Series 1000 / 1500 / 9000
Linearity Information
Sizing Flow Control Dampers
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For two-position or shutoff applications, where dampers are always either fully open or fully closed, flow control characteristics are largely irrelevant. Shut off dampers should be selected and sized to minimize pressure drop in the full open position, and to minimize air leakage and maximize thermal performance in the closed position. Tamco Dampers are ideal in these applications.
For applications where dampers are used to control flow over a range of air flows, it is necessary to size the dampers based on the pressure drop of the damper in different positions, the resistance of the duct’s system that the damper affects, and the characteristics of the control signal. The interactions between a flow control damper, the system in which it is installed, and the controls can be complex.
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It is critical for proper control and efficient system operation, that known and controllable factors be optimized. To this end, most control system manufacturers have developed sizing guidelines.
We recommend that the detailed sizing procedures provided by control manufacturers and ASHRAE, be used for sizing dampers, especially in complex or critical applications.
The following sections provide information on the characteristics of Tamco dampers to be used in these design methods and also provides some further general sizing guidelines that may be appropriate in simple systems. |
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The pressure drop and flow of Tamco Series 1000, 1500, & 9000 dampers were tested to AMCA Standard 500-89 at blade angles of 15, 30, 45, 60, 75 and 90 degrees.
The results confirmed that the performance characteristics (linearity curves) of Tamco Series 1000, 1500, and 9000 dampers are comparable to the assumptions used by major control system manufacturers and the ASHRAE handbooks.
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| Figures 1 and 2 show the tested “characteristic curves” for parallel and opposed blade Tamco dampers, and compare them to other published data. |
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General Sizing Guidelines
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Even for general sizing guidelines, one must understand the term "Damper Authority", which is defined as the following ratio:
pressure drop across a control damper in the full open position
total pressure drop of the portion of the system that the damper controls.
(Note that some manufacturers refer to characteristic ratios instead of damper authority. This is the ratio of the “series” resistance to damper resistance, where series resistance is the total system resistance less damper resistance.)
The characteristic curves shown in Figures 1 and 2 correspond to the 100% authority curve, where the entire pressure drop of the system occurs across the damper. As can be seen, this curve is not linear. As damper authority decreases (system pressure drop excluding the damper goes up), the shape of the flow versus blade angle curve changes. A family of curves can be plotted as in Figures 3 and 4, showing damper performance at various values of damper authority.
In most, but not all, flow control applications, a linear performance is desired. Proper sizing of the damper will result in economical damper selections, with performance close to linear. System characteristics, as well as damper characteristics, must be addressed.
Two approaches can be taken to size control dampers correctly.
The first method, often used by system designers, is iterative. The designer selects an arbitrary damper size, often the full dimension of the duct, determines pressure drop across the damper at full flow from manufacturer selection charts, and does a “check calculation” for damper authority. If the result indicates that the damper authority is insufficient for the application, smaller dampers are selected until required authority is achieved.
The second method consists of calculating the desired pressure drop across the damper at full flow, using the appropriate authority values and calculated duct system pressure losses. The procedure is:
- Calculate the duct system pressure loss at full flow, excluding the control damper loss, which is still unknown.
- Select the damper authority required to provide the desired control characteristics.
- Calculate the control damper pressure loss at full flow:
PD = (DA / (100-DA) x PS
where PD = damper pressure drop at full flow / DA = damper authority, selected by designer / PS = duct system pressure loss, calculated using standard methods.
- Select damper sizes from the calculated required pressure drop, PD, and manufacturer’s selection charts.
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When air flow must be controlled in direct proportion to a control signal, a linear performance characteristic is desirable. A typical application is throttling air flow to maintain a space temperature, such as with a VAV box. Referring to the authority curves, it is apparent that a parallel blade damper should be selected to have an authority of approximately 20% to 25%, and for an opposed blade damper, approximately 8% to 10%. An example using TAMCO selection charts is provided below.
Example of damper sizing for linear performance assumptions:
Sizing series 1000 / 1500 damper for linear performance in a system with a duct system (series) pressure drop, excluding damper, of 0.2 inches w.g. at full flow of 5000 cfm.
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Sizing Criteria - Other Cases
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MIXED AIR CONTROL
A typical application is the economizer section in an air handling system, where two air streams are mixed to maintain a fixed temperature setpoint. To achieve stable and predictable static pressures and improved energy efficiency and comfort, the dampers should be sized so that the total flow rate of the system remains constant. This is generally best achieved by selecting dampers that are sized to provide linear Performance in the range of interest. For dampers in ducted systems, this usually requires that the outside air, return air, and exhaust dampers all be selected with similar authority, in the order of 8% to 10% for opposed blade dampers, or 20% to 25% for parallel blade dampers. However, in many cases, the outside air damper is installed behind the intake louver and is the same size as the louver in an effort to reduce entry of snow and rain. This results in grossly oversized dampers. In this case, size the outside and exhaust dampers with the same authority. Opinions differ on sizing the return damper in this situation. A general guideline would be to select a parallel blade damper with a pressure drop similar to the pressure difference between the inlet and exhaust ducts at the return duct takeoff. The outside and exhaust dampers should be of the opposed blade type.
FACE & BYPASS CONTROL
For face and bypass applications, the pressure drop through both sections should be the same. Size the bypass damper so that the pressure drop is the total of coil plus coil damper pressure drops.
PRESSURE CONTROL
This application is no longer common since the advent of cost effective variable speed drives and fan inlet vanes. Both are more energy efficient and should be considered over damper control for these applications.
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