3. Check gauge pressures. The compound gauge should read in a vacuum, showing low-side system pressure. The high-pressure gauge should read above atmospheric pressure, showing high-side system pressure. 4. Momentarily turn the high-pressure gauge manifold hand valve just barely off the closed position. This will allow a small amount of refrigerant vapor into the central chamber of the gauge manifold. 5. While watching the compound gauge, momentarily turn the low-pressure gauge manifold hand valve just barely off the closed position. The compound gauge should rise slightly. 6. When the compound gauge reads 1 psig or 2 psig (6.9 kPa or 13.8 kPa), close the low- pressure gauge manifold hand valve. Another method of atmospheric balancing uses the liquid receiver service valve to throttle vapor into the low side of the system. 1. Check system connections and the compound gauge to ensure that the low side is in a vacuum. 2. With a clear view of the compound gauge, prepare to adjust the liquid receiver outlet valve. 3. Momentarily turn the liquid receiver outlet valve just barely off its seat, while monitoring the compound gauge. 4. When the compound gauge reads 1 psig or 2 psig (6.9 kPa or 13.8 kPa), close the liquid receiver outlet valve. When using the liquid receiver method of atmospheric balancing, remember that a small amount of liquid makes a large amount of vapor. The vapor breaks the vacuum by raising the low- pressure side to 1 psig or 2 psig (6.9 kPa or 13.8 kPa). When the suction side is opened for repairs, a small quantity of vapor will escape to the atmosphere. However, it is because of that refrigerant that air cannot enter the system. More refrigerant cannot escape because both refrigerant and atmospheric air are at the same pressure (0 psig). When a system is properly pumped down and the vacuum eliminated, the low-pressure side can be opened for minor, short-term repairs, such as changing a filter-drier. When repairs are completed, the part of the system that was opened completed. It may be necessary to use a “jumper” to electrically bypass the low-pressure safety control or otherwise force the compressor to continue running for a system pumpdown. The compressor evacuates (removes) all refrigerant from the low-pressure side and stores it in the liquid receiver. Then, the compressor is turned off and the liquid receiver inlet valve is immediately closed (frontseated). The low- pressure side of the system is in a vacuum. The major portion of the system refrigerant charge is now stored in the liquid receiver, but residual high-pressure refrigerant remains in the condenser and discharge line. The suction service valve is also frontseated to prevent backflow of the refrig- erant through the compressor valve reeds. Do not open the low-pressure side when it is in a vacuum, else atmospheric air will enter the system. The air will have to be removed (evacu- ated) before restarting the system. To prevent atmospheric air from entering the low-pressure side, release a small amount of refrigerant from the high-pressure side into the low-pressure side. This is called atmospheric balancing, as only enough refrigerant is allowed into the low side to bring pressure to just above atmospheric pressure, which would be around 1 psig or 2 psig (6.9 kPa or 13.8 kPa). With pressure raised to just above atmospheric, the least amount of refrigerant is used to prevent infiltration of contaminants. The low-pressure side of the system can then be opened for service and repair. Atmospheric balancing may also be known as pressure equalizing or balancing pressures. It can be done in several different ways depending on the system and the arrangement of instruments and equipment. One method of atmospheric balancing involves bypassing high-side vapor through the gauge manifold. See Figure 15‑18. With the gauge mani- fold connected across the compressor service valves, a small amount of vapor can be throttled into the low side of the system. 1. Check hose connections with Figure 15‑18. 2. Check valve positions with Figure 15‑18. Both gauge manifold hand valves should be closed. The suction service valve should be adjusted to prevent refrigerant from the compressor or high side from entering the low side. The discharge service valve should be adjusted to prevent refrigerant from the high side from entering the compressor. Chapter 15 Refrigerant Recovery and Recycling 275 Copyright Goodheart-Willcox Co., Inc.