The machinery and equipment in the engineering plant aboard ship are designed either to carry energy from one place to another or to change a substance from one form to another. The principles of energy transformations and some of the important energy changes that occur in the shipboard propulsion cycle are discussed in the following paragraphs.

Conservation of Energy

The basic principle dealing with the transformation of energy is the PRINCIPLE OF THE CONSERVATION OF ENERGY. This principle can be stated in several ways. Most commonly, perhaps, it is stated that energy can be neither destroyed nor created, but only transformed. Another way to state this principle is that the total quantity of energy in the universe is always the same. Still another way of expressing this principle is by the equation, Energy in = Energy out,

The energy out may be quite different in form from the energy in, but the total amount of energy input must always equal the total amount of energy output.

Another principle, the PRINCIPLE OF THE CONSERVATION OF MATTER, states that matter can be neither created nor destroyed, but only transformed. As you probably know, the development of the atom bomb demonstrated that matter can be converted into energy; other developments have demonstrated that energy can be converted into matter. Therefore, the principle of the conservation of energy and the principle of the conservation of matter are no longer considered as two parts of a single law or principle but are combined into one principle. That principle states that matter and energy are interchangeable, and the total amount of energy and matter in the universe is constant.

The interchangeability of matter and energy is mentioned here only to point out that the statement energy in must equal energy out is not strictly true for certain situations. However, any noticeable conversion of matter into energy or energy into matter can occur only under very special conditions, which we need not consider now. All the energy transformations that we will deal with can be understood quite simply if we consider only the principle of the conservation of energy-that is, ENERGY IN EQUALS ENERGY OUT.

Transformation of Heat to Work (Laws of Gases)

The energy transformation from heat to work is the major interest in the shipboard engineering plant. To see how this transformation occurs, we need to consider the pressure, temperature, and volume relationships that hold true for gases. Robert Boyle, an English scientist, was among the first to study the compressibility of gases. In the middle of the 17th century, he called it the "springiness" of air. He discovered that when the temperature of an enclosed sample of gas was kept constant and the pressure doubled, the volume was reduced to half the former value. As the applied pressure was decreased, the resulting volume increased. From these observations he concluded that for a constant temperature, the product of the volume and pressure of an enclosed gas remains constant. This conclusion became Boyle's law.

You can demonstrate Boyle's law by confining a quantity of gas in a cylinder that has a tightly fitted piston. Apply force to the piston to compress the gas in the cylinder to some specific volume. If you double the force applied to the

Figure 2-5.-Compressibility of gas.

piston, the gas will compress to one half its original volume fig 29.   

Changes in the pressure of a gas also affect the density. As the pressure increases, its volume decreases; however, no change occurs in the weight of the gas. Therefore, the weight per unit volume (density) increases. So, the density of a gas varies directly as the pressure if the temperature is constant.

In 1787, Jacques Charles, a Frenchman, proved that all gases expand the same amount when heated 1 degree if the pressure is kept constant. The relationships that these two men discovered are summarized as follows:

Boyle's law-when the temperature is held constant, an increase in the pressure on a gas causes a proportional decrease in volume. A decrease in the pressure causes a proportional increase in volume, as shown in figure 2-6 At sea level, the balloon has a given volume with respect to temperature and atmospheric pressure. As the balloon descends 1 mile below sea level, the volume of the balloon decreases due to increased atmospheric pressure. Conversely, as the balloon ascends to 1 mile above sea level, the balloon expands as the atmospheric pressure decreases.

Charles's law-when the pressure is held constant, an increase in the temperature of a gas causes a proportional increase in volume. A decrease in the temperature causes a proportional decrease in volume, as shown in figure 2-7. Balloons A and B have an outside pressure of 10 pounds per square inch (psi). Both have the same volume of air. Balloon A is at 40F and balloon B is at 100F. This shows that increased temperature causes the balloon size to increase.

Figure 2-6.-Pressure differential in respect to sea level.

Figure 2-7.-Pressure differential in respect to temperature.

