PRINCIPLES OF NAVAL ENGINEERING 



universe is always the same . Regardless of the 

 mode of expression, the principle of the conser- 

 vation of energy applies to all kinds of energy. ^ 

 Energy equations for many thermodynamic 

 processes are based directly upon the principle 

 of the conservation of energy. When the prin- 

 ciple of the conservation of energy is written in 

 equation form, it is known as the general energy 

 equation and is expressed as: 



energy in = energy out 



or, in more detail, it may be stated that the 

 energy entering a system equals the energy 

 leaving the system plus any accumulation and 

 minus any dimunition in the amount of energy 

 stored within the system. 



The first law of thermodynamics, a special 

 statement of the principle of the conservation 

 of energy, deals with the transformation of 

 mechanical energy to thermal energy and of 

 thermal energy to mechanical energy. The first 

 law is commonly stated as follows: Thermal 

 energy and mechanical energy are mutually 

 convertible, in the ratio of 778 foot-pounds to 1 

 Btu. 



The ratio of conversion between mechanical 

 energy and thermal energy is known as the 

 mechanical equivalent of heat , or Joule's equiva- 

 lent. It is symbolized by the letter J and, in 



12 



The principle of the conservation of energy and the 



principle of the conservation of mass have been basic 

 to the development of modern science. Until the estab- 

 lishment of the theory of relativity, with its implica- 

 tion of the mutual convertibility of energy and mass, 

 the two principles were considered quite separate. 

 According to the theory of relativity, however, they 

 must be considered merely as two phases of a single 

 principle which states that mass and energy are 

 interchangeable and the total amount of matter and 

 energy in the universe is constant . Nuclear fission, 

 a process in which atomic nuclei split into fragments 

 with the release of enormous quantities of energy, 

 is a dramatic example of the actual conversion of 

 matter into energy. Even in the familiar process of 

 combustion, modern techniques of measurement have 

 led to the discovery that a very minute quantity of 

 matter is converted into energy; for example, about 

 0.00007 ounce of matter is converted into energy when 

 6 tons of carbon are burned with 16 tons of oxygen. 

 In spite of the mutual convertibility of energy and 

 mass, the principle of the conservation of energy may 

 still be regarded separately as the cornerstone of the 

 science of thermodynamics. Machinery designed under 

 this principle alone still functions in an orderly and 

 predictable fashion. 



SECOND FLUID 



FIRST FLUID- 



FIRST FLUID - 



FIRST FLUID- 



FIRST FLUID- 



98,32 

 Figure 8-6.— Cross flow in heat exchanger. 



accordance with the first law of thermodynam- 

 ics, it is expressed as 



or 



J = 

 J = 



778 ft-lb per Btu 



778 ft-lb 

 1 Btu 



The mechanical equivalent of heat provides 

 us, directly or by extension, with a number of 

 useful numerical values relating to heat, work, 

 and power. Some of the most widely used values 

 are given here; others may be obtained from 

 engineering handbooks and similar publications. 



1 Btu = 778 ft-lb 



1 hp = 33,000 ft-lb per min = 



550 ft-lb per sec 

 1 kw = 1.341 hp 

 1 hp = 2545 Btu per hr = 



42.42 Btu per min 

 1 kw = 3413 Btu per hr 

 1 kw - 44,256 ft-lb per min 

 1 hp-hr = 2545 Btu 

 1 kw-hr = 3413 Btu 



The first law of thermodynamics is often 

 written in equation form as 



U2 - Ui = Q 



w 



where 



U. = internal energy of a system at the be- 

 ginning of a process 



U„ = internal energy of the system at the end 

 of the process 



Q = net heat flowing into the system during 

 the process 



W = net work done by the system during the 

 process 



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