In comparison with many other types of food plants, bakeries are relatively quiet. A typical bakery may have one or two areas where sound levels exceed the OSHA guidelines, impacting only a small percentage of the bakery employees. In a recent study published by OSHA, the estimated cost for noise abatement to bakeries was less than 1 per cent of the total estimated to be required for all food plants.
In noise surveys of four bakeries each producing different types of products, the following acoustical environments were found:
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1. In a medium size bread bakery, sound levels in the dough making area were 93-96 dBA. The noise exposure levels in all other areas were within the OSHA limits, although employees were occasionally exposed to intermittent sound levels above 90 dBA.
2. In a bakery producing cookies, sound levels due to normal operations exceeded 90 dBA at only one location, the pulveriser area. The unit was located in an isolated room and employee exposure time to the 96 dBA sound levels was extremely low. It was also found that at only one other operation within the plant did the noise levels exceed 85 dBA. This was a cutting machine operation and the noise was primarily due to old bearings which were replaced.
3. In a bakery operation producing pie shells, excessive sound levels were identified in three locations; all three operations involved air noise. In two locations, the exhaust of pneumatic powered machines exceeded 95 dBA. The sound levels due to air used for ejection of the product out of pans also exceeded 90 dBA.
4. In a plant producing frozen pizzas, excessive sound levels were measured in the pan washer area and the bake room. The highest sound levels in the plant were due to a bread grinder.
Where sound levels do exceed the OSHA limits, the bakery manager will find little consolation in the fact his noise problems may be less severe than those of other industries. Specific noise control solutions are required.
The following approaches to noise control may be considered for various items of bakery equipment:
1. Flour Hoppers:
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Excessive sound levels may be generated by the vibrations induced into the flour hoppers by air vibrators, as well as by the exhaust of the vibrator. These vibrations are, of course, necessary for proper material flow and cannot be eliminated. While maintaining the existing vibration level, the resulting sound levels can be reduced by acoustical lagging of the hopper.
The interior lagging treatment should consist of a 1″ acoustical foam or glass fibre. The exterior of the acoustical foam should be wrapped with a 0.7-1.0 pound per square foot barium-loaded vinyl. The vibrator body should also be included in the wrapping and only the inlet and outlet air hoses should penetrate the wrapping. All seems should be properly sealed to insure sanitation is maintained.
The use of new cushioned vibrations may be considered as direct substitutes for existing vibrators; however, these units often do not provide the same vibration characteristics. The exhaust of air vibrations always generates high noise levels and should be muffled. Electric vibrators may be used in place of pneumatic units to completely eliminate the exhaust problem.
An alternative method to the above would be to install an internal vibration system which would induce the vibration into the material directly, by-passing the hopper surface. Two schemes to accomplish this are outlined below.
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Either may be retrofitted to existing hoppers and will result in the elimination of hopper surface vibration as a noise source:
i. By placing a metal bar vertically in the hopper and vibrating it horizontally as a rigid body, the possibility of clogging would be decreased.
ii. By placing a screen of the same shape as the hopper interior inside the hopper and moving it to and fro, material would not be able to clog.
2. Conveyors:
Two types of conveyors are most commonly used to convey bakery products from the dough making stage through ovens to final packaging:
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i. Roller conveyors and
ii. Woven wire conveyors.
The noise of metal pans on metal rolls may generate noticeable impact noise. This noise is generally less than 90 dBA for pans with product but often exceeds 90 dBA for empty pans.
Three approaches to noise reduction may be considered:
i. Reduction of pan velocity (controlled by the incline angle of gravity feed conveyors).
ii. Rubber coating of the rolls.
iii. Substitution of the metal rollers with their plastic counterparts.
Woven wire conveyors are very quiet up to speeds of about 400 feet per minute, where a whine is observed due to the meshing of teeth with the belt. Such speeds are not encountered in bakeries. Noise may be encountered with the conveyor motor and may be reduced.
3. Pumps:
Pumps may be used, such as to pump dough from the mixer to a roll machine. Sound levels will depend upon the type of pump used. Gear driven, piston and lobe type pumps may be quite noisy. Noise reduction of these units is difficult and may require enclosure or replacement of the pump. The quietest pump is the progressive cavity type. Noise associated with pump motors may be silenced.
In many cases, most of the noise from pumps is radiated from the case of the pump itself. As such one of the most effective noise reduction measures is to totally enclose the pump. Often the enclosure design is rather simple since most pumps do not require air for cooling purposes and therefore only strict attention to detail is required for acoustically sealing the plumbing and shaft penetrations.
4. Dough Cutting and Product Ejection:
The thrust characteristics of air are commonly utilised for two functions in bakeries:
i. Ejecting product from pans.
ii. Cleaning the edge of dough cut.
A pressure of 40 psi is characteristic. The air is necessary to prevent a string of dough from trailing each cut from pan to pan. Open air jets are typically used for these functions. It should also be recognised that localised shields which block the line-of-sight path between a high frequency noise source such as an air jet and an operator may reduce sound levels by approximately 5 dBA. Such shields may be constructed of transparent plastic materials.
