The fans draw ambient air through an inlet grille in the end-bell and blow it out the back of the bell, along the cooling fins so the motor doesn’t overheat——-and stop!  For larger motors, the air may be blown through the windings inside the motor and then exits out the shaft end.

In either case, the fan, although providing a very important function/benefit, is also the primary source of noise.  For some well intentioned “noise stoppers”, putting an enclosure over the motor may appear to be a logical means to resolve the problem.  It is, however, fraught with several potential long-term side-effects that could prove disastrous.  1) It may limit or discourage maintenance; 2) Because any enclosure, unless properly ventilated, will most likely prevent proper cooling (which is why the fan was added in the first place) or 3) Be such a hassle to remove and re-install, that it often mysteriously disappears after the first or second time the motor requires maintenance.

Some forward thinkers, have utilized a baffle-type silencer, either prior to or built into a plenum that is attached to the inlet grille in the end-bell.  This can work acoustically, without a powered ventilation fan (such as might be needed for an enclosure.)  The problem with this approach is that the airflow area of the silencing elements has to be fairly large to keep the pressure drop across the “silencer”, very low.  The fans on the motors were designed to overcome the pressure needed to capture the ambient still air and move it via a torturous path and along or through the motor.  It doesn’t take too much added, eternal pressure drop to reduce the air flow, which is undesirable.  This can lead to overheating which then requires more frequent maintenance or shorter life-span of the motor……..if not stopping it.

Enter the spiral motor silencer!  This unique silencer improves on the previously described solution by notably reducing the physical size and hence requires far less space, wherever the motor is used.  Models are available for motors mounted in the vertical as well as the horizontal orientation.  The acoustical spirals that replace the conventional baffles, provide a better degree of noise reduction in the mid-to-low frequencies compared to the baffle-type and yet at a lower pressure drop.

The primary disturbing tone is usually related to the “blade-pass frequency”.  That is based on the number of blades on the fan, multiplied by the RPM (revolutions per minute) of the motor and that amount is divided by 60 (sec/min.) to yield the cycles/sec. now referred to as “Hertz.”  As the size of motors increases, the speed or RPM often goes down.  A small fractional horsepower motor may run at 3600 RPM; somewhat larger motors up to perhaps several hundred horsepower, more commonly turn at 1800 RPM.  Much larger motors up to thousands of horsepower may operate at 1200 or 900 RPM.

After running temperature-rise tests on various sized motors with and without the spiral intake silencers, it was found that the differential was only 1 or 2-degrees F.  It was also learned that about 60-70% of the fan noise normally comes out through the grille.  Hence, the spiral silencer on the intake usually provides enough noise reduction to resolve the problem without having to treat the air exhaust portion.

For any further questions or discussions on a specific application, please contact our office or refer to our website:  www.stanfordassociates.net.

An International Pharmaceutical Company

Issues:

• Re-tooling of an existing pharmaceutical production laboratory required installation of (5) new vane-axial fans on the rooftop of their facility in New Jersey.  These fans ranged in size from 25,000 CFM to 38,000 CFM and from 54 to 60” ID. The acoustical design required treatment both on the discharge side through up-turned exhaust stacks, (neighborhood noise issue) and intake side (for the noise levels in the lab below.)

• The fans had a limited availability for additional system pressures and were sensitive to disruption of laminar airflow at the intake/exhaust faces.

• Existing duct work limited the length of any proposed silencer unit to no more than 48” while high attenuation requirements for the silencer exceeded 15db at 63hz and 19db at 125 Hz.

• Structural mounting issues were also present. Any proposed silencer would have to be less than 500 lbs each to insure no additional structural modifications of the roof would be needed.

Solution:

• The acoustical consultant called upon their local Innovative Metal Industries (IMI) Representative for the correct silencer selection. This firm has had multiple applications with the Oxel® Spiral Silencer
line, including several with this same client.

• Custom flange sizes were developed allowing direct connection to the ducts that matched up with the flanges of the fans.

• The Oxel® Silencers provided the ability to couple the units close to the fan faces without disturbing the laminar airflow or adding additional pressure drops.  This in turn simplified the installation and allowed the client to use existing exhaust stacks.

Results:

• After installation, the measured noise levels were reduced dramatically below the requirements, both at the property line and inside the facility.

• At the rated 38,000 each silencer contributed less than 0.18” of additional system pressure. The same was true for the smaller units.

• During testing, the project engineer for the client, while on the roof with all units running, said it was so quiet he had to “place his hand on the fans to make sure they were on.”

