Download Noise Reduction in a Reciprocating Compressor by Optimizing the Suction Muffler

International Journal of Engineering Trends and Technology (IJETT) – Volume 16 Number 6 – Oct 2014
Noise Reduction in a Reciprocating
Compressor by Optimizing the Suction
Muffler
Katakama Nagarjuna ¹ K.Sreenivas²
¹ M.tech student, ²Professor, dept of mechanical engineering kits, markapur, A.P, INDIA
ABSTRACT
Fluid flow through acoustic mufflers is a
complex phenomenon that has been investigated for
many years due to its importance in compressor
efficiency.
The intake system of a compressor plays an
important role on the performance of reed type valves
of small hermetic refrigeration compressors,
especially suction valves designed without an
opening limiter. The valve affects directly volumetric
and energy losses and may have some influence on
the compressor noise level as well. A number of
methods have been proposed to analyze gas
pulsations in intake and discharge systems. If
pulsations are small compared to the mean pressure,
its behavior can be approximated by the acoustic
theory.
The project evaluates the effect of variables
like length of the insertion tube, cross sectional areas
of tubes, volume of expansion chamber, speed of
sound in the refrigerant media etc. on functional
parameters such as transmission loss characteristics
and pressure drop across the muffler.
INTROUCTION
The suction muffler of a reciprocating
compressor is used for reducing noise produced
by pressure pulsations. The suction muffler of a
hermetically sealed reciprocating compressor
serves a dual purpose. It acts as a reservoir of the
refrigerant and dampens the noise waves coming
out through the gas path due to fluttering of the
flapper valves. According to the shape, the
suction muffler is classified into one-chamber
type, two-chamber type, Helmholtz-resonator
type, pipe-resonator type, and so on.
The pressure pulsation generated at the
suction and discharge port of reciprocating
compressor is the main cause of a noise and
vibration. In addition, it has relation to
generation of a flow-induced noise and
ISSN: 2231-5381
compressor performance. Therefore, many
researches on suction muffler have been carried
out in view of noise reduction as well as flow
performance.
The pressure drop characteristics of the
muffler are analyzed using “ANSYS”. The noise
coming out of the hermetic compressor is related
to the suction side. Discharge side and other
electromagnetic components. Generally, the
suction side noise is in the range of 50-1500 Hz,
which is important from the perspective of the
end user of refrigerator. This renders the task of
designing a suction muffler much more critical
and iterative as the pressure drop needs to be
minimum on the suction side. An approach for
designing suction muffler for a commercial
refrigeration compressor model with R134A
refrigerant is discussed in this project.
METHODOLOGY
The whole analysis is carried out using CAE
tools – NX-CAD for modeling the muffler,
ANSYS for plotting Transmission Loss curve
and for calculation of Pressure Drop. This has
given the flexibility of quick iterations with fair
amount of accuracy as compared to the
experimental results. The advent of these tools
has helped in expediting the whole designing
process maintaining its effectiveness as well as
its purpose. The combination of software tools
helps to reduce the number of physical
prototypes for testing and also avoids the
repetitious testing
3D Model:
Two models were generated in NX-CAD with
baffles and acoustic analysis is carried out.
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International Journal of Engineering Trends and Technology (IJETT) – Volume 16 Number 6 – Oct 2014
Trial1:
Physical properties





Leak tendency and detection
Viscosity
Thermal conductivity
Reaction with oil
Cost
Refrigerant is taken as R-134A in our analysis.
The material properties are as given below
Material properties at inlet (material 1)
Density of fluid = 21.51 kg/m3
Velocity of sound (c) =180.3 m/sec
Material properties at outlet (material 2)
Fig 1: The geometric model without baffle used
for acoustic analysis
Density of fluid = 21.51 kg/m3
Density of fluid = 21.51 kg/m3
Inputs and material properties:
Refrigerant:
Refrigerant is a substance which has the
capacity to absorb heat from any other
substance. Refrigerant absorbs heat from source
and dissipate heat to sink either in the form of
Latent heat or Sensible heat.
Ideal refrigerant:
Ideal Refrigerant should have good
chemical,
Physical
and
thermodynamic
properties and in addition to above it should be
non-toxic, non-flammable, completely stable
inside the system, easily available and
environmentally friendly.
Properties of Refrigerant:
Absorption of sound at the interface is
accounted for by generating a damping matrix
using the surface area and boundary admittance
at the interface. Experimentally measured values
of the boundary admittance for the sound
absorbing material may be input as material
property MU. Ansys recommend MU values
from 0.0 to 1.0; however, values greater than 1.0
are allowed. MU = 0.0 represents no sound
absorption and MU = 1.0 represents full sound
absorption. DENS, SONC and MU are evaluated
at the average of the nodal temperatures.
