Too noisy? Some equipment might need an enclosure
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, 111⁄4-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.
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.
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 job-site 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 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.
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.
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.