The food process industry faces constraints in solving noise problems which are not imposed upon other industries. All noise control materials and systems installed in food plants must comply with strict sanitary standards.
The spectrum of noise sources which are encountered in food plants is also the greatest of any industry. In the textile industry for example, the majority of noise problems are confined only to looms, twisters and braiders. In the various types of food plants, literally hundreds of basic types of equipment, ranging from animal kill rooms to packaging, are commonly found to contribute to noise problems.
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This diversification of noise generating mechanisms makes any concentration of research efforts impossible. Each type of problem must be considered individually. The result is that the majority of recent research on noise control has been directed toward other areas (presses, looms, etc.).
Notwithstanding these challenges, significant advances have been achieved in recent years in many types of food process plants. It presents most of these recent advances in the form of solution approaches for noise problems in meat packing plants, poultry plants, dairies, vegetable and fruit canneries, bakeries, nut processing plants and beverage plants.
Governmental Sanitary Requirements:
The Food and Drug Administration (FDA) is responsible for regulating all foods.
In general, all materials in product contact zones should comply with the following requirements:
1. Non-Toxic:
FDA approval lists and procedures for approval of non-toxic materials are provided in the Food Additive Regulation.
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Approval may be obtained for other materials by providing-
(i) Trade or brand name;
(ii) Use; and
(iii) Chemical composition.
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2. Impervious:
The material must be impervious to water, meat juices, etc.
On investigation it has been indicated that the most compatible materials for noise control would be stainless steel and certain plastics such as Teflon, PVC, etc.
Regarding materials not in food contact, the primary requirement is the prevention of porous or irregular surfaces which permit dirt or bacterial build-up. The material must be cleanable.
Guidelines for Installation of Noise Control Systems:
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The following guidelines are applicable to the installation of noise control systems:
1. Avoid locating mechanisms directly above the product stream.
2. Cover the product stream.
3. Bearings and seals should be located outside the product zone. Parts in product zone must be sealed or self-lubricated.
4. Minimise loose items.
5. All filters must be easily replaced and cleaned.
6. Systems should be designed for thorough wash-down with high-temperature (140-180°F) water and detergents as required for proper cleaning. In some plants high pressure (600 psi) water is used.
7. Building contractors should be aware of necessity of keeping site clear to prevent attracting rodents and insects.
8. False walls and voids in walls should be avoided, particularly in process areas.
9. Avoid using glass in, above or near process areas.
10. Select or design equipment to be safe under processing conditions, easily cleaned and inspected.
11. Construction materials should be selected to resist wear and corrosion and to protect contents from external contamination.
12. Product contact surfaces must be inert, non-toxic, non-porous, smooth (no cracks, crevices or sharp corners), easily cleaned, non-peeling and inert to steam cleaning, hot water and sanitising solutions.
13. All inside corners should have internal angles of sufficient radius (¼” or greater) to provide easy cleanability.
14. All surfaces in contact with food should be visible for inspection or readily disassembled for inspection. It should be demonstrated that routine cleaning procedures eliminate contamination from bacteria, insects and soil.
15. Painting of product contact and product zone surfaces should be avoided.
16. All surfaces in contact with food should be readily accessible for manual cleaning. If not readily accessible, equipment should be easily disassembled for manual cleaning. If mechanical in-place cleaning is used, the results achieved without disassembly should be equivalent to those obtained with manual cleaning.
17. Interior surfaces in contact with food should be self-emptying or self-draining.
18. Exterior of equipment must be easily cleaned and not retain soil or wash water.
19. If equipment requires adjustment during operation, it should be designed so operators do not place their hands in product zone.
20. Floor attachments should be minimised.
21. Equipment should be at least 8″ off the floor, mounted on single pedestals wherever possible and with clearance of at least 18″ from ceiling. Use single point contact supports.
22. Equipment should be mounted at least 36″ from a wall and at least 36″ should be allowed between equipment.
23. All openings into equipment should be protected against entrance of contaminants as a function of the action of opening or the open condition.
24. Internal pipe caulking should be avoided.
25. Flexible piping should be non-porous, not affected by the food or cleaning compounds and in sections not over 3′ long.
26. Piping must be designed to operate ‘flooded’ during normal operation.
27. Avoid using screwed pipe in product lines, welded pipe must be inspected for proper penetration of weld before installation.
