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Taking The 'Less Is More' Approach
Energy conservation is always a goal, and is even more paramount in construction
today. When it comes to improving air flow in HVAC duct systems to optimize
efficiencies the mantra “less is more” rings true.
Over the years, there has been some controversy with mechanical contractors
over the use of single thickness versus double thickness turning vanes in square
throat
elbow applications. However, research conducted by the Sheet Metal and Air
Conditioning Contractors’ National Association (SMACNA) and ASHRAE Fundamentals
Handbook puts the controversy to rest. SMACNA test data clearly indicates that
using
double thickness turning vanes instead of single thickness vanes increases
the pressure
loss of square throat elbows which, in turn, decreases HVAC performance, air
flow and energy efficiency.
Single thickness vanes have a recommended installation length of 36 inches
as outlined in the SMACNA Duct Construction Standards, Metal and Flexible Third
Edition-2005. If a square throat elbow must exceed that length, it is recommended
that the single thickness be installed in sections or a tie rod be fastened
to
the face of the vane for reinforcement. This method will reduce additional
pressure loss.
To further maximize air flow efficiency, the SMACNA duct design committee
also suggests turning vanes be installed in all rectangular square throat elbows
and that contractors use vanes without trailing edges, because trailing edge
turning
vanes can be installed backwards due to careless workmanship. For many years
mechanical contractors, often with the system designer’s approval, eliminated
every other turning vane from the vane runners installed in rectangular square
throat elbows. This practice is not recommended because it more than doubles
elbow pressure losses.
This is proven by a SMACNA sponsored research performed by ETL Laboratories
of Cortland, New York, to test single thickness turning vanes with a radius
of 4
inches. The distance between the vanes varied from 3 inches to 6 inches.
The testing used standard turning vane rail runners, with airflow velocities
varying
from 1,000 to 2,500 fpm in the 24 x 24 inch test elbow. The loss coefficient
of 0.18 for the standard spacing was compared with the loss coefficient of
0.46 inch at a 6 inch spacing with every other vane missing.
After analysis, the test data clearly indicated that the pressure loss of
the elbow with missing vanes was more than two and a half times the pressure
loss
of a properly fabricated square throat elbow containing all the single
thickness vanes with 3 inch spacing. a 4-inch single thickness turning vane
is more
effective than both 2 inch and 4 inch double thickness vanes. According
to test results
a 4-inch single wall turning vane is two times more effective than 2 inch
double thickness turning vanes at all tested velocity fpm.
A variety of single vane products are available on the market for square
throat elbow applications. Sheet Metal Connectors, Inc., for example,
offers the E-Z
Rail II single-wall turning vane for improving HVAC system air flow through
square throat elbows. In addition to enhancing the HVAC performance,
use of the E-Z
Rail II reduces energy usage because the system uses less fan motor energy
to divert air flow.
The E-Z Rail II is a single-wall turning vane manufactured in 22 Gauge
G-60 Galvanized Steel with a vane radius of 4 inches, vane spacing
of 3 inches,
and extra long
tabs with self-locking buttons to hold the vane securely in place.
The vane and rail are packaged separately in 10 foot lengths and 10 pieces
per bundle.
Contractors
may also order single-wall vane and rail cut to length. ETL Testing
Laboratories has successfully tested the E-Z Rail II single-wall vane up to
3000 fpm
(test reports available upon request). The single-wall turning vane
should be inserted
at a 45-degree angle in the point heel of the elbow to the point throat
at the elbow.
There are a number of ways that mechanical contractors can improve
air flow in duct systems and decrease HVAC energy usage. Significant
savings
and efficiencies
are accomplished by utilizing single-wall turning vanes in square
throat elbow applications, installing vanes that are tangent to air flow in
square throat
elbows, and eliminating trailing edges in rectangular mitered duct
elbows.
Submitted by: Leon Schmidt
Sheet Metal Connectors, Inc.
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