In areas where moisture accumulation must be corrected, the system should first be cleared of refrigerant and air. The time required will depend upon the size of the system and the amount of moisture present. It is a good engineering practice to circulate heated air through a large dehydrator system for several hours, or as long as the dehydrator drying agent remains effective, before proceeding with the 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 on board tenders and repair ships. After the preliminary dehydration, the remaining moisture is evacuated by means of a two-stage, high-efficiency vacuum pump having a vacuum indicator. (These vacuum pumps are available on board tenders and repair ships.)

The vacuum indicator shown in figure 5-4 consists of an insulated test tube containing a wet-bulb thermometer with its wick immersed in distilled water. The indicator is connected in the vacuum pump suction line. The suction line from the vacuum pump is connected to the refrigeration system. The refrigerant circuit should be closed to the atmosphere and the charging connection opened to the vacuum pump. A two-stage vacuum pump is 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 55F (0.43 in.Hg, absolute), the pumps are placed in SERIES operation (where the discharge from the first step enters the suction of the second step pump). The dehydration process will produce a temperature drop of the vacuum

Figure 5-4.Dehydrator vacuum indicator.

indicator as shown in figure 5-5. Readings will initially reflect ambient temperatures, then show rapidly 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

Figure 5-5.Vacuum indicator readings plotted during dehydration.

decrease in temperature. When the temperature reaches 35F (0.2 in.Hg, absolute), dry air should be admitted through a chemical dehydrator into the system at a point farthest from the pump. Continue operating the pump so the dry air will mix with and dilute any remaining moisture. Secure the opening that feeds the dry air into the system. Continue evacuating the system until the indicator again shows a temperature of 35F. The dehydration process is complete. Close the valves and disconnect the vacuum pump.

Sometimes obtaining a temperature as low as 35F in the vacuum indicator will be impossible. The probable reasons for such a failure and the corrective procedures to take are as follows:

l Excess moisture in the system. The dehydration procedure should be conducted for longer periods.

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

l Leakage of air into the system. The leak must be found and stopped. You must then repeat the procedure required for detecting leaks in the system.

l Inefficient vacuum 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, replace the drying agent in the dehydrator. If a replacement cartridge is not available, reactivate the drying agent and use it until a replacement is available.

You can reactivate the drying agent by removing and heating it for 12 hours at a temperature of 300F to bake out the moisture. Place the drying agent in an oven or circulate a stream of hot air through the cartridge. Both methods are satisfactory for reactivating commonly used dehydrating agents such as activated alumina or silica gel. The specific instructions furnished by the manufacturer should be followed to reactivate special drying agents.

After reactivation, replace the drying agent in the dehydrator shell and seal it as quickly as possible. This prevents absorption of atmospheric moisture. When the drying agent becomes fouled or saturated with lubricating oil, replace it with a fresh charge, or dehydrator cartridge, taken from a sealed container. Remember that the dehydrators permanently installed in refrigeration systems of naval ships are designed to remove only the minute quantities of moisture unavoidably introduced into the system. You must be careful to prevent moisture or moisture-laden air from entering the system.

CLEANING THE SYSTEM

Systems may accumulate dirt and scale as a result of improper techniques used during repair or installation of the system. If such dirt is excessive and a tank-type cleaner is available, connect the cleaner to the compessor suction strainer. When such a cleaner is not available, a hard, wool felt filter about five-sixteenths inch thick should be inserted into the suction strainer screen. Run the plant with an operator in attendance for at least 36 hours or until the system is clean. The length of time required for a clean system depends upon the size and condition of the plant.