Elbow Design: Not All Elbows Are Created Equal

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When it comes to designing ductwork, particularly grease ductwork, small changes in the design can determine whether the system works efficiently or whether it works at all. Take for example a simple elbow. While square elbows are common in HVAC design, they can create nightmares in grease ductwork. They should only be used when absolutely necessary. The static pressure through a 10x24 duct is not the same as a 24x10 duct depending on the direction of the turn. When two elbows are in close proximity they can create what is referred to as system effect. The non-laminar flow resulting from the first elbow affects the flow through the second elbow, resulting in much higher static pressure than normal. This effect can be strong enough to prevent the system from ever operating as intended. The design also needs to be within the manufacturing and installation capabilities of the installers. It doesn't do any good to draw it if it can't be built.