning and refrigeration systems, there is very little printed information to show how proper evacuation should be performed. So, here is a brief description that you can pass on to your customers that are interested.
Evacuation is best done with a 4-knob, 4-hose manifold (gage set): Red (high side), Blue (suction side), Yellow (to refrigerant drum), and Oversize metal jacket (to vacuum pump).
With 3-knob manifolds, it's almost impossible to pull a proper vacuum, so I suggest 4-knob manifolds for all servicing.
To perform a vacuum on a system where the refrigerant charge has been lost and the leak repaired, connect the hoses, open the valves to the blue, red, and vacuum hoses, and turn on the vacuum pump. Then attach a digital vacuum gage (which I strongly recommend using) to the end of the yellow line and open the valve occasionally to check the suction. Don't leave the line open continuously, because the fewer the lines connected, the less chance of a leak.
When the desired vacuum is reached, leaving the yellow valve closed, disconnect the vacuum gage and connect the yellow hose to the refrigerant drum (drum valve closed), then open the yellow manifold knob to pull a vacuum all the way to the drum (a few seconds). Then close all the knobs and turn off the vacuum pump.
Next, turn the refrigerant drum upside down on an electronic refrigerant scale (which I strongly recommend), open its valve to discharge liquid refrigerant to the gage, zero the scale, then open the yellow, blue, and red manifold knobs to start charging the system (compressor OFF!). It only takes a minute or so. The system shouldn't be running, because the vacuum will usually pull in the full charge.
Triple EvacuationThe type of vacuum that is required depends on the situation. With a new installation where the condensing unit factory charge is intact, just the lines and evaporator need to be evacuated before opening the condensing-unit valves to release the full charge into the system. What we call a triple, or dilution, evacuation works here. You pull a quick evacuation (a couple of minutes), shut off the vacuum valve and the pump, then open the refrigerant valve and blow a little gas into the system (drum upright, to about 0-psig). Then shut off the refrigerant valve and restart the vacuum. You do this three times - it's very quick. At the end of the third vacuum you close all manifold valves, open the condensing-unit valves (first) to release the factory charge into the system, and then disconnect the gages. The system is ready to run.
In a triple evacuation, each time a vacuum is pulled and refrigerant is introduced, the dry refrigerant tends to dilute any air in the system, and it evaporates any moisture present. So, the vacuum goes much faster.
Deep EvacuationHowever, a triple evacuation just doesn't work and a single deep vacuum (which can take hours) is necessary whenever the compressor oil is contaminated with moisture due to previous poor servicing or where there has been a total charge loss and air has circulated through the running system. Humidity in the oil can only be removed from the oil with a long, deep evacuation and the installation of a liquid line filter drier.
What happens when air gets into a system is that its humidity attacks and enters the oil, causing it to turn milky as the compressor runs, and reducing its lubricating qualities. The moisture then forms on the compressor motor windings and causes them to short out. The most common cause of shorted and locked compressors, and of stopped metering orifices, is moisture-contaminated oil.
The reason why this method takes so long is that you are literally trying to boil the moisture out of the oil with the vacuum. However, a long triple evacuation may be sufficient if an oversized liquid-line filter drier is installed to clean up the oil.
Where you must open the refrigerant side of the system (say, to replace a compressor) and you do this quickly, a triple evacuation will pull a faster evacuation, but you'll need to pull a deeper vacuum all three times, and install a liquid line filter dryer.
The techniques shown above aren't anything new, but over the years, they have been largely forgotten by many non-union service technicians who think evacuation isn't necessary. I know this is true, because I have worked with dozens of them who don't even carry vacuum pumps on their trucks. And the fact that so few have 4-knob manifolds is further mute testimony that this is the true state of affairs in our industry. This situation absolutely must end, because new lubricants are far more affected by moisture than current mineral oils, so we must be more technically competent when installing and servicing systems that aren't chlorine based.