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SFEE.ppt .
1. Department of Mechanical & Manufacturing Engineering, MIT, Manipal 1 of 3
FIRST LAW OF THERMODYNAMICS
(Applied to open system)
THERMODYNAMICS - I
2. Department of Mechanical & Manufacturing Engineering, MIT, Manipal 2 of 3
THERMODYNAMICS - I
First Law of Thermodynamics for a closed system undergoing a change
of state is written as
Where E represents all forms of energy stored in the system
For a pure substance E= Ek+EP +U
When there is mass transfer across the system boundary the system is
called pen system.
Most of the engineering devices are open systems involving flow of
fluids across the system boundary.
We can have two types of open system analysis:
1. Focusing our attention about all energy interactions in the path of a
given mass of fluid.
2. Focusing attention upon a certain control volume through which certain
fluid moves.
Q = ∆E+ W
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The imaginary or real boundary around the open system is
called control volume.
A closed system boundary usually changes its shape,
position and orientation relative to the observer but the
control volume remains fixed and unaltered.
The matter usually crosses the control volume which is
accounted all the time during the analysis.
As the fluid crosses the control volume it takes some energy
into and out of the system.
If the rates of flow of mass and energy through the control
surface remains constant it is called as steady state system.
If the rates of flow of mass and energy through the control
surface not a constant it is called as unsteady state system.
4. Department of Mechanical & Manufacturing Engineering, MIT, Manipal 4 of 3
THERMODYNAMICS - I
Derivation of Steady flow energy equation
Assumptions:
• The mass flow through the system remains constant.
• There is no change in chemical composition of the fluid.
• The state of fluid at any point remains constant with
time
• Rate of heat and work interactions across the boundary
of control volume are at constant.
• Fluid is uniform in composition and there is no chemical
reaction within the control volume.
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THERMODYNAMICS - I
Derivation of SFEE
Consider a steady flow system with one stream enters
system and one stream leaves the control volume.
Assuming there is no accumulation of mass or energy
across the boundary and properties at any location within the
with in the CV are steady with time.
6. Department of Mechanical & Manufacturing Engineering, MIT, Manipal 6 of 3
1-1 represents fluid entry & 2-2 represents fluid exit
A1 and A2 represents cross sectional area in m2
u1 and u2 represents the specific internal energies in J/kg
V1 and V2 represents velocity in m/s
Z1 and Z2 represents the elevations with reference to a
datum line in m
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Equations 3 and 4 are known as Steady Flow Energy Equations (SFEE). All the terms
in equation 4 represent energy flow per unit mass (J/kg) and all the terms in equation
3 represent energy flow per unit time (J/s).
10. Department of Mechanical & Manufacturing Engineering, MIT, Manipal 10 of 3
Application of SFEE
All mechanical devices are basically are of two types
1. Energy transfer devices:
a. Work transfer devices
Power producing devices: IC engines, Turbines
Power absorbing device: Compressors and pumps
b. Heat transfer devices
Boilers, condensers, evaporators, heat exchangers
2. Energy transformation devices: nozzles, diffusers and
throttle valve
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THERMODYNAMICS - I
Application of SFEE
1. Turbines and Compressors
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THERMODYNAMICS - I
Application of SFEE
2. Nozzles and Diffusers: Definition of Nozzle and diffuser.
We have the SFEE
With no changes in PE, completely insulated, no work done, the
SFEE will be reduced to
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THERMODYNAMICS - I
Application of SFEE
3. Throttling Device
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4. Boilers and condensers
We have standard form of SFEE is
Neglecting changes in KE, PE and there is no work in boiler
the SFEE will be gets reduced to
For the condensers
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THERMODYNAMICS - I
Application of SFEE
5. Heat Exchanger
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NUMERICAL on SFEE
1.A centrifugal air compressor delivers 900kg of air per hour at
5 bar. The inlet conditions are velocity 5m/s, specific volume
0.8m3/kg. The discharge condition is with specific volume of
0.15m3/kg. The increase in enthalpy of air pressure through
the air compressor is 168kJ/kg and heat loss to the cooling
water and surrounding air is 15kW. The ratio of inlet to outlet
pipe diameter is 4. Find the power required to drive the
compressor
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2. At the inlet to a certain nozzle, the enthalpy of the fluid is 3025kJ/kg
and the velocity of 60m/s. At the exit the enthalpy of the fluid is
2790kJ/kg. The nozzle is horizontal and there is a heat loss of
100kJ/kg from it. Calculate the velocity of the fluid at nozzle exit.
Also find the mass flow rate of the fluid if the inlet area is 0.1m2 and
the specific volume of the fluid at the inlet is 0.19m3/kg
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3. A system comprising of 0.95 kg of pure substance has specific
internal energy of 16kJ/kg. The system is moving with a velocity
of 120m/s at an elevation of 1500m. Evaluate the energy of the
system relative the observer at rest at sea level.
The above system undergoes a process to a final specific internal
energy of 20kJ/kg, final velocity of 200m/s, final elevation of
270m. The work done on the system is 2200 N-m. Evaluate the
magnitude and direction of heat transfer during the process.
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The above system undergoes a process to a final specific internal energy of 20kJ/kg, final
velocity of 200m/s, final elevation of 270m. The work done on the system is 2200 N-m.
Evaluate the magnitude and direction of heat transfer during the process.
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4. A long well insulated pipe line consists of two pipes connected in series,
internal diameters of which are 90mm and 30mm respectively. A steady
flow of steam enters the 90mm diameter pipe at a pressure of 350kPa,
specific volume of 0.684m3/kg and an enthalpy of 2980kJ/kg. At a point in
the downstream in the 30mm diameter pipe, the pressure is 300kPa,
specific volume is 0.790m3/kg and the enthalpy is 2968kJ/kg. Find the
velocity of steam at the two points in the pipe line and mass flow rate of
the steam.
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5. A gasoline engine has a specific fuel consumption of 0.3kg/kWh. The
stream of air and gasoline vapor in the ratio 14:1 by mass enters the
engine at a temperature of 7900C. The net heat transfer rate from the
fuel – air stream to the jacket cooling water and to the surroundings is
35kW. Shaft power developed by the engine is 26kW. Evaluate the
increase in specific enthalpy of the fuel air stream assuming the
changes in kinetic energy and elevation to be negligible.
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6. In a cooling tower air enters at a height of 1m above the ground level and
leaves at a height of 7m. The inlet and outlet velocities are 20m/s and
30m/s respectively. Water enters at a height of 8m and leaves at a height of
0.8m. The velocity of water at entry and exit are 3m/s and 1m/s
respectively. Water temperatures are 800C and 500C at the entry and exit
respectively. Air temperature is 300C and 700C at entry and exit
respectively. The cooling tower is well insulated and a fan of 2.25kW
drives the air through the cooler. Find the amount of air per second
required for 1 kg/s of water flow. The values of CP of air and water are
1.005 and 4.187kJ/kg-K respectively.
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7. Air at 101.325kPa, 200C is taken into a gas turbine power plant at a
velocity of 140m/s through an opening of 0.15m2 cross sectional
area. The air is compressed, heated and expanded through a turbine
and exhausted at 0.18MPa, 1500C through an opening of 0.1m2 cross
sectional area. The power output is 375kW. Calculate the net amount
of heat added to air in kJ/kg. Assume the air obeys law
pv=0.287(t+273) where p is in kPa, v is in m3/kg and t is the
temperature in o C. For air Cp= 1.005kJ/kg K and enthalpy h= (Cp*t)
