FIBER-GLASS DUCT SYSTEMS

Throughout the Naval Construction Force (NCF) fiber-glass duct is becoming common on jobsites. It has the advantage of added insulating value, ease of fabrication and handling, as well as installation, and making it useful where traffic and handling/abuse are restricted.

DUCT CHARACTERISTICS

Fiber-glass ducts are manufactured of molded fiber-glass sheets covered with a thin film coating of aluminum, although thin vinyl or plastic coatings are sometimes used. In the NCF, we are primarily concerned with aluminum coated duct. Because it is fabricated of glass fibers, it is inherently insulated; therefore, it is used where insulation is a requirement.

Fiber-glass ducts can be molded into various shapes for special applications. The desired shapes can be ordered from the manufacturer's stock In the NCF, for all but special purposes, the duct is supplied in the flat form of a board that has V grooves cut into the inner surfaces to allow folding to fabricate rectangular sections (fig. 2-104, view A). The ends of the board is molded so when a rectangular/square duct is formed two sections of the same size will fit together in a shiplap joint (fig. 2-104, view C). This joint ensures a tight connection coupled with a positive alignment.

Figure 2-102.-Duct system with strap hangers from angle rails transverse to purlti

Figure 2-103.-Duct installed to a built-up steel roof.

Figure 2-104.-Fabricating rectangular/square fiber-glass duct from duct board.

Of extreme importance is the selection of the proper board size to fabricate the duct before cutting and grooving. In all applications the inside diameter of the duct is the determining factor of the board size. Use table 2-3 to determine board size.

Table 2-3.-Duct Board Length Selection Chart

NOTE: Within a heating system, the use of fiber-class duct is restricted by the adhesive used to affix the protective outer coating to the fiber glass. Check the specifications and ensure that it will not fail when exposed to heat over 250 degrees.

FABRICATION

To fabricate a rectangular/square duct, you must first measure the duct board accurately. Next, the grooves must be cut. Ensure they are at the proper locations and cut straight because this allows the board to be folded to create the desired rectangular/square shape. When cutting the board, you will need to leave an overlapping tab that is pulled tight and stapled (fig. 2-104, view A). Tape is then applied and the joint is heat-sealed (fig. 2-104, view B). Joints between sections are fabricated by pulling the shiplap end sections together and finished by stapling, taping, and heat sealing the joint (fig. 2-104, view C).

INSTALLATION

The very nature of fiber-glass duct requires that it be supported with 1-inch by 1/16-inch galvanized steel strap hangers. These must be supplied or fabricated to fit the duct precisely whether the duct be rectangular/square or round. Rectangular/square ducts up to 24 inches (span) can be supported on 8-foot centers. Ducts larger than 24 inches must be supported on 4-foot centers. For round ducts the supports must not be less than 6-foot centers.

SAFETY

Some of the safety precautions applicable to sheet-metal tools and equipment have been mentioned throughout this chapter. Here are a few additional precautions that should be carefully observed when you are working with sheet metal.

1. Sheet metal can cause serious cuts. Handle it with care. Wear steel reinforced gloves whenever feasible.

2. Treat every cut immediately, no matter how minor.

3. Remove all burrs from the metal sheet before attempting to work on it further.

4. Use a brush to clean the work area. NEVER brush metal with your hands.

5. Use tools that are sharp.

6. Keep your hands clear of the blade on all squaring shears.

7. A serious and painful foot injury will result if your foot is under the foot pedal of the squaring shears when a cut is made.

8. Do not run your hands over the surface of sheet metal that has just been cut or drilled. Painful cuts can be received from the burrs.

9. Get help when large pieces of sheet metal are being cut. Keep your helper well clear of the shears when you are making the cut.

10. Keep your hands and fingers clear of the rotating parts on forming machines.

11. Place scrap pieces of sheet metal in the scrap box.

12. Always remember to keep a clean shop. GOOD HOUSEKEEPING is the key to a safe shop.

13. Do not use tools that are not in first-class condition-hammer heads loose on the handle, chisels with mushroomed heads, power tools with guards removed, and so forth.

14. Wear goggles when in the shop.

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