First law of thermodynamic

engineering thermodynamic
first law of thermodynamic

1. Shroff S.R. Rotary Institute of Chemical Technology Principle Supporter & Sponsor-United Phosphorous Ltd(UPL)/Shroff family Managed By Ankleshwar Rotary Education Society Approved by AICTE, New Delhi, Govt. of Gujarat & GTU Affiliated SUBJECT :- ENGINEERING THERMODYNAMICS TOPIC :- FIRST LAW OF THERMODYNAMICS

2. First law of thermodynamics for a closed system undergoing a cycle : When system is made to undergo a complete cycle then net work is done on the system or by the system. Let us consider a cycle in which net work is done by the system. As per conservation of energy principle, energy can not be created, so this mechanical energy must have been supplied to system from some source of energy.  

3. The first law of thermodynamics can be stated as follows. 1). “When a system undergoes a thermodynamics cycle then the net heat added to the system from the surrounding is equal to net work done by the system on its surroundings” or §δ Q= §δ W ,where § Cyclic integration represents the sum for a complete cycle. 2) "Heat and work are mutually convertible but energy can neither be created nor destroyed, the total energy involved with an energy conversion remains constant".

4. 3). There is not any machine which can produce energy without corresponding source of energy".

5. Joule's experiment During 1843 to 1848 , Joule conducted several experiment and formulated first law of thermodynamics. His experiment’s consists of two process cycles carried out on a system. In process 1- 2, work W 12 was done on the system by means of a paddle wheel. The amount of work mg falling through height h, caused a rise in the temperature of the fluid. The system was initially at temperature t 1 , the same as that of atmosphere, and after work transfer, temperature rise t 2 at constant pressure 1 atm. The process. 1 - 2 undergone by the system is shown in figure.

6. The system was place in contact with surroundings by removing insulation as shown in figure Heat was transferred from the fluid to the surrounding in process 2-1, until the original state of the fluid was restored. The amount of heat transfer Q 21 from the fluid to the surroundings during process 2-1 shown in figure was estimated. Joule found W 12 is always proportional to the heat Q 21

7. There as system performs a thermodynamic cycle. If the cycle involved number of heat and work interaction, same result will be found. Where J is the joule's equivalent or mechanical equivalent of heat. The conclusion from the Joule's experiment leads to the statement of first law of thermodynamics as "During a cycle, a system undergoes the cyclic integral of heat added is equal to the cyclic integral of work done".

8. First law of thermodynamics for a closed system undergoing a change of state   In previous article, expression applies only to system undergoing cycles, in which there is no change of state, no change an exciting energy of system or no energy provided by system itself .But If a system undergoes a change of state during which both heat transfer and work transfer are involved,the net energy transfer will be stored accumulated within the system.

9. For example, a system receives 15 kJ Heat while paddle wheel does 8 kJ work on the system and system rejects7 kJ heat surrounding as shown in figure The net increase in energy of the system for this process will be 15+8-7=16kJ. This 16kJ energy will be stored in the system . The stored energy is neither heat nor work, and is called internal energy or the energy of the system. The generalized our conclusion as shown in figure the first law of thermodynamics for a closed system undergoes a change of state may be expressed as follows.

10. [Net energy transferred to or from the system is heat and work] = [Net increase or decrease in the total energy of the system] The total energy of a system is consists of three parts as internal energy kinetic energy and potential energy and Total energy of a system during a process can be expressed as sum of changes in its U,KE and PE. It is very important that the sign convention be observed heat and work interactions. Heat flow to a system, Q is positive Heat flow form a system, Q is negative Work done by a system, W is positive Work done on a system, W is negative

11. Enthalpy While analyzing processes we frequently come across certain combination of properties. for sake of simplicity and convenience this combination is defined as a new thermodynamics properties the enthalpy is combination of two properties as internal energy and product of pressure and volume. It express by Since u, p and v are all properties there for h also property it is an intensive property of a system because that is specific enthalpy multiplying both side by mass m of equation.

12. PERPETUAL MOTION MACHINE OF THE FIRST KIND : 1 Any device that violates either law is called a perpetual motion machine. A device that violates the first law of thermodynamics is called perpetual motion machine of the first kind (PMM1).

13. NON-FLOW PROCESS AND FLOW PROCESS :- Non-flow process : A process undergone by a closed system of fixed mass is called non flow process. This process does not permits the flow of mass in or out of the system. Flow process : In this process , the fluid(mass) enters the system and leaves after doing work. This implies open boundary which permits the flow of the mass to and from the system.

14. FLOW PROCESS AND CONTROL VOLUME :- A flow process constitute an open system through which the working fluid enters and leaves from the surface of the system. There may also be energy interaction in the form of heat and work, internal energy, can you take energy, potential energy, etc. Most of engineering devices such as internal combustion engine, steam engines, boilers, turbines, pumps, compressors, condensers, et cetera, the working flowed continuously flew in and out of the plant of the devices.