In our analysis we have taken Admittance = 1
Impedance = density*sonic velocity
= 21.51*180.3
= 3875.253 kg/m2-sec
Chemical Properties
 Flammability
 Toxicity
 Reaction with material
 Damage to refrigerated products
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International Journal of Engineering Trends and Technology (IJETT) – Volume 16 Number 6 – Oct 2014
Finite Element Model:
Fig 3: Various boundary conditions applied for
the acoustic analysis
Acoustic Analysis:
Fig 2: Finite element model meshed using fluid
30 element for acoustic analysis
The acoustic pressure in the fluid medium is
determined by the wave equation with the
following assumptions:
 The fluid is compressible (density
changes due to pressure variations).
 In viscid fluid (no dissipative effect due
to viscosity).
 There is no mean flow of the fluid.
 The mean density and pressure are
uniform throughout the fluid. Note that
the acoustic pressure is the excess
pressure from the mean pressure.
Analyses are limited to relatively small acoustic
pressures so that the changes in density are small
compared with the mean density.
The below figure is the plot of the contours of
pressure drop across the inlet and outlet. The
pressure drop is noted as 0.132. This value is
used for validation of other two suction mufflers
Fig 4: The contour plot of the pressure drop
Boundary conditions:
A unit pressure dof is applied at the acoustic
inlet of the suction muffler and the same is
useful to determine the pressure drop.
Impedance value of 3825kg/m2-sec is applied at
acoustic outlet of the suction muffler. Boundary
admittance of 1 for the sound absorbing material
is applied on the acoustic outlet
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Results obtained after performing acoustic
analysis
Pressure plot:
Transmission Loss:
Acoustic analysis is done in the
frequency range of 0-1200Hz to find the
transmission loss at various frequencies.
Transmission loss through the muffler is the
difference between the power incident on the
muffler and the power transmitted through it.
+ve transmission loss indicates sound
Attenuation and –ve transmission loss indicates
sound amplification
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International Journal of Engineering Trends and Technology (IJETT) – Volume 16 Number 6 – Oct 2014
Attenuation of the sound is the reduction
of acoustic energy from entrance to the exit of
the muffler tube.
In acoustic analysis the Y-axis shows
the transmission loss, + ve value indicates sound
attenuation where – ve indicates amplification
From the below figures it is observed
that transmission loss is _ve at a frequency of
150Hz
that the noise level peaks between 80-140Hz &
at 1000Hz. These results are used for further
validation.
The objective of this project is to
optimize the above muffler to increase
transmission loss the at the above said
frequencies. In this case, noise can be reduced
so much in a local frequency band, but the width
of the band is too narrow to reduce the noise
totally with varying temperature of the
refrigerant. On the other hand, it is reported that
a suction muffler with a pipe resonator can
reduce noises efficiently in a broad frequency
band. So a pipe resonator which we term here as
‘Baffle’ is used in this analysis. Two of such
baffles are developed with number iterations.
Trial2:
A baffle with the below dimensions is
inserted into the existing suction muffler and
acoustic analysis is carried out to determine the
transmission loss and the pressure drop.
MUFFLER WITH THE BAFFLE:
Fig 5: The transmission loss vs. frequency plot
over a frequency range of 0-600Hz for trial 1
model
From the below figures it is observed that
transmission loss is very less at a frequency of
1020Hz
Fig 8: The modified model of suction muffler
with baffle
Results obtained after carrying out acoustic
analysis in the frequency range of 0-1200Hz
Fig 6: The transmission loss vs. frequency plot
over a frequency range of 600-1200Hz for trial 2
model
The below figure is the plot of the contours of
pressure drop across the inlet and outlet. The
pressure drop is noted as 0.618. This value is
used for validation of other two suction
mufflers.
The above graphs are plotted as
transmission loss versus frequency. It is noted
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International Journal of Engineering Trends and Technology (IJETT) – Volume 16 Number 6 – Oct 2014
Transmission loss is plotted against
frequency. It is noted that the transmission
losses are increased with the introduction of the
baffle resulting in reduced noise levels.
Fig 9: Contour plot of pressure drop
Transmission Loss:
From the below figures it is observed that
transmission loss is -ve at a frequency of 80Hz
and 280Hz
From the above results it can be
concluded that the transmission loss has
increased over the frequency range of 3001200Hz compared to trial1 which indicates the
elimination of noise problem over this frequency
range. Pressure drop is increased from 0.132
units in trial 1 to 0.618 in trial 2 which may
result in less efficiency of the muffler.
So to increase the efficiency of the
muffler efforts are put to modify the baffle
dimensions so as to decrease the pressure drop.
A modified baffle is inserted into the suction
muffler and acoustic analysis was carried out to
determine pressure drop and transmission loss.
