Notes 5 pure substances

8/11/2019 Notes 5 pure substances

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Lec 5: Thermodynamic

properties, Pvtbehavior


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For next time: Read: 3-5

HW 3 due Sept 17

Outline: Buoyancy and stability

Pure substances and processes Property diagrams for pure substances

Important points:

How to calculate point of action of hydrostaticload

The general shape of the property diagrams

How to solve problems using the propertydiagrams


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Properties - Introduction

We have discussed extensive propertiessuch as U, m, and V (for volume) whichdepend on the size or extent of a system,

and Intensive properties such as u, v, T, and P

which are independent of system extent(engineers are ambivalent about

nomenclature for pressure, sometimesusing p and P interchangeably)


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Important questions...

How many properties are needed to definethe state of a system?

How do we obtain those properties?


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For a simple system,

We may write: p = p(v,T)

or perhaps: v = v(p,T).


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Pure

Puremeans of uniform and invariablechemical composition (but more thanone molecular type is allowed). Thisallows a single phase of air to be a puresubstance.

All our substances will be pure. We will

drop the use of the word. When werefer to a simple system we mean onefilled with a pure substance--a simple,pure system.


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For a simple, pure substance

y1 = f(y2,y3), or

p = p(v,T), v = v(p,T) and T = T(p,v)

What do these equations define, in space?

Equations used to relate properties arecalled equations of state


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Ideal gas law is a simpleequation of state

RTPvM

RR

u

Ru = universal gas constant

m = mass

n = number of moles

M = molar mass or molecular weight

mRTPV TnRPVu


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Behavior of real substances

Lets consider substances that do not obeythe perfect gas law

certainly not as a solid

certainly not as a liquid

sometimes not very well as a gas


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Phase Change Process of a PureSubstance

P=1 atmT=20C

P=1 atmT=100C

P=1 atmT=100C

P=1 atmT=250C

P=1 atmT=100C

liquid liquid liq/vap

vapor vapor


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Consider a constant pressureprocess (of water)


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T-v Diagram


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You should be able to discusscharacteristics of the following:

Liquid to liquid/vapor to vapor transition(begin with a constant pressureprocess).

Single phase regions--liquid, vapor,solid.

Two-phase regions--liquid/vapor andsolid/vapor.

Melting--solid to liquid (freezing)

vaporization--liquid to vapor(liquefaction)

sublimation--solid to vapor


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Notice the triple-stateline. Along this line all

three states exist inequilibrium

For water, the triple point is at273.16 K (32.018 F) and 0.6113 kPa

(0.0887 psia).


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Vapor Dome - regionencompassing the two-phase, vapor-liquidequilibrium region

Saturated vapor line

Saturated liquid line


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Phase Diagram (PT-coordinates)

P

T

Solid Phase

Region

Liquid

Phase

Region

Superheated Gas

Phase Region

Melting

Curve

Triple

State

Critical

State


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GAS @ gGAS

State d

Weight

LIQUID

GAS

Weight

LIQUID @ a

P

T

g

P

T

g

d

P

T

g

d

a

Superheated

Vapor

CompressedLiquid

Constant Temperature Process

Q


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P

T

b

Gas @ bGas @ b

Q

GAS

STATE f

LIQUID

Q

GASLIQUID

Q

P

T

bf

P

T

bfa

SuperheatedVapor

SubcooledLiquid

Constant Pressure Process


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P

v

SuperheatedRegion

SaturationRegion

SubcooledRegion

Critical Point

Sat. Vapor

Line

Sat. LiquidLine


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Saturation temperature

Temperature at which a phase changetakes place at a given pressure.


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Saturation pressure

Pressure at which a phase change takesplace at a given temperature.


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TEAMPLAY

Discuss what happens when water boils onthe stove at your residence. Start with apan of water at 70 F, 1 atm pressure.

QContinued onnext slide.


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TEAMPLAY (CONTINUED)

What is the pressure at various timesduring the entire process?

Does the temperature of the water

change?

If the vapor (steam) were contained inan elastic container, what would happen

as heat continued to be added after allthe liquid disappeared.

Sketch pV and TV diagrams.


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Saturation properties

Along thesaturated liquidline, properties areidentified by the

subscript f

Along thesaturated vaporline, properties areidentified by the

subscript g

Both sets of properties can be found in thetemperature and pressure tables in your

books appendices.Both tables give the same data.

There are different tables for different

substances.


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Temperaturetable (also

known as asaturationtable)

Table A-4


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Pressure table(also known

as asaturationtable)

Table A-5

[T o p ope ties a e not


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[Two properties are notindependent in the vapor dome

(the two-phase region)]

The temperature and pressure areuniquely related. Knowing a T defines theP and vice versa.


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TEAMPLAY

Find, for Refrigerant 134a, the followingproperties: the saturation pressure ata saturation temperature of -10 F.

and find for the same substance thesaturation temperature at a pressure of0.06 MPa.

Make sure everyone in your group

understands how to do this.

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Notes 5 pure substances