By John Constance, Professional Engineer, P.E.
The Engineers Collaborative
Industrial exhaust systems are designed to help protect plant personnel from exposure to nuisance and hazardous airborne vapors and dusts generated by production operations. The most common type of industrial exhaust systems usually includes dust and vapor contaminant capture hoods, connecting ductwork, an air pollution control device, and an exhaust fan. In the case of dust control, a safety monitoring High Efficiency Particulate Filter (HEPA) is sometimes included after a dust collector for extra protection of the environment. A good operating industrial exhaust system will have all these components working together to help produce a safe working environment.
What are the key factors that go into the design of industrial exhaust systems? How can you tell if an industrial exhaust system is working properly? How can changes in production operations affect the performance of an industrial exhaust system? These are just a few of the questions that need to be answered in order to help you understand industrial exhaust systems for dust control and vapor control.
The Design Phase For An Industrial Exhaust System
During the design phase of an industrial exhaust system, a number of factors are included to assure a successful system at start-up. Capture hood concepts were developed to satisfy airborne contaminant reduction goals without affecting production. Thought was given to the operation and maintenance of the system. Standards were set to measure the performance of the installed industrial exhaust system. These standards include an expected reduction in airborne contaminants generated by the production operation. Here is how this is accomplished:
Industrial Hygiene Air Sampling
The basic objective of most industrial exhaust systems is to reduce the amount of airborne contaminants in the work place. Even nuisance airborne contaminants can pose a health risk to personnel if the concentrations exceed those set by OSHA permissible exposure limits. Air sampling data taken prior to the design of the industrial exhaust system is used to determine the desired contaminant reduction to be provided by the exhaust system. Air sampling data can be used as an investigative tool rather than just measuring the production operator's exposure to the overall level of contaminant. This information helps determine which sources of contaminants need to be addressed and contributes to the most economical solution.
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Capture Hood Concept Development
The design of dust control, dust containment, and vapor capture hoods is probably the least understood and, as a result, the weakest link in the components that make up industrial exhaust systems. Capture hoods need to be adapted to the operations they are serving and should not interfere with the process or product.
Capture hood concepts are developed so that the capture hoods are positioned close to the contaminant source. Concepts using "textbook" hoods can be unsafe. For example, overhead canopy hoods are easy to design and install, but, if misapplied, they will draw contaminants past the operator's breathing zone. Lateral exhaust hoods get closer to the contaminant source and, when designed correctly, are not removed by production because they catch product or get in the way.
Engineering and Design Documents
Detailed engineering specifications and design drawings are prepared to give the industrial exhaust system construction contractor all the information that is needed to build the system. These documents are reviewed by production and health and safety personnel before the system is built to assure that all objectives are satisfied. These detailed documents are also used to provide guidance for performance checks, system maintenance and system upgrades.
Industrial Exhaust System Operation and Maintenance
During the design phase, care is taken to see that industrial exhaust system's components are positioned for ease of access and maintenance. Duct access doors are located for ease of use. Duct dampers are located to facilitate system air balancing while limiting accessibility by unauthorized personnel. Filter housings are positioned so that filters can be readily serviced. Capture hoods are placed so as not to interfere with production operations.
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The Industrial Exhaust System Construction Phase
If the industrial exhaust system's design documents are sufficiently detailed, it will not be difficult to follow the project during the construction phase to assure compliance with the design intent. A little effort during construction helps minimize future operating and maintenance problems. Here are some important checks that are made during the construction phase:
Fabrication Checks
The contractor's fabrication shop should be visited to inspect system components as they are being made. Materials of construction and metal gauges should be confirmed checked. All sheet metal components should be checked for sharp edges. Critical dimensions should be measured. Ask these important question, are capture hoods being fabricated according to the design documents or is the contractor becoming the air engineer by modifying hood designs to fit the contractor's needs?
Installation Checks for Industrial Exhaust Systems
Industrial Exhaust System components should be checked to see that they are not installed in such a way as to block access to each other or to other plant services and equipment.
Capture hoods should be checked to see that they are installed according to the design documents. A capture hood just a few inches out of position can result in a loss of dust or vapor contaminant control.
The dust collector filters and HEPA filters on-site storage should be inspected. These filters must be carefully protected from weather and construction activities. Insignificant damage to filters will cause major operational problems. "Invisible" holes burned through by welding sparks or punched through by poor handling and storage will allow dust to bleed through the filters. This will contaminate whatever is downstream of the filters.
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The Industrial Exhaust System Start-Up Phase
The effectiveness of dust and vapor capture hoods is measured by how well they reduce the airborne contaminant concentration at the sources of generation in the work place. With dust control systems, the system's ductwork needs to convey the captured dust to the dust collector with the proper transport velocity. This is important to minimize particle settlement within the ductwork. The efficiency of the dust collector is measured by the cleanliness of its discharge air. The safety monitoring filter (HEPA) needs to provide an effective barrier in case of dust collector filter media failure. All these system components are validated at startup:
System Air Balancing
The installing contractor is called upon to balance the air flow in the duct system so that the capture hoods exhaust the design air quantities. The procedures for air balancing as described in the Industrial Ventilation Manual published by the American Conference of Governmental Industrial Hygienists and are the standards to be followed. Pitot tube readings of duct velocity pressure in each duct connected to a capture hood provide the most accurate readings of exhaust air quantities. Air system balancing must be done with all capture hoods installed and connected to the exhaust system ductwork.