28. Select gasket materials to provide proper seals and not contaminate the product.
29. Continuously weld all support connections.
30. Seal all ends of support members.
31. Floors, walls and ceilings must be smooth, non-peeling, inert to process and easily cleaned.
32. Floor, wall and ceiling corners should be coved for easy cleaning. Suggest 4″ radius.
33. Caulk or seal all wall, floor and ceiling joints.
34. Structural members must be integral to the supported surface or caulked to it.
35. Avoid painted walls and ceilings, particularly where moisture is involved. Use prefinished, easily cleaned panels, insulated as necessary.
Poultry Plants:
Sound level surveys in poultry plants may indicate excessive sound levels due to three operations:
1. Picking.
2. Lung guns.
3. Packing.
Noise in picking areas is generated by picker brushes, gears and heating flames. Continuous attention by operators is generally not required and noise exposure times may be quite low. The picking noise, however, may carry over into adjacent areas, exposing other employees. This noise may be most easily controlled by confining it to the picking room.
Specifically, this may be achieved by the construction of a complete wall, such as of painted concrete block, isolating the picking area. Product should be passed through an opening in the wall, which must be of minimum area to reduce sound transmission. The opening may also be baffled.
Suction noise is a dominant source in lung guns. This noise may potentially be reduced by providing a complete seal around the opening in the bird to confine the noise. Air is then supplied to the interior volume by means of a separate air hose.
Noise in packing areas is often generated by vibrations of the stainless steel work tables due to impact of product and other objects. This noise may be reduced by treating the underneath side of the tables with a vibration damping treatment. Due to its remote location, the damping treatment should not prevent sanitation problems; however, precautions should be observed to select a material which is smooth, cleanable and non-toxic.
The acoustical environment of dairies consists of the noise of numerous items of mechanical equipment which is amplified by the reverberation of the work space due to hard wall, ceiling and floor surfaces. In some cases, reverberant noise levels may be reduced by sound absorptive treatment. Generally, however, noise reduction must be achieved by means of equipment modifications.
Approaches to equipment noise control are presented in the following sections:
Gas fired burners used on some fillers generate sound levels in excess of 90 dBA. Some companies have developed two quiet burner models which reduce the burner noise to 76 dBA at 3 feet. It should be pointed out that some fillers use electric heaters rather than burners, eliminating this noise source.
Another source of noise is the 92 and 94 dBA generated by using compressed air to discharge the cartons from the bottom forming section through a chute to the rear conveyor chains. A prototype machine developed is presently running in the field, which uses a mechanical stripper device, eliminating the air in this location. As a result, a reduction of noise was accomplished as a reading of 82 dBA was recorded at a distance of 3 feet while the machine was running.
Other noise sources on fillers include motors, air discharges, filler bowl vacuum pumps and vibrators.
2. Materials Handling—Automatic Casers and Stackers:
The primary noise sources of these items of equipment are air exhaust and metal-to-metal impacts.
3. Homogenisers:
One dairy reported a noise reduction from 107 to 97 dBA by boring the head and replacing the motor. Other potential machine design changes may include the use of a sound absorptive material within the hood, mufflers and vibration damping. These modifications could most easily be accomplished by the machine manufacturer and may not be practical for existing equipment.
The best approach for noise abatement of homogenisers already installed is the use of acoustical enclosures or to isolate the equipment in acoustical rooms.
The method of standardisation and clarification of milk presents the problem of the separator/clarifier location. In many new installations the separator/clarifier is an integral part of a high-capacity HTST system. Unfortunately, the noise level of most separator/clarifiers does not meet the minimum allowable noise standards.
To circumvent this problem, the new plant layout should isolate the separator/clarifier in a separate room that is near the short-time equipment, near the main control panel and adjacent to the processing room. Control of the machine is by the main panel operator on the processing room floor.
5. Ice Cream and Pop-Cycle Machines:
Sound levels ranging from 87-92 dBA were measured for two Vitaline machines. The major noise sources were identified to be: metal-to-metal impacts, exhaust from air cylinders and a vibratory bowl feeder.
Vegetable, Fruit and Prepared Food Canners:
The primary noise problem in most high speed canning plants is can-to-can impacts on conveyor lines. Where cans are partially filled, sound levels are a function of both the product in the can and the percentage it is filled.
Noise control for other food canning operations is discussed in the following sections:
The following sound levels were measured in a citrus process plant is shown in Table 15.1.
