ENTC 370: Announcements

1. ENTC 370: Announcements • Homework assignments No.2: • Assigned Problems: • 2.1, 2.19, 2.20, 2.24, 2.45, 2.53, 2.58, 2.80, 2.82, 2.83, 2.95. • Due next Tuesday, September 23rd before 10:50 am • For more information, go to: • http://etidweb.tamu.edu/classes/entc370

2. First Law of Thermodynamics • Known as Conservation of Energy Principle • Based on experimental observations: • Energy can be neither created nor destroyed during a process; it can only change forms • Energy Balance: “Basic accounting problem”

3. E= -5 kJ Qin =5 kJ Wout=? System Boundary Example A system (piston-cylinder device) receives 5 kJ of heat transfer and experiences a decrease in energy in the amount of 5 kJ. Determine the amount of work done by the system.

4. Closed System Undergoing a Cycle • Cycle: Initial and final states are identical Applying 1st Law:

5. Example A steam power plant operates on a thermodynamic cycle in which water circulates through a boiler, turbine, condenser, pump, and back to the boiler. For each kilogram of steam (water) flowing through the cycle, the cycle receives 2000 kJ of heat in the boiler, rejects 1500 kJ of heat to the environment in the condenser, and receives 5 kJ of work in the cycle pump. Determine the work done by the steam in the turbine, in kJ/kg.

6. Open System Example Air flows into an open system and carries energy at the rate of 300 kW. As the air flows through the system it receives 600 kW of work and loses 100 kW of energy by heat transfer to the surroundings. If the system experiences no energy change as the air flows through it, how much energy does the air carry as it leaves the system, in kW? System sketch: I:h

7. Energy Conversion Efficiencies A measure of performance for a device is its efficiency and is often given the symbol . Efficiencies (h) are expressed as follows: < 1 or 100% Efficiency Examples:Water Heater

8. Cb.e

9. CO2 H2O N2 Fuel CnHm Combustion Chamber Air Products PP, TP Reactants TR, PR Qout= HV Combustion Efficiency Qout: Heat transfer from combustion process HV: Heating Value of fuel The lower heating value, LHV, is the heating value when water appears as a gas in the products. The higher heating value, HHV, is the heating value when water appears as a liquid in the products.

10. Higher Heating Values of Common Fuels *More HHV/LHV can be found in Appendix A-27/A-27-E

11. Efficiencies Power Plant Overall Efficiency: ← Devices are connected in series Motor Efficiency:

12. Example A steam power plant receives 2000 kJ of heat per unit mass of steam flowing through the steam generator when the steam flow rate is 100 kg/s. If the fuel supplied to the combustion chamber of the steam generator has a higher heating value of 40,000 kJ/kg of fuel and the combustion efficiency is 85%, determine the required fuel flow rate, in kg/s. (diagram shows different quantities)

13. Lighting Efficacy/Effectiveness: • Other energy related issues to be discussed later in the semester: • Energy and Environment • Ozone and Smog • Acid Rain • Global Warming http://eartheasy.com/live_led_bulbs_comparison.html#a

14. Chapter 3Properties of Pure Substances • Matter can be subdivided into phases • Solids, liquids, and gases • In thermodynamics, we are mainly concerned with liquids, gases, and mixtures (liquid-gas phase) • Pure substances are homogeneous (chemical composition does not change) • Example: Air (made of Nitrogen, Oxygen, CO2) • Non-pure substances are heterogeneous • Example: Oil and Water (they don’t mix)

15. Chapter 3Properties of Pure Substances • Pure substance • A pure substance has a homogeneous and invariable chemical composition and may exist in more than one phase. • Homogeneous Substance • A substance that has uniform thermodynamic properties throughout is said to be homogeneous.

16. Examples of Pure Substances • Water (solid, liquid, and vapor phases) • Mixture of liquid water and water vapor • Carbon dioxide, CO2 • Nitrogen, N2 • Mixtures of gases, such as air, as long as there is no change of phase.

17. Examples of Phases Gas Solid Liquid http://upload.wikimedia.org/wikipedia/commons/0/0b/Phase_change_-_en.svg http://en.wikipedia.org/wiki/Phase_%28matter%29

18. p Critical Point Liquid Vapor- Liquid Vapor Solid Triple Point T The p-T plane for water Properties are used to distinguish among phases

19. Phase-Change Processes of Pure Substances P V

20. State 1 P = 1 atm T = 20° C Phases and Phase Change Processes • Phases: Solid, Liquid or Gas • Liquid • Compressed Liquid (CL) or subcooled liquid: Is not ready to vaporize (P > Psat, T < Tsat) • Saturated liquid (SL): Is about to vaporize Compressed Liquid Saturated Liquid State 2 P = 1 atm T = 100° C

21. State 3 Saturated Vapor P = 1 atm T = 100° C Saturated Liquid Phase Change • Vapor • Saturated: Is about to condense • Superheated: Is not ready to condense (T > Tsat) • Saturated liquid-vapor mixture • Liquid and vapor phases coexist Superheated Vapor Saturated Vapor State 4 P = 1 atm T = 100° C P = 1 atm T = 300° C

22. Heating Process of Water at Constant Pressure SHV CL T SV SL MX n

23. Phase Change: Liquid to Vapor • The energy required for solid-liquid or liquid-gas (vapor) transformations is called latent heat • For melting: Latent Heat of Fusion (LHF) • For vaporization: Latent Heat of Vaporization (LHV) • Examples: • Water: LHF is about 333.7 kJ/kg LHV is about 2,257.1 kJ/kg

24. Latent Heats Latent Heat of Vaporization (LHV): Amount of energy required to allow phase change (Liquid ←→ Vapor) Latent Heat of Fusion (LHF): Amount of energy required to allow phase change (Solid ←→Liquid)

25. Saturation Temperature and Saturation Pressure • The temperature at which water starts boiling depends on pressure. If pressure is fixed, so is the boiling temperature. • Tsat @ P • Psat @ T • During phase change, temperature and pressure are dependent properties Saturation Temperature ←→ Saturation Pressure

26. Water: Liquid-Vapor Saturation Temperature and Pressure Liquid Gas or Vapor

27. Liquid Gas or Vapor

28. Property Diagrams for Phase-Change Process Critical Point: Saturated Liquid and Vapor States are indistinguishable LHV decreases with Pressure Each curve is represented in Slide No. 18

29. Property Diagrams for Phase-Change Process (T-v Diagram)

30. P-v Diagram

31. P-T Diagram Note: Each substances has its own Triple Point (Table 3-3)

32. Questions • What is LHV and LHF? • What is saturation temperature, pressure? • Does saturation temperature increase, decrease or remain the same when saturation pressure goes up? • Does LHV increase, decrease or remain the same when saturation pressure increases?

33. Applications of Phase Change Process • Refrigeration: Pressure of liquid or vapor is controlled to induce phase change • During phase change process energy is released or absorbed • Examples: • Vacuum Freezing • Typical refrigeration and air conditioning systems • Power Generation: Water undergoes phase change process • Steam is used to power steam turbine

34. Property Tables • Enthalpy – A combination property • H = U + PV (kJ) • h = u + Pv (kJ/kg) • Thanks to Prof. Mollier who combined u + Pv to represent heat content or total heat in steam turbines h1 = u1 + P1n1 h2 = u2 + P2n2

35. Saturated Liquid and Saturated Vapor • Table A-4: water properties as a function of saturation temperature • Table A-5: water properties as a function of saturation pressure • Subscripts: • f: Saturated Liquid • g: Saturated Vapor • fg: Difference between vapor and liquid