The unit discussed in this section is a five-stage plant in which feedwater is flashed to vapor in five evaporator stages at successively lower pressures.

Connections (or passages) that exist between the evaporator stages are the feedwater and its distillate loop seals, which permit the flow of feed-water and distillate from stage to stage while preserving the varying degrees of vacuum in each stage.

Condensers are mounted on top of each stage between the front and rear water boxes. Feedwater flows through the tubes in six passes, entering at the lowest tubes at the front of the condenser, reversing direction at the water boxes three times, and leaving at the top of the tubes in the con-denser.

Each condenser has a pet cock for venting entrained air and noncondensable gases. The evaporator stages become larger in the direction of reduced pressure. The feedwater loop seals which extend from the bottom of evaporator stage one through four are visible as cylinders. An evaporator drain is located in the center of the dished bottom of each loop seal. The flanged brine outlet from the evaporator is at the bottom of the fifth stage.

The distillate loop seal between the distillate collection trough of one stage, and the condensers of the following stages, also protrude below the bottom of the evaporator.

If the salinity of the distillate reaches 0.065 epm per gallon, a warning device indicates the high salinity. The salinity cell shutoff valves per-mit withdrawal and descaling of the salinity cells without securing the unit.

Although each stage condenser produces an equal amount of distillate, the amount flowing from each stage is larger than the preceding. Con-sequently, the loop seal piping grows progressively larger.

The total distillate production of the five stages is withdrawn from the bottom of stage five and pumped into the shell of the distillate cooler, and on to the storage tanks.

The DISTILLATE COOLER is a heat exchanger of the shell-and-tube type, in which the heat of the hot distillate flowing around the tubes is transferred by conduction to the cooler feed-water flowing through the tubes.

Distillate flows into the shell space sur-rounding the tubes through an inlet near the feed-water outlet. The distillate is retained in the cooler long enough to efficiently transfer its heat through the tubes by vertically placed baffles, as it flows from top to bottom of the cooler. Thermometers are mounted on the inlet and outlet piping of the cooler and on the feedwater inlet piping.

As the distillate leaves the cooler, it is pumped to storage tanks, provided the salinity does not exceed 0.065 epm per gallon. (If the salinity exceeds 0.065 epm per gallon, a solenoid trip valve operated by a salinity indicating cell, dumps the distillate to the bilges or waste tank until the salin-ity is again back to or below 0.065 epm per gallon .)

Pet cocks are located on each end of the cooler

to bleed off any accumulation of air or non-condensable

gases.

The FEEDWATER PREHEATER is a gas or liquid heat exchanger of the shell-and-tube type, similar in design to the distillate cooler. The preheater is located in the feedwater line between the condenser of the first evaporator stage and the saltwater heater.

High pressure ships steam, first used by the air ejectors to evacuate the stage evaporators, is piped into the preheater shell. A series of five baf-fles, spaced closely together in the top steam outlet, reduce the velocity of the steam and let the steam condense on the outside of the heat transfer tubes.

Feedwater that has already been partially heated in the tubes of the distillate cooler and the five-stage condensers flows through the tubes of the preheater via the front water box in a single pass and acquires the heat of condensation of the air ejector steam before leaving the preheater at the rear water box outlet.

A salinity cell is set to energize at 0.10 epm. It operates as a shutoff valve in the piping below the condensate outlet to dump high salinity water to the bilge or the drain tank. A 6-inch loop seal in the condensate line ensures that the salinity cell is submerged at all times.

A thermometer is located on the front of the preheater, and a pet cock for venting is located on the water box.

The SALT WATER HEATER is a gas or liquid heat exchanger designed to raise the feed-water temperature prior to its entrance into the flash chamber of the first evaporator stage. The saltwater heater is mounted on the operating end of the evaporator and extends the full width of the unit. Feedwater enters and leaves the heater from the front water box after making four passes through the heater.

Four thermometers are installed on the heater: two to measure the feedwater inlet and outlet temperatures; a third, mounted on the heater shell, to measure the steam temperature sur-rounding the tubes; and a fourth, mounted on top of the heater shell, to measure the temperature of the desuperheating temperature in the steam side.

The steam supply to the saltwater heater flows from the auxiliary exhaust line, through the regulating valve (1-5 psig) and then through an orifice which provides, within limits, a uniform flow of steam. It then flows past the desuperheater nozzle, which reduces the steam temperature in the shell of the heater. Steam pressure is indicated by a pressure gage on the operating panel.

The entering steam is directed along the length of the tubes by impingement baffles. Steam con-denses on the tubes and falls to a condensate well at the bottom of the heater shell. (A drain regulator of the float type controls the level of the condensate in the well. A salinity cell, set to energize at 0.10 epm, controls a shutoff valve located in the ships piping between the drain pump and the regulating valve.) The desuperheater atomizes the heater condensate in the low pressure steam side of the heater.

The function of the saltwater heater is to pro-vide feedwater to the inlet of the first evaporator stage flash chamber. Since the amount of heat from the steam is constant, the feedwater flow through the heater must be adjusted according to the inlet temperature so that the feedwater flow is controlled by a valve on the outlet side of the heater.

The air ejector PRECOOLER is a gas or liquid heat exchanger which cools noncon-densables and condenses steam drawn from the first three evaporator stages and the saltwater heater by a two-stage, vacuum-producing air ejector.

The precooler receives its coolant from the feedwater pumped into the distilling unit. The flow of coolant is through the heat transfer tubes, where it makes six passes, entering and leaving at the front end of the cooler.

