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Section III

STEAM CYCLE OF THE PRESSURE CLOSED FEED SYSTEM
 

1. GENERAL DISCUSSION

With the continued increase in the strength and heat resisting qualities of steel, made possible by the development of alloy steels, came a corresponding increase in pressures and temperatures available for use in steam power plants. This increase allowed for greater power and higher efficiency of marine propulsion units. However, these increases carried with them correspondingly higher temperatures in the boilers or steam generating units of these plants. As the temperature in the boiler increases, the presence of any oxygen, entrained or absorbed in the feed water, becomes, in direct proportion to the temperature, the cause of rapid corrosion. Wherever water is exposed to the air, oxygen will be absorbed by the water and, if not removed, will be carried into the boiler. The necessity, then, for keeping the feed water from coming into contact with air caused the development of the closed feed system.

2. TYPES OF CLOSED FEED SYSTEMS

(a) General.-There are several types of closed feed systems in general use. All of these are basically the same except for the method of removing oxygen and air from the condensate, and the degree to which it must be removed. The amount of oxygen which must be removed is proportional to the temperatures employed in the boiler and, since these temperatures are proportional to the steam pressure, the higher the pressure used the more oxygen must be removed.

(b) Semi-enclosed Feed System.-The semi-enclosed feed system is used for the lower steam pressures and temperatures. The major difference between this system and the systems later described is that an open feed tank is utilized. Since the lower temperatures involved do not require the removal of much oxygen, the oxygen absorbed in this open feed tank does not have as harmful an effect on the boiler.

(c) Vacuum Closed Feed System.-The vacuum closed feed system is used where the middle

  range of steam pressures and temperatures are used. The boiler corrosion problem here is not sufficiently great to require the removal of all oxygen and air from the feed water, but the continued absorption of air obtained with the open feed system must be prevented. To prevent this, a completely closed feed tank is used, the top of which is vented to the main condenser to prevent the presence of air in contact with the surface of water in the tank. The presence of this vacuum will also have the effect of aiding in the partial removal of air from the water.

(d) Pressure Closed Feed System.-With a normal operating pressure in the boiler of 615 p.s.i. and the consequently high temperature in the boiler tubes, it is necessary to accomplish the removal of all oxygen and air from the feed water before it is delivered into the boiler. To accomplish this purpose the pressure closed feed system is employed. Here the feed tank is also entirely enclosed. The tank is placed under a pressure of steam and incorporates, also, a device for mixing steam and water to accomplish scrubbing of the water and freeing of all entrained and absorbed air and oxygen from the water. This system is the system employed in the main propulsion plants of all DD445 and DD692 class destroyers.

3. OPERATION OF THE PRESSURE CLOSED FEED SYSTEM

Figure 9 is a diagrammatic representation of the pressure closed feed system. In this system water and steam circulate throughout the entire cycle of operation without ever being exposed to the atmosphere. This cycle, being continuous and completely enclosed, will allow us to follow the flow of water and steam, discuss it at any point, and trace the return to the starting point. For the purpose of this text, it is considered desirable to start with the water as it rests in the main condenser in the form of condensate. In the main condenser, steam has just been converted into water (condensate) and is ready to be prepared for use as feed water. To remove the condensate from

 
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the main condenser the condensate pump is used. This condensate pump discharges the condensate through an air ejector where it serves as cooling water in the condensers of the air ejector. From here it passes into the deaerating feed tank. In this tank the water is so treated that all the entrained and absorbed oxygen and air are removed and the water is heated preparatory to pumping it to the boilers as feed water. In this condition it can now be called feed water. The feed water is removed from the deaerating feed tank by the main feed booster pump and discharged under pressure to the main feed pump, which further raises the pressure, and discharges it through an   economizer where the water is further heated and finally into the boiler. Circulating through the boiler, the water is heated by heat from the furnace and steam is formed. The steam is then directed through a superheater where its temperature is raised to a point as far above its saturated temperature as desired. It is then directed into the turbines where it is expanded and its heat transformed into the mechanical energy of rotation. All the heat possible having been removed from the steam, and it having been expanded to a vacuum of approximately 29 inches, it is exhausted into the main condenser, where it is condensed back into condensate.
Illustration of the steam cycle showing generation, expansion, condensate and feed.
THE STEAM CYCLE
FIG. 9
 
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