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Building Science Fundamentals
11. Understand Total Equivalent Length
Total equivalent length (TEL) is used in HVAC duct length measurements to account for bends, fittings (wyes and mixing boxes) and friction loss to find the installed sizes of ductwork. The reason is that if a HVAC system delivers 50,000 CFM of air output, the fact is that 50,000 CFM of air is not the total amount actually delivered to each room. The reason is because the air travels down the ductwork and some of it gets lost due to friction, bends and fittings. There are engineering manuals for mechanical engineers that have the shapes and sizes of ductwork and the associated friction loss with each one. For small commercial and residential purposes, Wrightsoft publishes duct sizing software that automatically accounts for these losses but we need to be aware they exist. Wrightsoft software is not always required though as a ductulator can be a suitable substitute for ductwork sizing. Sometimes utility companies require a Manual D (duct sizing) calculation be done, but if you are working with existing homes with no utility program requirements, go with a ductulator. If you want to see the science and formulas behind the losses, check out this manual for the HVAC industry.
The TEL of a 90 degree square to rectangular elbow for example is the equivalent length of 35 ft of straight duct. Rigid metal ductwork has about half the TEL of flex duct, so every 2 feet of straight flex duct is the equivalent of 1 foot of rigid ductwork. As a best practice, it is better to use two 45 degree bends or better yet a curve, than it is to use one 90 degree bend. The longer the TEL, the more the ductwork resists airflow and the bigger the blower has to be to deliver enough air to the rooms. TEL guidelines can be found in the ACCA manual.
The same restriction guidelines go for the PVC pipes in the suction and discharge sides of a pool pump. Long straight runs are much better and require the pump to work less, using less energy, than bends and elbows.
1c. Combustion science