The noise associated with container impacts and equipment of beverage bottling and canning lines poses a major noise problem in the food industry. The problem is particularly severe in high speed and multiple line operations and is amplified by reverberation due to hard wall and ceiling surfaces.
The following sections are designed to explain the mechanisms involved in noise generation of canning and bottling operations and to present solution approaches which may be applicable in many plants:
1. Empty Can Conveyors:
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High background ambient sound levels are observed due to empty can conveying. Typically, a cable conveyor is used to transfer cans from the depalletiser to the filler room. Although the sound levels three feed from a conveyor are generally only 95-100 dBA, very little attenuation is observed with increased distance from the conveyor due to its length (an acoustic line source) and usual location near walls and ceiling (acoustical radiation into a quarter-space provides a 6 dB amplification).
This source creates an ambient sound level throughout work areas which is typically near 90 dBA, with surges generating sound levels averaging 100 dBA. In operations involving multiple lines, the ambient sound levels may easily exceed 100 dBA.
The noise may be identified as resulting from four sources:
1. Impact resulting from the ‘start-stop’ motion of the cans on the line.
2. Sliding friction of can-against-can movement.
3. Sliding friction of the can against the guide rails.
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4. Repetitive impulse from can impacts against other cans and the guide rails.
The following approaches to noise control may be considered:
1. Do not route conveyor tracks through work areas – This approach provides the ideal solution for new plants, however, may not be considered practical for existing facilities. Moving existing conveyor tracks is quite costly and may require moving the depalletiser. In many plants, there simply may be no alternate routes.
2. Balance conveyor velocity to 1-2 cans per minute over the filler demand to minimise surges or adjust the speeds of the conveyor sections to move the can surges into a warehouse or other area away from employees.
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This solution will provide adequate noise reduction for some single line operations where the sections of the can track are located a reasonable distance away from employees. Where only a slight noise reduction is required, this is the simplest and least expensive answer for noise control.
3. Install new modulated speed control system – A modulated cable conveyor system utilises motion sensors, electronic controls and DC motor drives in modulating line speed to provide a constant container supply and keeps a space between cans, eliminating can-to-can contact.
In addition to reduced noise levels, reduced container damage and maximum productivity may also be achieved. A modulated conveyor system will not eliminate can-rail friction noise. This will not be a problem where only a few lines are involved; however, in plants with dozens of lines, ambient sound levels up to 96 dBA may occur due to friction noise alone.
4. Install plastic side rails – The theoretical noise reduction achievable by introducing a plastic (Teflon) to steel surface (μ = 0.07) to replace a steel to steel surface (μ = 0.2) is 4.5 dB. Noise reductions of 2 dBA have been reported due to the use of nylon side rails.
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5. Enclose the can line – Due to the many mechanisms involved noise, enclosure of the can line is the only solution which will provide adequate noise reduction for many lines. Costs are only moderate unless many lines are involved. Special design considerations must be observed to reduce potential maintenance and cleaning problems.
6. Install plastic or wooden support – A 2-3 dBA noise reduction has been reported by replacing the standard metal sheaves with a wooden cable track.
2. Can Drops:
Where can drops, 180 degree turns and ‘waterfalls’ are present in can lines, excessive noise may be generated by can impacts. Noise reduction may be achieved by enclosing or redesigning this section of the conveyor. Also, rubber guides may be installed to reduce can velocity.
3. Fillers:
Air is used to raise empty bottles for filling. When the bottle is filled, the cylinder mechanism is tripped at a stationary point and the air is released, generating noise.
The noise due to the filler cylinder discharges may be reduced by the installation of a box-shaped muffler at the discharge location. The front of the silencer should have a clear plastic door to provide cleanability and the back should be lined with acoustical foam. Baffles provide air diffusion in the design. The top and bottom of the silencer are open.
This discharge noise may also be reduced by modifications to each air valve. But due to the number of valves involved (typically 60) and the downtime required for modification, a fixed location silencer is the preferred solution.
The relief valve of fillers may generate sound levels of up to 115 dBA and may silenced using a conventional pneumatic silencer.
Significant noise is generated by the passage of air through the vent tube as measure to minimise foaming. This is also known as a ‘scavenger’. The only simple solution to this problem appears to be to review the necessity of blowing off the vent tubes and if it is necessary, to minimise the valve opening.
If this measure does not provide adequate noise reduction, the filler manufacturer should be contacted with regard to:
1. Changing vent tubes.
2. Achieving various settings in the production equipment to minimise foaming.
The impact noise of bottles feeding the filler may be reduced by means of an infeed worm screw. The noise associated with bottle handling may be reduced by the use of non-metallic parts.
There are several mechanisms of noise generation involved in filler operation and that noise reduction, while possible, is not easy. An alternate approach is to install a clear plastic sliding barrier in front of the filler. In one plant, a noise reduction from 106 dBA to 96 dBA was achieved by the installation of sliding Plexiglass doors.
Only a slight production interference problem was reported; however, the doors were frequently required to be left open for considerable time periods when filling problems were encountered.