2. Continuous Cookers and Retorts:
Sound levels ranging from 94-100 dBA may be generated by continuous cookers and retort equipment. This noise may be silenced by the installation of a silencer, such as available from the equipment manufacturer or a steam exhaust muffler, such as SM type. This muffler will withstand temperatures up to 1000°F. The ideal approach to retort equipment silencing seems to be to pipe away the steam.
The sound levels of a spaghetti produce forming machine were reduced to below 90 dBA by the installation of pneumatic mufflers.
Noise generated by meat ball forming machines was identified to be due to metal-to-metal impacts of machine mechanisms. For noise abatement, the use of non-metallic parts should be considered.
4. Vibratory Shake Tables and Conveyors:
Many vibratory shake tables are found to have noise levels below 90 dBA, especially where soft food products are involved.
Where excessive noise is observed, it may be due to:
1. Operation above the recommended design capacity.
2. Loose parts.
3. An imbalance of the conveyor; the system must be ‘tuned’ for optimum operation and minimum noise.
4. Motor noise.
Generally, a good maintenance programme is the best approach to noise control. The use of motor silencers may also be considered. Where conveyors are located overhead, a clear plastic barrier may be installed below them to shield employees from the noise.
Sound levels of vegetable cutters are generally due to the drive mechanism. Sound levels of older corn cutters were measured to be 94 dBA, while new units were below 90 dBA. Noise abatement may be achieved by gear damping, non-metallic gears or enclosure of the drive mechanism.
Feed Mills:
The major noise sources in feed mills are:
1. Flaking mills,
2. Hammer mills and
3. Pellet mills.
The noise levels of soyabean flaking roll machines have been measured to range from 93 dBA to 103 dBA. The primary mechanism of noise generation is structural vibration caused by the slight movement of the rolls as the product passes between them.
The following approaches to noise control may be considered:
1. All openings on the front and top of the mill should be closed with a nonporous material weighing no less than 0.5 pounds per square foot. Suitable materials would include plywood, sheet metal and barium-loaded vinyl. This feature will reduce the noise radiated outside the machine from the roller operation and the lower hoppers.
2. A sound absorbing panel of 2″ thick open-cell polyurethane foam with a film of 0.1 mil Tedlar may be installed on the inner side of one of the sheet metal panels of the lower hopper. The panel should be approximately ten square feet in area. The function of the panel is to reduce sound build-up within the machine structure.
3. The entire mill may be placed on vibration isolators to prevent vibrational energy transmission from the machine to the floor, lower hopper and other structures.
4. The sheet metal panels of the lower hoppers may be treated with a constrained layer damping treatment or may be constructed of commercially available vibration-damped panels.
5. The lower hopper may be installed with vibration isolation.
6. A lagging treatment may be applied to the entire face and back of the mill.
Sound levels of hammer mills may range up to 107 dBA. The dominant frequency of the noise is found to be at the hammer passage frequency –
f = n(rpm)/60
Where,
f = frequency, Hertz
n = number of hammers
rpm = mill speed.
The following approaches to noise control may be considered:
i. Use a lower speed motor (perhaps 1800 rather than 3600 rpm).
ii. Enclose the mill with an absorbent-line box.
iii. Provide some means whereby the access doors on the upper inlet duct can be closed when access in not required.
iv. Apply damping treatment to the exhaust duct.
Sound levels ranging from 87 to 100 dBA are typical of pellet mills and will be found to vary with operational conditions and the use of vibrators.
The following approaches to noise control may be considered:
1. Vibrators should be used only when necessary.
2. Air vibrators should have exhaust mufflers.
3. Motor silencers may be installed.
4. The pellet mill gears may be treated.
5. An acoustical curtain may be installed on the pellet exit chute.
6. Gear guards and chutes should be dampened.
7. A rubber coupling may be installed in the drive shaft.
8. The die door may be treated with lagging or damping.
Vibratory Bowl Feeders:
Vibratory bowl feeders are designed for a certain maximum design capacity and are generally quiet within their design range. When this bowl speed is exceeded, excessive noise is generated due to system non-linearities. It is often observed that bowls are set to feed parts faster than required by the line flow.
Maximum operating bowl speed should be determined for satisfactory operation and a limiter should be put on the potentiometer dial to prevent operation above this setting. The maximum operating point may be determined quiet easily by ear. If a limiter is not considered practical, the normal operational range should be marked on the potentiometer. In addition to noise level reduction, potentiometer control will result in both energy and machine wear savings.
The sound levels of vibratory bowl feeders may easily be reduced to below 85 dBA by the installation of an acoustical enclosure.
