So you think your HVAC shop is going to be highly productive. You have a coil line, a plasma table, an automated design system, good duct-forming and assembly equipment, and well-trained and highly motivated shop employees.
But wait - Why are those fitting patterns backed up like the freeway to downtown on Monday morning? The roadblock is usually the duct-lining area, aka "the Siberia" of HVAC shops.
This is the workstation often used to test the fortitude of new apprentices and the tenacity of skilled mechanics. Although duct-liner operations have a notoriously negative image throughout the sheet metal industry, they shouldn't. In fact, duct lining can be good, productive and satisfying work, and one of the most profitable shop operations.
Here's what it takes to set up a good duct-liner workstation and make it profitable.
The first task in the duct-lining process is cutting the insulation to fit the sheet metal pattern, which can be for a shape or straight duct. Many shops mistakenly apply the same technique on both, using a cut-and-slide method. This method incorporates laying the formed sheet metal pattern on the liner, cutting along two edges of the metal and re-positioning the metal to accommodate the seam and connector allowances prior to cutting the opposite two edges for the seam and connector deductions.
This is necessary for fitting patterns because of their random shape; however it is much more efficient to cut the full straight-duct joint patterns from a computerized insulation cut list. Such a list is standard in good-quality plasma-machine software. These lists can also be generated from a simple spreadsheet program.
DifferencesThe key differences in cutting straight-duct liner from a printed list as opposed to slide-and-cut are: the calculated cut list gives an exact fit, and it allows the liner to be cut, marked and held while the straight duct patterns are being cut and formed. This approach means that the insulation patterns can be waiting for the metal patterns, rather than vice versa. This significantly changes the process, greatly reducing the duct-liner bottleneck, improves fit quality and dramatically increases efficiency and profits.
The rectangular straight-joint insulation pieces can be most efficiently cut from a liner cut list using a commonly available liner-cutting table which has pizza cutter-style rolling blades for cross cutting and ripping, a straight-edge guide running down the worker side of the table, and an imbedded measuring tape attached to the straight edge.
The same technique can also be used with plasma machine-generated fitting patterns. Several plasma-table manufacturers offer various automated fitting-pattern liner-cutting methods, such as high-pressure water jet or high-speed "pizza wheel" cutters. These methods make fitting-liner cutting very efficient and profitable. All of the insulation fitting patterns are automatically generated from the plasma-machine software patterns, less seam and connector allowances.
It is important to note that another big advantage of this approach to liner cutting is that it enables the adoption of true batch processing by allowing the liner-cutter worker to keep a sizeable amount of cut liner pieces stocked in a holding area between the liner-cutting table and the gluing station. This batch processing is infinitely more efficient than the commonly used single-piece processing method.
GlueFollowing the liner pattern-cutting operation comes gluing, which can be very efficient - or not. The glue can be applied by brush, roller, spray gun or wet pinch rollers. Wet pinch rollers are much more efficient than rolling by hand or brushing, but require the use of the slower-drying water-based glue. The quick - 30 second - drying chemical-based glue is by far the most efficient type to use simply because the quick drying time allows the glued pieces to be immediately stacked in the next holding area for pinning, which continues the batch-processing advantage.
One problem that chemical-based glues have carried is the misconception that they all are highly flammable. The fact is that they are available nonflammable in the wet state by having a Trichloroethane-based solvent. This is critical because of safety for the applier.
If your present chemical-based glue has a red "flame" label on the container, it is highly flammable and should be replaced with a nonflammable product such as those made by Minnesota Mining and Manufacturing or PA-106 from Precision Adhesives.
The true cost of any adhesive is the coverage in square feet per gallon. When applied correctly using the appropriate spraying equipment, you can cover up to 350 square feet per gallon. The key is to have the proper tip size and air pressure at the spray gun, which will produce a 12- to 14-foot spray path to meet the 90 percent coverage required by Sheet Metal and Air Conditioning Contractors' National Association guidelines, while applying a very thin, opaque coat onto the metal. The coat should be thin enough to see the metal through the glue. That's why many chemical-based glues come in colors - so workers can see how much glue is being applied.
Changing productsIt is important also to note that whenever changing from any glue product to another, including chemical to chemical, water base to water base, or water base to chemical pump, hoses and guns must be completely flushed out using the appropriate cleaning liquid. Otherwise, the spray equipment could be damaged.
It is also very important to properly hang the spray gun with only the tip suspended in the solvent. This will prevent clogging between jobs. The solvent should never reach the gun trigger as it will damage the air seal on the trigger shaft (See sketch on page 20).
Either use a double-stage pump, which delivers a more even spray pattern with less wear on the pump, or a more efficient, lower maintenance diaphragm pump.
The duct-lining glue operation needs to incorporate a properly designed spray booth. Appropriate attributes of a well-designed glue-spray booth include a 3,000 cfm exhaust fan with an explosion-proof motor. This is necessary when using water-based or nonflammable materials in the wet-state chemical-based glues, because over time, the dust that accumulates in the overspray and exhaust system becomes highly explosive and flammable.
The design of an efficient glue spray booth should be similar to the sketch included with this article. Key features include left- and right-side gun hooks (for left-handed or right-handed workers) to allow sprayers to get rid of the gun quickly between patterns, attachment hooks to hold replaceable sheets of plastic to protect the spray-booth walls from overspray, disposable light-metal angles on the table to lay the metal patterns on, and the tip-soaking canister positioned at the table level for efficiency - never on the floor.
(Efficiency expert Jim Segroves is the author of Sheet Metal Shop Burden Recovery Guidelines. Contact Jim Segroves Consulting at 712 E. Walnut St., Garland, TX 75040; call (972) 494-6333; fax (972) 272-7062; see www.segrovescon sulting.com on the Internet.)