1 / 20

Pure Substances

Pure Substances. Thermodynamics Professor Lee Carkner Lecture 5. PAL # 4 First Law. Pumping water uphill and then running it back to produce energy Rate of power imparted to water = W’ water = (1000)(2)(9.8)(40) = But the pump and the turbine are only 75% efficient

liana
Télécharger la présentation

Pure Substances

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. Pure Substances Thermodynamics Professor Lee Carkner Lecture 5

  2. PAL # 4 First Law • Pumping water uphill and then running it back to produce energy • Rate of power imparted to water = • W’water = (1000)(2)(9.8)(40) = • But the pump and the turbine are only 75% efficient • hpump = W’water/W’pump • W’pump = 785/0.75 = • Cost per year = (1046)(10)(0.03)(365) = $114500 • hturbine = W’turbine/W’water • W’turbine = (785)(0.75) = • Revenue per year = (785)(10)(0.08)(365) = $171900 • Net Yearly Profit =

  3. PAL # 4 First Law • Should you build it? • No

  4. Pure Substances • A pure substance consists of either a single type of atom or molecule or a homogeneous mixture • Can exist as a mixture of phases • Gas • Liquid • Solid • Strong intermolecular forces, high density, fixed molecular position

  5. Types of Phases • Compressed liquid • Saturated liquid • Saturated Vapor • Saturated liquid and vapor can coexist together • Superheated vapor

  6. Saturation • A substance changes phase when it is at the saturation temperature for a given pressure • Can be compiled in tables for reference • Tsat increases with Psat • Need more internal energy to become vapor at higher pressure

  7. Change of Tsat with Psat

  8. Enthalpy • Defined as: h = u + Pv H = U +PV • Very useful for flow systems such as turbines

  9. Specific Volume and Quality • Mass of saturated liquid = mf • Mass of saturated vapor = mg • Since both phases often co-exist we define the quality, x: • The quality is the relative fraction of the mixture in the vapor phase

  10. Quality Relations • The total volume of a mixture is: • the volume occupied by each phase is: • We can use these relationships to write an expression for the combined specific volume of the mixture: vavg = vf +xvfg x = (vavg –vf )/vfg

  11. Property Diagrams • We will plot three key variables P, T, and v as: • Each region of the diagram represents a phase or mixture of phases

  12. Pv Diagram

  13. Pv Diagram Properties • Saturation curves define boundaries of liquid-vapor mixture region • Critical point is on the critical isotherm • Densities of both are the same • Above the critical point, no amount of pressure can condense the vapor to a liquid

  14. Regions on a Pv Diagram

  15. PT Diagram • Three curves can be drawn on the PT diagram • Fusion curve • Vaporization curve • Sublimation curve • The curves bound three distinct regions, one for each phase • Juncture of the three curves is the triple point where all three coexist

  16. PT Diagram

  17. Other PT Features • An isobar at standard atmospheric pressure intersects the normal boiling and melting points • The critical point is on the vaporization curve • Gas above critical T is called “gas”, below it is called “vapor”

  18. PvT Diagram for Water

  19. PVT Diagram • P, V and T relations can be formed to create a three dimensional diagram • The PT and PV diagrams are formed by projection

  20. Next Time • Read: 3.6-3.8 • Homework: Ch 3, P: 48, 54, 75, 86

More Related