Compressor Noise Control in Applications R

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International Compressor Engineering Conference School of Mechanical Engineering

1974 Compressor Noise Control in Applications R. J. Wilson Tecumseh Products Company

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Wilson, R. J., "Compressor Noise Control in Applications" (1974). International Compressor Engineering Conference. Paper 122. https://docs.lib.purdue.edu/icec/122

This document has been made available through Purdue e-Pubs, a service of the Purdue University Libraries. Please contact [email protected] for additional information. Complete proceedings may be acquired in print and on CD-ROM directly from the Ray W. Herrick Laboratories at https://engineering.purdue.edu/ Herrick/Events/orderlit.html COMPRESSOR NOISE CONTROL IN APPLICATIONS

R. J. Wilson Tecumseh Products company

This paper is concerned with current techniques of The control of compressor compressor sound control in refrigeration noise in the appli appli cation is generally limited cations. The compressors referred to the t~eatment of to are the these transmission paths. hermetic, reciprocating type, with On occasion the direct drive, source may benefit from damping 'two pole, induction motors. The operating applied to a speed localized housing resonance or range of these compressors is about the addition of 3300 to oil to the reservoir may help 3500 RPM. This generates a fundamental the case radiation. frequency These cases are exceptions to of 55 to 58 Hertz. Two cylinder versions the rule, however, woUld and do not have universal application. hav.e a pumping frequency of twice The t~at, or 110 to following techniques are applicable 116 Hertz. The compressors are designed in all cases with where noise reduction is required. internal mufflers at the intake and discharge, with_internal spring suspension between the crank Reducing ~Radiation case and outer housing and with a coiled, flexible steel tube to direct the discharge gas to the The case outside of radiation is an important factor in the the housing. The space within the sound hermetic case of residential, central air conditioners. contains the oil reservoir and the It is return refrigerant. also important, and may be the major sound in a household refrigerator. It may be of significance in window air conditioners. The hermetic refrigeration compressor is never operated alone. It is one of several components The sound waves generated at the surface in a refrigeration system that usually includes of the compressor housing are radiated outward one or two air moving blowers, a condenser, an and (1) reflected about, within the walls evaporator and some sort of cabinet that contains of the compartment and eventually are radiated and protects ~1 or part of these components. out through openings in the compartment, Sound that is generated by the compressor or (2) may be transmit some energy to the compartment radiated directly. from the steel case (case panels radiation) (causing vibration of the panels), or (3) lose or indirectly by exciting other com some ponents of energy in the form of heat due to the system (structure-borne noise). absorption. The compressor case radiation is independent of Figure 1 tells something the cabinet, or enclosure, in which about the nature of this it is mounted. case radiation. The sound of This sound is measurable in the laboratory a three ton air and conditioning compressor has the quantity is expressed in decibels, been analyzed in sound power octave, one-third octave and narrow level. The sound produced indirectly is not (6%) bandwidths. Note from the narrow band measurable by any test on the compressor alone. analysis that compressor case radiation is primarily The excitation can be measured as vibration or gas pressure line spectra. In the frequency range below pulse but these can give only a very about general indication 1000 Hertz, the harmonics are spaced far of the sound that will be enough apart produced in to be readily distinguished. Above an application. For this reason, the that, the final determination constant percentage bandwidth analyzer of the quantity and quality begins to loose resolution of the sound produced by a compressor and only prominent must be tones can be recognized. The one-third done in the system in which it will octave be operating. and octave analyzers provide decreasing resolution. The major paths by which compressor noise is trans mitted to the system are: (1) The air surrounding The chart of figure 2 is a composite of the compressor (case radiation), (2) The eight refriger compressor tests. Its purpose is ant within the tubing (discharge pressure to show the pulse), variation in sound spectra of a number (3) The tubing (discharge and of suction lines), and compressors. It is constructed (4) The mounting feet (vibration). by plotting the levels in each band of all the compressors in-

