A single butterfly damper disturbs the airflow considerably. It is therefore better, both for balancing during commissioning and for the automatic control of airflow, to use multi-leaf butterfly dampers which may be in parallel or opposed-blade configurations.
Every butterfly damper has an inherent characteristic relating airflow with blade position and Figure 13.18 shows two typical inherent curves. Such characteristics are for the case of a constant pressure drop across the damper, regardless of blade angle. It is evident that as the blades close in a real installation the pressure drop across the damper will not be constant and the characteristic will become different: it is then called an installed characteristic (see Figure 13.19) and its shape and position depend on the authority, a, defined in the way adopted for valves and related to the part of the duct system where variable flow occurs. It appears to be common practice to select automatic dampers in terms of another parameter,
5, defined as the ratio of the pressure drop through the system, excluding the damper, to that through the damper fully open, for the case of maximum airflow.
The pressure drop across a fully open damper depends on: damper construction, blade shape, damper dimensions, frame intrusion into the airstream, and the ratio of the crosssectional area of the fully open damper to that of the duct in which it is fixed. (The inherent characteristic is also a function of the damper-duct area ratio, but most significantly so only when this is less than 0.5.) Typical pressure drops, fully open, are of the order of 10 to 15 Pa for parallel blade dampers used in a mixing application, as shown in Figure 13.20.
Seals of various sorts can be provided to minimise leakage when a damper is nominally shut. A simple automatic damper, not having any special seals, has a typical leakage rate that can give a face velocity over its section of about 0.25 m s’1 for a pressure drop across it of 375 Pa, according to one manufacturer. A square law can probably be used to give a rough relationship between velocity and pressure drop. With special construction and seals a low-leakage damper can give velocities as low as from 0.01 to 0.025 m s 1 for the same pressure drop, it is claimed.
One of the commonest applications of motorised dampers in air conditioning is to vary the mixing proportions of recirculated and fresh air (see section 3.10), as shown in Figure 13.20. A controller Cl, positioned in the ducting to sense condition W, would be used to vary the mixing proportions of fresh and recirculated air in winter, so that the dry-bulb
Percentage damper rotation (0°-90°)
Fig. 13.18 Typical inherent damper characteristic.
Temperature of state W was maintained. The dampers would be moved by means of the damper motor (or group of damper motors) Rl. Consider the variations of pressure which may occur as the mixing proportions alter. If the discharge air and the variable fresh-air dampers have been chosen to have a pressure drop across them of 15 Pa, then, when 100 per cent of fresh air is handled the static pressure at state R is +15 Pa and the static pressure at state M is -15 Pa. The difference of pressure over the recirculation dampers is therefore 30 Pa. When the recirculation dampers are fully open, the discharge air and the variable fresh air dampers being fully closed, the pressure difference should still be 30 Pa if the volumes handled by the supply and extract fans are not to vary. Since virtually constant pressure operation is the rule, parallel-blades are a better choice than opposed-blades as they have a more nearly linear inherent characteristic. A flat S-shaped curve can be obtained if the dampers are properly selected. See Figure 13.19.
To get a good installed characteristic dampers are sometimes arranged so that they are never more than 45° open, because of the approximate linearity of the first half of characteristic curve (see Figure 13.19). To secure the correct balance of face velocity and pressure drop it is sometimes necessary to adopt a cross-sectional damper area less than that of the duct in which the damper is to be mounted. It is occasionally recommended that gradually expanding and contracting duct connections be made to the damper but this is scarcely ever a practical proposition and is probably not worth doing, in the majority of cases.
Posted in Engineering Fifth Edition