Elbow Losses

Submitted by jmcmanus on Fri, 2011-08-26 12:46

By:

Norm Grusnick

Besides improving the energy efficiency of HVAC systems, round ductwork cuts installed costs.

“High performance” has reached buzzword status in the HVAC industry. With energy costs soaring, designers and specifiers are searching for ways to maximize energy efficiency in buildings. The search naturally leads to consideration of the HVAC systems, which account for around 40 percent of the energy use in a typical building. Besides reducing costs, boosting the energy efficiency of HVAC systems helps building owners and operators meet the latest sustainability requirements spelled out in the LEED building certification system.

Of course, one way to meet sustainability objectives is to specify more efficient HVAC equipment. So when designing an HVAC system, engineers often focus on the more costly mechanical components such as chillers and air handlers. But there is another part of the system that has a major impact on performance and efficiency, and that’s the ductwork. That’s why today’s smart HVAC system designers are carefully considering their ductwork options.

There are three different but mathematically related pressures associated with a moving air steam in a duct. Static Pressure (SP) is defined as the pressure in the duct that tends to burst or collapse the duct and is expressed in inches of water gauge (wg). Velocity pressure (VP) is defined as that pressure required to accelerate air from zero velocity to some velocity (V) and is proportional to the kinetic energy of the air stream. Using standard air, the relationship between V and VP is given by V=4005√VP. VP will only be exerted in the direction of airflow and is always positive. Total Pressure (TP) is defined as the algebraic sum of the static and velocity pressures or TP=SP+VP. Total pressure can be positive or negative with respect to atmospheric pressure and is a measure of energy content of the air stream.

One of the options is to look at the round duct elbows. In commercial systems with a typical duct velocity of 1500 fpm, engineering specifications along with SMACNA Duct Construction Standards call for radiused elbows to have a minimum 1.5 centreline radius unless otherwise specified.

See the example in the diagram below of a 12-inch diameter duct system with four 90 degree elbows in the system.

1256 CFM flow rate, Velocity of 1598 fpm
4 Elbows at 1.5 r/D= 4 x 0.03= 0.12 in. wg
4 Elbows at 1.0 r/D= 4 x 0.07= 0.28 in. wg

So we can see from this example with a system of four elbows that if tighter radius elbows are used it would penalize the client by as much as 42 percent. This may lead to larger fans and higher energy consumption when the goal today is to reduce energy usage.

This article was written by Norm Grusnick, Engineer of Product Development at ECCO Manufacturing

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