Equivalent Diameter - Rectangular vs. Circular HVAC Ducts (2024)

FAQs

What is the equivalent diameter of a rectangular duct? ›

A measurement for a non-circular duct representing its shape in the same way a diameter represents a circle. Generally, the equivalent diameter can be calculated as D(ref) = square root of (4A/pi) where A is the cross sectional area, and D(ref) is the equivalent diameter.

The equivalent diameter of a circular duct that provides the same pressure loss as a rectangular duct (see figure below) is given by the Huebsher equation: de=(a+b)0.251.3(ab)0.625 de is the equivalent diameter ( mm or in). a is the rectangular duct width ( mm or in). b is the rectangular duct height ( mm or in).

Round duct performs with a lower pressure drop than rectangular duct. It is also more rigid, which keeps the drum effect from duct vibration lower compared to a rectangular duct. The noise level in rectangular ducts can be excessive if they are improperly supported.

One of the most used methods is the HVAC duct sizing rule of thumb. In fact, even professional AC contractors might use this method to size the AC ducts in your home. According to this rule, you need to install an air conditioner of one-tonne capacity for every 500 square feet of the floor area of your residence.

Although a 6" oval duct (called 6" oval) is typically sold as an “equivalent” to a 6" diameter round duct, they are not actually equivalent. The cross sectional area of a 6" oval duct is only 75% of that of a 6" round duct. An equivalent round duct would have a 5.1" diameter.

5.2 Rectangular Ducts

They provide flat surfaces for branch tap-ins and they are conveniently fabricated. For large plenums and duct sections containing many fittings, rectangular duct fittings are usually easier to assemble than round and oval fittings.

Spiral duct holds many significant benefits over rectangular duct. Using less material for fabrication and ease of installation, spiral duct can save up to 37% of installation and operating costs over the life of the HVAC system.

15-inch diameters in ½-inch-diameter increments, 16- through 38-inch diameters in 1-inch-diameter increments, and 40- through 90-inch diameters in 2-inch-diameter increments. order. Standard lengths of round longitudinal seam duct are 5 and 6 feet.

For circular ducts, the area is calculated as A = π × r², where r is the radius of the duct. For rectangular ducts, the area is calculated as A = l × w, where l is the length and w is the width of the duct.

The equivalent diameter is defined as the diameter of a circle with an equal aggregate sectional area, which is calculated by d = 2 Area / π . This is a classic method of an aggregate classification and used in many digital image processing programs.

What size duct do I need for 400 cfm? ›

Or working in reverse, if you know that a machine will require approximately 400 CFM to remove the waste, then you should design a 4" pipe for the application.

Rectangular ducts have uneven pressure gaps throughout the system which means lower efficiency and higher costs. The sharp turns and pressure differential in rectangular ductwork also creates noise and heavy reverberation.

The standard pvc rectangular ducting sizes are: 110mm x 54mm. 204mm x 60mm. 220mm x 90mm.

Circular ducts create less air resistance. That, in turn, results in a lower air speed, less noise and no draught.

The most typical register size is the 4x10. To give an idea of CFM's, if you are feeling a strong blast of air out of that size of register then you're feeling about 160 CFM.

RECTANGULAR END: 220mm x 90 Fits over rectangular duct pipe. ROUND END: 150mm (6 inch) Fits inside round pipe of flexible hose. Allows you to convert between round and rectangular ducting systems.