A Well Known Law School, Pennsylvania

Issues:

• A new lecture hall and auditorium was being built at a very prestigious law school in Eastern Pennsylvania.  It incorporated a cutting edge HVAC design that provides air distribution from under the floor rather than conventional overhead supply paths. This unique high efficiency system required large diameter Fiberglass Reinforced Plastic (FRP) duct work to be buried under the concrete slab of the facility.

• The silencer requirements included: High attenuation; small size and construction using the same materials as the direct burial ducts. Air volumes ranged from 9,500 to 14,000 CFM with duct sizes from 36” to 48” diameters.

• The silencers also had to withstand the significant, external pressures of direct burial under the slab.

• Corrosion resistance was critical and a zero maintenance requirement existed, due to no access after installation.

• High attenuation was required to handle the high air volumes to this acoustically engineered lecture hall.

• A specification requirement called for the installed system to remain pressurized for 4 hours with 0% leakage.

Solution:

• After a very successful installation of Oxel® Spiral Silencers at another campus of this same Law School, the New York City Mechanical Consulting Engineer providing the design, approached our local representative for this unique silencer requirement.  The rep and the factory engineering department, were very familiar with FRP construction due to IMI’s experience in providing silencer solutions in the Waste Water Treatment Plant industry. Most WWTP facilities typically use FRP silencers in the treatment of noise problems from scrubber systems.

• A casing that matched the FRP ducting in material, wall thickness, diameter and specifications was incorporated into the silencer design thereby eliminating any worries of potential corrosion or strength issues.

• The casings of the silencers were provided with mating flanges that the FRP Duct installer was familiar with, and thus reducing installation costs and special processes.

Results:

• The silencers were installed using the same system as the duct work (bonded on site).

• Higher attenuation was achieved than the specifications required and with lower increases in the original system static pressures.

The decision is decidedly less painful if the ROI is easily understood and favorable.  The problem arises when the proposed expenditure is tainted with the immediate presumption that it is a “non-productive capital expense!”  (Non-c-ense!)

Understandably, the dollar amount has an obvious impact on the level of angst generated.  Perhaps a $1,200 expense to replace your 3-4 year old computer may not be too painful and can be justified because:  it’s faster and will save you time; has a bigger hard drive and more RAM so it won’t be as prone to crash and just maybe there’s the subjective decision “that I want it!”  But now suppose you’re considering a decision to buy a new piece of production equipment for $100,000!  Oh, Boy!  The struggle within can cause a much higher level of gastric juice generation.  As mentioned previously, if it means your company can increase productivity and profits, that can ease the stress considerably.

Now compare that example to perhaps an adjunct expenditure of a similar amount to meet your Township’s Noise Code for your new diesel-generator that is being purchased to insure your computers won’t go down if/when your electric supplier’s system abruptly goes off-line!

Having dealt with managers faced with such a decision, since the late 1960s, I can tell you the first reaction of many is:  “Non-c-ense!”

The whole build-up to this point, is to suggest that maybe —- just maybe, there is a positive ROI for this type of investment.  Doesn’t avoiding taking more money out of your pocket, in effect, go equally to the bottom line as making the same amount of profit?!  How much in sales do you need, to generate to provide enough profit to offset a fine/fee/law suit you might otherwise have to deal with?

You might say, “Well, maybe ——- but is there any way it could be made a little better?”  And I say, “Oh, yes it can!”  Suppose you determine you need to expand your plant or install a good sized cooling tower.  To move ahead with your project you find out you need to go before your local Planning or Zoning Boards for approval.  What are your chances for gaining that approval if you’ve ignored neighbor complaints about noisy equipment, in the past?  If the Township Supervisors are getting calls during their dinner time, about the noise from your plant, how well disposed will they be towards your request?

Are these unlikely scenarios or scare tactics?  I can tell you based on all the years I’ve been in the noise control field; they’re not far-fetched at all!

You don’t get any ROI on your auto insurance —– until you have an accident.  You can’t easily put an actual value on the goodwill generated by something like contributing new lighting for the local high school football field.  Or, making sure any new equipment installed inside or outside your plant doesn’t increase the ambient noise levels.  Yet, most business people will acknowledge it’s usually worth it.

Wouldn’t it feel good when you walk into that Zoning Board meeting and you’re greeted with smiling faces on the Board members and on those of your neighbors?!  Believe it or not, understanding and acceptance go both ways.

That last statement extends to your noise control supplier, as well.  First, fully understanding your problem and then recommending an adequate fix, is paramount.  The supplier’s intent should not be to see how deeply he can jam his hand into your pocket!  Rather he should always be looking out for your best interests, because in actuality, in the long run that’s in his best interest as well!

