Two of the hottest markets today are laser marking and low-power lasers for cutting. Financial justification for these laser purchases is typically based upon a laser's ability to perform a number of specific, well-defined operations at a specified unit cost—date a plastic bottle, identify a manufactured part or cut an intricate design. These lasers produce a high-quality result that often leads to increased laser utilization and the introduction of multiple lasers.
This proliferation of laser use, specifically when multiple substrates or lasers are utilized, may lead to laser-generated air contaminant (LGAC) collection challenges that may affect the performance of the manufacturing process. When properly selected, small self-contained unitary air cleaners for marking and cutting applications provide efficient economic extraction. However, problems often arise when an extractor is purchased without considering the types or amounts of LGACs to be generated—potentially resulting in excessive downtime, noxious odors, high filter replacement costs and damage to expensive laser optics. These pitfalls can be avoided by proper planning in the design stage of the project or by the addition of system enhancement equipment retrofitted to existing air cleaning equipment.
The design of a filtration system can be broken down into three components: LGAC analysis, system specification and cost analysis. Each component is critical to the effectiveness of the extractor with implications on long-term efficiency.
To analyze the type and amounts of generated process particles, focus on the current application and any potential productivity increases or future material changes. The following is a recommended procedure.
Evaluate the material safety data sheets (MSDS) for the materials to be processed for any potential chemical reaction due to laser processing. Review the data for the presence of hazardous, toxic, flammable or explosive materials and determine potential odor-generating compounds and thresholds. Then establish the allowable OSHA exposure levels for all gases and dusts.
Determine the particle type and rate of generation by plume analysis—dry, wet, oily or electrostatic. Then determine the operating parameters: size of mark or weld, number of operations per hour and number of operations per day/week/year.
Figure 1. Cyclone efficiency, as show in a typical dust recovery curve.
This data is used to select the size and type of filter media required and it will highlight potential safety hazards, allowing time to implement proper design changes. The type and amounts of LGACs will form the basis for the second stage of the process—determining the size, type and options of the air cleaning system.
Unitary air cleaners
Due to limitations in blower size and available filtration capacity, a one-to-one laser-to-air-cleaner ratio is recommended for most unitary air cleaners. However, a well-designed system can be extended beyond this range, but usually not more than a two-to-one ratio. The viability of central units should be evaluated if multiple lasers are installed in the future or there are significant increases in production.
Unitary units are manufactured with blowers in the 100- to 250-cfm range with available static pressure of 4 in. to 35 in. of water. First determine the plume capture method, source or plenum. Source extraction utilizing a 2-in. or 3-in. flexible hose with a small "funnel hood" located 1 or 2 inches from the product has become common practice for most marking applications, while welding and cutting applications typically utilize a small plenum. Recommended airflow for source extraction is usually 60 to 100 cfm per pick-up point and larger volumes for plenum capture. The proximity of the air cleaner to the extraction point should not exceed 15 ft. Consult your air cleaner manufacturer for assistance and evaluation of the type of capture and length of ductwork and the required cfm and static pressure.
Next determine how to exhaust the treated air—recirculate into the facility or duct the treated air outdoors. Highly toxic gases should be terminated outdoors. Finally, determine the operational features of the unit, such as remote start/stop, PLC filter monitoring, VOC indicators and airflow verification.
Central units typically consist of blowers in the 300+ cfm range with available filter surface area of upwards of 220 ft2. Most central units are manufactured with centrifugal blowers that provide high airflow volumes with medium static pressure or with regenerative blowers that provide high static pressure with lower air volume. When sized correctly both types can extract from multiple pick-up points spaced across a large production environment.
The size and design of the ductwork and/or plenum is important, because effectiveness will be greatly reduced if improperly sized. Self-cleaning systems utilize a mechanical shaker or a burst of compressed air to clean the filters.
Once the analysis and design are completed a cost evaluation is possible. Unitary systems generally range in price from $500 to $5000 and central systems from $4,000 to upwards of $10,000.
Unitary systems typically consist of multi-stage filtration, including a dust-bag, a HEPA filter and an odor-controlling filter, each having a finite life and associated replacement cost that should be evaluated. A general guideline for filter life is approximately 1 to 3 months for the primary filter, 3 to 6 months for a HEPA filter and 3 to 9 months for an adsorbent cell. Average time for a filter change can range from 2 to 10 minutes and it is important to factor in the production downtime for filter maintenance.
Figure 2. A cyclone is used to separate large particles from the processing system's exhaust.
Central systems typically have lower maintenance costs, but the capital outlay is higher. However, they provide proportionately longer life. Replacement filters range in price to $300 depending on the size and media type. Washable and disposable filters can be specified and, depending on the filter media and the type of particle, a filter life of 6 to 12 months can be expected. Filters can be replaced on average in 5 to 10 minutes.
As outlined, the implementation of air cleaners for the capture and containment of LGACs is not difficult if sufficient planning is conducted in the design and budgetary stages of a project. Increases in particle loading—through additional lasers or increased throughput—of only 20 percent can greatly reduce the filtration capacity of an air cleaner, transforming a once low-cost, low-maintenance system into a "filter eater" with high maintenance costs and unacceptable downtime.
The introduction of new substrates can also cause unexpected problems. A change in the chemical composition of the material being processed can lead to premature "blinding" of filters or the ineffectiveness of odor-controlling cells. Fortunately, the extension of an air cleaner beyond the original design parameters does not necessarily signal the end of an extractors' usefulness as often utility can be restored with the addition of supplementary equipment.
A cyclone uses centrifugal force to separate larger particles from a process plume. This force holds the particles against the wall until they eventually fall into an easily emptied capture drum. Clean air at the center of the chamber passes into a tube and exits at the top outlet. Cyclones have long been used for collecting heavy amounts of large particles and, when coupled with a high-efficiency air cleaner, they substantially increase an air cleaner's performance and significantly reduce overall operating costs.
Most high-quality air cleaners utilize high vacuum blowers selected to allow flexibility across a wide range of applications that will provide for increases in future loading and will accommodate external filter attachments when required. Gases and odors can be controlled with 50- to 100-lb. drums filled with carbon or other adsorptive media. These attachments can usually be retrofitted to the existing units, thereby increasing capacity upwards of 5 times with very little additional costs.
A variety of filter media is available for both unitary and central extraction systems. If changes in material from the original specification are made an evaluation of the MSDS or an examination of a particle sample will provide the needed information to correctly select the appropriate filter media. Most times the new filter media can be retrofitted with no additional costs or equipment.
Fortunately there is no shortage of quality air cleaners that can efficiently and cost effectively accomplish the required task. The challenge is in identifying potential problems early in the design stage or purchasing a system with the flexibility to meet the changing requirements of today's manufacturing.
Kevin East is the sales and marketing manager for Fumex. He can be contacted at _keast@.fumexinc.com or via telephone at (800) 432-7550.