The concentration of brine in the evaporators, to a certain extent, has a direct bearing on the quality of the distillate, and since varying quan-tities of brine discharged overboard may affect the operating conditions, the quantity of brine discharged and the brine density must be kept as constant as possible.

If the brine concentration is too low, there will be a loss in capacity and economy. If the brine concentration is too high, there will be an increase in the rate of scaling of the evaporator heating surfaces, and the quality of the distillate will be impaired.

The brine density, which should never exceed 1.5/32, is dependent mainly on the quantity of brine pumped overboard and the amount of freshwater being produced. The density must be checked frequently during each watch and adjusted to the required density. On older distill-ing plants, the brine density is adjusted by means of a hand-controlled valve located in the discharge line of the brine overboard pump. In plants equipped with wire control valves and in basket type plants, the brine density is controlled by ad-justing the first-effect feed valve. Increasing the rate of feed decreases the brine density, and decreasing the rate of feed increases the brine density.

Frequent changes of brine density have a tendency to disrupt steady performance of the plant; therefore, only very small changes should be made. The proper setting for a specific plant should be learned from experience, and this setting should be maintained as practicable.

Use of the Salinometer

The salinometer is an instrument (on the principle of a float) for measuring the degree of salinity or the concentration of the brine. It is a hollow, metal vessel weighted at the bottom, and having a projecting stem which is graduated in four scales to read the salinity for various temperatures of the brine. The graduations are marked in thirty-seconds. When the salinity of a sample of brine is to be measured, the temperature of the sample should be brought to a temperature corresponding to that of one of the scales on the instrument in order that an accurate reading may be obtained. The accuracy of the salinometer should be checked occasionally by placing it in distilled water; if it is accurate, it should sink to the zero mark on the scale corresponding to the temperature of the water.

A pot is provided for holding the sample of brine. The pot must be amply deep so that there is no danger of breaking the bottom of the salinometer when it is placed in the pot. To use the salinometer, proceed as follows:

1. Draw off a sample of brine, from the test cocks on the discharge side of the brine pump, then insert the thermometer into the sample.

2. Allow the sample to cool to the temperature of one of the scale temperatures.

3. Put the salinometer in the pot and read the degree of salinity.

4. Remove the salinometer and wipe off all moisture since accumulations of salt or dirt will result in false readings.