What you have to know when applying blast gate damper

 

Introduction

A damper is a device that adjusts the volume of airflow passing through the outlet, inlet or duct.

By proper adjustment of all dampers in a system, a desired distribution of airflows in all the branches can be obtained, while minimizing the total airflow of the system to save energy— this is also termed air balancing.

The most common dampers used in industrial ventilation and dust collection systems to balance airflows are slide gate dampers (The slide gate, also called “blast gate or cut-off” damper).

In a slide gate damper (see Figure 1 of a straight flat slide gate damper), the slide is inserted perpendicular to the flow.

Figure 1 A sample of straight flat slide gate damper

As the gate is sliding into the duct, it will add resistance to the airflow and consequently reduce the volume of airflow, while increasing airflows in all other branches. Withdrawing the gate has the reverse effect on that branch and all the others in a system.

Interaction between a fan and dampers

As the damper opens or closes to adjust the airflow in a system, fan operation moves to a new point on the fan curve.

Dampers increase the total resistance of a system; the fan airflow output decreases while increasing the total static pressure produced by the fan. Fan efficiency may also be affected if the fan was not selected with the additional resistance by dampers in mind.

However, if dampers are employed only to produce the desired distribution and the fan speed is adjusted properly, the pressure requirement for the system will be nearly always lower than without the dampers. This is also a partial reason why air balancing can save energy cost.

When designing a ventilation or dust collection system, the calculated amount of air leakage of the whole system should be added to the selected fan capacity and so noted either in the specifications or on the drawings. It should be done the same for the additional resistance caused by dampers.

Many fan drives, or even the fans themselves, have had to be changed because duct system air leakage or extra resistance by dampers was ignored by the system designer.

Selecting straight flat blast gate dampers as an example, Airvate calculated the resistances for various ratios of insertion depth to duct diameter.

If you want to know the resistance by a curved slide gate (Concave or Convex), you are encouraged to email Airvate directly.

In Table 1, a damper is used to increase the resistance of the branch but still keep the same duct velocity to avoid settlement of dust, so a smaller diameter of duct doesn’t have to be selected. Then, the pressure loss will be added to the total pressure loss of that branch in the calculation for the whole system design. The pressure losses under different insertion ratio and duct velocity are tabulated.

For example, when the ratio of insertion is 0.4 and the duct velocity is 4000 FPM, then static pressure loss caused by the blast gate is 2.88”wg.

Table 1: Damper Resistance at Different Insertion Ratio and Duct Velocity

In Table 2, when a damper is adjusted and the static pressure at the branch end doesn’t change dramatically (this is normally true in a large ventilation system) both the airflow rate and the duct velocity in the branch will be reduced. This is the reason why dust settlement becomes a concern every time a damper closes partially or fully.

For example, in the case of the static pressure being -1”wg at the branch end, the damper is fully open, the duct velocity is 4000 FPM, and after the gate is inserted to the duct, and the ratio of insertion is 0.4, the duct velocity will become 2030 FPM in this branch.

Table 2: Reduced Duct Velocity by Damper at Different Insertion Ratio

Things to keep in mind when using dampers

There are some issues associated with the use of dampers we have to pay attention to.

Many times when a dust generated point is turned off, users simply cut off the branch by closing the blast gate completely, and believe this will save energy and other branches can have more airflow. Actually, this is strongly not recommended, since the amount of saving in heat and power is not great enough to offset the disadvantages. Especially in the design stage, the suction main must be designed for unduly high velocities to avoid dust settlement in case that some gates are closed.

After blast gates are put into use in a dust collection system, if the dust is abrasive, the sharp edge of the damper will become smooth gradually, and this can reduce the resistance of the damper at a given insertion depth.

The damper resistance definitely will be affected, more or less, if there is any accumulation of dust on the slide gate or just downstream or upstream of it. Accumulation of dust on the downstream side would greatly reduce the resistance. On the contrary, if the accumulation of dust is at the position beyond the reach of the gate, this will increase the resistance of the branch (or section).

So, slide gate dampers definitely need care, inspection, and maintenance.

• The friction between the slide and the damper should not be too high.

• Dampers are usually avoided when sticky materials are in the ducts.

• Damper slides should be inspected periodically and cleaned regularly as sticky or stringy dusts can be caught on the slide of the damper. Plugging might occur due to rags, pieces of paper and other material that are sucked into the duct by the fan. Flammable materials caught on a damper could create a fire hazard. Hence, care should be taken to avoid flammable materials.

• The location of the damper on a duct is also important for proper operation of the system. Whenever space is available on-site, a damper should never be located near elbows and other disturbances that may influence the pressure drop. Dampers can become sources for material build up if not located properly. 

• Moreover, blast gates should be locked after the air balancing work is finished.

• If slides are lost or damaged, they should be replaced immediately, so the damper and the whole system can function properly as expected.

Industrial Ventilation and Dust Collection Services by Airvate that you can afford:

• Industrial Ventilation and Dust Collection systems testing and  Air Balancing

• Whole system trouble-shooting

• Dye Testing for Bag Leak: Once a baghouse collector is installed or if there’s any hint that bag leak is possible in an existing baghouse, Airvate can do a Dye testing step by step.

• Technical Services: Field Survey / Dust Collection System Evaluation

• New System Start-up: Step by step, including system balancing, baghouse controller optimization, and fan belt tension adjustment, etc.

Keywords

Slide gate damper, blast gate damper, cut-off damper, industrial ventilation, dust collection system, resistance by slide gate damper, resistance by blast gate damper, air balancing