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So, let's do an example.įor Imperial unit, let's say the duct length is 12″ (1 ft), the duct area of the 40″ by 4″ rectangular duct is:ĭuct Area = (Duct Width x Duct Length x 2) + (Duct Height x Duct Length x 2)ĭuct Area = (40″ x 12″ x 2) + (4″ x 12″ x 2)įor SI unit, let's say duct length is 0.Too large or too small HVAC ductwork sizing can cause problems similar to what happens when technicians install an improperly sized HVAC unit. To calculate the duct area, you first need to have a duct length. Whether you are using a physical ductulator or software, you'll need to manually calculate and compare the duct area of the sizes you've selected to find the most cost-optimized sizes. #Free download duct sizer mcquay freeHowever, not all of them are free to use. Besides, there are also many mobile app duct sizers out there where you can download and use. But, beware of viruses and malware.įurthermore, there are many online ductulators you can use to calculate duct sizes. You can google it and many websites provide free downloads. Calculate Duct Size Using SoftwareĪfter about 3 years of working, one of my seniors gave me a softcopy ductulator called McQuay Duct Sizer. But, you might want to check the air velocity (orange color section) if you're using round ducts. It will be displayed at the round duct section (white color section) as you rotate the ductulator. #Free download duct sizer mcquay how toLater, I'll guide you on how to select the best duct size.įor round ducts, it is very straightforward. A not-so-align duct size such as 30″x5″ is considered slightly oversized/undersized and not optimal in terms of the cost.Īs you might have noticed, there are many different duct sizes to choose from. You always want to use the most aligned duct sizes because they fit the best for that airflow. Then, you see the bottom part of the ductulator to find the rectangular duct size (green color section). ![]() With a ductulator, all you have to do is mark the friction loss at 0.1 in.wg per 100 ft (0.816 Pa per meter) and then rotate your airflow until it aligns with the marked friction loss (blue color section). I was told that engineers generally use a friction loss of 0.1 in.wg per 100 ft (0.816 Pa per meter) when sizing ducts. Particularly, the friction (head) loss was marked at 0.1. The ductulator I had was a used one with a few markings on it. After all, it was more than 7 years ago since I had my ductulator. It looks something like this on Amazon but the Amazon one is way better than mine. I still remember that the ductulator is made by Trane. It has a mechanism that allows its middle part to rotate which gives us the duct sizes based on a few parameters. It is round in shape with many words and numbers written on it. When I first started working as a fresh-graduate project engineer, my manager gave me a ductulator. Finally, the maximum airflow must be the same for both round ducts and rectangular ducts. Next, the rectangular duct ratio must not exceed 1:4. Then, the air velocity is also limited at 1500 fpm and below. So, I calculated the equivalent duct diameter for each rectangular duct sizes as follow:įor the conversion between round ducts and rectangular ducts, the friction loss must maintain at 0.1 in.wg and below. Simply use the same duct area to convert from round to rectangular ducts is often inaccurate. Round ducts and rectangular ducts are sized differently. Round to Rectangular Duct Conversion Chart Just match the airflow with the duct size. ![]() The following duct CFM chart can help you speed up the process. ![]() Now that you know how to calculate duct size manually. In this case, the air velocity is around 830 fpm which is well under the 1500 fpm limit.Īir velocity usually is not a concern unless the airflow is approaching and exceeding 6000 cfm.Then, pull the line diagonally to the left to find the corresponding air velocity.Using 16″ round duct, pull the duct diameter line diagonally down to meet with the 1200 cfm vertical airflow line. ![]()
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