Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGuniverse is always the same . Regardless of themode of expression, the principle of the conservationof energy applies to all kinds of energy. ^Energy equations for many thermodynamicprocesses are based directly upon the principleof the conservation of energy. When the principleof the conservation of energy is written inequation form, it is known as the general energyequation and is expressed as:energy in = energy outor, in more detail, it may be stated that theenergy entering a system equals the energyleaving the system plus any accumulation andminus any dimunition in the amount of energystored within the system.The first law of thermodynamics, a specialstatement of the principle of the conservationof energy, deals with the transformation ofmechanical energy to thermal energy and ofthermal energy to mechanical energy. The firstlaw is commonly stated as follows: Thermalenergy and mechanical energy are mutuallyconvertible, in the ratio of 778 foot-pounds to 1Btu.The ratio of conversion between mechanicalenergy and thermal energy is known as themechanical equivalent of heat , or Joule's equivalent.It is symbolized by the letter J and, in12 The principle of the conservation of energy and theprinciple of the conservation of mass have been basicto the development of modern science. Until the establishmentof the theory of relativity, with its implicationof the mutual convertibility of energy and mass,the two principles were considered quite separate.According to the theory of relativity, however, theymust be considered merely as two phases of a singleprinciple which states that mass and energy areinterchangeable and the total amount of matter andenergy in the universe is constant . Nuclear fission,a process in which atomic nuclei split into fragmentswith the release of enormous quantities of energy,is a dramatic example of the actual conversion ofmatter into energy. Even in the familiar process ofcombustion, modern techniques of measurement haveled to the discovery that a very minute quantity ofmatter is converted into energy; for example, about0.00007 ounce of matter is converted into energy when6 tons of carbon are burned with 16 tons of oxygen.In spite of the mutual convertibility of energy andmass, the principle of the conservation of energy maystill be regarded separately as the cornerstone of thescience of thermodynamics. Machinery designed underthis principle alone still functions in an orderly andpredictable fashion.SECONDFIRSTFLUIDFIRST FLUID -FIRSTFIRSTFLUID-FLUID-FLUID-98,32Figure 8-6.— Cross flow in heat exchanger.accordance with the first law of thermodynamics,it is expressed asorJ =J =778 ft-lb per Btu778 ft-lb1 BtuThe mechanical equivalent of heat providesus, directly or by extension, with a number ofuseful numerical values relating to heat, work,and power. Some of the most widely used valuesare given here; others may be obtained fromengineering handbooks and similar publications.1 Btu = 778 ft-lb1 hp = 33,000 ft-lb per min =550 ft-lb per sec1 kw = 1.341 hp1 hp = 2545 Btu per hr =42.42 Btu per min1 kw = 3413 Btu per hr1 kw - 44,256 ft-lb per min1 hp-hr = 2545 Btu1 kw-hr = 3413 BtuThe first law of thermodynamics is oftenwritten in equation form asU2 - Ui = Q wwhereU. = internal energy of a system at the beginningof a processU„ = internal energy of the system at the endof the processQ = net heat flowing into the system duringthe processW = net work done by the system during theprocess172