Metal to Metal Impact Noise:
Where structures or parts are impacted with metal-to-metal surface contact, large portions of the impact energy is converted to vibrational energy and in turn sounds.
The acoustical energy generated by part impacts is directly proportional to the kinetic energy at the impact point.
Ia α K.E. = ½ mv2
Where,
Ia = acoustical energy
K.E. = kinetic energy
m = part mass
v = part velocities.
One method of reducing impact noise is to modify the system to reduce the impact velocity. Another method is to interrupt the metal-to-metal contact with a cushioning material, which serves to reduce the momentum transfer of the impacting structure.
Four types of impact surfaces may be considered and it is found that the strongest of the materials will offer the least isolation. Selection of an optimum material must be made on an experimental basis.
Three classes of materials to be considered are as follows:
1. The highest degree of impact isolation is offered by the rubber surface. It is found that rubber will wear very well in many situations, but is unacceptable in others.
2. Of great interest for highly stressed mechanical components are the plastics and their characteristics listed in Table 15.2.
3. An impact energy absorbing foam may also be faced with an exterior layer of damped sheet metal to serve as a protective facing.
Boilers:
Energy is available in various forms in nature. Normally energy is utilised in a form different from the one in which it is available. We therefore require equipment which converts energy from one form to a more conveniently usable form.
The steam generator, or boiler, is a combination of systems and equipment for the purpose of converting chemical energy from fossil fuels into thermal energy and transferring the resulting thermal energy to a working fluid, usually water, for use in high-temperature processes or for partial conversion to mechanical energy in a turbine. The boiler complex includes the air-handling equipment and ductwork, the fuel-handling system, the water-supply system, the steam drums and piping, the exhaust-gas system, and the pollution-control system.
Steam generators are the most common vapourisers.
They fall into two general categories, namely:
(i) Industrial; and
(ii) Power-utility steam boilers.
Industrial steam boiler types include fire-tube boilers and waste-tube boilers. Fire-tube boiler (or steam generators) are characterised by the containment of the products of combustion within the tubes of the boiler. The water being vapourised surrounds the tubes and is contained by a larger shell with front and rear tube sheets.
In boiler design utilises a central cylindrical combustion chamber with return of the gases through smaller tubes. Baffles are frequently provided to return the gases through more tubes for a third or fourth pass. These units are usually completed as a shop-erected package boiler with all combustion equipment and controls installed and ready to operate after electrical and piping connections are made at the place of utilisation.
Fire-tube boilers are rated for capacity in terms of boiler horsepower. One boiler horsepower is defined as the ability to evaporate 34.5 lb water from and at 212°F. Such units are available in size from 5 to 600 hp (about 20000 lb/hr. steam) and in pressures to 250 lb/sq. inch.
For the size and pressure range they are very competitive in price and installation costs. They do not lend themselves readily to the installation of superheaters and are thus limited to the generation of saturated steam only. They are applicable to oil and gas-fuel firing only.
Water-tube boilers are characterised by the containment of the vapourising water within the tubes while the products of combustion are on the outer surfaces. The furnace chamber may consist of containment by either refractory walls or special configurations of the water tubes. For economic reasons, size of 10000 to 1,20,000 lb/hr. are generally of ‘package’ shop-erected construction.
These are currently available only for oil and gas-fuel firing, with active work underway to develop similar coal-fired units. Larger-capacity oil and gas-fired units and almost all coal-fired units are generally ‘field-erected’, or constructed in place. Since the water tubes are of small diameter compared with the shells of fire-tube boilers, there is no similar limitation on their operating pressures.
Many older water-tube boilers are of the straight-water-tube construction, but modern units are almost universally of bent water-tube design. The use of bent tubes allows extreme flexibility in design. The shaping of the tubes to enclose the combustion space has sharply reduced the amount of refractory used and has permitted higher rates of burning within the combustion space.
Superheaters may be readily fitted to such units. Industrial units in excess of 2,50,000 lb/hr. capacity are rare and operating pressures in excess of 600 lb/sq. inch are also seldom encountered. Higher-pressure units are sometimes installed where electric power is generated within the plant, but the economic trends had been away from power generation with a few notable exceptions.
Utility boilers include conventional boilers and unconventional boilers. Conventional boilers used in power utility plants are those units operating below the critical pressure of the steam with widely accepted modes of operation. They may include features that yesterday were unconventional such as forced circulation, reheat, divided furnaces, once-through flow, and pressurised furnaces.
