Loren Cook Logo airflow and static pressureWhat is the Relationship Between Airflow and Static Pressure in a Ducted Air System?

We dropped in on Chris Curry at Loren Cook for this coffee break, who offered an excellent demonstration that definitively answered this question.

If you couldn’t join us on the webinar, here’s some of what you missed.

The goal with ducted airflow is to design a system that minimizes the amount of static pressure (resistance to airflow) while not oversizing the duct to the point where it won’t fit or becomes too expensive for the project (due to the amount of metal needed). The less static pressure, the less energy the fan consumes to deliver the required airflow.

Curry illustrated that by pointing out the Third Fan Law: When increasing a fan’s RPM, the horsepower required by the motor to turn the fan wheel increases by the cube of the change in motor speed. For example, if you want to increase fan RPM by 10%, the energy required to turn the fan increases by 33.1%.

Keep this in mind when you consider energy optimization in a ducted system. It’s good to seek out equipment with high EER ratings and motor efficiency. But also make sure that your duct design is optimized. This includes the size of the ductwork (which affects the velocity of the air moving through it) and the fittings that you specify in the design (which affects both the pressure drop and system effect associated with the installed duct system).

It only takes one improper fitting to increase static pressure at a magnitude that wastes all the efficiency you’re gaining from a premium fan motor.

Which duct fittings have the most significant effect on static pressure?

Curry arranged a 12-inch utility set fan with 2 ½ duct diameters of straight duct on the discharge end to achieve laminar flow. He then measured the drop in static pressure in the system when he attached fittings to the end of the duct.

Straight Elbow

Contractors assume the pressure drop of this fitting will be about .15 inches (an old rule of thumb says an elbow equals roughly the same static pressure as 100 feet of duct). Surprisingly, it’s more than double that (around .3). This fitting creates an eddy behind the throat of the fitting, drawing air into the duct and creating turbulence.

Straight Elbow with Turning Vanes

The addition of turning veins in a straight elbow brought the static pressure drop to only about .07. The downside is that the vanes can gather debris inside the duct, so Curry recommends installing an access door upstream of any elbow with turning vanes.

Short Radius Elbow

Curry admitted this fitting is controversial and sometimes not approved for installation. He demonstrated that the 13” radius elbow had a .06–.07 drop in static pressure, as good or better than the straight elbow with turning vanes. The problem with that elbow is that you need to have the space for the short radius elbow to curve — lots of mechanical rooms don’t have that space.

Square Throat Radius Heel Elbow

You might think that combining the square throat of the straight elbow with a radius heel would give you the benefits of both elbows. You might expect a static pressure drop between .06 and .3 (something around the standard guess of .15).

It’s worse than both. There’s a misconception that the airflow follows the radius heel’s curve. Instead, that same negative eddy created by the square throat is made worse because this elbow creates a pinch point (it has a smaller diameter in the middle than either end).

45-Degree Radius Elbow

This is the solution to the previous fitting — modifying the throat from a 90-degree angle to two 45-degree angles to remove that pinch point and keep the cross-section distance consistent through the elbow. That’s a truer compromise: the static pressure drop is close to the .15 standard guess.

Long Sweeping Radius Elbow

The one everyone wants — a long, graceful, gentle arc. Curry used a 20” radius elbow, one and half times the duct diameter. And sure enough, the static pressure drop is negligible (roughly .02–.03). But the practical problem remains. This elbow costs more and takes up even more leasable space than the short radius elbow. That’s a significant amount of money and real estate throughout an entire building’s ductwork.

Radius Elbow with a Splitter

Here’s the best of all worlds: Static pressure drop was .02–.03, significantly better than the short radius elbow. The split inside the duct turns this 13” short radius elbow into two long radius elbows of 5” and 8”, giving you the airflow benefits of a long curve in a short space.

Curry also discussed tee fittings and fittings on the fan’s inlet side. Are you curious why a fan’s submitted performance doesn’t always match a test and balance report? Or whether a forward-curved vs. backward-curved fan wheel makes any difference in the efficacy of these fittings? Contact an SVL sales engineer, and they’ll let you in on what you missed.

Speaking of which…

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