Applications for Source Capture Vs. Ambient Air Capture

March 7th, 2025 | Posted in Air Cleaners

At Air Quality Engineering, we understand that air cleaning isn’t free. The less air you have to clean, the less money you’ll spend keeping the plant air breathable. Since source capture (also called “local exhaust ventilation” in the textbooks) allows one to capture and clean the minimum amount of air necessary to achieve that goal, source capture is the preferred solution when it can be implemented. One of the better examples of source capture is applying an air cleaner to a machining center cabinet; the cabinet is already in place to control coolant spray from filling the shop, so adding an air cleaner to the cabinet is often an easy installation. A cabinet-mounted air cleaner also has the benefit of allowing the collected coolant to drip back into the enclosure for reuse.

Source Capture Applications

Multi Machining

When there are multiple machining centers to be addressed, another take on the same idea is to use a large centrally mounted air cleaner with ductwork running to the various machine tools. Since economies of scale apply, a single large unit will clean more air per purchasing dollar than will several individual air cleaners. This initial economy at purchase time is appealing even though installation requires a carefully designed and installed ductwork system.

The ductwork system is the downside of the central system approach: they always seem to leak, dripping coolant on the shop floor and creating a slip hazard for people and forklifts. It should also be borne in mind that adding a new machine tool requires more than just adding a drop to the ductwork. That new drop will adversely affect the carefully engineered balance of the airflow through the ductwork and result in ineffective airflow and reduced mist capture efficiency. Adding an additional machine tool to a ducted system requires redesigning the ductwork system to ensure balanced airflow.

By comparison, adding a machine tool in a facility with cabinet-mounted air cleaners simply requires mounting an air cleaner to the new machine tool without having to re-engineer a ductwork system. In fact, cabinet-mounted air cleaners are often specified when purchasing new machine tools.

Welding Fume

Another application of source capture for air cleaning is welding fume. Since the source of the fume (the welding arc) is highly localized, a source capture system is the preferred means of capturing the welding fume before it fills the shop with that haze that seems to be the rule in many shops.

Capturing the fume at the source can be problematic, however. This is because the effective capture zone for the air cleaner is relatively small – about the size of a soccer ball or even smaller depending on the intake diameter and airflow. For a source capture application to be effective for welding operations, the ventilation intake must be moved frequently to keep the arc within the effective capture zone.

Care must also be taken to avoid placing the intake too close to the arc and thereby disturbing the gaseous shield that protects the molten weld pool from contact with the atmosphere and consequent decreased weld quality. Due to the need to keep the intake “not-too-close-but-not-too-far,” the intake is often not ideally placed, and fume capture efficiency is compromised. Recognizing this conundrum, it may be necessary to install ambient capture air cleaners as well as source capture air cleaners.

Air Capture Applications

Ambient air capture air cleaners are hung from the shop ceiling, often in daisy-chain fashion, and continuously clean and circulate the filtered air through the shop. Ambient capture is useful when there are fugitive emissions from welding operations or machining operations that cannot be collected by source capture applications. Ambient air capture is easy to install – just hang the air cleaners from the ceiling structure in a pattern that provides for thorough mixing of the shop air.

The downside of ambient air capture is that it requires cleaning much more air than source capture and therefore results in higher purchase costs for the multiple units necessary to create the desired circulation pattern.

Sometimes there is just no reasonable way to capture the contaminant at the source: cigarette smoke is an example. There are several (expensive) ashtrays available that claim to capture cigarette smoke, and they may work as claimed, but what about exhaled cigarette smoke? Is the smoker expected to exhale slowly into the ashtray? Hardly. Ambient air cleaning has been found to be the most realistic solution to cigarette smoke.

There are applications that exhibit features of both source capture and ambient air capture, examples being canopy hoods working with vertical airflow air cleaners (e.g. Air Quality Engineering, Inc. F 62B) over electrical discharge machining (EDM) units or over injection molding machines.