Chapter 4 Energy and Energy Balance

8/12/2019 Chapter 4 Energy and Energy Balance

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Chapter 5

Energy and

Energy Balance


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Forms of Energy

Threecomponent of total energy of asystem

Kinetic energy (Ek)

energy due to the translational motion of thesystem as a whole relative to some frame ofreference (usually the earths surface) or to

rotation of the system about some axis. Potential energy (Ep)

energy due to the position of the system in a

potential field (such as a gravitational or


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Transfer of Energy

In closed system (i.e. no mass istransferred across the system boundarieswhile the process is taking place), energy

may be transferred between such asystem and its surroundings in two waysas heat or work.

Heat Energy that flows as a result of temperature

differencebetween a system and its

surroundings.


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First Law of Thermodynamics

Law of conservation of energy, which statethat energy can neither be created nordestroyed.

General form of first law ofthermodynamics

Inlet Energy + Heat - OutletEnergy Work = Accumulation


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Kinetic Energy Equation (Ek)

Kinetic energy, Ek (J) of an object of massm(kg) moving with velocity u(m/s)relative to the surface of the earth is

If the fluid enters a system with a massflow rate (kg/s) and uniform velocity u(m/s), the rate at which kinetic energy(J/s) is transported into the system is

2

2

1muEk 2

2

1umEk

kEm


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Potential Energy Equation (Ep)

Gravitational potential energy, Ep

if the fluid enters a system with a mass

flow rate (kg/s) and an elevation zrelative to the potential energy referenceplane.

mgzEp m

gzmEp

)( 1212

zzgmEEE ppp


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Example 7.2-1

Water flows into a process unit through a 2cm ID pipe at a rate of 2 m3/h. Calculate thekinetic energy transport in this stream in

unit J/s.

Solution:

Ek= 0.870 N.m/s= 0.870 J/s


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Class Discussion

Example 7.2-2


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Energy Balances on Closed

System Closed system

no mass is transferred across the systemboundaries while the process is taking place

Energy balance

Final System Energy Initial SystemEnergy

= Net Energy Transferred to theSystem

=

WQEEU pk


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Energy Balances on Closed

System When applying energy balance equation to a given process, the following point must

be aware;

1. The internal energy of a system depends almost entirely on the chemicalcomposition, state of aggregation (solid, liquid, or gas), and temperature of thesystem materials. If no temperature changes, phase changes, or chemicalreactionsoccur in a closed system and ifpressure changes are lessthan a few

atmospheres, thenU 0.

2. If a system is not accelerating, thenEk= 0. If a system is not rising or falling,thenEp = 0.

3. If a system and its surroundings are at the same temperatureor the system isperfectly insulated, then Q = 0. The process is then termed adiabatic.

4. Work done on or by a closed system is accomplished by movement of thesystem boundary against a resisting force or the passage of an electrical currentor radiation across the system boundary. If there no moving parts or electricalcurrentat the system boundary, then W = 0.


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Class Discussion

Example7.3-1


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Energy Balances on Open

System In open system, mass is transferred

across the system boundaries while theprocess is taking place.

Therefore work must be done on opensystem to push mass in and work is doneon the surrounding by mass that emergesfrom the systems.

Both work terms must be include in theenergy balance for open system

The net work done by an open system

sW

flW


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Energy Balances on Open

System ^ symbol is used to denote the specific

property ( property divided by mass or bymole) such as specific internal energy (

kJ/kg), specific volume ( m3/kg) and soon.

One important property for energybalance on open system is specificenthalpy ( kJ/kg).

V

VPUH


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Class Discussion

Example 7.4-1


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Energy Balances Equation for

Open System

spk WQEEH

streaminput

jj

streamoutput

jj

streaminput

jj

streamoutput

jj

k

streaminput

jj

streamoutput

jj

gzmgzmpE

umumE

HmHmH

22

22


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Class Discussion

Example 7.4-2


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Reference States and State

Properties It is not possibleto know the absolute valueofand for a process material, but we candetermine the changeinand change in

corresponding to a specific change of state(temperature, pressure, phase).

A convenient way to tabulateand is tochoosea temperature, pressure and state of

aggregation (i.e. phase) as a reference state.

Since cannot be known absolute, forconvenience we may assign a value o=0 to be

reference state. Then1= 1-0;2= 2-0


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Class Discussion

Example 7.5-1


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Steam Table

Class Discussion

Example 7.5-2


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Class Discussion

Example 7.5-3


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Energy Balance Tips

When labeling flowchart, write downtogether the temperature, pressure andstate of aggregation of the process

material.

Normally (depend on the processdescription) for chemical process unit;shaft work, kinetic and potential energychange tend to be negligible compared to

heat flows, internal energy and enthalpy

UQ HQ


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Class Discussion

Example 7.6-1


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Class Discussion

Example 7.6-2


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Class Discussion

Example 7.6-3


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Mechanical Energy Balance

Important in the operations involve theflow of fluids to, from and between tanks,reservoirs and process unit.

Mechanical energy balance for steadystate flow of an incompressible fluid;where F is friction loss

m

WFzg

uP s

2

2

0

2

2

zguP


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Class Discussion

Example 7.7-1


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Class Discussion

Example 7.7-2


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Class Discussion

Example 7.7-3


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THANK

Chapter 4 Energy and Energy Balance

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