Hvac Duct Fittings Equivalent Length

1 psi equals 27 7 inches of water gauge. 120sc snap collar round 35 el. When you add up the effective lengths of all the fittings and then add that number to the length of the straight sections in the most restrictive runs in the return and supply ducts you find the total effective length tel.

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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.

Hvac duct fittings equivalent length. 10 6 duct locations 27 10 7 duct fittings and transitions 28 10 8 aspect ratio 32 10 9 standard duct sizes 32 11 0. 23 offset starting collar 10 el no. What is equivalent length. Ordinary heating ventilating and air conditioning duct systems read air pressures at 0 4 psi or less often much less.

Smacna hvac systems duct design manual fourth edition december 2006 table a 1 pg a 4. Fittings such as elbows tees and valves represent a significant component of the pressure loss in most pipe systems. This restriction friction loss adds to the tel total equivalent duct length of the system. The strength of the equivalent length method is that it is very simple to calculate.

This article details the calculation of pressure losses through pipe fittings and some minor equipment using the equivalent length method. Ductwork design considerations 33 11 1 duct frictional resistance 33 11 2 duct equivalent length 35 11 3 ductwork system effect 35 11 4 installation issues 37 11 5 ductwork insulation 38 11 6 ductwork air leakage 39. Each type style and configuration of fittings elbow angle takeoff reducer diffuser outlet register boot etc is listed in acca s manual d with an assigned equivalent length this value is the comparison length of this fitting to an equivalent length of straight pipe or duct. 90 degree turns in forced air ducting are to be avoided whenever possible.

Each fitting has an effective length that equates its pressure drop to an equivalent amount of straight duct. A common duct pressure of 0 25 inches water column is equal to 0 25 divided by 27 7 in wc psi 0 009. The weakness of the equivalent length method is that. 20 straight starting collar 35 el no.

A common value to remember is 0 0003 ft for standard galvanized material which is what the friction chart is based on. Therefore the tel of the ducting must be minimized for the best performing comfort system. Equivalent lengths plenum duct fittings no. 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.