Charles's law is also stated-when the volume is held constant, an increase in the temperature of a gas causes a proportional increase in pressure. A decrease in the temperature causes a proportional decrease in pressure, as shown in figure 2-8 Tanks A and B are of the same size and have an equal volume of gas. Tank A has a pressure of 10 psi when heated to 40F. Tank B has a pressure of 12 psi when heated to 100F. Unlike the balloons, the steel tanks do not expand to accommodate the changes in temperature and pressure. This shows that changes in temperature are inversely proportional to changes in gas pressure when the volume is held constant.

Figure 2-8.-Interaction of gases in respect to temperature and pressure.

Suppose we have a boiler in which steam has been formed. With the steam stop valves still closed, the volume of the steam remains constant while the pressure and the temperature are both increasing. When operating pressure is reached and the steam stop valves are opened, the high pressure of the steam causes the steam to flow to the turbines. The pressure of the steam thus provides the potential for doing work. The actual conversion of thermal energy to work is done in the turbine section.

Steam

Steam is water to which enough heat has been added to convert it from the liquid to the gaseous state. When heat is added to water in an open container, steam forms. However, it quickly mixes with air and cools back to water that is dispersed in the air, making the air more humid. If you add the heat to water in a closed container, the steam builds up pressure. If you add exactly enough heat to convert all the water to steam at the temperature of boiling water, you get saturated steam. SATURATED STEAM is steam saturated with all the heat it can hold at the boiling temperature of water.

The boiling temperature of water becomes higher as the pressure over the water becomes higher. Steam hotter than the boiling temperature of water is called SUPERHEATED STEAM. When steam has 250 F of superheat, the actual temperature is the boiling temperature plus 250 F. At 600 psi the boiling temperature of water is 489 F. So if steam at 600 psi has 250F of superheat, its actual temperature is 739F. WET STEAM is steam at the boiling temperature that still contains some water particles. DESUPERHEATED STEAM is steam that has been cooled by being passed through a pipe extending through the steam drum. In the process, the steam loses all but 20F to 30F of its superheat. The advantage of desuperheated steam is that it is certain to be dry, yet not so hot as to require special alloy steels for the construction of the piping that carries the desuperheated steam about the ship.

Steam use will be discussed later in chapters 3 and 4 of this textbook. We will describe the steam cycle and typical boilers used on naval ships.

Combustion

Combustion refers to the rapid chemical union of oxygen with fuel. Perfect combustion of fuel would result in carbon dioxide, nitrogen, water vapor, and sulphur dioxide. The oxygen required to burn the fuel is obtained from the air. Air is a mechanical mixture containing by weight 21 percent oxygen, 78 percent nitrogen, and 1 percent other gases. Only oxygen is used in combustion. Nitrogen is an inert gas that has no chemical effect upon combustion.

The chemical combination obtained during combustion results in the liberation of heat energy. A portion of this energy is used to propel the ship. Actually, what happens is a rearrangement of the atoms of the chemical elements into new combinations of molecules. In other words, when the fuel oil temperature (in the presence of oxygen) is increased to the ignition point, a chemical reaction occurs. The fuel begins to separate and unite with specific amounts of oxygen to form an entirely new substance. Heat energy is given off in the process. A good fuel burns quickly and produces a large amount of heat.

Perfect combustion is the objective. However, this has been impossible to achieve as yet in either a boiler or the cylinders of an internal-combustion engine. Theoretically, it is simple. It consists of bringing each particle of the fuel (heated to its ignition temperature) into contact with the correct amount of oxygen. The following factors are involved:

Sufficient oxygen must be supplied.

The oxygen and fuel particles must be thoroughly mixed.

Temperatures must be high enough to maintain combustion.

Enough time must be allowed to permit completion of the process.

Complete combustion can be achieved. This is accomplished by more oxygen being supplied to the process than would be required if perfect combustion were possible. The result is that some of the excess oxygen appears in the combustion gases.

Units of Heat Measurement

Both internal energy and heat is measured using the British thermal unit (Btu). For most practical engineering purposes, 1 Btu is the thermal energy required to raise the temperature of 1 pound of pure water to 1F. Burning a wooden kitchen match completely will produce about 1 Btu.

