Duct sizing is a challenge contractors, designers, and engineers face every day. There are three primary methods used sizing ductwork and each has specific applications and uses. Duct sizing can be complicated, but in many situations all you have to do is take a step back and consider the application and specific goals. Today we’re going to break down the three primary methods used for sizing commercial ductwork and provide a couple common applications for each. In the end it’s up to the person responsible to determine the appropriate method and if you need specific advice about an application contact us for professional guidance.

 

1) Velocity method
Using duct velocity to size ductwork is simple and can be quickly calculated in the field. Different environments and applications require different velocities. For example, medium pressure duct mains over corridors and other occupied spaces should be limited to 1,500 feet per minute (FPM). Below is a table with common duct velocities that can be used for many applications; however, it is up to the designer of record to make the final velocity determination for each specific application.
  Commercial Systems Industrial Systems High Velocity Systems
  FPM   FPM   FPM
Mains   1000-1500   1500-2400   1700-3250
Primary Branches   600-1000   1000-1500   1100-2000
Branches and Runouts   200-500   600-1000   800-1300
2) Equal friction loss method

The equal friction loss method is the most commonly used method because it is simple, flexible, and accurate. Most commercial, industrial, and even residential systems are sized using this method. Duct systems sized using this method generally use the least amount of space and have the lowest initial cost. The equal friction method is based upon a constant pressure loss per unit length (i.e. 0.08 in.wc. per 100 ft.) plus the pressure loss through each fitting. The fan is selected based upon the airflow and the pressure drop through the duct run with the highest pressure drop. When using this method the system is not self-balancing and requires a testing, adjusting, and balancing technician to balance the system. Below is a table with common friction losses that can be used for many applications; however, it is up to the designer of record to make the final determination for each specific application.

  Commercial Systems Industrial Systems High Velocity Systems
  in.wc.  /100 ft.   in.wc.  /100 ft.   in.wc.  /100 ft.
Mains   0.15   0.20   0.30
Primary Branches   0.10   0.15   0.20
Branches and Runouts   0.06   0.10   0.12
Many contractors, designers and engineers will us a combination of the first two methods by limiting the velocity and limiting the friction loss.
3) Static regain method
The final method is the most complicated and as a result it’s not used that often. However, it is more energy efficient and is self balancing when designed properly. The most common application is in a gymnasium or hockey arena, where manually balancing the system is not practical due to height or space limitations.  When using the static regain method ducts are designed so that the available static pressure is used to offset the friction loss on the subsequent section of duct. This ensures that the static pressure available at the end of each branch is equal throughout the system resulting in equal airflow at each terminal. More complicated calculations, procedures and diagrams are required to ensure this method is executed properly. There is also little room for error because the system cannot be manually balanced or corrected after construction.

Leave a Reply

Your email address will not be published. Required fields are marked *

You may use these HTML tags and attributes:

<a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <s> <strike> <strong>