How to Measure Airflow in a Duct

Measuring the airflow in ducts is an important task in maintaining an HVAC system. Every system is designed to work under a given “load”. Lower loads lead to an underperforming system and higher loads can result in increased running cost and malfunctions. Because of that, it is important for us to have a way of measuring airflow in ducts and do it accurately. It all depends on the place we decide to take our measurement from, and the type of pipes used in the HVAC system. Why does that matter? Because different shapes of ducts would have different velocity profiles. The highest measurements of airflow would be in a laminar flow. This means there will be no friction present. And different shapes of ducting have different velocity profiles because they have more or fewer edges, which create friction.

Round Duct Airflow Measurement

Round ducting is considered the most efficient type of ducts when it comes to airflow. In order to take measurement of airflow rate in a duct, we need to traverse it correctly. Using a couple of points in this traverse, we will later be able to calculate the nominal airflow in ducts. For circle ducting, we will need to do a cross-section of the duct pipe. Imagine we cut it in six equal pieces, like a pie. We will need to take measurements of airflow rate in a duct from three points alongside our “cuts” in the pie, each point further away from the wall and closer to the centre. This will give us enough data to calculate the true airflow. Because of friction near the walls of the pipe, the airflow there and that in the centre will differ. But why won’t we just take three measurements and be done with it? Well, because air is moving like a fluid, it avoids places with higher pressure, and we have to measure the velocity of the air while flowing through the full volume of the pipe. So we take three measurements, for every single “cut” and we end up with eighteen points for measurements. We can also divide the cross-section into four pieces, and take five measurements from each, giving us twenty points for measurements. The method with six equal pieces gives us a more accurate average.

Measuring airflow in a duct with a square or rectangular shape

A rectangular duct pipe will require a minimum of twenty-five sample points alongside the cross-section to give us an accurate measurement. A maximum of forty-nine sample points can be used in pipes with larger dimensions. Most of those points will be next to the walls of the pipe, where friction will affect the airflow. In fact, almost 70% of the points will be near walls, compared to 40% in circle ducting pipes. This is where it becomes clear, why round ducts are preferred. However, this is not true for all of the cases, there are still applications where rectangular and square ducts prevail over round ones.

Modern anemometers can give us figures about the airflow in ducts in cubic feet per minute. Those devices make the everyday life of HVAC professionals a lot easier. However, now that you are familiar with the overall methods of gathering data about the airflow, you can see how and why some readings may be slightly off the real numbers. It has become standard for LEV indicators to be installed alongside ductworks to constantly monitor the pressure inside (1000 Pa / 2500 Pa) and use those readings to overlook the overall state and efficiency of the system.

As always, if you are having trouble choosing the right LEV indicator for your system our customer support team can assist you. All you have to do is give them a call at 01455 616444.