4. Install Overhead Distribution Duct
A. If duct has been previously installed, trim away any tape flush with the end of the duct.
B. Secure duct to supports with screws.
C. Prepare the tape for joining duct sections per one of the following procedures.
NOTE: Retaping over old tape may be made four times at any one joint. If additional taping must be made, cut away taped portion of duct and replace with a short duct section of the same size.
(1) Procedure 1 (Ducts installed without GE RTV 133 adhesive)
WARNING: PREVENT SOLVENT TO SKIN CONTACT; AVOID INHALATION OF VAPORS; USE ONLY IN AREAS WITH ADEQUATE LOCAL EXHAUST VENTILATION.
CAUTION: THIS PROCEDURE DOES NOT APPLY TO DUCTS REPAIRED OR INSTALLED WITH GE RTV 133 BLACK SILICONE ADHESIVE. USE PROCEDURE 2 FOR DUCTS WITH THIS TYPE ADHESIVE.
(a)
Clean faying surfaces of ducts with the solvent using dry clean cheesecloth or wipers. Blot dry excess solvent.
(b)
Thin BMS 5-7 Type II adhesive by adding one part of toluene to two parts of adhesive, by volume. Mix thoroughly.
(c)
Install tape joint using one, or a combination of the following three methods. For all three methods the following is applicable: 1) Ensure that there is a minimum of 1.5 inch of tape on each duct end. 2) Overlap the tape ends 1-2 inches at the tape joint except for ducts with nozzle
openings, no overlap of the tape ends is required. Pull out wrinkles in the tape and smooth out any voids or air pockets between the tape and the ducts.
(d)
Method I: Apply a moderate to heavy coat of adhesive, approximately 0.020-0.030 inch thick to both the tape and to the end of the duct. While the adhesive is still wet, apply the tape to the duct. Apply a liberal topcoat of adhesive to ensure saturation of the tape.
(e)
Method II: Apply a 3-inch wide coat of thinned adhesive, approximately 0.040-0.060 inch thick to the outside surface of each duct end. While the adhesive is still wet, apply the tape to the adhesive coated surface of duct. Apply a liberal topcoat of adhesive to ensure saturation of the tape.
(f)
Method III (For locations with limited access): 1) Place the tape on a suitable flexible installation aid.
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21-23-11 Page 402 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. Feb 20/92
533 Overhead Duct Installation
Feb 15/72 Figure 401 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 21-23-11 Page 403
2) Apply a coat of thinned adhesive approximately 0.040-0.060 inch thick to the tape. The adhesive will soak through and hold the tape temporarily to the installation aid.
3) While the adhesive is still wet, position the installation aid against the duct joint with the adhesive side toward the duct. Pull the tape away from the installation aid onto the duct surface. Smooth out any wrinkles and trapped air to ensure that the tape is contacting the duct. If possible, insulation blankets should be prevented from contacting the adhesive to allow air circulation until the adhesive is dry.
(g) Allow the adhesive to cure under contact pressure for a minimum of 4 hours at 65-100°F before pressurizing the air conditioning system.
(2) Procedure 2 (ducts using GE RTV 133 adhesive)
(a)
Coat the tape on one side with adhesive. Open time is approximately 30 minutes before a surface skin will begin to form. The tack-free skin interferes with adhesion. Extreme caution must be exercised during application to ensure that no adhesive is applied outside the bond area if subsequent finishing is required.
(b)
Wrap one layer of coated tape (with adhesive down) over the joint with a minimum of 1 inch on each duct. Overlap the tape ends 1-2 inches at the tape joint except that for ducts with nozzle openings, no overlap of the tape ends is required.
(c)
Pull out wrinkles in the tape and smooth out any voids or air pockets between the tape and the ducts.
(d)
Apply positive pressure to ensure contact of the adhesive to the duct. A bondline thickness of 0.04 to 0.08 inches is required. This is approximately 2.0 ounces of adhesive per foot of 4-inch wide tape.
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