CHAPTER 7 ENERGY BALANCES SYSTEM SYSTEM. * What is energy? * Forms of Energy. - Kinetic energy (KE) - Potential energy (PE) PE = mgz

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1 SYSTM CHAPTR 7 NRGY BALANCS SYSTM nergy & 1st Law of Thermodynamcs * What s energy? * Forms of nergy - Knetc energy (K) K 1 mv - Potental energy (P) P mgz - Internal energy (U) * Total nergy, K + P + U

2 Change n knetc energy: 1 K K K1 m(v V Change n potental energy: Note: Δ means change and s always calculated as fnal value mnus ntal value P P P1 mg(z z1) Change n potental energy: U U U 1 Types of Work ) How energy can be transferredbetween a system and ts surroundngs? Heat energy that flows as a result of temperature dfference between a system and ts surroundng ; heat s defned postve when t s transferred to the system from the surroundngs. Work energy that flows n response to any drvng force other than a temperature dfference; work s defned postve when t s done by the system on the surroundngs. 7-6 nergy Converson Unts Flow work (W fl )-energy carred across the boundares of a system wth the mass flowng across the boundares (.e. nternal, knetc & potental energy) Shaft work (W s )-energy n transton across the boundares of a system due to a drvng force other than temperature, and not assocated wth mass flow (an example would be mechancal work due to a pston, pump or compressor)

3 Recall 1. s always measured relatve to reference pont! And Reference Ponts Reference plane for P Reference frame for K Reference state for Û or Ĥ(.e. usually, but not necessarly Û or Ĥ 0) 1. Changes n are mportant, not total values of. depends only on begnnng and end states 3. and W depend on process path (could get to the same end state wth dfferent combnatons of and W) General Balance quaton A balance on conserved quantty (.e. mass, energy, momentum) n a process system may be wrtten as: Input + generaton - output - consumpton accumulaton nergy Balance on Closed Systems There s no mass transfer nto a closed system The only way energy can get nto or out of a closed system s by heat transfer or work W s How do you descrbe a closed system control volume? What effect does ths have on the mass and energy balances? a. Heat transfer (): b. Work (Ws): Note: * Work s any boundary nteracton that s not heat (mechancal, electrcal, magnetc, etc.)

4 Frst Law of Thermodynamcs Closed System In a closed system, nergy can nether be created nor destroyed ; It can only change forms Input + generaton - output - consumpton no mass crosses the boundary, hence the nput & output terms are elmnated energy can be transferred across the boundary as heat & work, hence the accumulaton term may be defned as the change n total energy n the system,.e. accumulaton Input - output accumulaton 0 0 Fnaltotal nergy Intal n the System n the System Totalnergy Changenthetotal system energy f - -W s Δ Δ K f + W s P K+ P+ U + heat transferred to the system W s work done by the system K+ P+ U W s U U + P + K W Note: W s W s, (Summaton of all heat transfer across system boundary) (Summaton of all work across system boundary)

5 For a closed system what s equal to? K + P + U Is t steady state? (f yes, 0) Is t adabatc? (f yes, 0) Are there movng parts, e.g. do the walls move? (f no, W s 0) Is the systemmovng? (f no, K 0) Is there a change n elevatonof the system? (f no, P 0 ) Does temperature, phase, chemcal composton change, or pressurechange less than a few atmospheres? (f no to all, U 0) W s xample 1 A closed system of mass 5 kg undergoes a process n whch there s work of magntude 9 kj to the system from the surroundngs. The elevaton of the system ncreases by 700 m durng the process. The specfc nternal energy of the system decreases by 6 kj/kg and there s no change n knetc energy of the system. The acceleraton of gravty s constant at g9.6 m/s. Determne the heat transfer, n kj. 7.4 nergy Balances on Open Systems 7-17 Some common open system steady flow devces How are open systems control volumes dfferent from closed systems? What effect does ths have on the energy balance? Only one n and one out

6 Applcaton of Frst Law - Conservaton of energy for a control volume General nergy Balances on Open Systems at Steady State tme rate of change net rate at whch net rate at whch net rate of energy energy s beng energy s beng transfer nto the of the energy contaned wthn transferred n transferred out control volume + the control volume at by heat transfer by work at accompanyng mass flow m & tme t at tme t tme t a Types of Work Recall. How energy can be transferred across boundares of a closed system? an open system?