December 10th, 2025 | Posted in Air Cleaners

What You Need to Know About Effective Source Capture

Maintaining air quality in many manufacturing facilities can be a challenge. Often, the sources of airborne contaminants are spread around the shop making general, or ambient, air cleaning infeasible. In these cases, in the air cleaning industry, we often use Source Capture (also known as Local Exhaust Ventilation or LEV) to capture airborne contaminants. Effective source capture air cleaning is particularly appropriate in the case of exceptionally harmful contaminants such as hexavalent chromium created by welding chromium-containing alloys like stainless steel. Obviously, if we’re dealing with exceptionally harmful substances, we want the LEV system to be really effective or we’re just wasting our money and potentially over-exposing our employees.

To meet this need for effective airborne contaminant control, Air Quality Engineering Inc. offers a wide range of both fixed and portable air cleaning equipment designed for source capture of particulate airborne contaminants such as welding fume. The figure a few paragraphs down shows an illustration of a representative source capture air cleaner. The air cleaning equipment is relatively simple in design concept but is quite demanding in terms of design detail and manufacturing execution. Basically, these units consist of:

• an air intake designed to be reasonably close to the contaminant source
• a cabinet (in this case on wheels)
• an appropriate filtration system
• a correctly sized blower

The Misunderstood Science Behind Airflow

The primary concern in operating this equipment and achieving effective source capture is often operator misunderstanding of how airflow really works. There seems to be a tendency to rely on “Suction” to “reach out” and capture the contaminant. However, have you noticed that so-called “Suction” doesn’t reach out very far? Seriously, think about it: when using a vacuum cleaner don’t you have to get the intake nozzle on top of whatever you ‘re trying to clean up? Like within a couple of inches. When using an industrial LEV system, the effective capture area is seldom greater than about 1 to 1.5 capture hood intake diameters away from the hood intake. Usually this in the range of about 20” or so. If the intake is much farther than that that, the capture effect just seems to dissipate, the contaminants float away from the LEV intake, and the ventilation system is no longer useful beyond that distance.

If you use your hand as gauge of airflow, you’ll note the same thing: If you place your hand really close to the intake, the airflow is quite noticeable, but at about 1 to 1.5 hood intake diameters away, you can no longer feel the airflow very much, if at all, However, if you place your hand in the exhaust stream from the blower (or vacuum cleaner) you can sense that air flow for quite some distance; Perhaps as much as 30 hood intake diameters away. Why is this? Is there really any such thing as “Suction?”

Well, no, there really is no such thing as “Suction.” “Suction: is a word to describe a phenomenon that doesn’t exist, like “spare time” or “extra money.” Have you had either of these lately? Your blogger sure hasn’t. If there is no such thing as “Suction” what going on? The figure below, from the ACGIH (modified to show an actual Air Quality Engineering, Inc. Model M66V LEV) illustrates the phenomenon.

What Really Happens Inside an LEV System

In this unit, the blower (or fan) is in the upper portion of the M66V. The fan blades are mechanically grabbing masses of air. Each fan blade “takes a bite” out of the air in the duct on the intake side of the fan/blower. That air is pushed out of the unit by the fan. In the drawing this air is labeled as “Exhaust”. That describes what happens on the “blow side.” Notice in the drawing that sensible airflow is available up to 30 duct diameters (30 D) from the fan outlet. However, we’re more interested in the intake side because that’s where we have our source of airborne contaminant that we want to control.

On the intake side, we only get sensible airflow velocity up to one to one-and-a-half intake diameters from the hood intake. Why the huge difference? It’s because the fan has NOT imparted energy to the air on the intake side. On the intake side, all the fan/blower has done is remove some air from the duct, creating a low-pressure area due to the removal of some of the air – Again, No energy is imparted to air, there is just less air at the intake because the fan/blower blades are doing their job. Because there is a low-pressure area at the intake, the surrounding air just flows into the low pressure area from ALL directions, as can be visualized by the airflow arrows around the intake.

If you observe water draining out of a bathtub, you’ll note that there is truly little significant water movement until the water is very near the drain, then the water speeds up and rushes in the drain. In the case of the drain, the water is propelled by gravity. In the case of our LEV , the air is propelled by the pressure differential between the outside air and the air in the duct. The air actually gets PUSHED into the duct from all directions without the benefit of having any energy imparted to it. The air isn’t “Sucked in”: It’s PUSHED in! That’s why there’s very little “reach” in an LEV contaminant capture scenario.

How to Ensure Real Source Capture Performance

Clearly then, effective use of source capture equipment requires paying attention to the placement of the air cleaner intake. This is true regardless of brand (physics is not brand sensitive!) and whether or not the captured air is blown out through the roof or cleaned and recirculated, saving on utility bills. Any way you look at it, effective source capture is dependent on intake placement relative to the contaminant source.

Another factor that can adversely affect effective source capture, especially in welding shops, is the use of shop comfort fans arranged to blow on the welder and work piece. While having a breeze blowing on the welder on a hot humid day is a good thing, excessive air flow over the work piece often disturbs the gaseous shield necessary for adequate weld quality and is not a good thing.