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  • Writer's pictureAnne Lee

Opposed vs Parallel Blade Air Dampers: How to Choose?

Updated: Aug 9


What are Ventilation Dampers?

In order for a HVAC system to operate satisfactorily, it is crucial to determine the design and selection of a ventilation damper in relation to the other components within the system.

The main purpose of a ventilation air damper is to either throttle or stop the flow of air through a duct or opening, regulating the flow of air within the air filter systems to the surrounding.

In the market nowadays, Rectangular Dampers have been named as the most prominent form of damper, specifically designed for proportional controlled applications.

The modulation of damper blades will occur in one of two ways, depending on how the damper linkage is being set up.

Types of Air Dampers

1. Parallel Blade Air Dampers

All blades of a damper connected with a parallel type of linkages, moving together at the same frequency and in the identical direction.

2. Opposed Blade Air Dampers

Adjacent blades of a damper with an opposed type linkage turn in opposite directions.

Parallel & Opposed Blades Dampers

Figure 1.1: Rectangular Air Dampers (Parallel & Opposed Blades Dampers)

Opposed vs Parallel Blade Air Dampers

Due to their unique blade orientation installed in a control air damper in air filter systems, they exhibit a non-linear characteristic in the relationship between flow rate and the position of the blades.

They are made up of several amount of blades, each of which is directly connected to a central spindle so that they are able to move together while adjusting the angle of inclination, Ɵ, or to the axis of duct.

With two blades or more, the spindles are linked to provide either parallel or opposed movement.

air flow of parallels blades dampers and opposed blades dampers

Figure 1.2:(A) Parallel Blades; (B) Opposed Blades

For Parallel blade air dampers, it possesses a considerably steeper and much more aggressive flow rate to damper position curve.

Parallel oriented blades will move the same way when opening and closing, and during closure, the front edge of each blade will come together with the back edge of the blade below it.

These edges overlapped to shut-off the damper, providing the blades a flat profile when fully closed. Practically, parallel blades damper characteristics are better suited for volume control air filter systems range from wide open to 75% initiate opening.

When opening and closing motion of the parallel blades, they tend to minimize the pressure drop through the damper’s blades, allowing the best orientation for maintaining air flow.

Due to the parallel blade air dampers being more sensitive to arm swings, even a slight change in damper angle may result in significant impact on the temperature paradigm.

As a result, parallel ventilation dampers are commonly applied for open / close air filter applications, directing airflow in a quick burst.

Benefits of Parallel Blade Air Dampers

Few of the benefits of parallel blade ventilation air dampers are:-

1. Least complex design

2. Cost-effective

3. Air flow increases more rapidly due to the parallel blades orientation

Unlike parallel blade dampers, opposed to ventilation blades, dampers have a shallower curve with a pronounced slow increase in flow rate, commonly used for volume control modulating air filter applications ranging from wide open 100% open to 25% open.

The pressure drop is increased by the arm swing with opposing damper blades, due to it exhibiting a more proportional and regulated damping effect.

This can put undue strain on the air filter system every time the damper operates.

Benefits of Opposed Blades Air Dampers

As a choice to consider on opposed ventilation blades dampers application:-

1. Able to modulate airflow and provide more closing torque for the damper

2. Gap between each ventilation air damper blades has tighter seals, which minimize air leakage in the air filter

3. Size of the opposed blade damper (Twice the size of a parallel blade’s)

Regardless, the commercial dampers have a variety of cross-sectional blade shapes including flat plates with crimped edges, aerofoil and wedge. In an empirical relationship, the damper loss coefficients, KdƟ is being defined as:-

Dampers Pressure Loss Coefficient Exponential Decay Graph

Figure 1.3: Dampers Pressure Loss Coefficient Exponential Decay Graph

loge KdƟ = a + bƟ

; where Ɵ refers to the blade angle ranges:

Single & Opposed blades : 10 degrees ≤ Ɵ ≤ 60 degrees

Parallel blades : 10 degrees ≤ Ɵ ≤ 60 degrees

a & b are constants related to parameters such as blade size and shape, projections on blades, relationship between the duct wall and blade configuration.

For a given design system flow rate and pressure drop, the choice of air damper will then depend on factors such as:-

  1. Ductwork connections

  2. Costs of alternative arrangements

  3. Length of duct acquired for full static pressure regaining

  4. The ability to mix two airstreams at two temperatures levels and prevent stratification in the total air volume flow before the next section of the system.

In A Nutshell

By choosing the right ventilation blades orientation will ensure air dampers are designed for peak HVAC efficiency, allowing us the save cost over the lifetime of the air filter system. Blade orientation will directly determine how the damper closes and may affect air performance during closures.

parallels and opposed blades air dampers comparison table

As an OEM HVAC manufacturer, TCW Group has a team of R&D which will always be ready to provide the appropriate advice on the ventilation blades orientation which suits all kinds of designs for HVAC air filter systems.

Our experienced Production team coalesced with highly sophisticated machines capability further made the manufacturing of dampers in different types of materials such as Galvanized Steel (GI), Stainless Steel (SUS), and Aluminium Extrusion (Al) viable.

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