Its In the Bag!

I have had an eventful two weeks – first in Colorado Springs at the 2013 HVACR & Mechanical conference and next in Las Vegas at the 2013 HVAC Excellence Educators and Trainers Expo. Even though I am very glad to be home I always am energized by these events. The educational sessions are great, put on by industry professionals who are tops in their field. Although I love ogling the latest techie toys, there are often inexpensive ideas and tips that help demonstrate how systems work. For example: you can use a large plastic garbage bag and a stop watch to demonstrate airflow. You flatten the bag, place it over a register, and time how long it takes to fill up. Then a quick calculation gives an idea of the CFM. The formula works like this – there are approximately 7.5 gallons per cubic foot, so a 55 gallon trash bag = 55 gal/7.5 gal/ft³ = 7.33 ft³. The flow in cubic feet per second is determined by dividing 7.33 ft³ by the seconds it takes to fill the bag. That multiplied by 60 gives you CFM. Written out it looks something like (7.33 / seconds to fill bag) x 60 = CFM. You can even do return air by filling the bag and then holding it over a return grill so that it completely covers it. You time how long it takes to collapse the bag. Now I am not recommending this as a means of checking system performance, but it is great for demonstrating exactly what is meant by cubic feet per minute. What I love about this is it demonstrates both the volume by seeing the bag fill up, and the time by clocking how long it takes. After demonstrating the concept of air flow you can show how to measure it properly with accurate instruments. Hopefully, the students will have a better idea of exactly what is meant by CFM and the measurements will mean more to them.