The range of the pressure maintained in the first-effect tube must be between 16 inches of mercury (in.Hg), with clean tubes, to 1 to 2 in.Hg as scale forms. The output of a submerged-tube type of distilling plant is not greatly reduced until the deposits on the tubes have caused the vacuum to drop to about atmospheric pressure. When the first-effect tube nest vacuum is lost entirely, the reduction in output becomes very great. Assuming the reduction in vacuum is due to scale and not to improper operating conditions, the tubes must be cleaned.

Keeping the vacuum in the first-effect tube nest as high as possible reduces scale formation to a minimum, enabling the plant tooperate at full capacity. A vacuum reduction that results from any factor other than deposits on tube surfaces should be corrected to reduce deposits and greatly extend the intervals between cleanings. The primary factors affecting the first-effect tube nest vacuum are air leakage, low water levels in the evaporator shells, improper venting of the evaporator shells, scale or other deposits on the tubes, and improper draining of the evaporator tube nests. Loss of vacuum resulting from deposits on evaporator tubes should be gradual. Under normal conditions, there will be no major loss of vacuum for any one days operation. Any sudden drop in vacuum can be traced to causes other than scale deposits.

The generating steam circuit operates under vacuum and is subject to air leaks. Leaks from the steam side of the first-effect tube nest to the first-effect shell space cause losses of capacity and economy. Loss of vacuum and loss of capacity may be due to air leaks. The air leaks may he from the atmosphere into the generating steam line (downstream from the orifice plate); from the first-effect tube nest front header; or from the first-effect tube nest drain piping. Air leaks in this part of the distilling plant may be less noticeable than air or water leaks elsewhere, because the effect on the plant is similar to the scaling of the tube surfaces.

Proper Water Levels

A reduced first-effect tube nest vacuum can result from low water level in any evaporator shell. On older plants, the water levels are controlled by manually regulating the feed valves. On newer plants, the water levels are automatically controlled by weir-type feed regulators. Inability to feed the first effect is usually due either to scale deposits in the seawater sides of the air ejector condenser and the vapor feed heater or to obstructions in the feed line. Inability to feed the second or third effects is due to air leakage or heavy scale deposits in the feed lines between the effects. It is important that you keep the gauge glass and the gauge glass fittings free from scale and air leaks. Air leaks or scale will result in false water level indication readings. Once the distilling plant is in operation, the feeding must be maintained at a steady rate. A sudden rise of the water levels or too high a water level will cause carryover of small particles of brine within the vapor (priming). Maintain the level of water in the shell at the highest level that can be held and still prevent the carrying over of saltwater particles within the freshwater vapor. If this constant water level is not maintained, scales will form rapidly on the exposed tube surfaces.

The pressure differential between the first and second effects permits the second-effect feed to be discharged into the second-effect shell. A partial or total loss of pressure differential indicates that air leaks have occurred between the first-effect and second-effect shells in the two-effect distilling plants. Large air leaks between the first effect and second effect can be readily detected, because the vacuum gauge for the first effect will read approximately the same as the vacuum gauge for the second effect. Large air leaks of this type will disrupt the operation of the plant and must be located and repaired before the plant will operate properly.

Improper Venting of Evaporator Tube Nests

Improper venting of the evaporator tube nests can cause either an accumulation of air in the tubes or an excessive loss of tube nest steam to the distilling condenser. A loss of tube nest air or steam results in a loss of capacity or a loss of economy. Problems of this type usually result from improper operations, rather than from material failures.