Chapter 8- INTRODUCTION TO THERMODYNAMICS 



processes are (1) internal energy, (2) heat, (3) 

 mechanical potential energy, (4) mechanical 

 kinetic energy, (5) work, and (6) flow work. 



The first five of these energy terms are 

 familiar, but the last one may be new. Flow 

 work , sometimes called displacement energy , is 

 the mechanical energy necessary to maintain 

 the steady flow of a stream of fluid. The numeri- 

 cal value of flow work may be calculated by 

 finding the product of the absolute pressure {in 

 pounds per square feet) and the volume of the 

 fluid (in cubic feet). Thus. 



flow work = pV ft-lb 



or, more conveniently, using specific volume 

 rather than total volume, 



flow work = pv ft-lb per lb 



The product py will, of course, have a nu- 

 merical value even when there is no flow of 

 fluid. However, this value represents flow work 

 only when there is a steady, continuous flow of 

 fluid. Flow work may also be expressed in 

 terms of Btu per pound, as 



flow work = ^Y Btu per lb 



As mentioned before, the steady-flow equa- 

 tions take various forms, depending upon the 

 nature of the process under consideration. How- 

 ever, the terms for internal energy and flow 

 work almost invariably appear in any steady- 

 flow process. For convenience, this combination 

 of internal energy and flow work has been 

 given a name, a symbol, and units of measure- 

 ment. The name is enthalpy (accent on second 

 syllable). The symbol is' H for total enthalpy 

 or h for specific enthalpy— that is, enthalpy per 

 pound. Total enthalpy, H, may be measured in 

 Btu or in foot-pounds. Specific enthalpy (en- 

 thalpy per pound), h, may be measured in Btu 

 per pound or in foot-pounds per pound. The 

 enthalpy equation may be written as 



H = 



pv 



+ Btu 



where 



H = total enthalpy, in Btu 

 U = total internal energy, in Btu 

 p = absolute pressure, in pounds per square 

 foot 



V = total volume, in cubic feet 

 J = the mechanical equivalent of heat, 778 

 ft-lb per Btu 



Since it is frequently more convenient in 

 thermodynamics to make calculations in terms 

 of 1 pound of the working substance, we should 

 note also the equation for specific enthalpy: 



pv 

 h = u + -^ Btu per lb 



here h^ u^ and v are specific enthalpy, specific 

 internal energy, and specific volume, respec- 

 tively. When it is desired to calculate enthalpy 

 in foot-pounds, rather than in Btu, it is only 

 necessary to drop the jJ from the equations. 



The terms heat content and total heat are 

 sometimes used to describe this property which 

 we have designated as enthalpy. However, the 

 terms heat content and total heat tend to be mis- 

 leading because the change in enthalpy of a work- 

 ing fluid does not always measure the amount of 

 energy transferred as heat, nor is it necessarily 

 caused by the transfer of energy in the form of 

 heat. Also, the transferred energy that causes 

 a change in enthalpy is not entirely "contained" 

 in the working fluid, as the terms heat content 

 and total heat tend to imply; although the internal 

 energy, u^, is stored in the working fluid, the pv 

 cannot in any way be considered as "contained" 

 in the fluid. 



Type of State Change 



Thus far we have considered processes class- 

 ified as non-flow or steady-flow. The nature of 

 the state changes undergone by a working fluid 

 provides us with another useful way of classify- 

 ing processes. The terms used to identify cer- 

 tain common types of state changes are defined 

 briefly in the following paragraphs. 



ISOBARIC STATE CHANGES.-An isobaric 

 state change is one in which the pressure of and 

 on the working fluid is constant throughout the 

 change. In other words, an isobaric change is a 

 constant-pressure change. Isobaric changes oc- 

 cur in some piston-and-cylinder devices in which 

 the piston operates in such a fashion as to main- 

 tain a constant pressure. Isobaric state changes 

 are not typical of most steady-flow processes, 

 but they are approximated in some steady-flow 

 processes in which friction and shaft work are of 

 insignificant magnitude. 



177 



