To understand steam generation, you must know what happens to the steam after it leaves the boiler. A good way to learn the steam plant on your ship is to trace the path of steam and water throughout its entire cycle of operation. In each cycle, the water and the steam flow through the entire system without ever being exposed to the atmosphere. The four areas of operation in a main steam system are generation, expansion, condensation, and feed. After studying this chapter, you will have the knowledge and ablity to describe the main steam cycle and the functions of the auxiliary steam systems.

MAIN STEAM SYSTEM

The movement of a ship through the water is the result of a number of energy transformations. Although these transformations were mentioned in the last chapter, we will now discuss these transformations as they occur. Figure 3-1 shows the four major areas of operation in the basic steam cycle and the major energy transformations that take place. These areas are A-generation, B-expansion, C-condensation, and D-feed.

GENERATION-The first energy transformation occurs in the boiler furnace when fuel oil burns. By the process of combustion, the chemical energy stored in the fuel oil is transformed into thermal energy. Thermal energy flows from the burning fuel to the water and generates steam. The thermal energy is now stored as internal energy in steam, as we can tell from the increased pressure and temperature of the steam.

EXPANSION-When steam enters the turbines and expands, the thermal energy of the steam converts to mechanical energy, which turns the shaft and drives the ship.

For the remainder of the cycle, energy is returned to the water (CONDENSATION and FEED) and back to the boiler where it is again heated and changed into steam. The energy used for this purpose is the thermal energy of the auxiliary steam.

The following paragraphs will explain the four major areas of operation in the basic steam cycle shown in Figure 3-1

GENERATION

When a liquid boils, it generates a vapor. Some or all of the liquid changes its physical state from liquid to gas (or vapor). As long as the vapor is in contact with the liquid from which it is being generated, it remains at the same temperature as the boiling liquid. In this condition, the liquid and its vapors are in _equilibrium contact with each other. Area A of Figure 3-1 shows the GENERATION area of the basic steam cycle.

The temperature at which a boiling liquid and its vapors may exist in equilibrium contact depends on the pressure under which the process takes place. As the pressure increases, the boiling temperature increases. As the pressure decreases, the boiling temperature decreases. Determining the boiling point depends on the pressure.

When a liquid is boiling and generating vapor, the liquid is a SATURATED LIQUID and the vapor is a SATURATED VAPOR. The temperature at which a liquid boils under a given pressure is the SATURATION TEMPERATURE, and the corresponding pressure is the SATURATION PRESSURE. Each pressure has a corresponding saturation temperature, and each temperature has a corresponding saturation

pressure. A few saturation pressures and temperatures for water are as follows:

Pounds Per Square Inch Degrees

Absolute (psia) Fahrenheit (F)

We know that atmospheric pressure is 14.7 psia at sea level and lesser at higher altitudes. Boiling water on top of a mountain takes a lot longer than at sea level. Why is this? As noted before, temperature and pressure are indications of internal energy. Since we cannot raise the temperature of boiling water above the saturation temperature for that pressure, the internal energy available for boiling water is less at higher altitudes than at sea level. By the same lines of reasoning, you should be able to figure out why water boils faster in a pressure cooker than in an open kettle.

A peculiar thing happens to water and steam at an absolute pressure of 3206.2 psia and the corresponding saturation temperature at 705.40F. At this point, the CRITICAL POINT, the vapor and liquid are indistinguishable. No change of state occurs when pressure increases above this point or when heat is added. At the critical point, we no longer refer to water or steam. At this point we cannot tell the waterer steam apart. Instead, we call the substance a fluid or a working substance. Boilers designed to operate at pressures and temperatures above the critical point are SUPERCRITICAL boilers. Supercritical boilers are not used, at present, in propulsion plants of naval ships; however, some boilers of this type are used in stationary steam power plants.

If we generate steam by boiling water in an open pan at atmospheric pressure, the water and steam that is in immediate contact with the water will remain at 212F until all the water evaporates. If we fit an absolutely tight cover to the pan so no steam can escape while we continue to add heat, both the pressure and temperature inside the vessel will rise. The steam and water will both increase in temperature and pressure, and each fluid will be at the same temperature and pressure as the other.

In operation, a boiler is neither an open vessel nor a closed vessel. It is a vessel designed with restricted openings allowing steam to escape at a uniform rate while feedwater is brought in at a uniform rate. Steam generation takes place in the boiler at constant pressure and constant temperature, less fluctuations. Fluctuations in constant pressure and constant temperature are caused by changes in steam demands.

We cannot raise the temperature of the steam in the steam drum above the temperature of the water from which it is being generated until the steam is removed from contact with the water inside the steam drum and then heated. Steam that has been heated above its saturation temperature at a given pressure is SUPERHEATED STEAM. The vessel in which the saturated steam is superheated is a SUPERHEATER.

The amount by which the temperature of superheated steam exceeds the temperature of saturated steam at the same pressure is the DEGREE OF SUPERHEAT. For example, if saturated steam at 620 psia with a corresponding saturation temperature of 490F is superheated to 790F, the degree of superheat is 300F (790 - 490 = 300).

Most naval propulsion boilers have superheaters. The primary advantage is that superheating steam provides a greater temperature differential between the boiler and the condenser. This allows more heat to be converted to work at the turbines. We will discuss propulsion boilers and component parts more extensively in the next chapter. Another advantage is that superheated steam is dry and therefore causes relatively little corrosion or erosion of machinery and piping. Also, superheated steam does not conduct or lose heat as rapidly as saturated steam. The increased efficiency which results from the use of superheated steam reduces the fuel oil required to generate each pound of steam. It also reduces the space and weight requirements for the boilers.

Most auxiliary machinery operates on saturated steam. Reciprocating machinery, in particular, requires saturated steam to lubricate internal moving parts of the steam end. Naval boilers, therefore, produce both saturated steam and superheated steam.