Tam 201 Thermodynamics Lecture 2

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Tam 201Thermodynamics Lecture 23
2
Conservation of Mass
Mass can not be created or destroyed
System boundary (arbitrary boundary around a
system)

• Rate (kg/s)-
• rate that mass accumulates

Mass flow rate Out of the system
Mass flow rate Into the system
3
Conservation of Mass
For a ridged system (does not expand or
contract) And incompressible fluids

• rate that mass accumulates

Otherwise it will Explode!
0
Mass flow rate Out of the system
Mass flow rate Into the system
5kg/s
2kg/s
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1st law of Thermodynamics
Energy can not be created or destroyed ,but can
be changed From one form to another (you cant
get better than breaking even)
2nd law of Thermodynamics
Anything that happens will increase the entropy
(disorder) of the universe. (You cant break
even)
3rd law of Thermodynamics
There is zero entropy in a perfect crystal at
absolute zero
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1st Law

• Amount Format (if no mass flow though boundaries)

Used for batch type processes
Change in potential Energy of System from Time
1 to time 2
Change in internal Energy of System from Time 1
to time 2
Change in kinetic Energy of System from Time 1
to time 2
Heat transferred Into system From time 1 To time
2
Work done On system From time 1 To time 2
1Q2
1W2
z1
z2
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1st Law

• Amount Format (if no mass flow though
  boundaries) how to simplify

No shaft Work or rigid boundaries
adiabatic (perfectly insulated no heat transfer)
No change in temperature
No change in height
No change in velocity
0
0
0
0
0
Change in potential Energy of System from Time
1 to time 2
Change in internal Energy of System from Time 1
to time 2
Change in kinetic Energy of System from Time 1
to time 2
Heat transferred Into system From time 1 To time
2
Work done On system From time 1 To time 2
1Q2
1W2
z1
z2
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1st Law example
Time 1
Time 2
45m/s baseball
Baseball at rest Same height and temperature as
time 1
How much heat is transferred from time 1 to time
2?
No shaft Work or rigid boundaries
No change in temperature
No change in height
0
0
0
Plug in
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In class assignment 2
1W2
A mass of 100 kg is dropped 1m to Provide shaft
work for an electric Generator- assume no heat
transfer And no change in temperature g9.8m/s2
Time 1 mass at rest
1m
Time 2 mass at rest
How much shaft work is produced from time 1 to
time 2 (simplify first law expression and plug
in)?
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1st Law - Energy Conservation

• Rate Format

1Q2
1W2
J/sWatt
z1
z2
boundary transports
Rate of energy transport In the mass coming into
System (W)
system energy
Rate of energy transport In the mass coming out
of system (W)
0
for steady state (occurring at a constant
rate) Processes
Power input To the system (W)
Heat transfer rate into system (W) Through walls
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1st Law - Energy Conservation

• Rate Format
• simplifications

No change in velocity from Inlet to outlet
No change in height from Inlet to outlet
0
0
0
Closed system No mass flow
system energy
0
for steady state steady flow SSSF (occurring
at a constant rate) Processes
0
No power input To the system (W)
No Heat transfer into system (W) Through walls
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Rankine Power Cycle

• We generally think of the Rankine cycle as the
  common steam power plant cycle.
• It was not that long ago that the steam cycle was
  a major automotive power plant
• How about the future???

• 1906 Stanley Steamer
• -127 mph Stanley Rocket

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Rankine Cycle Components
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Abbott Power Plant
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Turbine (unit 3 at Abbott, 1.5MW)
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Condenser
The condenser is a shell and tube heat exchanger
located directly underneath the turbine
Cooling water from cooling towers
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Boiler Feedwater Pump (steam driven)
steam turbine
pump
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Boiler 6 stories
Main floor coal-fired boiler Coal feeding section
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1st Law - Closed System (no mass flow in or out)
Rankine Cycle Power Plant
Turbine Condenser
Boiler
Steady
Pump
0
Closed system No mass flow
0
for steady state (occurring at a constant
rate) Processes
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Basic Rankine Cycle Example
Set up 1st Law Analyses
All components assumed to operate under SSSF
Conditions (therefore, no change of system mass,
energy, and entropy) assume no change in height
or velocity
Pump
Boiler
Conservation of mass
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Basic Rankine Cycle Example
Pump
Boiler
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In class assignment 3
All components assumed to operate under SSSF
conditions (therefore, no change of system
mass, energy, and entropy) assume no change in
height or velocity
Turbine
Condenser
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Basic Rankine Cycle Example
Set up 1st Law Analyses
All components assumed to operate under SSSF
conditions (therefore, no change of system mass,
energy, and entropy) assume no change in height
or velocity
Pump
Boiler
Turbine
Condenser
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Basic Rankine Cycle Example
Note from mass conservation we have
Pump
Boiler
Turbine
Condenser
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General 2nd Law

• Entropy Balance Rate Format

Entropy flow with heat transfer
Rate of system entropy change
Entropy generated is the left over after
calculating all other quantities
Boundary Temperature
Entropy Gen. Always
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General 2nd Law simplifications
Reversible No entropy generated
0
0
0
0
SSSF
Closed system
Adiabatic (no heat transfer well insulated)
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Basic Rankine Cycle Example
2nd Law Relations
Pump
reversible adiabatic
Use this
Boiler
Turbine
reversible adiabatic
Use this
Condenser
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Rankine Power Cycle ExampleBasic Cycle
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Pump
Boiler
Turbine
Condenser
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How much can we sell the electricity for?
What is the cost of the fuel?
Natural gas 0.75/therm (100 cubic feet of
natural gas)
Coal 0.25/therm (60/ton) In the same manner
find 270,000/year
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Power Plant Efficiency
From last example
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Carnot efficiency - cant get better than
this (no entropy generation and reversible)
Power
Use absolute Ts!
TH
QH
Work
Refrigeration AC
QL
TL
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In class assignment 2
1W2
A mass of 100 kg is dropped 1m to Provide shaft
work for an electric Generator- assume no heat
transfer And no change in temperature g9.8m/s2
Time 1 mass at rest
Time 2 mass at rest
1m
How much shaft work is produced from time 1 to
time 2 (simplify first law expression and plug
in)?
No change in velocity
No heat transfer
No change in temperature
0
0
0
Simplified expression
Plug in