Not all of the seawater feed that is fed into an evaporator is changed into distillate. That portion of the feed that does not become distillate is called brine. The concentration of sea salts in this water is called brine density, which has a direct bearing on the quantity and quality of the distillate produced by the evaporator. A low brine density indicates that not enough of the feedwater is being converted to distillate. A low brine density means poor efficiency in the plant, a condition that results in a reduced output capacity.

A high brine density indicates that too much of the feedwater is being converted into distillate. Operation with a high brine density causes excessive scale on heat transfer surfaces. This causes poor quality distillate due to excessive vapor formation. There is possible carryover and reduced efficiency due to the loss of heat energy carried over with the brine.

The ideal value of brine density is slightly less than 1.5/32. The density should never exceed this level. Since the average seawater contains approximately 1 part of dissolved sea salt to 32 parts of water (1/32 by weight), the brine density should be just under 1 1/2 times the density of average seawater, or 1.5/32.

Brine density is measured with a salinometer, which works on the same principle as a hydrometer. The float, or hydrometer, is a hollow metal shell attached to a square stem, weighted with shot at one end. Each side of the stem is graduated in thirty-seconds, from 0/32 to 5/32, and calibrated for four different temperatures: 110, 115, 120, and 125F.

Figure 15-14 shows a salinometer and a brine sampling pot. A brine sample is drained into the pot from a test valve and the temperature is measured. The temperature must be adjusted to correspond with one of the temperature scales on the salinometer to accurately measure the density. The salinometer is then placed in the pot, weighted end down, and allowed to float in the brine. The brine density is read where the brine water level crosses the scale on the side of the graduated stem that corresponds to the brine temperature.

You should check salinometers each month by measuring the density of distilled water. You can

Figure 15-14.-Salinometer and sampling pot.

obtain a sample at the proper temperature by bypassing the distillate cooler, or you can heat a sample of the distillate. The reading of the pure water should always be zero (0/32 at a specified temperature). Follow the instructions and pro-cedures in the EOSS for your plant and ship for adjusting the brine density. A properly operated and maintained distilling plant will give many years of satisfactory service.