Determining Damper Size - Honeywell AUTOMATIC CONTROL Engineering Manual

CHARACTERISTIC RATIOS = SERIES RESISTANCE
100
20 10 5 3 2
0
0
% DAMPER OPENING
Fig. 29. Damper System Characteristics of
Parallel Blade Dampers.
CHARACTERISTIC RATIOS =
100
20 10
0
0
% DAMPER OPENING
Fig. 30. Damper System Characteristics of
Opposed Blade Dampers.
To achieve performance closest to the ideal linear flow
characteristic, a characteristic ratio of 2.5 for parallel blade
dampers (Fig. 29) and 10 for opposed blade dampers (Fig. 30)
should be used. The percent of the total resistance needed by
the damper can be determined by:
Total resistance (100%) = damper resistance + series
Characteristic ratio
(Fig. 29 and 30) =
Substituting (total resistance – damper resistance)
for series resistance:
DAMPER RESISTANCE
1
INHERENT
CHARACTERISTIC
100 C1498
SERIES RESISTANCE
DAMPER RESISTANCE
5 3 1
INHERENT
CHARACTERISTIC
100
C1499
resistance
series resistance
damper resistance
DAMPER SELECTION AND SIZING
total resistance – damper resistance
Characteristic ratio =
or
For parallel blade dampers:
2.5 =
Damper resistance = 29% of total resistance
or
29
100 – 29
For opposed blade dampers:
or damper resistance = 9% of total resistance
9
100 – 9
For example, if a coil (Fig. 31) with a pressure drop of 0.55
(in. wc) is located in series with an opposed blade damper, the
damper should have a pressure drop of 0.06 (10 percent of
0.55 = 0.06).
0.06 IN. WC
DROP
Fig. 31. Typical Pressure Drop for Coil and
Damper (Series Resistance).

DETERMINING DAMPER SIZE

The desired relationship of damper resistance to series
resistance developed in DAMPER CHARACTERISTICS is
used to determine the desired damper pressure drop. This
pressure drop is then used in the damper sizing procedure in
Table 4.
For example, for a 36 by 64 in. (2304 in
airflow of 20,000 cfm and a pressure drop of 0.06 across a
parallel blade damper, determined the damper size as shown
in Table 5.
465 ENGINEERING MANUAL OF AUTOMATIC CONTROL
damper resistance
total resistance
= – 1
damper resistance
100
– 1
damper resistance
= 41% of series resistance
100
10 = – 1
damper resistance
= 10% of series resistance
0.55 IN.WC
DROP
C1497
2
) duct with an