15. Analyses of flow process in such devices is done with the concept of control volume and control surface. For example, consider a portion of steamed turbine plant in the figure. The high pressure and temperature steam enters the steam turbine at section 1 and after doing work on the rotor due to expansion of steam, and steam comes out at low pressure at section 2. In these types of flow process, the analysis like in close system (non-flow process), for finding the heat and work interactions is not feasible.

16. Hence the concept of control volume and control surface is used which is very convenient and feasible. Control volume is an imaginary envelope is supposed to exist around equipment. The space bounded by control volume is called control surface. But there is a difference between system boundary and control surface. The boundary of the system may change shape, position and orientation, and it may movable, but the control surface is fixed and the Mars (matter) flows across the control surface.

17. The concept of control volume helps to relate the energy interaction as work and heat with average change of state of fluid from entrance to exit section. 

18. STEADY AND UNSTEADY FLOW PROCESS : STEADY FLOW PROCESS :- It is a flow process in which fluid parameters at particular point of the control volume remain constant during entire process. The steady means no change with time. This means fluid parameters like velocity , pressure, temperature etc., are functions only of location and do not very with time as shown in figure.

19. UNSTEADY FLOW PROCESS :- It is a flow process in which fluid parameters at any point of the control volume is not constant during entire process. This means fluid parameters like velocity, pressure, temperature etc., are not only functions of location but also function of time as shown in figure.

20. The steady flow process is characterized by the following conditions in a control volume: (1) The mass flow rate remains constant within the system, i.e. mass entering the control volume must be equal to the mass leaving it and do not ready with time. (2) The state of fluid at any fixed point in control volume is same and do not very with time. (3) The state of energy of the fluid at the entrance and the exit of the control volume does not vary with the time. (4) Heat and work transfer rate across the control surface does not vary with time.

21. (5) Chemical composition of the fluid within the control volume is fixed. So change in the chemical energy is not involved. When any flow process does not satisfy the above conditions of steady state, and process is known as unsteady flow process.

22. STEADY FLOW ENERGY EQUATION (SFEE) :- Consider flow of fluid through a generalized open system as shown in figure. The working fluid enter the system at section 1 and leave the system at section 2 and passing at a steady rate. Let, m= mass flowing through the control volume, kg/s Q= heat entering the control volume, kJ W= work transferred from control volume, kJ C 1 ,C 2 = velocity of fluid at entrance and exit, m/s

23. p 1 ,p 2 = pressure of fluid at entrance and exit, N/m 2 u 1 ,u 2 =internal energy per kg of fluid at entrance and exit, kJ/kg v 1 ,v 2 = specific volume of fluid at entrance and exit m 3 /kg h 1 ,h 2 = enthalpy of fluid at entrance and exit, kJ/kg C.V= Control volume q= heat entering the control volume per kilogram of fluid, kJ/kg w= work transferred from the control volume per kg of fluid, kJ/kg z 1 ,z 2 = elevation of entrance section and exit section, m.

24. The energy balance required in open system for flow process may be written as follows  Energy entering to the C.V + Heat entering to the C.V = Energy leaving the C.V + Work transferred from the C.V + Increase of stored energy within the C.V For steady flow process,increase of stored energy within the control volume is zero.

25. Energy entering to the C.V Heat entering to the C.V Energy leaving the C.V Work transferred from the C.V + + = ∴  ∴ Internal energy at sec 1 Flow work at sec 1 Kinetic energy at sec 1 Potential energy at sec 1 Heat entering to the C.V + + + + = Internal energy at sec 2 Flow work at sec 2 Kinetic energy at sec 2 Potential energy at sec 2 + + + + Work transferred from the C.V

26. ∴ m 1 u 1 +p 1 v 1 + +z 1 g + Q = u 2 +p 2 v 2 + +z 2 g m 2 + W It is applicable to compressible and incompressible fluid is, liquids and gases, ideal and real fluids. If m 1 = m 2 = m

27. m u 1 +p 1 v 1 + +z 1 g + Q = u 2 +p 2 v 2 + +z 2 g m + W m h 1 + +z 1 g + Q = h 2 + +z 2 g m + W ∴ ∴ This equation is called steady flow energy equation(SFEE).

28. THROTTLING DEVICE : Function : It is the flow restricting device as shown in figure, that cause a large pressure drop in the fluid. Throttling process : It is an irreversible process in which fluid flows across the restriction in a manner that cause the reduction in pressure and increase volume. In this process no heat flows from and to the system, no change in enthalpy of the fluid, no change in kinetic energy and potential energy and there is no work transfer.

29. The throttling process is mostly used to regulate speed of turbine, to determine the condition of steam (dryness fraction) and to reduce the pressure of refrigerant before entry into the evaporator in the refrigeration plant.

30. We know that SFEE, m h 1 + +z 1 g + Q = h 2 + +z 2 g m + W For throttling device, ∆KE ≈ 0,(C1 = C2 ),(negligible change in KE) ∆PE = 0,(z1 = z2 ),pipe is horizontal Q = 0, (pipe is thermally insulated) W = 0, (no shaft work involved) ∴ mh 1 = mh 2 ∴ h 1 = h 2 , i.e., enthalpy of fluid remains constant during throttling process

31. For ideal gas h= C p T ∴C p T 1 = C p T 2 ∴ T 1 = T 2 For ideal gas, throttling process takes place at constant enthalpy , constant temperature and constant internal energy. For real gas, h ≠ f (T) ∴T 1 ≠ T 2 ∴ u 1 ≠ u 2 ∴ h 1 = h 2 Or u 1 + p 1 v 1 = u 2 + p 2 v 2 Internal energy + Flow energy = constant Therefore change in internal energy is not zero in real gas.