Major Design/Build firm for a Co-Generation Facility, Ontario, Canada

Issues:

The expansion of an existing Gas Turbine driven power plant Heat Recovery system was under construction as a “design/build” project.

The customer was installing two auxiliary blowers that would be exhausting outdoors. Each fan was rated at over 90,000 CFM. After analysis by an acoustical consultant, it was found that the units would exceed property line noise limits.

The fans were sensitive to additional static pressure, which would diminish the efficiency of the recovery system. This would translate into increased operating costs per kilowatt generated.

The duct-work design limited the length of any proposed silencer unit to no more than 72”. The acoustical specifications called for a silencer Dynamic Insertion Loss of 28 db at 63 Hz, 38 db at 125 Hz and 57db at both 250 & 500 Hz!

The silencers would also need to be mounted in an elevated location and weight was a major concern.  The casings of the silencers also had to meet very stringent Transmission Loss (TL) requirements since they were mounted inside the occupied area of the plant.

Solution:

The acoustical consultant provided the requirements to the factory for silencer selection. After analysis, our Dual Spiral unit was selected. This Dual unit incorporated (2) Oxel® spirals in-line with a resonating chamber space between the two.

A heavy casing was also utilized to prevent breakout noise from the silencer.

Even with the high attenuation provided by the dual units, they were less than half the length of a standard baffle-type silencer and had a smaller face area for the same given pressure drop.

Results:

The silencer provided the specified attenuation, low pressure drop and did not require any additional structural supports outside of the ones present for the duct-work.

The facility was able to use the originally designed locations for the duct exhausts without having to modify the entire wall of the facility already under construction. This saved the facility the substantial cost of redesigning and modifying the structure.  Some of the Advantages of using An Innovative Solution from Innovative Metal Industries:

The high attenuation, combined with lower pressure drops that the Oxel® Spiral Silencers provide, address many requirements that a standard, heavier baffle silencer cannot.

The physically smaller size and weight of the Oxel® Silencer, typically eliminates increased project costs for additional structural modifications and support normally required for standard baffle silencers.

Energy savings realized by reducing the required blower/fan horsepower needed, due to the lower system static pressure.

Prior to installation of a tempering line, the managers of a U.S. window manufacturer with a single plant determined they needed an enclosure. They had two main objectives: one, to protect their operators from the noise associated with this type of process; two, to avoid adding the heat load to their air-conditioned plant.

The blower room is in a separate lean-to building outside the plant wall. The air and noise from the blower come from heavy duty connecting duct work through the wall and distribute along the length of the quench and cooling sections.  More noise comes from air forced through many small holes in the multiple pant-leg sections staged down the length of the conveyor system between where the glass emerges from the oven and the take-off section of the conveyor. The BHF-type tempering line was supplied by Tamglass of Finland.

Enclosure Layout and Specifications

The three-sided enclosure has overall dimensions of 46 feet, 11 1⁄4-inches wide by 24 feet, 4 inches deep on one end and 22 feet 21⁄4 inches deep on the other—to accommodate the plant wall configuration—by 17 feet, 6 inches to 18 feet 1⁄2 inch tall to adapt to the plant roof slope.

The enclosure extends from the intake end of the oven to the end of the cooling section in length. The front wall is approximately 5 feet out from the side of the conveyor to allow for a cart and then extends back to the plant wall on each end. The enclosure height is from the floor to within 6 inches of the building roof. The remainder of space between the top of the wall panels and the roof is covered with a double layer of loaded vinyl, flexible curtain material. This allows for roof deflection and still provides an acoustical barrier.

Panel Construction

The wall panels are 4 inches thick constructed with an 18-gauge galvanized, solid outer skin and a 22-gauge galvanized, perforated inner skin. There is 4-pound density mineral fiber fill between the two skins. The panels are internally framed with 18-gauge galvanized channels and include similar channel vertical stiffeners at a maximum of 16 inches on center. The vertical edges of the panels are roll-formed into a tongue-and-groove form, so the mating panels are joined together without an intermediate H-joiner.

There are two single 3-by-7-foot doors in each end wall and one 6-by-7-foot double door in the front wall. All doors are 4 inches thick with 18-gauge galvanized, solid skins inside and outside. The hinges are a heavy-duty cam-lift type that eliminates the need for a threshold at the bottom. This feature makes it easier to roll a cart inside and avoids a potential trip point. The latches are a heavy-duty meat-locker type with a safety release paddle on the inside.  All windows in the walls and in the doors are double-paned with a nominal 4-inch air gap between 1⁄4 inch thick and 3⁄8 inch thick glass supplied and tempered by the customer.

