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 ships, 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 second or third effects is due to air leakage or heavy scale deposits in the feed lines between the effects. It is impor-tant that the gage glass and the gage glass fittings be kept free of scale, otherwise false water level indications will be given. Air leaks around the gage glass will also result in false level indications in the gage glass.

Once the distilling plant is in operation, the feeding must be maintained at a steady rate. Sud-den rising of the water levels or too high a water level will cause carryover of small particles of brine with the vapor (priming). The level of water in the shell must be carried at the highest level that can be held and still prevent the carrying over of saltwater particles with the freshwater vapor, otherwise scale will form rapidly on 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 par-tial or total loss of pressure differential indicates that air leaks have occurred between the first and second-effect shells in the two-effect distilling plants. Large air leaks between the first effect and second effects can be readily detected because the vacuum gage for the first effect will read approx-imately the same as the vacuum gage for the sec-ond 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.