32. ENGINEERING APPLICATION OF STEADY FLOW ENERGY EQUATION Nozzle Function : It is a device that increases the velocity o f a fluid at the expense of its pressure drop. pressure energy Kinetic energy (pressure) (velocity) conversion

33. Nozzle is a passage of varying cross section by means of which pressure energy of following fluid is converted into kinetic energy. We know that SFEE, For nozzle, W=0, Q=0, Z 1 – Z 2 = 0

34. Hence, SFEE When inlet velocity c 1 is small compared to the exit velocity c 2 , c 1 = 0  

35. We get If the working fluid is a perfect gas, then and  

36. (2) Diffuser Function : It is a device that increase pressure of a fluid at the expense of its velocity drop. A diffuser is a passage of varying cross section by mean of which kinetic energy of flowing fluid is converted into pressure energy. Kinetic energy Pressure energy (velocity) (pressure) The cross section area of diffuser increase in the Flow direction for subsonic flow as shown in fig. Decrease for supersonic flow. We know that SFEE, conversion

37. For Diffuser, W=0 Q =0 Z 1 -Z 2 =0 Hence,

38. In case of diffuser, C 2 C 1  

39. (3) Boiler Function : It is a device or equipment in which heat is produce by combustion of fuel is utilize to produce steam from water. at desired temperature and pressure as shown in fig. Fuel + Air Heat Water Steam combustion

40. W=0 Z 1 -Z 2 =0 Hence, mh 1 + Q = mh 2  

41. Hydraulic or water turbine Function : It is hydraulic machine, converts hydraulic energy into mechanical energy as shown in fig. Water (with high P.E) Mechanical energy We know that SFEE

42. For Hydraulic turbine, Q=0 W=+W U 1 =U 2 h 1 -h 2 = u 1 + p 1 v 1 - u 2 - p 2 v 2 = p 1 v 1 - p 2 v 2 Hence,

43. Steam or Gas turbine Function :- It is device for producing work output from a flow of fluid which is expanding from high pressure to low pressure. The work output from the turbine may be used to run a generator and produce electric power as shown in fig.

44. We know that SFEE, For turbine, 1. = , ∆PE = 0 2. , ∆KE = 0 3. Q =0 4. W ≠ 0(shaft work is done by the system,so W is +ve) i.e, W = m

45. Compressor Function :- It is device used to increase the pressure of a fluid. Rotary compressor:- We know that SFEE, For rotary compressor, 1. 2.

46. 3. Q =0 4. W ≠ 0(shaft work is done on the system,so W is -ve) W = m , where,

47. (2) Reciprocating compressor :- We know that SFEE, For reciprocating compressor, 1. = , ∆PE = 0 2. , ∆KE = 0 3. Q ≠ 0 4. W ≠ 0, shaft work is done on the system, W is –ve. i.e,

48. Centrifugal water pump Function :- The pump is mechanical device which conveys liquid from one place to another place. It is the hydraulic machines which converts the mechanical energy into hydraulic energy. We know that SFEE,

49. For pump, (1) Q = 0, no heat transfer (2) W = - W,work supplied (3) Change of internal energy i.e,

50. Heat exchanger It is a device in which transfers heat from one fluid to another fluid.In heat exchanger, under steady state operation, the mass flow rate of each fluid stream flowing through a heat exchanger remains constant. The condenser, evaporator and radiator are example of heat exchangers,

51. 1. Condenser :- Function:- It is a device used to condense the steam by rejecting heat from steam to cooling medium. We know that SFEE, For condenser, 1. ∆PE = 0, 2. ∆KE = 0, 3. W = 0 ,no shaft work

52. i.e,

53. 2. Evaporator :- Function:- It is used to extract heat from the space to be cooled at low temprature. - The refrigerant liquid enters the evaporator ,which absorbs latent heat from the space to be cooled, and it is converted into vapour.

54. We know that, For evaporator, 1. ∆PE = 0, 2. ∆KE = 0, 3. W = 0 ,no shaft work i.e,

55. UNSTEADY FLOW PROCESS : A s previous article steady flow analysis, there is neither a change in the energy of control volume nor in the properties of fluid at any section with respect to time. H owever in many of thermodynamic processes like the filling and emptying process, the internal energy and mass of tank changes with respect to time.

56. T he filling and emptying process and variable flow processes and can be analysed by the control volume technique.

62. B ott l e or tank emptying process T he tank emptying process is the reverse if tank filling process. T here is flow of fluid from tank to the surroundings as shown in figure. I f the surroundings is too much large compared to the system,energy of fluid leaving the tank of entering the surroundings is constant.

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