ENTC 370: Announcements

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ENTC 370 Announcements

• Yes, labs this week
• Lab manual has been uploaded
• Go to E-learning to download copy
• Homework assignments
• Assigned Problems
• 1.9, 1.20, 1.24, 1.28, 1.34, 1.39, 1.47, 1.49,
  1.55.
• Due next Tuesday, September 9th before 1050 am
• For more information, go to
• http//etidweb.tamu.edu/classes/entc370

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First Law of Thermodynamics

• Known as Conservation of Energy Principle
• Based on experimental observations
• Energy can be neither created nor destroyed
  during a process it can only change forms
• Energy Balance Basic accounting problem

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Energy Change of a System

• Energy Energy at Energy at
• change final state initial state
• DEsystem Efinal Einitial
• DEsystem DU DKE DPE

For stationary systems ? DKE 0 DPE 0 ?
DEsystem DU
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1st Law Examples for Closed Systems
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1st Law Examples for Closed Systems
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1st Law Examples for Closed Systems
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Mechanism of Energy Transfer, Ein and Eout

• Heat Transfer Just adding or removing heat, Q
• Work Not caused by temperature difference, W
• Requires motion
• Piston
• Movement of electrons in a wire
• Mass flow Mass carries energy, either dumps it
  or removes it, m.

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Mechanism of Energy Transfer, Ein and Eout
For Open and Closed Systems
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Energy Balance (1st Law) for a Closed System

• General Form
• Q W DE
• For Closed System
• Q - W DU DKE DPE, since DKE, DPE are
  negligible, then
• Q - W DU
• For closed systems ?

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Example
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Example
A system (piston-cylinder device) receives 5 kJ
of heat transfer and experiences a decrease in
energy in the amount of 5 kJ. Determine the
amount of work done by the system.
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Closed System Undergoing a Cycle

• Cycle Initial and final states are identical

Applying 1st Law
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Example
A steam power plant operates on a thermodynamic
cycle in which water circulates through a boiler,
turbine, condenser, pump, and back to the boiler.
For each kilogram of steam (water) flowing
through the cycle, the cycle receives 2000 kJ of
heat in the boiler, rejects 1500 kJ of heat to
the environment in the condenser, and receives 5
kJ of work in the cycle pump. Determine the work
done by the steam in the turbine, in kJ/kg.
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Example
Air flows into an open system and carries energy
at the rate of 300 kW. As the air flows through
the system it receives 600 kW of work and loses
100 kW of energy by heat transfer to the
surroundings. If the system experiences no
energy change as the air flows through it, how
much energy does the air carry as it leaves the
system, in kW? System sketch
Ih
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Energy Conversion Efficiencies A measure of
performance for a device is its efficiency and is
often given the symbol ?. Efficiencies are
expressed as follows
lt 1 or 100
Efficiency Examples Water Heater
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Cb.e
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Combustion Efficiency
Qout Heat transfer from combustion process HV
Heating Value of fuel
The lower heating value, LHV, is the heating
value when water appears as a gas in the products.
The higher heating value, HHV, is the heating
value when water appears as a liquid in the
products.
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Example A steam power plant receives 2000 kJ of
heat per unit mass of steam flowing through the
steam generator when the steam flow rate is 100
kg/s. If the fuel supplied to the combustion
chamber of the steam generator has a higher
heating value of 40,000 kJ/kg of fuel and the
combustion efficiency is 85, determine the
required fuel flow rate, in kg/s.
(diagram shows different quantities)
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Efficiencies
Power Plant Overall Efficiency
? Devices are connected in series
Motor Efficiency
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Lighting Efficacy

• Other energy related issues to be discussed later
  in the semester
• Energy and Environment
• Ozone and Smog
• Acid Rain
• Global Warming

Ch.3
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Chapter 3Properties of Pure Substances

• Matter can be subdivided into phases
• Solids, liquids, and gases
• In thermodynamics, we are mainly concerned with
  liquids, gases, and mixtures (liquid-gas phase)
• Pure substances are homogeneous (chemical
  composition does not change)
• Example Air (made of Nitrogen, Oxygen, CO2)
• Non-pure substances are heterogeneous
• Example Oil and Water (they dont mix)

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Chapter 3Properties of Pure Substances

• Pure substance
• A pure substance has a homogeneous and invariable
  chemical composition and may exist in more than
  one phase.
• Homogeneous Substance
• A substance that has uniform thermodynamic
  properties throughout is said to be homogeneous.

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Examples of Pure Substances

• Water (solid, liquid, and vapor phases)
• Mixture of liquid water and water vapor
• Carbon dioxide, CO2
• Nitrogen, N2
• Mixtures of gases, such as air, as long as there
  is no change of phase.

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Examples of Phases
Liquid
Gas
Solid
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Phase-Change Processes of Pure Substances
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Phases and Phase Change Processes

• Phases Solid, Liquid or Gas
• Liquid
• Compressed Liquid (CL) or subcooled liquid Is
  not to vaporize
• Saturated liquid (SL) Is about to vaporize

Compressed Liquid
Saturated Liquid
State 2
P 1 atm T 100 C
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Phase Change

• Vapor
• Saturated Is about to condense
• Superheated Is not about to condense
• Saturated liquid-vapor mixture
• Liquid and vapor phases coexist

Superheated Vapor
Saturated Vapor
State 4
P 1 atm T 100 C
P 1 atm T 300 C
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The p-T plane for water
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Heating Process of Water at Constant Pressure
SHV
CL
SV
SL
MX
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Phase Change Liquid to Vapor

• The energy required for solid-liquid or
  liquid-gas (vapor) transformations is called
  latent heat
• For melting Latent Heat of Fusion (LHF)
• For vaporization Latent Heat of Vaporization
  (LHV)
• Examples
• Water
• LHF is about 333.7 kJ/kg
• LHV is about 2,257.1 kJ/kg