02 part5 energy balance

1. Energy Balance
S.Gunabalan
Associate Professor
Mechanical Engineering Department
Bharathiyar College of Engineering & Technology
Karaikal - 609 609.
e-Mail : gunabalans@yahoo.com
Part - 2

2. Energy accounting
• Energy accounting is a system used to
measure, analyze and report the energy
consumption of different activities on a
regular basis.
• It is done to improve energy efficiency

3. Energy Balances
Energy Balances is the law of conservation of
energy,
Unit II
13) Discuss briefly the energy balance for closed
and open system (Nov 2011)
OR explain or derive energy balance equation
OR Explain or derive Steady Flow Energy equation

4. Energy Balances on Closed Systems
• Example for a Closed System
– Storage tank
ΔU + ΔEk + ΔEp = Q - W
K – kinetic energy
P- Potential energy
• ΔU = 0 if there are no temperature changes, phase changes, or
chemical reactions.
ΔEk = 0 if the system doesn't accelerate
• ΔEp = 0 if the system doesn't change in height
• Q = 0 if the system doesn't exchange heat with the surroundings,
that is, if the system is adiabatic or insulated
• W = 0 if the system has no moving boundry (ex. piston), or if there
are no moving parts, electrical current, or radiation exchange with
the system and the surroundings.

5. Thermodynamic System

6. Thermodynamic System
Open system
http://chemwiki.ucdavis.edu/Physical_Chemistry/Thermodynamics/A_System_And_Its_Surroundings
A open system is defined when a fixed volume is under
study. There can be mass transfers as well as energy
transfers across the boundary.

7. Thermodynamic System
• Truly isolated systems cannot exist in nature,
• The only possibility is the universe itself,
• So its is a hypothetical concepts
http://chemwiki.ucdavis.edu/Physical_Chemistry/Thermodynamics/A_System_And_Its_Surroundings
isolated systems

8. Thermodynamic System
http://chemwiki.ucdavis.edu/Physical_Chemistry/Thermodynamics/A_System_And_Its_Surroundings
Closed systems
• A closed system is a system that exchanges only energy with its
surroundings, not matter.
• Matter can no longer transfer because the lid prevents matter from
entering the pan and leaving the pan. Still, the pan allows energy transfer.
A closed system always contains the same matter. There can be no
mass transfers across the boundary. There may be energy transfer
across the boundary.

9. First law of thermodynamics
=
= +
= +
∆ + ∆ + ∆ = − ( ℎ + )
∆ = ∆ +
(∆ + ) + ∆ + ∆ = − ( ℎ )
∆ + ∆ + ∆ = − ℎ
Energy Balances on Open Systems

10. ∆ + ∆ + ∆
= − ℎ
( 2 − 1) + 2 − 1 + ( 2 − 1) = − ℎ
1 + 1 + 1 +
= 2 + 2 + 2 + ℎ
Energy Balances on Open Systems

11. = 1
2
= mgH
m – mass, g – gravity,
H – Height or height from datum (z)
We better use z instead of H
= mgz
Energy Balances on Open Systems

12. 1 +
1
2 1
2 + 1 + = 2 +
1
2 2
2 + 2 +
Convert in to specific term ie /Kg
ℎ1 +
1
2 1
2 + 1 +
= ℎ2 +
1
2 2
2 + 2 +
Energy Balances on Open Systems

13. nergy transfer per unit mas ( )
Work transfer per unit mas ( )
ℎ1 + 1
2 + 1 + = ℎ2 + 2
2 + 2 +
Steady Flow Energy equation
Energy Balances on Open Systems
Also Called
is Velocity
This equation based on mass flow rate

14. ℎ1 + 1
2 + 1 + = ℎ2 + 2
2 + 2 +
X by
(ℎ1 + + 1) + = (ℎ2 + + 2) +
represented as w
(ℎ1 + 1
2 + 1) +
= (ℎ2 + 2
2 + 2) +
Energy Balances on Open Systemsis Velocity
Derive the steady flow energy equation for open system on time basis (Apr/May 2010)
This equation based on Time

15. Mass Balance
• Conservation of mass
– The mass flow rate of a system at entry equal to
mass flow rate at exit of the system
=
1 1
1
=
2 2
2
This is Equation of Continuity
– Velocity of flow
=
/
/
=
= mass flow rate

16. Steady Flow systems
• Steady Flow systems where mass flow in
equals mass flow out.
• In unsteady flow systems parameters such as
pressure, mass, temperature etc. will
• change with time. (hence ‘unsteady’)
• In steady flow systems parameters such as
pressure, mass, temperature etc. will remain
• constant with time. (hence ‘steady’

17. Reference
• Rajput, R. K. 2010. Engineering thermodynamics. Jones and Bartlett
Publishers, Sudbury, Mass.
• Nag, P. K. 2002. Basic and applied thermodynamics. Tata McGraw-Hill, New
Delhi.
• http://blowers.chee.arizona.edu/201project/EBopensys.pg1.HTML
• http://chemwiki.ucdavis.edu/Physical_Chemistry/Thermodynamics/A_Sys
tem_And_Its_Surroundings