## 100% Effective Duct Length

The concept of 100% effective duct length is an important one in the HVAC industry. When air exits a fan, the flow profile is distorted by the action of the fan wheel flinging the air towards its perimeter, causing more air to be concentrated at the outside of the fan scroll and thus, a higher velocity at that location.

As the air moves down the duct, this distorted profile interacts with the duct wall, causing the faster air to slow down and the slower air to speed up. In addition, some of the velocity pressure is converted to static pressure, a phenomenon called "regain".

The point in the system where a uniform velocity profile has re-established itself is called the 100% effective duct length. If transitions or fittings are introduced into the fan discharge duct before the 100% effective duct length point, the fan performance will be adversely affected and the losses in the fittings will be higher than predicted by the loss coefficients, a phenomenon called System Effect. If you want to know more about system effect, you may find the blog post titled

Thus, it is desirable to be able to compute the 100% effective duct length and if at all possible, avoid placing any fittings between that point and the fan. The formula for 100% effective duct length is as follows.

As the air moves down the duct, this distorted profile interacts with the duct wall, causing the faster air to slow down and the slower air to speed up. In addition, some of the velocity pressure is converted to static pressure, a phenomenon called "regain".

The point in the system where a uniform velocity profile has re-established itself is called the 100% effective duct length. If transitions or fittings are introduced into the fan discharge duct before the 100% effective duct length point, the fan performance will be adversely affected and the losses in the fittings will be higher than predicted by the loss coefficients, a phenomenon called System Effect. If you want to know more about system effect, you may find the blog post titled

*System Effect–Dealing with the Point Where the Fan Meets the Duct*to be helpfulThus, it is desirable to be able to compute the 100% effective duct length and if at all possible, avoid placing any fittings between that point and the fan. The formula for 100% effective duct length is as follows.

The download below will give you to the .wmf file that is discussed above so you can use it to document your math in a spreadsheet, report, or presentation. You may also find the following related information on the website to be helpful:

- As you may have observed, calculating effective duct length requires that you know the equivalent duct diameter if you are dealing with a rectangular duct. You will find the AMCA equation for equivalent duct length here. That page also contains some ASHRAE equations that have a similar sounding name but are a different concept and thus not interchangeable with the AMCA term in the Effective Length equation.
- The Effective Duct Length spreadsheet tool has the equations for 100% effective duct length, equivalent diameter, circular equivalent of a rectangular and flat oval duct, and hydraulic diameter built into it to allow you to enter a flow rate and your duct dimensions and calculate the various metrics.

effective_duct_length.wmf |