Tests were conducted to determine the pressure drop characteristics of 20-inch diameter, 4-mil thick, polyethylene tubing and both factory and shelter fabricated 90-degree elbows. The tests were performed at flow rates ranging from 1300 to 4100 cubic feet per minute. These plastic components are part of a portable ventilation system that was developed for Civil Defense fallout shelters, Specification MIL-V-40645. Fully inflated 20-inch diameter plastic tubing has about three-quarters of the pressure drop of sheet-metal duct. However, the last fifty feet of a plastic duct system, which is not completely inflated, has 1-1/2 to 3 times the pressure drop per foot of fully inflated plastic tubing. The result is that for duct systems over 100 feet long the pressure drops for sheet-metal and plastic tubing are approximately the same. The friction losses for both factory fabricated and shelter fabricated elbows were established. A 40-inch, smooth radius, 90-degree factory fabricated elbow is recommended for use with the Civil Defense Package Ventilation Kit. This elbow develops a pressure drop equivalent to 50 feet of straight tubing. The best shelter fabricated elbow is a three-piece elbow with a radius of 60 inches that can be fabricated from the straight tubing and tape stocked in the Package Ventilation Kit. This elbow develops a pressure drop equivalent to 90 feet of straight tubing. (Author)
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