All the data taken during the system air balancing is recorded and filed for future reference as it is the base line against which future system performance will be measured.
System Safety Components
If collected, dust is flammable and even explosive, the dust collector will have been installed with fire and explosion protection components. These can include a wet sprinkler system, mechanical vents and/or a chemical suppression system on the collector body and isolation valves or pressure relief vents on the dust collector inlet and outlet ductwork per NFPA 68. These systems are thoroughly checked out by the equipment manufacturers to assure they will operate as intended.
Electrical bonding of the dust collector filters to building ground is also confirmed.
System pressure gauges and low flow and overpressure alarm systems are checked out to assure compliance with the design documents.
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Component Baseline Documentation
In addition to the system air balancing data recorded during the startup phase, additional baseline information concerning all the system components is recorded:
- Capture hood static pressures are measured and recorded for future reference. If a hood's static pressure changes later on, there will be no doubt that the hood exhaust air quantity has changed. If the exhaust air quantity has lowered, then the effectiveness of the hood has decreased. Knowing what the start-up static pressure was after air balancing will provide the information necessary to help correct the problem.
- Operating static pressures of a dust collector and safety monitoring filter system (HEPA) are recorded to signify their "clean" static pressure base lines. The static pressure drops across the filters will increase over time. However, any deviation from an expected pressure drop at startup indicates a problem that may prematurely shorten the life of that component.
- A velocity pressure measurement in the main duct just before or after the air pollution control device or exhaust fan can be converted to the total system exhaust air quantity. This velocity check is carefully taken so that the resultant total exhaust air quantity can be compared to the design quantity.
Industrial Hygiene Air Sampling
Effective dust and vapor capture hoods properly used can reduce the contaminant concentration caused by the source by an average of 95%. To assure that this goal is satisfied by the industrial exhaust system, air sampling is done immediately after system start-up at each contaminant source covered by a capture hood. Industrial hygienists supervise this function to assure that the air sampling is properly done.
Measuring Particulate Filtration Effectiveness
For dust control systems, special testing is necessary to validate the installed efficiency and to maintain the high efficiency expected of HEPA filters. Tests at start-up should be made to confirm that the filtration equipment has been installed properly and that no components have been damaged (easily done when handling filters). Periodic testing needs to be performed to assure that the components are operating at the expected high efficiency.
Proper testing checks the condition of the HEPA filters, the seal between the HEPA filters and the filter bank housing as well as the housing itself. In the past, challenging HEPA filter installations with cold dioctyl phthalate (DOP) has been the accepted practice of confirming HEPA filter installed efficiency. Concern about the safety (carcinogenicity) of DOP has resulted in active research for acceptable alternatives for DOP. In the mean time, proper precautions should be taken when working with DOP.
Efficiency testing of dry dust collectors and wet scrubbers is not as rigorous as the challenge of HEPA filters. Periodic particulate sampling at the dust collector inlet and outlet will measure the efficiency of particulate collection. Continuous checking of dry dust collector emissions can be accomplished with broken bag detectors. This equipment detects filter failures which can occur due to failed filter media. An increase in particulate emissions from the dust collector can be detected before visible emissions are seen at the exhaust system outlet. With a HEPA filter bank downstream of the dust collector, a broken bag detector system could prevent HEPA filter plugging if immediate action is taken to replace the dust collector filter media. A plugging HEPA filter will reduce air flow resulting in reduced contaminant control at capture hoods.
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Periodic Safety Checks of Your New
Industrial Exhaust System
Dust Explosion Protection Components
If the industrial exhaust system handles dry particulate and if the collected dust is organic or metallic in nature, an explosion relief or suppression system most likely was installed on the dry dust collector. Check your material safety data sheets. Periodic inspection and maintenance of these components should be done in accordance with the manufacturer's recommendations and applicable safety guidelines such as those set by the National Fire Protection Association (NFPA).
An important activity often neglected is the periodic sampling of the collected dust for an explosivity determination. If the process has changed so that the particle size or shape of the collected dust has changed, dust explosivity may be affected. If the chemistry of the processed product has changed, dust explosivity may again be affected. If the collected dust shows an increase in explosivity above the level for which the installed explosion venting or suppression system was designed, immediate action must be taken to correct the deviation from the design condition.
Processed Product Changes
Changes in the nature and quantity of the processed product can also affect the performance of the industrial exhaust system. Raw material changes can dramatically change a well performing exhaust system into a maintenance nightmare. New hygroscopic ingredients can plug filter media and drastically reduce air flow through the system. Finer powder can promote even finer collected dust that may bleed through filter media or pass through wet scrubbers and cause an air pollution or return air system problem. Maintenance personnel must be included in the processed product information loop so that they do not waste time looking for defective system components that are in reality functioning according to design.
Industrial Hygiene Air Sampling
In order to help maintain a healthy atmosphere in the plant, periodic air sampling should be performed by qualified industrial hygiene personnel. It is an overall way of checking to see that the system is in good working order. If air sampling shows that the operation is not running up to the standards recorded just after The Startup Phase, an investigation by engineering, production and maintenance personnel should be performed to determine the causes.
Periodic air sampling at production operations serviced by the industrial exhaust system has many benefits:
- A continuous level of safety will be maintained.
- Maintenance costs will be minimized.
- Costly shutdowns of the production line may be prevented.
- Good relationships with state and local agencies will exist.
- Corporate image and reputation with the public will be enhanced.
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