An insufficient flow of circulating water is indicated when the temperature of the water rises more than 20F while passing through the condensing section of the distiller condenser. The last-effect shell pressure is directly dependent upon the distiller condenser vacuum. The vacuum is dependent upon the temperature and quantity of the circulating water and the proper operation of the air ejectors. Too low an overboard discharge temperature of the distiller condenser circulating water is accompanied by efficiency losses in the distilling plant. The overboard discharge temperature should be kept as high as possible, without exceeding the desired 20F temperature rise through the distiller condenser. In addition, limiting the quantity of circulating water tends to prolong the service life of the tubes and tube sheets. When troubles occur which are not caused by improper operating procedures, you should inspect the condenser circulating water system to determine the true cause of the faulty operation. You must carry out preventive maintenance procedures to ensure that the circulating water pump is maintained in good material condition. You should also carry out routine procedures to ensure the proper setting and maintenance of the back-pressure regulating valve. A regulating valve that is not working properly must be disassembled and repaired before its faulty operation interferes with the operation of the distilling plant. You should inspect the condenser circulating water system pipings at regular intervals for cleanliness as well as for scale or foreign matter. The operators of the distilling plant should inspect and clean the strainers according to the PMS.

Improper Drainage

If the distilling plant fails to produce the designed output when the pressure above the orifice is 5 psig and the first-effect tube nest is several inches of mercury, this is an indication of improper drainage of the distiller condenser or of one of the evaporator tube nests subsequent to the first effect. Complete flooding of the flash chamber gauge glass is also a positive indication of improper draining of the condenser. Because the level appears to be in the gauge glass or below is not necessarily an indication of improper drainage. Air leaks at the gauge glass fittings may indicate a false liquid level.

Brine Density

Proper brine density should be maintained at 1.5/32. If the brine concentration is too low, there will he a loss in capacity and economy. If the brine concentration is too high, there will an increase in the rate of scaling of the evaporator heating surfaces.

FLASH-TYPE DISTILLING PLANTS

Many maintenance procedures for flash-type distilling plants are similar to the maintenance procedures required for submerged-tube distilling plants. Both types of plants are subject to air leakage, saltwater leakage, and malfunctioning of pumps and other auxiliary equipment.

HEAT-RECOVERY DISTILLING PLANTS

Heat-recovery distilling plants are single-effect distilling plants with a submerged-tube heat exchanger. This heat exchanger uses heat energy contained in the jacket cooling water circulated through diesel main propulsion engines and ships service diesel generators. This unit requires no steam for air ejectors because feed is used as the motive power to operate eductors for air and brine removal To supplement the heat in the jacket cooling water when engines are running at low rates, the plant has electric heating modules and steam heaters. This ensures that the jacket cooling water will be at the required temperature when it enters the submerged-tube heat exchanger. The jacket water passes through all the heat exchangers (whether energized or not) to the inlet of the submerged-tube bundle. Here the heat is transferred through the tubes to the feed in the boiling compartment. The jacket water then exits the tube bundle and returns to the engine.

The heat-recovery system is fitted with a circulating pump and an expansion tank Most heat-recovery distillers aboard Navy ships have a secondary heat exchanger between the engine jacket cooling water system and the distiller unit. This heat exchanger isolates the engine coolant, with all its chemical additives, from the distiller. Systems not having this secondary heat exchanger get heat directly from the engine coolant to support the distiller. This is called a single-loop system. A single-loop system must be monitored continuously to ensure that no engine coolant leaks through the distiller submerged-tube heat exchanger. For more information on the monitoring requirements, refer to NSTM Chapter 233, Diesel Engines. For cleaning heat-recovery plants, follow the applicable instructions as you would for cleaning the submerged-tube or the flash-type distilling plants. For more detailed information concerning the distilling units the Navy uses, refer to the manufacturers manual and NSTM, Chapter 531, Volumes 1, 2, and 3, Desalination Low-Pressure Distilling Plants.