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FIRST LAW OF THERMODYNAMICS - h1012v1_79
Figure 14    First Law of Thermodynamics

Thermodynamics Heat Transfer and Fluid Flow Volume 1 of 3
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FIRST LAW OF THERMODYNAMICS Thermodynamics The  First  Law  of  Thermodynamics  is  referred  to  as  the  Conservation  of  Energy  principle, meaning  that  energy  can  neither  be  created  nor  destroyed,  but  rather  transformed  into  various forms as the fluid within the control volume is being studied.  The energy balance spoken of here is maintained within the system being studied.  The system is a region in space (control volume) through which the fluid passes.  The various energies associated with the fluid are then observed as they cross the boundaries of the system and the balance is made. As discussed in previous chapters of this module, a system may be one of three types:   isolated, closed, or open.   The open system, the most general of the three, indicates that mass, heat, and external work are allowed to cross the control boundary.   The balance is expressed in words as: all energies into the system are equal to all energies leaving the system plus the change in storage of  energies  within  the  system.   Recall  that  energy  in  thermodynamic  systems  is  composed  of kinetic energy (KE), potential energy (PE), internal energy (U), and flow energy (PL); as well as heat and work processes. (all energies in) = (all energies out) +  D(energy stored in system) S S S Ein S Eout DE  storage For  most  industrial  plant  applications  that  most  frequently  use  cycles,  there  is  no  change  in storage (i.e. heat exchangers do not swell while in operation). In equation form, the balance appears as indicated on Figure 14. where: = heat flow into the system (Btu/hr) Q = mass flow rate into the system (lbm/hr) min uin = specific internal energy into the system (Btu/lbm) Pinnin = pressure-specific volume energy into the system (ft-lbf/lbm) = kinetic energy into the system (ft-lbf /lbm) where V2in 2gc =   average velocity of fluid (ft/sec) Vin gc =   the gravitational constant (32.17 ft-lbm/lbf-sec2) =    potential energy of the fluid entering the system (ft-lbf/lbm) where g gc Zin Zin =   height above reference level (ft) g =   acceleration due to gravity (ft/sec2) gc =   the gravitational constant (32.17 ft-lbm/lbf-sec2) HT-01 Page 54 Rev. 0







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