Steam and noncondensables are drawn into the cooler at the top near the rear of the cooler. Impingement baffles at the inlet and seven ver-tical baffles, through which the transfer tubes run, direct the flow of hot gases around the tubes for efficient heat transfer.

Condensate collects on the tubes and drops to the bottom of the shell. A salinity cell operates a shutoff valve in the precooler condensate line to dump to the bilge or drain tank when the salin-ity is greater than 0.65 epm.

The outlet for noncondensables is mounted on the top of the shell and flanged to the suction chamber of the first ejector of the two-stage air ejector system. The two air ejectors produce the vacuum in the precooler which causes the flow of steam and noncondensables from the evaporator. A thermometer is mounted on the feedwater inlet of the cooler.

Cooling water from the air ejector precooler flows into the AFTER-CONDENSER, which is the fifth of the heat exchangers mounted on the evaporator. The after-condenser completes the condensation of any air ejector steam not con-densed in the precooler and cools noncondensable gases before venting them to the atmosphere. The after-condenser enables the unit to operate without emission of steam from the evaporator. Cooling water enters and leaves the after-condenser through an inlet pipe in the front and an outlet pipe in the rear of the condenser. Air ejector steam and noncondensable gases enter the shell side through an inlet in the front of the unit. Noncondensable gases are vented through a valve on the rear of the unit. A series of vertical baffles direct the steam around the tubes on which it condenses. Condensate is re-moved through bottom outlets on both ends of the condenser.

A salinity cell set to operate at 0.10 epm con-trols a shutoff valve below the condenser. Three high-pressure steam-operated vacuum-producing AIR EJECTORS are mounted on the precooler side of the evaporator unit. The ejec-tor system consists of a single-stage (booster) air ejector and a two-stage air ejector arrangement in which the steam outlet from one air ejector is flanged to the suction side of the other.

The single-stage ejector uses ships steam to draw vapor and noncondensables from evaporator stages four and five. Gases are drawn from the evaporator through a vapor duct in each distillate collection trough so that a minimum of steam is withdrawn. Pipes from stages four and five lead to a bronze tee flanged to the ejector. The single-stage ejector steam and entrained gases leave the ejector outlet tubing, flow through a check valve, and reenter the evaporator shell through the top of stage three, from which they are piped into the bottom of the stage three con-denser section.

The purpose of this arrangement is to enable the single-stage ejector to produce the high degree of vacuum required in stages four and five. An ejector discharging into a vacuum is able to achieve a higher degree of vacuum than one discharging to atmosphere. A vacuum of 28 inches of mercury is required in stage five.

The two-stage ejector draws noncondensables from the saltwater heater and the first three evaporator stages and, since the noncondensables from stages four and five are directed back into stage three, the two-stage ejector actually handles all noncondensables within the unit.

The suction chamber of the first stage of the second ejector (first two-stage) is flanged to the noncondensables outlet of the precooler through which the gases pass before entrainment in the air ejector steam. The two-stage ejectors use ships steam and produce a vacuum in the precooler slightly greater than in the first evaporator stage. Orifice plates of varying size are flanged into the piping from the evaporator stages and the saltwater heater leading to the air ejectors. These plates prevent the air ejectors from withdrawing any undue amount of steam from the evaporator along with the noncondensables.

The discharge of the first stage of the second ejector is flanged to the suction chamber of the third (second two-stage) ejector. The discharge of the third ejector is flanged to piping, containing a check valve, which runs diagonally across the top of the evaporator shell to the air ejector steam inlet of the preheater shell near the front water box.

The pressure of ships steam piped to the ejec-tors is indicated on the independently mounted pressure gage panel. Line pressure to the air ejec-tors must be maintained at or above 135 pounds per square inch gage (psig), as a lower pressure will cause unstable operation of the ejector and will affect the vacuum in the evaporator.

A DUPLEX STRAINER, located in the ships feedwater inlet piping, removes solid matter from seawater by filtering through one of two per-forated and screened bronze baskets. Basket wells are located in the body or housing of the strainer on either side of the centrally located flanged in-let and outlet.

A lever handle between the wells directs the feedwater into the left- or right-hand well. When one basket becomes clogged, flow is switched to the other and the clogged basket is ready to be removed and cleaned.

An inlet and outlet angle-type RELIEF VALVE is flanged into the feedwater inlet be-tween the feedwater pump and the air ejector precooler. The valve is set to open at 75 psig to prevent pressure buildup from an obstruction in the feedwater lines or accidental operation of the feedwater pump with the feedwater control valve closed.

Two FLOWRATORS are mounted on the unit to measure the amount of feedwater and cooling water pumped into the system. Since the amount of fluid to be measured in both cooling and feedwater lines is large, the flowrators are mounted in bypass piping arrangements, measur-ing a small portion of the actual main stream flow and providing a reading on the graduated scale of the cylinder for the entire flow. Main stream and range orifices are provided for each flowrator.

The flowrators serve as manometers. The pressure drop across the manometer is equal to the pressure drop created by the constriction of the main stream orifice. The range orifice at the inlet of the flowrator constricts the bypass flow so that a maximum main stream flow will register a maximum reading on the flowrator scale. It is, therefore, essential that main stream and range orifices be in good condition and of proper bore diameter, if correct readings are to be obtained on the flowrators. The size of the orifice bore should be checked regularly. When cleaning orifice plates and checking bore diameter (stamped on the plates), be careful not to damage the metering edge (the upstream edge). It must be square and sharp, free of either burrs or rounding so that the corner does not reflect light when viewed with magnification. Piping should also be inspected to see that scale deposits have not decreased the inside diameter.