Understanding HVAC Air Flow Efficiencies

Pressure Loss with Duct Size Changes  

For those calculating HVAC duct sizes the equation Quantity = Area X Velocity should be familiar.  By using it you can calculate duct size changes while maintaining the same quantity (CFM) and velocity (FPM) in the duct.  

Unfortunately, the equation does not take pressure loss due to friction into account.

Pressure loss is the result of friction as the air moves along the inside of the duct.  Keep air quantity and velocity equal and a duct with greater duct/air contact area will suffer greater friction loss.

How to compensate?  While Q = A X V remains an important tool, there's more to consider, and SNIPS Magazine has covered what you need to know in an informative article titled Understanding airflow: ductwork sizing explained.

SNIPS Magazine delivers in-depth, insightful journalism that covers the sheet metal industry, heating, air conditioning, ventilation, indoor air quality, architectural sheet metal, and more.  

Learn why round duct has the least friction loss, why next to round duct square duct is best, and why you should avoid using duct with an aspect ratio greater than 3 to 1.

Read the Snips Magazine article and then contact ZEN Industries to learn how we can better meet your HVAC project needs.

Joining Equivalent Sized Duct

Challenges in Joining Round to Square Duct

Sometimes the job site and the engineering specs just don't see eye to eye.  

What happens when the duct specified by the engineer simply won't fit in the ceiling space?  You may be forced to consider switching from the preferred round duct to rectangular duct under the circumstance.

To keep the air flow calculations valid, you consider that the cross-sectional area of the rectangular substitution needs to remain within 1% of the area of the original round duct.  That should work out OK, right? 

Unfortunately, this is another instance where friction loss rears its ugly head.  The greater the surface area of the duct, the greater the friction loss. 

To get out of this sticky situation correctly, you need to consider both the math and your contract very carefully.  First, you need a better formula when calculating the equivalent diameter of rectangular duct.  Second, you want to get written approval of the size change before fabricating the replacement duct.

To walk through the math and review a few good examples, you can read the article Taking the mystery out of equivalent size of duct on the Snips Magazine website.

At ZEN Industries, our goal is to help your business succeed. No matter how big or small your project, ZEN can provide all or part of your HVAC needs.  

No matter how big or small your project:  

ZEN Industries provides all or part of your HVAC needs.  HVAC ductwork manufactured according to your schedule, meeting 3rd Edition 2005 SMACNA standards.  Contact Drew McCaffrey - Sales & Estimating at 877-600-0274.   

Success is about Service   

Success in our business is about service. The quality of our HVAC duct work is of the highest caliber and our price competitive with the best in the industry. Nonetheless, quality and price are default items today in sheet metal fabrication. They give us access to the playing field. Service is where ZEN Industries Inc. gains its edge, and service means people - it's about our experienced personnel and the relationships we build with each and every customer.  

Contact ZEN Industries today.

Return to Top