The triple evacuation method is a multiple evacuation procedure based on diluting the non-condensibles and water in the system with dry nitrogen. The nitrogen is then vented from the system, carrying with it some o f the contaminants. As the procedure is repeated, the remaining contaminants are reduced until a contaminant-free system is available. This method relies on the moisture-absorbing ability of nitrogen to absorb (blot up) moisture in the system during the time period between each evacuation. When the nitrogen is removed, the moisture absorbed is “swept” out of the system. Field experience has shown that three repeats o f this procedure will usually assure a contaminant-free system, hence the name triple evacuation method. The triple evacuation method is also called the triple sweep method, the dilution method, and the blotter method. Figure SP-3-3 shows the sequence of the triple evacuation process.
As shown in Figure SP-3-4, the triple evacuation process takes about three hours when performed as recommended. This time can vary depending on how deep a vacuum is drawn in each step, how large the system is, and how great a capacity the vacuum pump has. As shown, the system is evacuated to 29.72 inches of mercury (in. Hg) vacuum, or about 5,000 microns during the first two evacuations. The vacuum pump continues to run at this level or lower for at least 15 minutes.
Between the evacuations, the system is pressurized with dry nitrogen to about 10 psig and allowed to sit for about an hour so that the nitrogen can absorb the moisture. Be aware that using a pressure higher than 10 psig only wastes nitrogen. A pressure of 10 psig provides enough nitrogen to adequately absorb moisture in the system. At this time, it is a good practice to install a filterdrier in the system to prevent future moisture problems.
For the third and last evacuation, a deep vacuum o f 29.92 in. Hg or 500 microns is drawn and the system is leak tested in the manner previously described for the deep vacuum method.
One benefit of using the triple evacuation method is that the majority o f moisture is swept out rather quickly in the first two evacuations. The third and deeper evacuation takes care of the liquid water that the shallow evacuations do not remove. Another benefit is that the dry nitrogen used between evacuations absorbs moisture well and is readily available and inexpensive. Also, it can be vented from the system to the atmosphere after each o f the waiting periods.
Using nitrogen to pressurize systems for triple evacuation requires that certain precautions be followed:
• Never use oxygen, compressed air, or flammable gas to pressurize a system! An explosion will result when oil and oxygen are mixed.
• Nitrogen is a high-pressure gas (about 2,000 psig). A t full cylinder pressure, nitrogen can rupture a refrigerant cylinder and/or the refrigeration system.
• To prevent personal injury and to control the system test pressure to the safe test pressure limits established by the system manufacturer, connect the nitrogen to the system using a gauge-equipped accurate pressure regulator on the nitrogen tank and a pressure relief valve in the pressure feed line to the system. The relief valve should be adjusted to open at about 2 psig above the system test pressure, but never more than 150 psig.
