07-10-2010, 11:28 AM
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OPTIMUM DESIGN OF AN ACOUSTIC ENCLOSURE FOR DIESEL GENERATOR
Project presentation on OPTIMUM DESIGN OF AN ACOUSTIC ENCLOSURE FOR DIESEL GENERATOR
What is an acoustic enclosure
An acoustic enclosure is a containerized vessel used for housing generators to :
Reduce noise levels.
Improve reliability and durability of generator sets (gensets).
Easy serviceability.
High uptime of genset.
Improved aesthetic of genset.
Better working conditions.
Acoustic enclosures
Problem statement
Optimum design of an acoustic enclosure for a diesel generator set (Ratings: 125 kVA) which satisfies the following clauses:
The enclosure should act as a containerised portable vessel along with the generator set (DG set) feasible to be used for frequent mobile applications.
Proper air ventilation for DG set should be available.
Noise reduction achieved should be achieved as specified under the CPCB Norms.
Customers :
DEUTZ ENGINES PVT. LTD, CUMMINS INDIA LTD, ASHOK LEYLAND LTD, FULL MARK MARKETING PVT LTD, SANDVIK ASIA, GODREJ & BOYCE MANUFACTURING CO. LTD, R & DE (E), PUNE, VRDE- AHMEDNAGAR.
Design data specifications:
Genset model : Cummins DB 125
(diesel)
Genset rating : 125kVA
Current (Amp) : 179
Fuel consumption: 149 gm/kW-hr
Length (mm) : 2225
Width (mm) : 925
Height (mm) : 1600
Weight (Kg) : 1370
Engine BHP : 154
No.of cylinders : 6
RPM : 1500
Cooling : Water cooled
Presentation details
Results
ENCLOSURE STRUCTURE DESIGN
NEED FOR STRUCTURE DESIGN
Preventing escape of sound to surrounding
Earlier enclosure has to be installed on site around the fixed generator
Now a days enclosure are made such that they contain the generator inside
Thus can be moved as a single unit
To optimize proper lifting points
Selection of proper beams and columns
DESIGN PROCEDURE
Raw material selection
Material selection initially can be arbitrary.
For acoustic enclosures, as per design analysis, the material selected should be such that it has:
. Good strength and rigidity preventing failures lifting, transporting, and working conditions.
. Ease of manufacturability (fabrication, assembly)
. Structural reliability for a prolonged life with minimum maintenance
cost
. Structural compliance with IS standards (including safety regulations,
FOS, Pollution norms)
STEELS , preferably heat treated steel AISI1020 is used for rigid structures because of their good strength, ductility, availability,
weld ability and highest scrap value.
Selection of frame columns & beams
Columns and beams are selected based upon their cross-sections.
By using identical dimensions for different cross-sectional columns or beams like I-section, L-section and []- section, the one with better strength can be selected
Material is same ,therefore bending stresses are assumed to be same
FINITE ELEMENT METHOD
ANSYS 10 is used
Model created in ANSYS (fig showing structure model using BEAM44 of hollow rectangular C/S)
METHODOLOGY OF FEA
Discretize the continuum
Select interpolation function
Find the element properties
Assemble the element properties to obtain the system equations
Impose the boundary
conditions
Solve the system
equations
Taking the genset load at predefined nodes and lifting supports at 4 end nodes of enclosure we can analyze the system for maximum and minimum stresses as follows:
OPTIMIZATION PROCESS
The sub-problem approximation method is used
Max allowable stress is 84 MPa
Maximum feasible and infeasible sets for iteration are chosen 100 and 80 respectively
Quadratic curve fitting polynomial function is chosen for design and state variables for next iteration
Design variable: 5 = t = 12.2
0 = LOCX1 = 2000
State variable: 10 = MAXSTRESS = 84
Objective function: weight=dens *(2*B*t+2*(D- 2*t)*t*(4000*4+1600*6+2000*4))
Where, dens =density of AISI 1020 i.e. 7850 kg/m3
• From above analysis we obtain maximum stress in the entire structure as 53.511 MPa and the feasible nodes are for lifting are at a distance 482 mm from all base nodes.
• The optimum thickness value obtained is 6.2 mm.
• The analysed result obtained can be formulated on graph as follows:
OPTIMIZATION RESULT
referring to ISMC codes
The optimum channel selection is 152 x 76 RSC of material AISI 1020
AIR VENTILATION
Cross ventilation and fresh air is must for satisfactory operation of genset. Air should flow from alternator to engine end.
As air flows through enclosure, its temperature increases. This increase is difference between temperature measured at alternator and at outdoor. The maximum allowed temperature rise is limited to 5oC to 10oC.