The framed window openings were pre-installed at the factory. All glass stops also were factory installed. The trim that holds the glass in place was pre-cut and mitered at the factory and field installed. The wall panels were sized and designed with factory framed notches to accommodate penetrations for conduits, sprinkler pipes and building steel in the roof and along the plant walls.  Due to site conditions, it was necessary to provide a removable panel section at the intake end of the enclosure, spanning the width of the conveyor. This is to allow access to the end of the oven for maintenance.

Ventilation

Typically, tempering lines have one or more blowers for the quench and cooling sections. For this project, there is 90,000 cubic feet per minute of air being supplied to the tempering line.  For some projects, the enclosure has a panel roof with the same construction as the walls. The quench and cooling air needs to be exhausted either into the plant or outdoors.

Depending on jobsite conditions, air and noise exhausted outdoors can be ducted outside unattenuated. This spent air, or a least a portion thereof, is usually returned to the plant for energy conservation. This must be passed through a silencer section to maintain the acoustical integrity of the enclosure. At the same time, silencers must be designed to keep their pressure drop low enough to avoid any of this air being forced back into the oven and adversely affecting the quality of the glass.

When the plant roof is low enough, it might require less square footage of panel to enclose the equipment from floor to roof than 84 Glass Magazine® • April 2006 the amount required for a panel roof. This approach would therefore offer a more economical design. When the enclosures have a panel roof, the exhaust from the quench and cooling air is either ducted to the plant roof or walls, or directly back into the plant.

For energy conservation, a damper can be installed in the duct going to the roof or wall to divert some or all of that air back into the plant during the heating season. Whatever portion is returned to the plant must go through silencers to prevent noise from being broadcast into the plant along with the heated air.  All enclosures include viewing windows for safety reasons; to showcase the tempering line; the plant’s own tempered glass was used in the windows and the enclosure itself.

Performance

The typical noise reduction of these enclosures is about 25 dbA. This measurement—dbA—indicates a reading of the intensity of the noise to the human ear in decibels on an “A” scale for compliance with laws governing workplace environment. For this project, the following table shows noise levels just inside and outside of the enclosure and at several locations.

The enclosure for the tempering line did not interfere with the operators’ normal activities. The operator at the control panel explained that prior to installation of the enclosure, he could not converse with a co-worker 2 feet away.

Once the tempering line was contained, he could carry on a normal conversation. The cost of noise control varies in direct proportion to the decibel reduction required; the degree of access required to the equipment; how close or far the employees are to the noise source and the duration of the exposure. As can be seen in the table above, the dbA reduction brought the levels safely below 85 dbA and the cost for this unit, and ones similar to it, are generally in the $30,000 to $40,000 range. The variation will be determined by the enclosure size and whether any of the spent air is exhausted back into the plant, a situation requiring exhaust silencers.

Who Determines What’s Noisy?

The two organizations with widely used noise standards include the U.S. Occupational Safety and Health Administration in Washington, D.C., and the American Conference of Governmental Industrial Hygienists in Cincinnati. The threshold noise level is 85 dbA for eight hours exposure. At 85 dbA and above, the employer must institute a hearing conservation program. This will include annual audiometric testing of all exposed employees. At the OSHA action level of 90 dbA, the employer is to reduce the employee’s exposure below that limit by:

1. Engineered control, preferred, unless proven not feasible

2. Administrative control, by, for example, rotating employees out of noisy work environments to reduce their overall exposure

3. Personal hearing protection such as ear plugs or muffs, if engineered or administrative controls are determined not to be feasible.

The Threshold Noise Level is 85 dbA for Eight Hours Exposure.

The enclosure for the tempering line did not interfere with the operators’ normal activities. The operator at the control panel explained that prior to installation of the enclosure, he could not converse with a co-worker 2 feet away. Once the tempering line was contained, he could carry on a normal conversation.  The cost of noise control varies in direct  proportion to the decibel reduction required; the degree of access required to the equipment; how close or far the employees are to the noise source and the duration of the exposure.

As can be seen in the table above, the dbA reduction brought the levels safely below 85 dbA and the cost for this unit, and ones similar to it, are generally in the $30,000 to
$40,000 range. The variation will be determined by the enclosure size and whether any of the spent air is exhausted back into the plant, a situation requiring exhaust silencers.

Noise Sources

Other noise sources in the plant still had some impact on the readings outside the enclosure. They included the blowers associated with the washer and the cutting tables. Intake silencers for these blowers will reduce the noise from their current levels of 86-89 dbA, down below the required 85 dbA and speech interference levels, making the workplace infinitely more efficient and productive for the plant’s employees and managers alike