Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGheat) from one substance to another is normallyreflected in a temperature change in eachsubstance— that is, the hotter substance becomescooler and the cooler substance becomeshotter. However, the flow of heat is not reflectedin a temperature change in a substancewhich is in process of changing from one physicalstate ' to another. When the flow of heat isreflected in a temperature change, we say thatsensible heat has been added to or removedfrom a substance. When the flow of heat is notreflected in a temperature change but is reflectedin the changing physical state of a substance,we say that latent heat has been addedor removed.Since heat is defined as thermal energy intransition, we must not infer that sensible heatand latent heat are really two different kinds ofheat. Instead, the terms serve to distinguishbetween two different kinds of effects producedby the transfer of heat; and, at a more fundamentallevel, they indicate something about themanner in which the thermal energy was or willbe stored. Sensible heat involves internal kineticenergy and latent heat involves internal potentialenergy.The three fundamental physical states of allmatter are solid, liquid, and gas (or vapor). Thephysical state of a substance is closely relatedto the distance between molecules. Themolecules are closest together in solids, fartherapart in liquids, and farthest apart in gases. Whenthe flow of heat to a substance is not reflected ina temperature change, we know that the energyis being used to increase the distance betweenthe molecules of the substance and thus changeit from a solid to a liquid or from a liquid to agas. In other words, the addition of heat to asubstance that is in process of changing fromsolid to liquid or from liquid to gas results inan increase in the amount of internal potentialIn thermodynamics, the physical state of a substance(solid, liquid, or gas) is usually described by the termphase, while the term state is used to describe thesubstance with respect to all of its properties—phase,pressure, temperature, specific volume, and so forth.Thus the phase of a substance may be considered asmerely one of the several properties that fix the stateof the substance. While the precision of this usagehas some obvious advantages, it is not in standard useamong engineers. In this text, therefore, the termphysical state (or sometimes state) is used to denotethe molecular condition of a substance that determineswhether the substance is a solid, a liquid, or a gas.energy stored in the substance, but it does notresult in an increase in the amount of internalkinetic energy. Only after the change of statehas been fully accomplished does the addition ofheat result in a change in the amount of internalkinetic energy stored in the substance; hence,there is no temperature change until after thechange of state is complete.In a sense, we may think of latent heat asthe energy price that must be paid for a changeof state from solid to liquid or from liquid togas. But the energy is not lost; rather, it isstored in the substance as internal potentialenergy. The energy price is "repaid," so tospeak, when the substance changes back fromgas to liquid or from liquid to solid; duringthese changes of state, the substance gives offheat without any change in temperature.The amount of latent heat required to causea change of state— or, on the other hand, theamount of latent heat given off during a changeof state—varies according to the pressure underwhich the process takes place. For example, ittakes about 970 Btu to change 1 pound of waterto steam at atmospheric pressure (14.7 psia)but it takes only 62 Btu to change 1 pound ofwater to steam at 3200 psia.Figure 8-2 shows the relationship betweensensible heat and latent heat for one substance,water, at atmospheric pressure." If we startwith 1 pound of ice at 0°F,we must add 16 Btuto raise the temperature of the ice to 32° F. Wecall this adding sensible heat. To change thepound of ice at 32°F to a pound of water at 32°F,we must add 144 Btu (the latent heat of fusion ).There will be no change in temperature whilethe ice is melting. After all the ice has melted,however, the temperature of the water will beraised as additional heat is supplied. Again, weare adding sensible heat. If we add 180 Btu—that is, 1 Btu for each degree of temperaturebetween 32° F and 212°F-the temperature of thewater will be raised to the boiling point. Tochange the pound of water at 212°F to a poundof steam at 212°F, we must add 970 Btu (thelatent heat of vaporization ). After all the waterhas been converted to steam, the addition ofmore heat will cause an increase in the temperatureof the steam. If we add 42 Btu to theThe same kind of chart could be drawn up for othersubstances, but different amounts of thermal energywould of course be required for each change of temperatureor of physical state.168