The major cause of system failures is moisture (H2O) which is brought in through air leaks. When H2O combines with R- 12 it forms sulfuric acid which will attack the entire system. Good engineering practice dictates that evacuation, dehydration and fixing all air leaks will in the long term prevent an acid attack upon the system. Where moisture accumulation must be corrected, the system should first be cleared of refrigerant and air. The time required for these processes will depend upon the size of the system and the amount of moisture present. It is good engineering practice to circulate heated air through a large dehydrator for several hours, or as long as the dehydrator drying agent remains effective, before proceeding with the

Figure 6-9.Dehydrator vacuum indicator.

evacuation process. If possible, the dehydrated air should be heated to about 240F.

Large dehydrators, suitable for preliminary dehydration of refrigeration systems, are usually available at naval shipyards and aboard tenders and repair ships.

After the preliminary dehydration, the remaining moisture is evacuated by means of a two-stage high-efficiency vacuum pump. (These vacuum pumps are available aboard tenders and repair ships.) A vacuum indicator (figure 6-9) is attached to the two-stage high efficiency pump. It consists of an insulated test tube containing a wet bulb thermometer with its wick immersed in distilled water. This indicator is connected to the vacuum pump suction line, which in turn, is connected to the refrigeration system. The refrigerant circuit should be closed to the atmosphere and the charging connection should be opened to the vacuum pump.

Two-stage pumps are started for operation in PARALLEL so that maximum displacement may be obtained during the initial pump-down stages. When the indicator shows a temperature of about 55 F (0.43 inch Hg, absolute), the pumps are placed in SERIES operation (wherein the discharge from the first step enters the suction of the second step pump). The dehydration process will be reflected in the temperature drop of the vacuum indicator (as shown in figure 6-10). Readings will initially reflect ambient temperatures. Then they will show rapidly

Figure 6-10.Vacuum indicator readings plotted during dehydration.

falling temperatures until the water in the system starts to boil.

When most of the evaporated moisture has been evacuated from the system, the indicator will show a decrease in temperature. As soon as the temperature reaches 35 F (0.2 inch Hg, absolute), admit dry air through a chemical dehydrator into the system at a point farthest from the pump. As the pump continues operating, the dry air will mix with and dilute any remaining moisture. Secure the opening which feeds the dry air to the system. The system must continue evacuating until the indicator again shows a temperature of 35 F. At this time, the dehydration process is complete. Close the valves and disconnect the vacuum pump.

Sometimes it is impossible to obtain a temperature as low as 35 F in the vacuum indicator. The probable reasons for this and the corrective procedures to take, are as follows:

1. Excess moisture is present in the system. Because of the acid being formed, the dehydration procedure should be conducted for longer periods.

2. Absorbed refrigerant is present in the lubricating oil contained in the compressor crankcase. Remove the lubricating oil from the crankcase before proceeding with the dehydration process.

3. Air is leaking into the system. The leak must be found and stopped. It will be necessary to repeat the procedure required for detecting leaks in the system.

4. Inefficient vacuum pump or defective vacuum indicator. The defective unit(s) should be repaired or replaced.

Immediately after each period of use, or after the system has been opened for repairs, the drying agent in the dehydrator should be replaced. If a replacement cartridge is not available, the drying agent can be reactivated and used until a replacement is available.

Reactivation is accomplished by removing the drying agent and heating it, for 12 hours, at a temperature of 300F to bake out the moisture. The drying agent may be placed in an oven, or a stream of hot air may be circulated through the cartridge. These methods are satisfactory for reactivating commonly used dehydrating agents such as activated alumina and silica gel. However, when special drying agents are employed they should be reactivated in accordance with the specific instructions furnished by the manufacturer.

After reactivation, the drying agent should be placed back in the dehydrator shell and sealed as quickly as possible, in order to prevent absorption of atmospheric moisture. When the drying agent becomes fouled or saturated with lubricating oil, it must be replaced with a fresh charge, or with a dehydrator cartridge, taken from a sealed container.

Remember that the dehydrators that are permanently installed in refrigeration systems of naval ships are designed to remove only the minute quantities of moisture unavoidably introduced in the system. Extreme care must be taken to prevent moisture, or moisture-laden air, from entering the system.