When large amounts of thermal energy are involved, it is usually more convenient to use multiples of the Btu. For example, 1 kBtu is equal to 1000 Btu, and 1 MBtu is equal to 1 million Btu. Another unit in which thermal energy maybe measured is the calorie. The calorie is the amount of heat required to raise the temperature of 1 gram of pure water 1C. One Btu equals 252 calories.

Sensible Heat and Latent Heat

Sensible heat and latent heat are terms often used to indicate the effect that the flow of heat has on a substance. The flow of heat from one substance to another is normally reflected in a temperature change in each substance-the hotter substance becomes cooler, the cooler substance becomes hotter. However, the flow of heat is not reflected in a temperature change in a substance that is in the process of changing from one physical state (solid, liquid, or gas) to another. When the flow of heat is reflected in a temperature change, we say that sensible heat has been added to or removed from the substance (heat that can be sensed or felt). When the flow of heat is not reflected in a temperature change, but is reflected in the changing physical state of a substance, we say that latent heat has been added or removed.

Does anything bother you in this last paragraph? It should. Here we are talking about sensible heat and latent heat as though we had two different types of heat to consider. This is common (if inaccurate) engineering language. So keep the following points clearly in mind: (1) heat is the movement (flow) of thermal energy; (2) when we talk about adding and removing heat, we really mean that we are providing temperature differentials so thermal energy can flow from one substance to another; and (3) when we talk about

Figure 2-9.-Relationship between sensible heat and latent heat.

sensible heat and latent heat, we are talking about two different kinds of effects that can be produced by heat, but not about two different types of heat.

As previously discussed, the three basic physical states of all matter are solid, liquid, and gas (or vapor). The physical state of a substance is closely related to the distance between molecules. As a general rule, the molecules are closest together in solids, farther apart in liquids, and farthest apart in gases. When heat flow to a substance is not reflected in a temperature increase in that substance, the energy is being used to increase the distance between the molecules of the substance and to change it from a solid to a liquid or from a liquid to a gas. You might say that latent heat is the energy price that must be paid for a change of state from solid to liquid or from liquid to gas. The energy is not lost. It is stored in the substance as internal energy.

The energy price is repaid, so to speak, when the substance changes back from gas to liquid or from liquid to solid, since heat flows from the substance during these changes of state.

Figure 2-9 shows the relationship between sensible heat and latent heat for water at atmospheric pressure. The same kind of chart could be drawn for other substances; however, different amounts of thermal energy would be involved in the changes of state for each substance.

If we start with 1 pound of ice at 0F, we must add 16 Btu to raise the temperature of the ice to 32F. We call this adding sensible heat. To change the pound of ice at 32F to a pound of water at 32F, we must add 144 Btu (the LATENT HEAT OF FUSION). No change in temperature will occur while the ice is melting. After all the ice has melted, however, the temperature of the water will be raised as additional heat is supplied. If we add 180 Btu-that is, 1 Btu for each degree of temperature between 32F and 212F-the temperature of the water will be raised to the boiling point. To change the pound of water at 212F to a pound of steam at 212F, we must add 970 Btu (the LATENT HEAT OF VAPORIZATION). After all the water has been converted to steam, the addition of more heat will cause an increase in the temperature of the steam. If we add about 44 Btu to the pound of steam that is at 212F, we can super heat it to 300F.

The same relationships apply when heat is being removed. The removal of 44 Btu from the pound of steam that is at 300F will cause the temperature to drop to 212F. As the pound of steam at 212F changes to a pound of water at 212F, 970 Btu are given off. When a substance is changing from a gas or vapor to a liquid, the heat that is given off is LATENT HEAT OF CONDENSATION. Notice, however, that the latent heat of condensation is exactly the same as the latent heat of vaporization. The removal of another 180 Btu of sensible heat will lower the temperature of the pound of pure water from 212F to 32F. As the pound of water at 32F changes to a pound of ice at 32F, 144 Btu are given off without any accompanying change in temperature. Further removal of heat causes the temperature of the ice to decrease.