Steam Engine

Sliding valve

Steam enters chamber fromleft, leaving from right both valves closed, piston moves to the right

Steam enters from right,leaving from leftpiston moves back to the left

Thermodynamics Properties

Thremo Porperties-1.ppt

Two independent properties are needed to define the state of a pure substance (Ch. 2, YAC)

Pure Substance: A homogeneous substance with a fixed chemical composition.Does not have to be a single element or in a single phase.

Helium, Nitrogen, Water, Sulfur-di-Oxide are pure substances.

Pressure (p), Temperature (T), specific volume (v) internal energy (u) andenthalpy (h) are among the most common thermodynamic properties.

The variation or behavior of properties of a pure substance and their relationshipto each other is most commonly and easily examined via property diagrams and property tables.

Property diagrams are also used to analyze the thermodynamic processes in thermal systems and thermodynamic cycles under which many systems operate.

Simple steam power plant cycle

Heat in

Work out

Condensing back to liquidfor easy transportation

Heat in

Work out

Heat out to cooling water

Work inby pump

Property Diagram

Superheated steam(or vapor)

Saturated waterliquid+vapor

Compressed(subcooled)liquid

Phase Change

Above critical point,there is no cleardistinction betweenliquid and vapor phases

Melting

vaporization

sublimation

Constant pressure process

Saturated (liquid+vapor)

p

v

p

T

T

v

Constanttemperaturelines

Constant pressureline

(a) P-v diagram: Along the constant temperature line, increasing vapor pressure will lead to higher density (lower specific volume) due to compression. After it reaches the saturation pressure at that temperature, the vapor will condense into liquid while the pressure remains constant until all vapor condense into liquid.

(b) T-v diagram: Along the constant pressure line, increasing liquid temperaturewill lead to lower density (higher specific volume) due to thermal expansion.At the saturation temperature, the liquid will vaporize into gaseous form at a constantpressure until all liquid vaporize.

p

T

(c)P-T phase diagram

solidliquid

vapor

Melting/freezing

Substances that contract on freezingSubstances that

expandon freezing ex. Water.Higher pressure makesit more difficult to freeze, thus requiring lower freezing temperature. The reverse is true for substances that contract on freezing

Critical point

Vaporization/condensation

Sublimation Triple point: all three phasescoexist in equilibrium

Critical point: A limiting state above which there is no clear distinction betweenliquid and vapor phases.

Vaporization: As pressure goes down, vaporization occurs at lower temps That’s why water boils at lower temperatures at higher elevations.