5. Pan and Tray Noise:
The impacts of pans being loaded onto conveyors or being stacked may generate considerable noise and presents a problem which is quite difficult to solve. Where loading is accomplished manually, operator technique is very important. It may also be feasible to develop non-metallic guides to reduce the velocity of pans before the impact occurs. Where pans impact metal stops, non-metallic materials may be installed.
6. Pan Washer Areas:
In addition to the noise of pans, fan noise is generally present in washer areas. This noise may be reduced by means of fan inlet and outlet silencers.
7. Dough Makers:
Several noise sources may associates with dough makers:
i. In one bakery the drive unit generated sound levels of 95 dB at 250 Hz. This noise was reduced to 88 dB by enclosing the motor and drive unit.
ii. Gears in the dough maker head induce vibrations into the housing, which in turn radiate noise. This noise may be contained by installing an enclosure or lagging treatment over the head. The lagging treatment would consist of a 1″ thick glass fibre or foam in contact with the housing and completely covered by an isolated stainless steel skin. All joints should be scaled to prevent flour saturation. Existing grease fittings should be extended outside the panel to allow maintenance access.
iii. Air may be used to prevent a string of dough from trailing each cut.
8. Panner:
Noise may be generated by the rattling of levers, drives and guides for the panner, divider and associated equipment. This noise may potentially be reduced by replacing metal parts with impact resistant plastic parts in order to reduce rattle from metal-to-metal contact.
9. Ovens:
Combustion noise in ovens is generally of low intensity and does not present noise exposure problems. The acoustic energy of the combustion process is broadband in nature, with peak energy in the 125 -500 Hz range – the combustion roar frequencies. This frequency range relates to the flame propagation speed divided by flame thickness.
Where sound levels are excessive, burner operation should be reviewed to identify if combustion oscillations are causing the problem. From measurements it is relatively simple to detect whether pulsations are present in the fuel and air supplies to a burner. These can be removed either by changing the fuel/air supply equipment or fitting appropriate acoustic silencers.
In addition, potential abatement of combustion roar must be achieved by confining the noise to the firebox. It is not feasible to enclose the furnaces, since this would impede the air flow required for the combustion process; however, localised baffles may be considered.
10. Grinders:
Sound levels of a bread grinder which exceeded 100 dBA were reduced by the installation of a quieter drive mechanism. Additional noise reduction could be achieved by the treatment of interior surfaces of panels of the housing with a damping treatment to reduce vibrations induced by the drive mechanisms. In some cases it may be possible to locate the grinder in an isolated room. Product may be fed by means of a conveyor.
11. Pulverisers:
Sound levels of pulverisers may be as high as 100 dBA. In very few instances is constant attendance by operators required. Thus, location of pulverisers in isolated rooms or complete acoustical enclosures are the most direct approach to noise control. The use of localised enclosures may also be considered.
In addition to the direct radiation of sound, pulverisers will induce vibrations into the floor, which are transmitted throughout a building structure. These vibrations may annoy occupants of adjacent offices. This problem may be solved only by the installation of vibration isolators beneath the pulveriser.
12. Machinery Noise:
Mechanism noise may be identified in various items of bakery machinery, such as slicers, wrappers, etc. Generally, higher sound levels are associated with older or higher speed machinery. It is impossible to provide specific recommendations for such machinery, since specific noise sources will vary from machine to machine. Most frequently, this noise will be due to motors, pneumatic controls, metal-to- metal impacts or maintenance problems.
13. Mixers:
Mixers are used to mix pastes and high viscosity fluids in the food industry. Designs vary depending upon types of fluids being mixed, their flow rate ratios, viscosity ratios and the non-Newtonian characteristics of the fluid. Applications vary from high to low shear rates and running speeds may range from a few rpm to 1800 rpm.
Mixers generally do not present a major noise problem because:
1. The direct noise of the process is contained in a bowl or drum.
2. Most mixers do not require constant operator attendance, reducing noise exposure time.
3. The paste or fluid serves to reduce impacts of the rotor-stator and dampen vibrations.
Where mixer noise levels are excessive, the following approaches may be considered:
1. The motors may be the dominant noise source and should be silenced.
2. The maintenance condition of the rotor, stator and drive mechanism should be checked.
3. The structural rigidity of the mixer should be increased if the noise results from frame and panel vibration.
4. Where vibrations are due to panel resonances, vibration damping treatments should be applied.
5. If operator attendance is not required, the mixer should be isolated with a wall.
6. The mixing drum may be enclosed or lagged. The lagging treatment may consist of a 1″-2″ layer of acoustical foam in contact with the drum, faced with an isolated exterior layer of 1.0 (minimum) stainless steel or barium-loaded vinyl. The foam, of course, must be completely sealed for sanitary purposes.