Trial 3:
MUFFLER WITH MODIFIED BAFFLE:
Fig 10: Transmission loss vs. frequency plot
over a frequency range of 0-600Hz for trial 2
model
From the below figures it is observed
that transmission loss is +ve over a frequency
range of 600 -1200Hz
Fig 12: Muffler with modified baffle
The muffler baffle dimensions are as listed
below:
Fig 11: Transmission loss vs. frequency plot
over a frequency range of 600-1200Hz for trial 2
model
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INLET COMPARTMENT:
Baffle tube diameter at the inlet= 6mm
Length of the baffle tube = 10.1 mm
OUTLET COMPARTMENT:
Baffle tube diameter at outlet= 9mm
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International Journal of Engineering Trends and Technology (IJETT) – Volume 16 Number 6 – Oct 2014
Length of the baffle tube = 14.2 mm
Results obtained after carrying out acoustic
analysis in the frequency range of 0-1200Hz
From the below figure it can be seen that the
maximum pressure drop is 0.245units]
The above graph indicates the
transmission loss versus frequency curve. It is
noted that by introducing the modified baffle
into the suction muffler, the transmission losses
are dramatically increased over a wide
frequency range.
CONCLUSION
After the initial run for the same flow and
acoustic boundary conditions the Trial-3 muffler
model looks better than other combinations,
which further need to be analyzed for reversible
flow and its effect.
Discussion on the Transmission loss-TL
Results:
Fig 13: Contour plot of pressure drop.
The pressure drop value is 0.245199 which is
almost near as the trial1 (0.134) and less than
that of case2.
Transmission Loss:
From the below figures it is observed that
transmission loss is _ve at a frequency of 130Hz
and 310Hz
1. As can be seen from the graphs for Trial1 to
Trial 3, the insertion of baffle has considerable
effect on the magnitude of transmission loss.
2. Increase in the length of the baffle gives more
pressure drop and decreasing the efficiency of
the muffler.
3. Decrease in the insertion length of baffle tube
gives better magnitude of transmission loss in
higher frequency range with marginal decrease
in performance in the lower frequency range.
The results show us that the Trial3
muffler gives us the best results from the
parameter analysis. The transmission loss
characteristics of the muffler are as per the
requirement. The muffler is further checked for
the pressure drop characteristics to see the effect
of the muffler on the compressor performance.
These values are used to find the pressure drop
across the muffler. This pressure drop is used in
the equations of compressor performance.
Assumptions made for the analysis:
Fig 14: Transmission loss vs. frequency plot
over a frequency range of 0-1200Hz for trial 3
model.
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1. The sound pressures inside the muffler are
small compared to the overall level of pressure
inside the system.
2. Only plane waves of pressure are considered.
(Imaginary/Complex waves are not considered.)
3. Viscosity effects of the fluid are neglected
(The medium is in viscid and stationary).
4. The tailpipe of the muffler has its
characteristic impedance value (ρ X C) i.e. it
will act as a perfect absorber.
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International Journal of Engineering Trends and Technology (IJETT) – Volume 16 Number 6 – Oct 2014
Method of plotting Transmission Loss curve
in ANSYS:
1. Each Trial was analyzed in ANSYS for
Transmission Loss Curve. These TL curves were
analyzed for the required criterion. As seen from
Figure 2, Trial3 has the best TL curve.
2. The response curves and the interaction
effects plotted for Trial3.
3. The optimized combination of parameters
were derived and then analyzed for the pressure
drop in ANSYS.
REFERENCES
1. Singh, R. and Soedel, W., “A Review of Compressor Lines
Pulsation Analysis and Muffler Design Research Part I – Pulsation
Effects and Muffler Criteria,” International Compressor
Engineering Conference at Purdue, pp112-123, 1974.
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2. Singh, R. and Katra, T., “On The Dynamic Analysis and
Evaluation of Compressor Mufflers,” International Compressor
Engineering Conference at Purdue, pp372-381, 1976.
3. Koai, K. L., Yang, T. and Chen, J., “The Muffling Effect of
Helmholtz Resonator Attachments to A Gas Flow Path,”
International Compressor Engineering Conference at Purdue,
pp793-798, 1996.
4.Kinsler, L. E., Frey, A. R., Coppens, A. B, and Sanders, J. V.,
“Fundamentals of Acoustics,” John Wiley & Sons,Inc., 1982.
5.Bassi, F., Pelagalli, L., Rebay, S., Betto, A., Orefice, M., Pinto,
A., “Numerical Simulation of a Reciprocating Compressor for
Household Refrigerators”, Proc. Compressor Engineering
Conference at Purdue, West Lafayette, Indiana, USA, p. 97-104,
2000.
6.Choi, J.K., Joo, J.M., Oh, S.K., Park, S.W., “Smart Suction
Muffler Design for a Reciprocating Compressor”, Proc.
Compressor Engineering Conference at Purdue, West Lafayette,
Indiana, USA, p.619-626, 2000.
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