Noise abatement for peanut and pecan plants is considered together since they involve the direct processing of nuts and have similar noise sources.
The following approaches to solving noise problems for these operations are based on experiences with four plants:
1. Peanut Cleaners:
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In one noise study involving cleaners by two manufacturers, it was found that cleaners in good operating condition and in proper maintenance radiated only 85 dBA; however, easily traceable maintenance problems on other machines generated noise levels in excess of 90 dBA and up to 104 dBA. Thus, maintenance should be recognised as a basic approach to noise control.
Each cleaner should be periodically inspected in operation while all other machines in the room are off. The inspection should include measurement of the noise level of the machine at 3′ on all sides. If the noise level exceeds 88 dBA, the machine elements generating the excessive noise levels should receive the required maintenance. The induced and forced draft blowers of cleaners are the primary noise sources.
There are two principle sources of noise in a blower – The source responsible for the generation of the tone is the pulse created each time a blade passes the cut-off point. The frequency of the tone is equal to the number of times each second a blade passes the cut-off. The second major noise source in a blower is the turbulence created by the blower while adding energy to the air stream.
High frequency turbulent noise is typically produced close to the blade tips while low frequency noise is usually produced in the fan inlet and outlet ducts. There are three primary paths by which noise leaves a blower. The first of these is the blower inlet. Noise travels from its source inside the shell out through the inlet opening and then through the air to the observer.
Noise also leaves the blower by way of the discharge passage, with some of this noise radiating through the walls of the blower discharge duct and toward the observer. The third noise radiating area is the housing itself. The noise created inside the housing excites the housing causing it to vibrate and the vibrating of the housing causes sound to be radiated into the room.
In order to adequately solve a blower noise problem, it is necessary to consider all these areas and treat each in accordance with the amount of sound power radiated by it.
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Four approaches to noise control should be considered:
i. Inlet or outlet silencers.
ii. Lagging of the ducts.
iii. Vibration damping of the housing or ducts.
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iv. Decoupling ducts.
2. Peanut Shellers:
Sound levels of 96 dBA are typical for shellers. The primary noise sources in shellers are the inlet noise of the husking fan and the outlet noise of the lower blower. Noise control by use of blower silencers would be the ‘ideal’ solution; however, the machine structure and space constraints do not allow for this approach.
Another consideration would be a tuned, reactive silencer; however, dust, complexity and cost factors make this approach unsatisfactory. Another approach to blower noise control is the use of sound absorptive plenums for both inlet and outlet air flows. The existing machine hood presently served as a small volume blenum for both blowers.
Noise control may be achieved by lining the hood with open cell polyurethane foam. The foam should be four inches thick (due to the low frequency noise spectrum of the blowers). A facing of 1 mil Tedlar should be applied to the outer surface of the foam. The side opening of the hood should be closed with a ¼” thick hinged window.
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It may also be necessary to isolate the outlet duct from the husking fan with a compliant coupling or to apply a damping treatment or lagging to the duct.
Maintenance of the shellers is also important to achieve minimum noise. In particular, loose rods and metal parts in the vicinity of the shaking tables may be found to generate significant noise.
3. Pecan Graders:
For effective noise control of an inshell grader, the entire outer surface area of the hoppers and chutes may be treated with a damping compound or sheet damping, applied to a thickness of not less than 1½ times the thickness of the metal. The easiest damping treatment technique is the application of adhesive-backed damping sheets, applied directly to the metal surfaces after cleaning.
These materials can easily be painted for cleaning purposes. As an alternative to the use of sheet damping, a spray-on or trowel-on damping treatment may be used. Specific application instructions may be obtained from these manufacturers.
4. Pecan Pop Remover, Washers, Conveyors:
The noise generated by this equipment can nearly always be traced to the impact of pecans on sheet metal panels.
For noise abatement, the following guidelines should be observed:
1. The back side of the panel impacted by the pecans should be treated with a constrained layer damping treatment.
2. The front side of the impacted panel may be treated with a wear retardant rubber material.
3. Machine maintenance should be carried out to insure minimum noise levels.
4. A layer of nylon may be positioned between the metal surfaces associated with the machine impacts. Experiments with one cracker indicated a 2-3 dB reduction with this modification. While nylon does not provide optimum impact isolation, it seems to be the only material available which would withstand the wear for any length of time.
5. The exterior surfaces of the pecan hoppers may be treated with a vibration damping material.
6. A sound absorptive material may be installed in the cracking room if reverberant sound build-up is identified as a problem.