How to Size a Spark Trap

September 3, 2021

Screenshot of DuctQuote showing how to insert a spark trap into your ductwork design drawing.You’ve got a ductwork system with 18-inch pipes and need to install a spark trap to extinguish sparks from the grinding, welding, and cutting done in your shop. So you get an 18-inch spark trap, right? Wrong.

Why is the Spark Trap’s Size Important?

Improperly sized spark traps can result in significant pressure loss issues, causing the system to lose suction at its pickup points. A correctly sized spark trap will keep the velocity of the air flowing through the trap between about 3,000 feet per minute and 3,500 feet per minute, so the pressure loss of the spark trap itself is only about 0.8 inches of water column.

Properly Sizing a Spark Trap

The two essential pieces of information you need to know to size your spark trap are:

  • Air velocity of the system
  • Trunk size of the ductwork

For example purposes, we’ll imagine we’re designing a ductwork system for a metal shop. The shop’s airflow velocity is 4,500 cubic feet per minute, and the ductwork is 18 inches in diameter.

1. Calculate CFM Down to 3,000 and 3,500

Using a CFM table (found towards the bottom of the resources section of our website), we can find the pipe diameter we need to get the CFM between 3,000 and 3,500.

We should pick the 22-inch diameter because:

  • It gets us the closest to the actual amount of air we'd be drawing through the 18-inch pipes (7,960)
  • It gives you a little bit more capacity in case anything happens in the future

For this system, we will need a spark trap with a 22-inch inlet diameter for our application.

2. Get the Dimensions of Your Spark Trap

The next step is to refer to the spark trap spec sheet, also available in the resources section of our website (under the “performance components” heading), to find the spark trap’s maximum diameter and length.

The US Duct spark trap with a 22-inch inlet has an overall length of 85 inches and a maximum diameter of 40 inches.

3. Add the Spark Trap into Your DuctQuote Drawing

First, insert a piece of equipment into the drawing that represents the spark trap. A generic, inline device with one inlet and one outlet will work. Then, set the equipment’s shape and size. The horizontal cylinder shape most closely matches the shape of the spark trap. Set the size to 85 inches in length and 40 inches in height (our maximum diameter). Finally, add a label so you know that piece is the spark trap and adjust the color if needed.

4. Connect the Spark Trap to the System

Start by selecting and deleting a piece of pipe that’s the overall length of the spark trap and add connections to the spark trap. In this case, we’d delete an 85-inch section of pipe and add round connections with both inlet and outlet diameters set at 22 inches. Change the mounting setting to hanging. When you drag the spark trap to align with the pipes, the component will “snap” into place.

Pro tip: The inlet and outlet connections of the spark trap must align with the airflow direction. Adding the connections will also indicate to DuctQuote that the spark trap is part of a single system.

5. Let DuctQuote Do the Rest of the Hard Work


Not only will DuctQuote automatically add reducers into the drawing, but it will also add them to the bill of materials and remove the cost of the 85 inches of pipe you eliminated to make room for the spark trap. (Nifty, right?)

Ready to Build a Better Ductwork System?

From ductwork design to safety components to installation support, our Duct Guys are ready to help you provide your customers with reliable, effective ductwork systems. Get in touch with a Duct Guy to discuss your application.

Steve Rayburn, US Duct's DuctQuote coach and duct design expertYou've got enough things to worry about. Duct design shouldn't be one of them.

View the entire 12-minute webinar: Sizing US Duct's Patented Spark Trap

Steve Rayburn, our resident DuctQuote coach and expert, wants to help you get the most out of our proprietary design and quoting tool. Access the webinar to have Steve walk you through the process of sizing our spark trap and inserting it into DuctQuote — ultimately helping you make the sale.

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