Honeywell AUTOMATIC CONTROL, Engineering Manual

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ENGINEERING MANUAL OF AUTOMATIC CONTROL
DAMPER SELECTION AND SIZING
458
M10418
DAMPER
BAFFLE
Fig. 16. Baffle Inside Duct Surrounds Damper.
TYPICAL DAMPER CONSTRUCTION
Figure 17 shows construction of an opposed blade damper
with linkage. A parallel blade damper is essentially identical
except for placement of blade linkage and rotation direction of
alternate blades. Higher leakage dampers have either no blade
side seals or less effective sealing elements, e.g., adhesive-
backed cellular foam strips.
PERFORMANCE DATA
Performance data for dampers can vary in content and form
due to the many types of dampers and the philosophies of their
manufacturers. When casually compared, performance ratings
of different dampers may seem equivalent but are not due to
differences in criteria on which the data is based. The following
paragraphs discuss the types of performance data and their
variations.
LEAKAGE RATINGS
Leakage ratings of dampers are the most widely publicized
damper performance data. Figure 18 graphically shows typical
leakage performance of dampers with side seals, but without
blade edge seals. The torque necessary to achieve the indicated
leakage ratings is specified at the top of the graph as 5 lb-in.
per square foot of damper area. The wide performance band is
explained by the note at the bottom of the graph.
For typical dampers, leakage increases more significantly with
the number of blades than with the length of the blades. The data
shown applies to a combination of damper heights and widths.
For example, a damper 48 in. high x 12 in. wide is the area
equivalent of a damper 12 in. high x 48 in. wide. However, there
is significantly more leakage with the 48 in. high x 12 in. wide
damper, due to the increased number of blades. Using the leakage
performance graph in Figure 18, the range of leakage for these
two dampers (4.0 ft
2
each) at 1.0 in. wc is:
Leakage = Area x Rating/ft
2
Minimum Leakage = 4.0 x 10.0 (min)
= 40 ft
3
/min
Maximum Leakage = 4.0 x 26.0 (max)
= 104 ft
3
/min
Performance characteristics for low leakage dampers differ
from standard dampers. Figure 19 shows typical pressure drop/
leakage rating relationship for low leakage dampers. Specific
leakage ratings for horizontal (A) and vertical (B) damper
dimensions are listed and the torque required has been increased
to a flat value for any single section to accommodate light
compression blade edge seals.
Using the leakage ratings in Figure 19, the leakage of the
two dampers in the previous example at 1.0 in. wc is:
12 in. (A) + 48 in. (B) = 2 cfm + 43 cfm
= 45 cfm
If the A and B dimensions are reversed, the leakage is as follows:
48 in. (A) + 12 in. (B) = 7.2 cfm + 12 cfm
= 19.2 cfm
Other methods of conveying leakage ratings are a tabular
format (e.g., leakage per area in cfm/ft
2
) and a percentage basis.
The tabular format lists specific ratings for each individual
damper size. Leakage per area must include sizes of interest.
The percentage basis leakage rating is meaningless unless
complete conditions including total airflow that the data is based
on are also stated.
In many cases, damper data indicates that it is certified by an
industry association. Certification means that tests were done
under conditions defined by an association of manufacturers
but is not a rating by an approval body. For applications other
than smoke and fire, there are no approval bodies governing
damper leakage or any other performance characteristics.