# Why Ductcalc.Ca is more accurate and easier to use than cardboard Ductulators and other free software simulators?

Published on: November 24, 2019

**Mathematical approximation:** Most cardboard ductulators (and free software simulating them) rely on an approximation of the Colebrook equation to solve for friction factor. This approximation can be up to 2% far from the real solution of that logarithmic equation.

Ductcalc.Ca uses numerical iteration to solve the real Colebrook equation as per the latest ASHRAE Handbook 2017 chapter 21 for Duct design.

**Flexible duct sizing:** All cardboard ductulators are based on a fixed absolute roughness of anything between (0.0003 and 0.0005 feet). Flex duct material absolute roughness is anything between (0.004 and 0.0003 feet) which is completely outside all cardboard ductulators range. To make up for this physical limit, they multiply the static pressure drop by a fixed value of 1.76 which can produce a result very much higher or lower than the real static pressure drop of this type of duct.

Ductcalc.Ca uses the exact absolute roughness of flexible duct and uses the latest ASHRAE 2017 equations to calculate based on accurate absolute roughness.

**Flexible duct compression:** The above mentioned constant of 1.76 for flexible duct is based on an ideal installation of flexible duct and does not account for a compression ratio higher than 5% which is not the case in most flexible duct installations.

Ductcalc.Ca can take an input of duct compression ratio up to 100% and it calculates the PDCF (pressure drop correction factor) based on ASHRAE 2017 formula. So far, I haven't seen any free online ductulators that have this input which means that they are using the 1.76 number just like cardboard ductulators.

**Duct material list and custom absolute roughness:** Most ductulators use a fixed absolute roughness value of around 0.0005 feet which accounts for only specific type of galvanized steel sheet. Duct inner material can be anything from galvanized sheet to duct liner to flex non-metalic to concrete which all have very different absolute roughness values between (0.0000015 and 0.015 feet as per ASHRAE fundumentals handbook).

Ductcalc.Ca uses the ASHRAE 2017 material list and goes farther to let the user to input their own absolute roughness value provided by the duct material supplier.

**Rectangular duct air velocity calculation:** This one is very critical and very much a common mistake done by designers who use conventional ductulators to design their rectangular ducts based on fixed air velocity. The velocity shown on all cardboard ductulators is only for round ducts and the sizes shown for rectangular ducts are actually based on the same static drop of that round duct. So they are not getting the correct velocity this way.

Ductcalc.Ca calculates different static pressure drops for each type of duct round and rectangular when fixed velocity method is chosen.

## How to get Ductcalc.Ca?

You can subscribe to Ductcalc.Ca/Premium for free. Before the trial period ends, you can either cancel your subscription or pay a symbolic amount per month to use this high quality engineering tool and support its development.

Use the link below to take advantage of the premium ductulator offer for Black Friday before November 30, 2019.

The good news is that it is very easy to cancel if you don't like the tool from your premium account and without paying any penalty.