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Thermodynamics Lecture Series First Law – Control Volume Applied Sciences Education Research Group (ASERG) Faculty of Applied Sciences Universiti Teknologi MARA email: [email protected] http://www5.uitm.edu.my/faculties/fsg/drjj1.html First Law - Quotes Quote “Education is not the piling on of learning, information, data, facts, skills, or abilities--that's training or instruction--but is rather a making visible what is hidden as a seed... To be educated, a person doesn't have to know much or be informed, but he or she does have to have been exposed vulnerably to the transformative events of an engaged human life... One of the greatest problems of our time is that many are schooled but few are educated.” Author:Thomas Moore Source:he Education of the Heart by Thomas Moore Introduction - Objectives Objectives: 1. State the conservation of mass principle. 2. State the meaning of steady-flow process and the implications on a system’s properties. 3. Write the unit-mass basis and unit-time basis (or rate-form basis) energy balance for a general steady-flow process. 4. Write the unit-time basis (or rate-form basis) mass balance for a general steady-flow process. Introduction - Objectives Objectives: 4. State the assumptions for steady-flow devices such as nozzles, diffusers, turbines, compressors, throttle valves, heat exchangers and mixing chambers. 5. State the purpose for each of the steady-flow device noted above. 6. Write the unit-mass basis and unit-time basis (or rate-form basis) energy balance for each of the steady-flow devices noted above Introduction - Objectives Objectives: 7. Write the unit-time basis (or rate-form basis) mass balance for each of the steady-flow device. 8. Use the energy and mass balance to solve problems related to each of the steady-flow device. First Law - General Control Volume Open System Steady-flow Devices First Law - General How to relate changes to the cause Mass in Qin Qout Properties will change indicating change of state System E1, P1, T1, V1 To E2, P2, T2, V2 Dynamic Energies as causes (agents) of change Win Wout Mass out First Law – Energy Balance Energy Energy Change of Entering - Leaving = system’s a system a system energy Energy Balance Amount of energy causing change must be equal to amount of energy change of system First Law – Energy Balance Energy Energy Change of Entering - Leaving = system’s a system a system energy Energy Balance Ein – Eout = Esys, kJ or ein – eout = esys, kJ/kg or E in E out E sys ,kW First Law – Energy Balance Energy Balance –General system Qin – Qout+ Win – Wout+ Emass,in - Emass,out = U+ KE + PE, kJ qin – qout+ win – wout + qin – qout = u+ ke + pe, kJ/kg Q in Q out W in W out E mass ,in E mass ,out U KE PE , kW First Law – Stationary Closed Energy Balance –Stationary Closed system Qin – Qout+ Win – Wout+ 0 – 0 = U+ 0 + 0, kJ qin – qout+ win – wout + 0 – 0 = u+ 0 + 0, kJ/kg Qin Q out W in W out 0 0 U 0 0, kW First Law – Mass Balance Mass Mass Change of Entering - Leaving = system’s a system a system mass Mass Balance min – mout = msys, kg or m in m out msys , kg / s First Law - Mass flow rates Mass Balance – Mass & Volume Flow Rate The volume of the cylinder is V A m , m 3 The velocity of the mass is ,m / s t The volume flow rate is 3 V m m A , t t t 3 s m