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Reading Materials: Chapter 7

Reading Materials: Chapter 7. Energy Balance. LECTURE 22. Shaft Work vs Flow Work . Illustration 1 (from L. 21). Power generation system Steady state, Flow rate of working fluid (eg. Water) is . Turbine. Boiler. Generator. Condenser. Pump. Boiler . Illustration 22-1.

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Reading Materials: Chapter 7

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  1. Reading Materials: Chapter 7 Energy Balance LECTURE 22 CHEM ENG 1007

  2. Shaft Work vs Flow Work CHEM ENG 1007

  3. Illustration 1 (from L. 21) Power generation system • Steady state, • Flow rate of working fluid (eg. Water) is Turbine Boiler Generator Condenser Pump CHEM ENG 1007

  4. Boiler CHEM ENG 1007

  5. Illustration 22-1 1000 kg/h water at 75oC and 1 bar is fed to a pump, where the pressure of the water is increased to 20 bar. The high-pressure is fed to the boiler to produce steam at 250oC and 20 bar. The steam fed to a turbine. The exhaust steam exists in the turbine is saturated steam at 1 bar. The exhaust steam is cooled further, leaves the condenser as a subcooled liquid at 75oC, and the cycle continues. Calculate (a) the work produced by the turbine, (b) the heat input in the boiler, (c) the work required by the pump, (d) the heat removed by the condenser, (e) the efficiency of the heat engine. CHEM ENG 1007

  6. Power Generation System CHEM ENG 1007

  7. Power Generation System From Steam Table 4 3 2 1 CHEM ENG 1007

  8. Solution a) Work produced by the turbine b) Heat input to the boiler CHEM ENG 1007

  9. Solution c) Work input to the PUMP d) Heat removed from the condenser CHEM ENG 1007

  10. Solution e) The efficiency of the heat engine CHEM ENG 1007

  11. EB associates with Phase Changes • Chemical processes often include the change of a material between the three phases: solid, liquid, and vapour. • For example, water can be in the form of solid (ice), liquid, or vapour (steam). • Such processes involve the addition or removal of heat. CHEM ENG 1007

  12. EB associates with Phase Changes The three combinations of phases between which a change can occur are as follows: • Liquid to vapour (vaporization) or vapour to liquid (condensation) • Solid to liquid (melting) or liquid to solid (freezing) • Solid to vapour (sublimation) or vapour to solid (solid condensation) CHEM ENG 1007

  13. Phase Diagram CHEM ENG 1007

  14. Conversion of water to steam in a boiler CHEM ENG 1007

  15. Conversion of water to steam in a boiler • Initially the water at ambient temperature is subcooled. • As heat is added its temperature rises steadily until it reaches the saturation temperature corresponding with the pressure in the cylinder. The volume of the water hardly changes during this process. At this point the water is saturated. CHEM ENG 1007

  16. Conversion of water to steam in a boiler • As more heat is added, steam is generated and the volume increases dramatically since the steam occupies a greater space than the water from which it was generated. • The temperature however remains the same until all the water has been converted into steam. At this point the steam is saturated. • As additional heat is added, the temperature of the steam increases but at a faster rate than when the water only was being heated. The volume of the steam also increases. Steam at temperatures above the saturation temperature is superheated. CHEM ENG 1007

  17. Conversion of water to steam in a boiler If the temperature T is plotted against the heat added Q the three regions namely subcooled water, saturated mixture and superheated steam are clearly indicated. CHEM ENG 1007

  18. Conversion of water to steam in a boiler Three stages are evident: (i) “Sensible heat”: T of water raised to boiling point. (ii) “Latent heat”: constant T and P CHEM ENG 1007

  19. Conversion of water to steam in a boiler (iii) “Superheat”: Heat added to dry saturated steam raises its T, at constant P. CHEM ENG 1007

  20. CHEM ENG 1007

  21. How to determine Phase Enthalpy Change? Example: Water Solid  Liquid (Melting/Solidification) Liquid  Gas (Vaporisation/Condensation) CHEM ENG 1007

  22. How do we calculate Enthalpy (H)? • Another Example of steam table Ĥ=2675-417.4 = 2257.6 kJ/kg CHEM ENG 1007

  23. Example 10.3 A steady-state boiler produces steam from a waste-water stream. The water enters the boiler as saturated water at 5.7 atm and 430 K (the boiling point of water at 5.7 atm), and the steam exists the boiler as saturated steam at that same temperature and pressure. The properties of importance are: Mass flow rate : 8150 kg/hr Heat of vaporization at 5.7 atm and 430 K: 2091 kJ/kg How much heat must be added to the process? CHEM ENG 1007

  24. Example 10.3 boiler Saturated water (liquid) 5.7 atm, 430K Saturated steam (vapour) 5.7 atm, 430K How much heat must be added to the process? Notices that heat is being added to the process, so Q is a positive value CHEM ENG 1007

  25. Illustration 22-2 Conversion of liquid water into “superheated” steam 100 kg/h of water at 25oC & 1 atm is vaporised into steam at 120oC and 2 atm. Calculate the heat input required. Given that: CHEM ENG 1007

  26. Solution • Path 1: ignore the effect of pressure (assume outlet pressure =1 atm) H [L, Tb( 100oC)] H [V, Tb( 100oC)] H (L, 25oC) H (V, 120oC) CHEM ENG 1007

  27. Solution • Path 2: ignore the effect of pressure (assume 2 atm at inlet) H [L, Tb( 105oC)] H [V, Tb( 105oC)] H (L, 25oC) H (V, 120oC) CHEM ENG 1007

  28. Solution • Path 3 (by using steam table) From Steam Table CHEM ENG 1007

  29. Illustration 22-3 A small evaporator fitted with three identical electrical heating elements connected in parallel, is used to vaporize alcohol on a continuous basis. Liquid alcohol is fed in at a temperature of 24oC and at a rate of 0.018 kg s-1. Alcohol vapour is removed at the same rate, at a temperature of 82oC. Because heat is lost to the surrounds only 80% supplied is available to heat the alcohol, determine the required power input (kW) Data for alcohol: Mean specific heat 3 kJ kg-1K-1 Enthalpy of vaporization at 82oC: 650 kJ kg-1 CHEM ENG 1007

  30. Solution CHEM ENG 1007

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