Why and How to do Air Balancing on Industrial Ventilation and Dust Collection Systems?

 

Introduction

The performances of all the hoods in the whole system normally decide the success of industrial ventilation and dust collection systems from the point of a system owner’s view. The appropriate hood design and its needed airflow rate require the application of good ventilation practice, mastery of the technical knowledge and the ability and patience to do the mathematical calculations.

When more than one hood is connected to one ventilation system, the design engineer has to assure that each hood can and will receive the volume of airflow equal or above its designed value and the whole system operates with the minimum possible airflow rate to save energy. This task is termed air balancing of the system or duct balancing.

Unbalanced systems cause problems

On the contrary, examine the case where one or more hoods are unable to receive desired volume of airflow from an unbalanced ventilation system.

It is not uncommon that many hoods are receiving airflow volumes below their goal values, while some others with extra airflows are wasting energy in a system with sufficient total airflow. Consequently, airflow distribution later becomes worsened gradually by partial or even whole plugging.

Moreover, an unbalanced duct system may give misleading signals to the control system, if there is one, causing the performance and energy efficiency of the system degraded.

Sketch of an Industrial Ventilation System

Two procedures for air balancing

The balancing of industrial ventilation systems is generally accomplished by one of two procedures.

• Static pressure balance – Balance by Design

As shown in the Design chapter in the Industrial Ventilation manual, the "static pressure balance" attempts to balance air flows in the system by judicious selection of duct size and length and fittings so that the pressure losses in each branch duct entering a junction are equal. This technique can present some difficulties when maintaining a conveying velocity is required, and the airflow distribution cannot be changed easily. Also, any deviation from the design during construction will affect flow distribution in the entire system.

•  The slide blast gate adjustments – Balancing with Dampers

The preferable technique for accomplishing air balancing in industrial ventilation systems is “the slide blast gate adjustment”. In this procedure the ductwork is sized to achieve the required conveying velocity through the system and the pressure loss of each branch is adjusted by a slide gate damper located at the branch.

Air balancing with dampers is commonly implemented by partially closing the dampers to regulate the excess proportions and varying fan speeds to achieve overall air flow rate as designed within tolerance. Application of this technique requires extensive field adjustments involving velocity pressure and static pressure measurements and the trial-and-error setting to determine the final slide gate positions.

Air balancing is the last and most important means of guaranteeing the airflow distribution in the duct as designed. It could fix the problem caused by unavoidable estimation error during the duct design and the construction modifications.

Different methods for Balancing with Dampers

There are three methods presented in the Industrial Ventilation, a manual by ACGIH.

1. Basic method
2. Modified Basic method
3. Target method

Normally when applying the Basic and Modified Basic methods, the practitioners have to do the adjustments two or more rounds to obtain a satisfactory air balance. However, in the field, many are unwilling to do a second round, let alone more, and leave the adjustments as is after the first round adjustment. Because of their inefficient trial-and-error nature, the results are largely dependent on the practitioner’s skill and experience.

Many plant managers are reluctant to perform TAB regularly, as they think TAB is costly and time-consuming and largely interrupts normal operating.

However, the Target method is different. It is really not time-consuming and also doesn’t cost that much for the system owners. But it needs more mathematical calculations in the procedure, and even some tricks. It does require the practitioner to have not only such knowledge, but also experience. I personally believe these are the reasons preventing many practitioners from using it. Especially for large industrial ventilation / dust collection systems, it has its own inherent advantages:

1. It does not require the fan speed to be adjusted before adjusting blast gate dampers
2. Normally, it only requires about one and a half rounds to obtain a satisfied air balancing and should seldom, if ever, require more than two full rounds.

Air Balancing Recommendation by Airvate

In fact, it is suggested that balancing should be performed regularly for all constant/variable air volume, small or big, industrial ventilation / dust collection systems.

Most systems may have to be balanced several times over their useful life, even with stable operating conditions and no major duct changes.

If there are changes to operating conditions or major duct changes, systems are very likely to require re-balancing, and such changes include:

• The airflow requirement for one or more hoods changed.

• A branch is added or removed from a system.

• A duct is replaced with one having a larger or smaller diameter or other changes are made that dramatically change resistance to flow

• One or more hoods are re-positioned in a manner that substantially changes the lengths and number of elbows in branch ducts, or if it changes how and where the branch and submains are connected to each other.

• The insertion depth of one or more blast gate dampers changed itself and influenced the whole system airflow distribution. 
• Other reasons that will gradually influence the whole system airflow distribution: 
• Abrasive wear of a previously sharp edge of a damper can reduce the resistance of the damper at a given insertion depth. 
• It is also possible for the damper resistance to vary due to accumulation of contaminants on the slide or just upstream or downstream of the slide. Accumulation of contaminants on the downstream side would substantially reduce the resistance. On the other hand any accumulation that extends beyond the reach of the gate could increase the resistance.

• A blast gate damper was adjusted by one operator in some special conditions, but forgot to restore the original position after the special conditions were gone. This may have caused some other operators  to adjust their dampers, consequently hurting the whole system’s air balance.

Therefore, testing, adjusting and balancing (TAB) plays a critical role in balancing the system to achieve desired air distribution.

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

1. Industrial Ventilation and Dust Collection systems testing and  Air Balancing
2. Whole system trouble-shooting
3. 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.
4. Technical Services: Field Survey / Dust Collection System Evaluation
5. New System Start-up: Step by step, including system balancing, baghouse controller optimization, and fan belt tension adjustment, etc.

Keywords

Air balancing, duct balancing, industrial ventilation, dust collection system, dust collection system trouble-shooting, target method, basic method, modified basic method, slide blast gate, blast gate damper, blast gate adjustment, static pressure, static pressure balance, hood static pressure, centerline velocity pressure, balancing with dampers, balancing by design