V m A, s Then, mass flow rate is A kg m , s First Law – Steady - flow Energy Balance – Control Volume Steady-Flow Steady-flow is a flow where all properties within boundary of the system remains constant with time First Law – single stream CV How to relate changes to the cause Mass in, State 1 Properties will change indicating change of state E mass ,in m1 1 System E1, P1, T1, V1 To Qin E1, P1, T1, V1 W in W out Mass out, State 2 E mass ,out m2 2 Q out Single Stream CV First Law – Steady - flow Energy Balance – Control Volume Steady-Flow Steady-flow is a flow where all properties within boundary of the system remains constant with time Esys= 0, kJ; esys= 0 , kJ/kg, E sys 0, kJ/s Vsys= 0, m3; msys= 0 m sys 0 , kg/s First Law – Energy Balance CV Mass & Energy Balance– Steady-Flow CV Mass balance msys 0, So, min mout or m in m out , kg/s Energy balance E sys 0, So, E in E out kJ/s Qin Win m Qout W out in m , kW out q w in qout wout out , kW First Law – Energy balance CV Mass & Energy Balance–SteadyFlow: Single Stream Mass balance IN, 1 OUT, 2 msys 0. So, min mout , kg/s Energy balance E sys 0. So, E in E out , kJ/s Qin Qout W in W out m m , kW out in qin – qout+ win – wout = qout – qin, kJ/kg First Law - Single Stream Mass & Energy Balance–SteadyFlow: Single Stream Energy balance Mass balance m1 m 2 m, kg/s Qin Qout W in W out mout in , kW Qin Qout W in W out mh2 ke2 pe2 mh1 ke1 pe1 , kW mh2 h1 ke2 ke1 pe2 pe1 , kW mh ke pe, kW First Law Energy balance CV Mass & Energy Balance–SteadyFlow: Single Stream Mass balance: m1 m 2 m, kg/s Energy balance: qin – qout+ win – wout = qout – qin, kJ/kg = h2 – h1 + ke2 – ke1 + pe2 – pe1, kJ/kg = h + ke + pe, kJ/kg First Law – Energy Balance CV Mass & Energy Balance–SteadyFlow: Single Stream Energy balance: qin – qout+ win – wout = h + ke + pe, kJ/kg 2 2 where kJ ke kefinal keinitial and 2 1 2000 , kg pe pefinal peinitial g y 2 y1 , kJ 1000 kg First Law of – Ideal Gas Mass & Energy Balance–SteadyFlow CV: Ideal Gases Use Ideal Gas Equation of State for real gases that behave like ideal gases. Criteria: Pgas << Pcrit and/or Tgas >> Tcrit. P = RT Where is the specific volume,m3/kg, R is gas constant, kJ/kgK, T is absolute temperature in Kelvin For known P and T, use to determine , and hence the mass flow rate. First Law of – Ideal Gas Mass & Energy Balance–SteadyFlow CV: Ideal Gases Use of property table for real gases that behave like ideal gases. Knowing T, read value for h and viceversa. If T or h not found, do interpolation First Law - Nozzles Nozzles Purpose: Increase velocity Effect: Pressure drops Mass balance m1 m 2 m, kg/s In Out State 1 Energy balance State 2 A2 << A1 Qin Qout W in W out mout in , kW Qin Qout W in W out mh ke pe, kW 0 0 0 0 mh ke 0 , kW First Law - Nozzles Nozzles Purpose: Increase velocity Effect: Pressure drops 1 A1 2 A2 , kg/s m1 m 2 Mass balance: 1 2 Energy balance: 0 0 0 0 mh ke 0 , kW 0 h ke, kJ/kg 0 h2 h1 ke2 ke1 , kJ/kg 2 2 1 2 h2 h1 ke1 ke2 , kJ/kg 2000 First Law - Nozzles Nozzles Purpose: Increase velocity Effect: Pressure drops 2 2 1 2 , kJ/kg Energy balance: h2 h1 ke1 ke2 2000 Air:Use energy balance to find h2 and use table A17 (& interpolation technique) to determine T2. Air: Use table A-17 to find h1 In (& interpolation technique) State 1 for a given T1. A2 << A1 Out State 2 First Law - Diffusers Diffusers