208 eluded. The maximum and minimum levels in each In applications where 10% or more of the cabinet band are connected to form an envelope within panel area is open for ventilation, the reduction which all of the compressor spectra fall. The in compressor case radiation is limited to about compressors were all tested at the same load 3 to 5 dB(A). If greater reductions in case conditions. A wide range of load conditions radiation are needed, it may be possible to would produce a further scatter in the data. An completely enclose the compressor. If the indication of this is shown by the "X" plots in enclosure is carefully sealed, the reduction in the 315 and 400 Hertz bands which indicate the case radiation is only limited by the trans levels attained as the cpmpressor speed was mission loss of the enclosure and the total sound lowered to near stall point. absorption within it. Figure 3 is an example of the reduction in case radiation that was obtained Where reduction of compressor case radiation is with a sealed enclosure. The higher sound spec needed, it must be done within the enclosure trum was produced by a compressor operating in before the sound is radiated out of the cabinet the open. The lower spectrum is the same and is beyond control. This is accomplished by compressor enclosed in a sheet metal box having absorption. Acoustical absorbing material, such a layer of one inch thick fiber glass covering as fiber glass or an open cell plastic foam, is five of the six sides in the enclosure. Signi applied to the compressor side of the enclosure ficant sound reduction was obtain~d 'at fre panels. To be effective, the sound must be quencies well below lOOO.Hertz. directed into the material repeatedly. This is done by enclosing the compressor as much as A disadvantage to completely enclosing the possible. Baffles may be placed between the compressor is the loss in performance that compressor and cabinet openings to reflect a results. The ambient compressor temperature portion of the sound back into the enclosure. increases which increases the temperature of the r-eturn refrigerant in the housing. The increased The effectiveness of the material used for sound refrigerant temperature .entering the cylinder absorption varies with the construction of the results in less mass flow of refrigerant without material with respect to density, fiber or pore an equal decrease in power consumption. The size, its thickness and the method of application. performance loss in the E.E.R. may range from It is also dependent on the frequency and the about 1% to as much as 5%. There will also be a angle of incidence of the sound to be absorbed. loss in "high-load, low voltage" performance. It The supplier of acoustical materials rates the may be possible to recover the performance loss absorption value on a scale of 0 to 1.0 called by a change in the refrigeration system, An the sound absorption coefficient. The value of increase in condenser surface can reduce the these materials in small enclosures is subject discharge pressure, for example. This entails to some limitations: an increase in cost, however.

(l) It is not possible, in a small compartment, The compressor motor protection will be affected to obtain significant absorption of low fre by the higher compressor temperatures. The quency sound because of space limitations. For effect is minimized where internal motor pro sound of low frequency (long wavelength) absorp tectors are used since the internal compressor tion relies heavily on the reactive effect of temperature will not increase as much as the the space between the surface of the absorbing ambient temperature. Many compressors in the material and the reflecting surface in back of window air conditioning sizes (below two ton it. For this reason, the absorption coefficients capacity) employ externally located motor pro of a material will show improvement when the tectors. The increase in protector temperatures depth of the mounting is in the range of 1/4 may prevent the system from meeting its full wavelength of the sound. The available space performance specification. The tendency would in most enclosures limits the depth of material be to "de-rate" the protector causing over to about one inch or less. The absorption co protection of the motor. In some cases, this efficients fall off rapidly for frequencies can be handled with a change in the motor below about 1000 Hertz. protector.

(2) The absorption coefficients given for the Reduction Q! Discharge Pulse material are obtained in a diffuse sound field. This condition is approximated in an enclosure The "discharge pulse" is a pressure fluctuation that is large in relation to the wavelength of in the superheated refrigerant leaving the the sound and in which the total absorption is compressor. The pulse is actually an acoustic not large. In the usual compressor enclosure wave that is generated by the large, rapid this may be true in the frequency range above pressure changes that occur when the discharge about 1000 Hertz. valve opens. The pressure pulse is confined to the refrigerant gas in the discharge piping and (3) The primary limitation on the use of ordinarily does not contribute directly to the absorbing material is the total area to which case radiation. Its contribution to noise in the the material can be applied. The total refrigeration system normally occurs because absorption within the enclosure is the product pressure waves in the refrigerant excite the of the effective absorption coefficient times condenser tubing causing vibration of the tubing its area. and the attached unit base and cabinet panels.

