1 / 15

Lesson 3 ENERGY, WORK, AND HEAT

Lesson 3 ENERGY, WORK, AND HEAT. DEFINE the following: Heat Latent heat Sensible heat Units used to measure heat DEFINE the following thermodynamic properties: Specific enthalpy Entropy. Energy. The capacity of a system to perform work or produce heat Categories Potential Energy

Download Presentation

Lesson 3 ENERGY, WORK, AND HEAT

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Lesson 3ENERGY, WORK, AND HEAT • DEFINE the following: • Heat • Latent heat • Sensible heat • Units used to measure heat • DEFINE the following thermodynamic properties: • Specific enthalpy • Entropy

  2. Energy • The capacity of a system to perform work or produce heat • Categories • Potential Energy • Kinetic Energy • Specific Internal Energy • Specific P-V Energy • Enthalpy • Work • Heat • Entropy

  3. Potential Energy • Energy of position • PE = mgz/gc where PE = potential energy (ft-lbf) m = mass (lbm) z = height above some reference level (ft) g = acceleration due to gravity (32.17 ft/sec2) gc = gravitational constant = 32.17 ft-lbm/lbf-sec2

  4. Kinetic Energy • Energy of motion • KE = mv2/2gc where: KE = kinetic energy (ft-lbf) m = mass (lbm) v = velocity (ft/sec) gc = gravitational constant = 32.17 ft-lbm/lbf-sec2

  5. Specific Internal Energy • Internal energy per unit mass • Equal to total internal energy (U) divided by the total mass (m). u = U/m where: u = specific internal energy (Btu/lbm) U = internal energy (Btu) m = mass (lbm) • Internal energy includes • Energy due to rotation, vibration, translation, and interactions among molecules • Can not be measured or evaluated directly

  6. Specific P-V Energy • Energy per unit mass. • Equals the total Pressure times the Volume of a liquid divided by the total mass m • Also equals the product of the pressure P and the specific volume v, and is written as Pv Pv = PV/m where: P = pressure (lbf/ft2) V = volume (ft3) n = specific volume (ft3/lbm) V m m = mass (lbm)

  7. Enthalpy • A property of a substance, like pressure, temperature, and volume, • Cannot be measured directly • Normally given with respect to some reference value. • Usually used in connection with an "open" system problem in thermodynamics • Specific enthalpy (h) h = u + Pv where u is the specific internal energy (Btu/lbm) P is the pressure of the system (lbf/ft2) v is the specific volume (ft3/lbm) of the system.

  8. Work • Energy in transit • Not a property of a system – it is a process done by or on a system • Defined as the action of a force on an object through a distance W = Fd where: W = work (ft-lbf) F = force (lbf) d = displacement (ft)

  9. Heat • Like work, is energy in transit • Occurs at the molecular level as a result of a temperature difference • Denoted by the letter Q • Heat transferred per unit mass denoted by q q = Q/m where: q = heat transferred per unit mass (Btu/lbm) Q = heat transferred (Btu) m = mass (lbm)

  10. Heat (continued) • Sensible Heat - The heat added to or removed from a substance to produce a change in its temperature • Latent Heat - The amount of heat added to or removed from a substance to produce a change in phase. • Latent Heat of Fusion - the amount of heat added or removed to change phase between solid and liquid. • Latent Heat of Vaporization - the amount of heat added or removed to change phase between liquid and vapor

  11. Heat Capacity • The ratio of the heat (Q) added to or removed from a substance to the change in temperature (ΔT) produced • Denoted by Cp • Specific heat (cp) is heat capacity of a substance per unit mass • Applies when the heat is added or removed at constant pressure

  12. Heat Capacity (continued) Cp = Q/ΔT cp = Q/mΔT cp = q/ΔT where: Cp = heat capacity at constant pressure (Btu/°F) cp = specific heat at constant pressure (Btu/lbm-°F) Q = heat transferred (Btu) q = heat transferred per unit mass (Btu/lbm) m = mass (lbm) ΔT = temperature change (°F)

  13. Entropy (S) • A property of a substance • Quantifies the energy of a substance that is no longer available to perform useful work ΔS = ΔQ/Tabs Δ s = Δq/Tabs where: Δ S = the change in entropy of a system during some process (Btu/°R) ΔQ = the amount of heat transferred to or from the system during the process (Btu) Tabs = the absolute temperature at which the heat was transferred (°R) Δs = the change in specific entropy of a system during some process (Btu/lbm -oR) Δq = the amount of heat transferred to or from the system during the process (Btu/lbm)

  14. Power • Power - The time rate of doing work. • Units are energy per unit time • English system - ft-lbf/sec or ft-lbf/hr • Horse Power - hp • British thermal units per hour - Btu/hr • Electrical units - watts (W) or kilowatts (kW)

  15. Energy and Power Equivalencies • 1 ft-lbf = 1.286 x 10-3 Btu = 3.766 x 10-7 kW-hr • 1 Btu = 778.3 ft-lbf = 2.928 x 10-4 kW-hr • 1 kW-hr = 3.413 x 103 Btu = 2.655 x 106 ft-lbf • 1 hp-hr = 1.980 x 106 ft-lbf • 1 Joule = 778 ft lbf/Btu • 1 ft-lbf/sec = 4.6263 Btu/hr = 1.356 x 10-3 kW • 1 Btu/hr = 0.2162 ft-lbf/sec = 2.931 x 10-4 kW • 1 kW = 3.413 x 103 Btu/hr = 737.6 ft-lbf/sec • 1 hp = 550.0 ft-lbf/sec

More Related