4. ‘Soft’ Bottle Handling Equipment:
Non-metallic (plastic or phenolic) bottle feed mechanisms are commonly used for noise reduction. The theoretical noise reduction achieved by cushioning bottle impacts would be in proportion to the increase in material elasticity.
5. Crowner:
Air jets are used on crowners for two functions:
1. To assist flow in the hopper.
2. To move crowns along the feed chute.
The hopper air jets may be replaced by agitator springs for slow speed lines. At higher speeds, a low pressure air jet will be required to assist agitation.
The silencer will provide a major noise reduction for free air jets (without crowns); however, high noise levels will be generated by turbulence as the air jet passes the crown edge. This noise cannot be eliminated by design, but may be isolated by means of a localised enclosure over the crown chute or by the installation of a clear plastic barrier in front of the crowner.
6. Can Seamers:
The noise of can seamers is generally the highest noise source in a beverage canning plant, typically measuring 100 dBA at the operator position. The primary noise source is the impact of the code wheel upon the can lid. Significant noise is also attributed to the cover separator mechanism. In addition, a loud double impact may occur when the lid turrent is out of time.
Recently a company designed an experimental lined stainless steel barrier to fit and enclose the cover separator above the cap feed plate and to partially enclose the cap feed turret below the cap feed plate. A baffle was added under the cap feed plate.
The following noise reductions were reported:
The replacement of the code wheel may be considered as a possible approach to noise reduction.
7. Filler Room Acoustics:
A sound level build-up of 3-7 dBA is often found in filler rooms due to reverberation caused by hard wall surfaces. Recognising sanitation requirements, it may be possible to install sound absorptive panels with a thin plastic facing for cleanability.
8. Labelers:
Noise studies of three labelers indicate that the following sound levels are typical:
There are numerous noise producing sources associated with labelers and an extensive design study would be required to develop mechanism modifications for noise control. Such designs may also interfere with the machine operation. Enclosure of the machine is not considered feasible.
Observations of labeler operations indicate that the operator is not required to spend all of his time directly at the machine and can observe his operation from a distance of a few feet at some times. The most effective method of noise exposure reduction would be to limit the exposure of the operator to a maximum of 6 hours per day within a 3′ radius of the machine. A line painted at this distance may assist in implementation of this operational control.
9. Bottle Conveyors and Combiners:
Noise is generated on conveyors and combining tables due to bottle-to-bottle and bottle-to-rail contacts.
The following approaches may be considered for noise reduction:
1. Smoothen bottle flow.
2. Eliminate unnecessary bottle pile-ups.
3. Reduce bottle impact velocity.
4. Utilise plastic guides and rails.
5. Redesign transfer points.
6. Use cradle-belts.
7. Install acoustical line covers.
10. Cradlebelts:
To eliminate impact noise due to bottle or can contact on conveyor lines, the use of cradle-belts may be considered.
11. Bottle Washers:
The sound levels generated as the bottles exit a washer may be significantly reduced by treating the guides with plastic. A remaining noise is the impact of the bottles as they drop approximately 1″ onto arms. The impact sound level (slow meter response) is 100 dBA in front of the washer.
This noise may be reduced by reducing the drop height of the bottle. The law of conservation of energy indicates that the energy associated with the fall will be totally converted into other energy forms. Vibrational and acoustical power is the primary resultant energy forms.
Thus-
Ia α wh
Where,
Ia = acoustical energy
w = bottle weight
h = drop height
Thus, the expected noise reduction would be directly proportional to the bottle drop height. To achieve a noise reduction of 10 dBA, the drop height should be reduced from 1″ to 0.1″.
This may be accomplished by:
1. Moving the arms upward by adjustment or by spacers.
2. Modifying the guides to allow the bottle to slide onto the arms.
It is reported that the top of the bottles may break if the guides are raised too high. It may be necessary to modify the machine slightly to prevent such breakage.
Noise is also generated by bottle contact at the washer discharge. The use of barrier may be considered for noise reduction.
12. Inspectors:
Bottle inspectors are frequently exposed to excessive noise due to adjacent equipment.
Reduction of their noise exposure may be achieved by:
1. Isolation of adjacent equipment by a wall.
2. Noise reduction of adjacent equipment.
3. Enclosure of the inspectors.
4. Rotation of the inspectors.
13. Drop Packers:
The drop of cans or bottles into trays or cartons results in high impact noise levels. The use of a rubber pad on the drop plate provides an evident solution; however, only a 1-2 dBA noise reduction is generally achieved.
Some can packers do not involve dropping cans and are inherently quiet. One design involves forming the tray around the cans.
Bottle drop packers may be replaced with lowering head case packers with a pressure controlled infeed for inherently quieter operation.
The use of an acoustical barrier may be considered for noise control. In one plant, a reduction from 107 dBA to 96 dBA of bottle impact noise was achieved by the use of an open-front clear plastic slitted curtain.
The enclosure of an infeed can packer provided a noise reduction from 104 dBA to 88-90 dBA.
The noise sources were identified as:
1. Can-can contacts and infeed surges.
2. A pneumatic rocker arm.