The range of the pressure maintained in the first-effect tube must be between 16 inches of mer-cury, with clean tubes, to 1 to 2 inches of mer-cury as scale forms. The output of a submerged tube type 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 to operate at full capacity.

A vacuum reduction which results from any factor other than deposits on tube surfaces should be corrected to reduce deposits and greatly pro-long the interval of time between cleanings. The primary factors affecting the first-effect tube nest vacuum are air leakage, low water level 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 nor-mal conditions, there will be no large change of vacuum for any one days operation. Any sud-den 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. Losses of vacuum and capacity may be due to air leaks from the atmosphere into the generating steam line (downstream from the orifice plate), from the first-effect tube nest front header, and 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.