The Required air flow rate to keep specific temperature rise in control can be calculated as :
ma =
where,
ma = mass flow rate of air into enclosure
Q = heat rejection into enclosure (25kW)
Cp = specific heat (0.017 kW/°C)
T = temperature rise in enclosure (5 °C)
= density of air (1.099 kg/m3)
So, for T=5 o C, we require mass flow rate of air of about 4469.30 lit/sec. To allow this air flow in the enclosure, we need proper inlet and outlet openings without any compensation in soundproofing.
Also, care has to be taken that, at high velocities of radiator fan, there should be no suction of rain water, dirt, debris or any other unwanted elements with the air .
To achieve such opening, which allows only air to pass with minimum foreign particles ‘LOUVERS’ are used.
A louver is a ventilation product that allows air to pass through it while keeping out unwanted elements
The basic consideration for louver section are :-
Free Area
Water penetration
Resistance to air flow
Free area is derived by taking the total open area of a louver and dividing by the overall wall opening. Generally, it is taken as 35% to 60%.
First Point of Water Penetration is the point at which a louver allows the passage of water through the louver.
Every obstruction in the airstream creates resistance, which reduces velocity of air flow. The resistance of the louver can be measured by running air through the louver and measuring the pressure differential at various free area velocities. Lower blade angles or more aerodynamic shapes create less resistance.
For proper enclosure ventilation our objective is to select louver dimensions for both inlet and exit keeping in mind :
The company standards
Prescribed static pressure
Louver considerations
Selection of louver material
Apart from steel most commonly used louver material today is the aluminium AISI 6063 T5 because of the following reasons:
Light weight construction.
Feasible mounting for various opening types.
Architectural surface finishes provides minimum drag against air flow.
It can sustain higher wind loads of up to 100mph.
Able to withstand high temperatures of air about 600 to 650°C.
Calculation procedure
Final louver dimensions
SOUND PROOFING
TARGET
• Reduce the maximum sound power level of 97dB to 71 dB at distance of 1 meter from all sides of enclosure.
NOISE PREDICTION METHOD (theoretical)
• This method can predict level of noise with acceptable engineering accuracy over the frequency range 50-5,000 Hz using simple and readily available expressions.
• According to experiments performed by National Research Council (NRC) in Canada the difference between practical measurement and theoretical noise prediction is less than 0.5dB and 90% of results were found to lie within ± 2.5 dB.
CONCEPT AND FORMULA USED
ADVANTAGE & LIMITATION
CONSIDERED WITH STEEL &ALUMINIUM PLATE
MATERIAL FOUND DESIRABLE BASED ON THE ITERATION
Future scope
Study and development of optimization code by combining the following aspects for acoustic enclosure:
Optimum Weld thickness to be used
Computational fluid dynamics of flow of air through enclosure
Thermo-structural analysis to study effective heat transfer around the DG set in the enclosure
Development of insertion loss model for acoustical treatment at the sound source. At present transmission losses of 25 dB have been achieved from the various acoustical materials applied to the enclosure body. But, additional sound reduction of up to 10 dB can be achieved using insertion loss phenomenon.
REFERENCES:
BOOKS
Mechanical Engineering Design, by Joseph E. Shingley, Pg. 933, 983.
Design of Machine Elements, by V. B. Bhandari, Pg. 1 to 82, 22 to 57, 71 to 85.
Structural Design and Drawing, by N. Krishna Raju, Pg. 199 to 203.
Structural Engineering for Architecture, by A. P. Dongre, Pg. 13.1 to 13.8.
Statics and Strength of Material (Foundation for Structural Design), by Barry Onouye, Pg. 151to 261, 415.
Mechanical Vibrations and Noise Engineering, by A. G. Ambekar, Pg. 343 to 384.
JOURNALS
Diesel Engines Technical Data, published on Aug 1998(Company Confidential).
Reciprocating internal Combustion Engines (British Standard Controlled).
Application Manual-Liquid Cooled Generator Sets (Company Confidential).
Engine Installation Recommendation (Company Confidential).
Genset Installation Recommendation (Company Confidential).
PAPERS
Optimal Design of an Enclosure for a Portable Generator, by
Joseph E. Blanks.
Development of Technique to Predict Level of Dynamic Noise of Hydraulic Excavators, by Mikio Iwasaki.
Accuracy of Prediction Methods for Sound Transmission Loss, by K. O. Ballagh.
SITES
http://efundaMaterials/Alloys/carbon_steels/ show_carbon.cfmID=AISI_1020&prop=all&Page_Title=AISI%201020.
http://conpargroup.co.uk/evenchan.htm
http://engineersedgebeam_bending/beam_bending5.htm
http://sail.co.in