209 Small window air conditioners appear to be more Reduction ~Mounting Vibration susceptible to this problem than most other applications. This follows from the design of The transmission of noise these units as they through the compressor utilize a common cabinet to mounting feet to the unit house both the base is common to all condenser and evaporator sections applications. From of the system. the smallest household refrigerator to the largest air conditioner, some type of isolator is used to reduce this noise. Vibration induced in the condenser tubing is readily conducted to the indoor portion of the There are three types of vibration associated unit where the inside blower is quieter and the with the compressor: compressor case radiation is at least partially blocked by the weather wall of the unit. (1) Unbalance (rotational, reciprocating). The speed of the acoustic noise in the discharge (2) Torque reaction (load generated). gas is about 600 ft. per second in R22 and about (3) Flexural case vibration (audible frequency 500 ft. per second in Rl2. It varies with the range). temperature and pressure of the refrigerant. An important characteristic of the discharge noise The first two forms of is its harmonic vibration occur at low content. Frequencies as high as frequency and appear primarily 1000 Hertz at the first have been observed. The amplitude of two harmonics. The the pressure motion originates at the wave may range up to 5 psi or more, compressor crankcase and under some conditions. the internal spring suspension is in the transfer path to the housing. The internal suspension Reduction of the is usually pressure pulse is accomplished adequate to reduce the vibration by inserting a muffler to acceptable in the discharge line levels. The third type of vibration between the compressor occurs at and the condenser. The higher frequencies, It is this vibration best location for the that muffler is usually as close generates the case radiation discussed to the compressor outlet earlier. as possible to minimize The compressor internal spring suspension the excitation of the is discharge tubing as well ineffective here as much of this energy as the condenser tubing. is transferred to the housing through the refriger ant, the discharge tube, and the oil reservoir. The most practical muffler, for this purpose, is the single expansion chamber. It has the Synthetic rubber isolators are advantages of reasonable applied to the cost, availability and compressor mounting feet and are effective a relatively broad frequency in range of attenua reducing the transmission of high tion. This is a "reactive" frequency type muffler and the noise to the unit base. In greatest attenuation many cases, these occurs at the "tuned" isolators are soft enough to offer frequency and at odd multiples some reduction thereof: lf, 3f, in the low frequency vibration also. etc. The attenuation at l/2f and l l/2f is only 3dB less, making the muffler suitable for the The choice of rubber full range isolator is based upon the of compressor load conditions. The following criteria: length of the muffler can be found from l ~ 3c/f; where l is the length in inches, c is the speed 1. Transmissibility- The degree of isolation of sound in the refrigerant and f is the fre required. quency at which maximum attenuation is desired. 2. Life - The rate of deterioration The maximum attenuation due to is a function of the temperature and stress. cross-sectional area ratio (m) of the muffler 3. Shipability - The ability to to the discharge line. As restrain a rule, a muffler compressor motion during shipment. diameter of 4 to 5 times the tube diameter is 4. Physical size, shape and ease of assembly. sufficient. 5. Cost.

Reduction Of ~ Transmission The transmission loss of an isolator is related to the static deflection of the The discharge and suction isolator under lines are a possible the weight of the vibrating mass. The path for conducting compressor allowable noise to the static deflection of a rubber isolator is system. The transmission path, limited in this case, is to about 10% of its free (unloaded) height. the wall of the tube. The applications that are most susceptible to this noise are household While the rubber isolator is the choice on nearly refrigerators and freezers where steel, rather all applications, coil springs may be used to than copper, tubing is likely to be used. The achieve greater transmission loss at low fre desired result of the tube design is flexibility. quencies. In some window air conditioners, the limitation on space is a problem. In these cases, the tubing can be lengthened by providing a coil in the horizontal plane of at least 270 around the compressor.

210 Determining ~ Transmission Path of the test data must take into account the alterations made to the system. In the event that a sound test discloses an objectionable noise in the refrigeration system, After the compressor case radiation has been it remains to identify the source and the major measured, the structure-borne paths may be path. If a sound analysis of the system is added so that their effect can be evaluated. available, it may give an indication of the type The procedure might be: of compressor noise involved. All compressor noise is at one or more harmonics of the pumping 1. Case radiation only rate so that frequency alone is not definitive. However, the noise from various causes does tend 2. Case radiation and mounting to be limited to certain frequency ranges, as follows: 3. Case radiatipn, mounting and discharge.

1. Case radiation: 200 to 10,000 Hertz 4. Complete system •.

2. Discharge noise: 100 to 1,000 Hertz At any step in the program., th~ appropriate means of attenuation for that path may-be 3. TUbing: 55 to 1,000 Hertz inserted and evaluated before the complete system is re-tested. 4. Mounting: 55 to 350 Hertz

The frequency ranges shown are rough indications only and assume that normal techniques of compressor installation are used. If the objectionable noise is in the frequency range above 1,000 Hertz, it is likely that case radia tion is the cause. If below about 200 Hertz the compressor mounting is likely to be the cause. In the frequency range between the.se, some investigation is required to separate the possibilities.

The best procedure is to attempt to operate the compressor, in the system, at the load conditions at which the noise is present and with all other sound sources eliminated. In addition, it should be possible to remove all the structure-borne paths for noise transmission so that the effect of case radiation can be measured alone.

To operate the compressor, alone, an external load source is required. This does not need to be a refrigeration system with condenser and evaporator.

It is sufficient to keep the refrigerant in the gaseous state and use a throttling valve to obtain the required discharge and suction pressures. In addition to the throttling valve, some means of cooling the return refrigerant to the compressor is needed. If the compressor discharge and suction pressure, return gas temperature and voltage supply is the same as the original test conditions, the case radiation will be duplicated.

It is seldom possible, of course, to separate the compressor structure-borne noise paths from the system without compromising the sound radiation to some extent. The refrigerant p~p~ng must be brought out through the cabinet. If there are openings in the cabinet, these may be used. If not, openings must be cut and patched. The compressor mounting is never easily eliminated. It may be enough to change from a rubber to a soft, spring mount in some cases. In any case, ingenuity is often required, and interpretation

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