The water-making capacity of any distilling plant that uses heat to boil seawater is affected primarily by five factors: (1) the number of stages, (2) the temperature of the seawater through which the ship is passing, (3) the rate of flow of seawater feed through the plant (which is the only factor the operator can control), (4) the pressure (vacuum) in each stage, and (5) the temperature of the steam supplied to heat the feedwater. A change in any factor will upset the heat balance of the plant, and cause it to make water at less than rated capacity.

One factor which cannot be controlled is the injection temperature of seawater, which can change drastically as a ship travels from one part of an ocean to another. Therefore, when you are assigned as an operator of a distilling plant, you must be alert to such changes, which will be indicated by temperature and pressure gauges. To this end, you should have a good understanding of the meaning of saturation, which is simply a condition that can occur only for a given heat flow rate and value of temperature or pressure, as shown in table 15-1.

Saturation of water is a condition, which simply means that for any given operating pressure or temperature, the water has absorbed all of the Btus it can without changing state. In other words, the water is saturated with heat. Now, if the water absorbs any more Btus, the saturated water must change state by boiling into a vapor (steam). O-other hand, when steam vapor is at a saturation pressure or temperature, by losing the same number of Btus, the saturated steam must cool and change state back into saturated water. You must understand, however, that these changes cannot occur at any other value of pressure, or temperature, as indicated in table 15-1. To look at it another way, the seawater feed entering the first stage of a distilling plant has been heated to its saturation temperature, but the pressure is high enough to keep the water in its liquid state. Now, when this heated water enters the very low pressure area of the first stage, it flashes into saturated vapor and will continue to do this as long as the temperature of the incoming feedwater, the pressure in the first stage, and the flow rate of vapor being converted to condensate do not change. All that is required to change the vapor in the first stage back into water is to remove heat from the vapor at a fast enough rate, which is done by causing the vapor to flow continuously through a lower temperature area created by the heat exchanger in the upper part of the first stage. Because the seawater feed gives up some of its heat to the first stage, feedwater entering the second stage will have a slightly lower temperature. Therefore, the operating pressure in the second stage must be lower (higher vacuum), because the saturation point of the feedwater has been lowered. In any case, the saturated seawater feed entering the second stage will also flash into vapor (change state to steam). When the saturated vapor is directed to flow over the tubes carrying seawater feed passing through the upper part of the second stage, the vapor gives up its heat and changes state back into liquid water that we call distillate. Flash-type plants are designed to operate with a feed temperature of 170F (76C) at a certain feed rate when the temperature of the seawater is 85F (29C). By using the feed control valve to regulate the rate of feed to the first-stage flash box, you can maintain the temperature of the feed entering the first stage at the required 170F ( 5F). When the seawater injection temperature is lower than 85F, reduce the feed rate so that the 170F feed temperature is maintained. The feed temperature should never be allowed to exceed 175F (79C). A higher operating temperature will greatly increase the amount of scale formation. All other valves in the feed line should be opened wide to prevent their interfering with the proper flow of feed through the plant.

Do not attempt to control the feedwater temperature after it leaves the feedwater heater and enters the first-stage flash box. Allow the temperature to adjust itself to the varying plant conditions. Full capacity will result from proper feed flow, proper vacuums throughout the plant, and proper steam pressure above the orifice for the seawater heater.

Remember, the capacity of the flash-type distilling plant depends on both the quantity of evaporator feedwater entering the first-stage feed box and the difference in temperature between the feedwater entering the first stage and the vapor in succeeding states; but you can change the capacity only by increasing or decreasing the amount of heat added to the seawater by controlling the flow of seawater through the feed-water heater.