Purpose: Increase Pressure Effect: Velocity drops 2 2 1 2 , kJ/kg Energy balance: h2 h1 ke1 ke2 2000 Air:Use energy balance to find h2 and use table A17 (& interpolation technique) to determine T2. Air: Use table A-17 to find h1 (& interpolation technique) for a given T1. In State 1 A2 >> A1 Out State 2 First Law - Turbines Turbines Purpose: Produce Work Effect: Pressure Drops 1 A1 2 A2 , kg/s Mass balance: m1 m 2 1 2 Energy balance: qin – qout+ win - wout = h2 – h1+ ke2 – ke1+ pe2 – pe1 Qin Qout W in W out mh ke pe, kW In 0 0 0 W out mh2 h1 0 0 , kW – wout = h2 – h1, kJ/kg wout = h1 – h2, kJ/kg Out First Law - Compressors Compressors Purpose: Increase Pressure Sacrifice: Work supplied 1 A1 2 A2 , kg/s Mass balance: m1 m 2 1 2 Energy balance: qin – qout+ win - wout = h2 – h1+ ke2 – ke1+ pe2 – pe1 Qin Qout W in W out mh ke pe, kW Out 0 0 0 W in mh2 h1 0 0 , kW win = h2 – h1, kJ/kg In First Law – Increase Pressure Pressure Increasing Devices Compressors: Increase Pressure of gas to high P Fans: Increase Pressure of gas slightly to move air Pumps: Increase Pressure of liquid to high P Sacrifice: Work supplied First Law - Throttle Throttle First Law - Throttle Throttle Purpose: Reduce Pressure Effect: Temp drops 1 A1 2 A2 , kg/s m1 m 2 Mass balance: 1 2 Energy balance: qin – qout+ win - wout = h2 – h1+ ke2 – ke1+ pe2 – pe1 Qin Qout W in W out mh ke pe, kW 0 0 0 0 h2 h1 0 0 , kJ/kg h2 h1 , kJ/kg isenthalpic process In Out First Law – Mixing Chamber Mixing Chamber Purpose: Mixing Mass balance: m1 m2 m3 , kg/s Energy balance: qnet,in – wnet,out = q3 – q1 - q2, kJ/kg (m1 m 2 )h3 ke3 pe3 m 2 h2 ke2 pe2 m1h1 ke1 pe1 , kW ke3 ke2 ke1 ke3 0; pe3 pe2 pe1 pe3 0 Then, m2 ( h3 h2 ) m1 (h1 h3 ), kW 1 3 2 First Law – Heat Exchanger Heat Exchanger Boundary has 2 inlets and 2 exits First Law – Heat Exchanger Heat Exchanger Boundary has 1 inlet and 1 exit First Law of Thermodynamics In, 3 Heat Exchanger -no mixing -1 inlet and 1 exit In, 1 Exit, 2 Exit, 4 First Law of Thermodynamics In, 3 Heat Exchanger -no mixing -2 inlets and 2 exits In, 1 Exit, 2 Exit, 4 First Law of Thermodynamics 3 Heat Exchanger Case 1 1 2 4 0 4 - 3 2 - 1 m1 m2 , m3 m4 , kg/s First Law of Thermodynamics 3 Heat Exchanger Case 2 1 2 m1 m2 , kg/s Qout 4 m3 m4 , kg/s Qnet ,in 0 2 - 1 First Law of Thermodynamics Heat Exchanger Energy balance: Case 1 Purpose: Remove or add heat 0 0 0 0 m 4 h4 ke4 pe4 - m3 h3 ke3 pe3 m 2 h2 ke2 pe2 m1 h1 ke1 pe1 , kW 3 Mass m1 m2 , m3 m4 , kg/s balance: m1 h2 h1 ke2 ke1 pe2 pe1 m3 h4 h3 ke4 ke3 pe4 pe3 where ke pe 0 1 2 4 First Law of Thermodynamics Heat Exchanger Mass balance: Purpose: Remove or add heat m1 m2 , m3 m4 , kg/s Energy balance: Case 2 Q in Q out 0 0 m2 h2 m1 h1 , kW Qout 0 0 m2 h2 h1 , kW where 3 h2 h1 1 2 m3 h3 h4 m1 Qout4 First Law of Thermodynamics Heat Exchanger Mass balance: Purpose: Remove or add heat m1 m2 , m3 m4 , kg/s Energy balance: Case 2 Q in Q out 0 0 m2 h2 m1 h1 , kW Qin Qin 0 0 0 m2 h2 h1 , kW where 3 h2 h1 1 2 m3 h3 h4 m1 4 First Law of Thermodynamics Concluding remarks - Quotes “The illiterate of the 21st century will not be those who cannot read and write, but those who cannot learn, unlearn, and relearn.” Author: Alvin Toffler Source: Lessons from the Art of Juggling; How to Achieve Your Full Potential in Business, Learning and Life by Michael Gelb and Tony Buzan