They are usually custom-designed and built, with the trend to larger units in the multi-million pounds per hour capacity. Their superheated output is taken directly to a single turbine generator in recent designs for the production of electricity. They are universally of water-tube design, and all fuels are fired, with coal being the most common.
Unconventional boilers are those which are experimental or extremely rare in their application. Currently, mercury binary, combination gas turbine, supercritical, and nuclear boilers are in this category. Supercritical units are conventionally fired once-through forced-fluid-flow designs where the entire system is pressurised over the critical vapour pressure.
Units of over 5000 lb/sq. inch operating pressure have been constructed and are in operation. Mercury-binary-system boilers impose a mercury-vapour topping system on a conventional steam cycle. The mercury is vapourised at a high saturated vapour temperature by conventional fuel firing.
After electricity is produced in a mercury-vapour turbine generator the latent heat of the mercury is transferred to steam in a condenser-boiler. A number of such plants have been built and are in operation. Combination gas-turbine and steam-turbine plants have been built for many years (Velox boilers).
New designs are currently being built which incorporate a more active part by the gas turbine. These designs include those where the exhaust gases of large gas turbines are re-heated by supplemental fuel firing to serve as the combustion gases of conventional steam boilers.
Nuclear units have been built for power utility steam boilers in the United States and other parts of the world. It does not appear that they will compete economically with fossil-fuel plants until fundamental problems in fuel preparation and waste disposal are solved.
In hot-water-radiator heating systems, based on boilers, there will be the noise from such ancillary equipment as fuel injectors and pumps (especially in the increasingly popular small-bore systems) and this noise will be transmitted both through the floor of the boiler house and so into the fabric of the building, as well as through the pipework of the building.
Consideration of some of the problems involved can begin in the boiler house, where noise can be generated by the burners of oil or gas-fired appliances and by fans providing induced draft for the burner systems. Manufacturers of burner systems are now paying more attention to the need for quieter operation but it may still be necessary to fit burners on isolated mountings and attenuators may be required in flues linked with induced draft fans.
In the larger central-heating installations especially, the question of induced draft may be of particular importance, since vibratory energy can be imparted into the flue, travel upwards with the products of combustion and be discharged from the top of the flue to the annoyance of neighbours.
All noise generated in the boiler house and not controlled at source, is available for transmission through the pipework to the remainder of the building. Wherever possible, it is preferable for all noise transmitted in this way to be controlled as near the source as practicable, if it cannot be wholly controlled at source. The best method of achieving this is to fit flexible connections to all pipelines, a less difficult undertaking than isolating the pipework from the walls throughout the building.
A certain measure of isolation can, of course, be achieved by isolating the hangers, brackets and other methods of fixing pipework to walls, but these procedures are never so satisfactory as isolation at or near the source of the noise. If isolated fixing is to be employed, it should be concentrated on those fasteners of the pipe runs.
Other problems may arise from the connection of pipework to passive items of equipment, such as storage tanks and calorifiers, the latter being particularly important in large systems supplying both central heating and hot water. Vibratory energy injected into such passive items can be then transmitted into those parts of the building fabric which are in contact.
To overcome this difficulty, passive items of equipment can be mounted on rubber mats—a procedure easily carried out before the equipment is installed but may be difficult to effect afterwards, particularly with the very large tanks and high-capacity calorifiers now being installed in commercial and industrial premises.
All the isolation techniques to which reference has been made will control structure-borne noise but there will also be an element of airborne noise from the boiler house which may also have to be dealt with. This can be done by acoustic treatment of the walls and ceiling of the boiler house itself and possibly, in the areas adjacent to the plant room.
The treatment may also have to include double-glazing on windows and the installation of sound-resistance or sound-proof doors from the boiler house to the interior of the building; and if nearby neighbours are likely to be affected, from the boiler house to the surrounding area.
In addition to pipework, the radiators associated with a hot-water central-heating system can also prove effective vehicles for the transmission of unwanted sound, although if adequate measure has been taken to control the noise at source and to isolate all pipework, the radiators should give rise to no problems on their own account.
Any noise which does enter the radiator, however, will immediately transmitted to the building fabric if the radiator is bolted solidly to the floor and fixed by brackets to the wall. If the floor to which the radiator is fixed is itself an isolated construction, fixing the radiator solidly to the wall will clearly reduce the efficiency of the sound control arrangements.