Rectangular or Round?

The advantages of round air-ducts

Required Material

For a same cross section less steel is required (approx. 13%) to manufacture a round duct than a rectangular one (1).

Duct-wall Thickness

Rectangular air ducts usually require a thicker material gage in order to achieve a stability comparable to that of a circular duct.


Less material means less weight. Round ducts are therefore usually lighter and easier to handle than rectangular ducts.

Number of Connections

Rectangular ducts are available in segments which are usually between one and two meters long. Round ducts come in segments up to 6 meters in length. Round ducts require consequently less connections than rectangular ducts.


Circular air ducts are tighter (or not as labor intensive to make them tight) than rectangular ducts, mostly because of the absence of sensitive areas such as the gaps around the corner-pieces, and also because of the reduced number of required connections.


Circular ducts can always be adjusted on site, while rectangular ducts cannot usually be modified without the necessary tools.


Even the time-saving 4-bolt rectangular connection system cannot compete with the 1-bolt system of the METU round duct connections (SR joining clips). Furthermore, circular ducts require less hangers than the heavier rectangular ducts.


Many round ducts, fittings and the corresponding connections are standard items usually available ex-stock. Rectangular ducts on the other hand are usually custom made: they must be precisely planned and manufactured according to the project specifications.

Effectiveness of the Ventilation System

The air-ducts' round shape has the following advantages:

  • It reduces friction losses
  • A less turbulent air-flow in fittings
  • Minimize the energy consumption
  • Simplify the planning through standard parts


When using METU round duct connections the use of self-drilling screws can be avoided. The result is a dirt-retardant, smooth inner duct surface generating less friction and pressure drops and increasing the overall efficiency of the ventilation system. For the sealing classes C and possibly D, the energy losses and contaminations are reduced. All these features translate into lower operating and maintenance costs.

What to expect

The requirements for ventilation systems will continue to rise:

  • More energy efficiency
  • Better hygiene
  • Lower installation and maintenance costs

To achieve the above, one needs:

  • A higher air-duct tightness (D / ATC1)
  • Smooth inner duct surfaces
  • Thinner duct walls with sufficient stability to minimize material consumption
  • A quick and trouble-free assembly

The METU-FORM system of ducts, fittings and connections has been designed to meet these criteria. That way you, the customers, automatically fulfill the higher requirements for our future.

Source book for efficient air
duct systems in Europe,
AIRWAYS 4.1031 / Z / 99-158, © 2002