Sponge or Glass? The Difference Between ePTFE Membrane Cartridges and Classic Cartridges.

The difference between deep filtration and surface filtration is just a single line in the catalogs. In the field, however, it's the entire lifespan of the unit.

Consider two cartridge filters. Both boast impressive efficiency figures in their catalogs, both fit into the same housing, and from the outside, they are virtually indistinguishable. You install one, and the unit operates with consistent performance for months. You install the other, and it works flawlessly for the first few weeks; then the pressure loss gradually increases, suction weakens, jet-pulse cleaning doesn't work as well as before, and finally, one day, the welding operator complains, "This machine isn't sucking up the fumes." The difference isn't the quality of the filter paper; it's the physics of filtration. And this difference determines everything if you're dealing with submicron particles like welding fumes.

Depth Filtration: It Works Like a Sponge

Classic cellulosic or polyester cartridges work on the principle of depth filtration. Particles penetrate into the media, which consists of interwoven fibers, and are trapped there, between the fibers. Imagine immersing a sponge in dirty water: The dirt doesn't stay on the surface of the sponge; it penetrates deep into the pores. Even if you squeeze the sponge, some of it remains inside. Most particles in welding fumes are smaller than one micrometer, typically between 0.05 and 1 micrometer. Particles of this size are so deeply embedded in the fibers that the jet-pulse pulse cannot completely dislodge them. After each cleaning cycle, a certain amount of permanent charge accumulates in the media. Consequently, the base pressure loss starts slightly higher each month. In engineering, we call this the “aging” of the filter. This is an irreversible process, and the end is always the same: filter replacement.

Surface filtration: Like snow on glass

In an ePTFE membrane cartridge, a very thin layer of expanded PTFE, whose pores are smaller than most of the particles, is laminated onto the media. The particles cannot penetrate the media, accumulating on the surface of the membrane to form a dust cake.

Like snow falling on a glass surface: it stays on top, it doesn't penetrate the glass. When the jet-pulse pulse hits, the cake separates from the surface in a block and falls into the dust collection hopper. This has three practical consequences.

1. Cleaning actually cleans; 1. Base pressure loss remains close to its initial value for a long time.

2. Efficiency is high from the start; the low-efficiency initial phase that depth filters go through "until cake formation" is absent in membranes.

3. Less energy is consumed for the same separation efficiency. This is because the resistance the fan has to overcome is lower, and the cost of pressure loss is repaid every month in the electricity bill.

So why doesn't everyone use ePTFE membranes?

To be honest, membrane cartridges are more expensive and not the right choice for every application. In oily, sticky fumes (for example, in processes where dense oil vapor is mixed), the membrane surface can become dull and lose its advantage; different solutions are needed there. Also, in a comparison looking at the purchase price, the classic cartridge always wins. But when looking at the total cost of ownership (including cartridge replacement frequency, downtime, energy consumption, and waste disposal), the picture reverses in most resource applications.

We always recommend the same calculation to our customers: Compare the cost of one cubic meter of clean air over three years, not the price of the cartridge.

Single stage or two stages?

At this point, another design question arises. For years, the common practice was a two-stage system: a pre-filter at the front (e.g., class F9), and a HEPA cassette at the back. In theory, this seems logical. The pre-filter removes the coarse load, and the HEPA provides the final finish.

However, this architecture has a hidden weakness for welding fumes:

Since the actual mass of the fumes is sub-micron, it largely passes through the pre-filter and is loaded directly into the HEPA cassette. The HEPA cassette, however, cannot be cleaned with a jet-pulse system; it is discarded when full. Thus, the most expensive component of the system suddenly becomes the fastest-consuming consumable.

The high-efficiency membrane cartridge changes this equation: It is both cleanable and can be manufactured in a class that alone can carry the system efficiency required by the W3 level.

For this reason, we have based our designs on a single-stage membrane architecture. In applications where air is returned to the environment (recirculation) (some countries' regulations may require this separately), an additional H13 safety cassette can be placed at the outlet. In the industry, this is called a "police filter," and its primary function isn't filtration; it's to be the last line of defense in case of cartridge damage.

Once the filter selection is correct, the next step is determining how much air the fan behind it actually draws in. We'll explain in the next article why the flow rate figure in the catalog is never exactly the same in the field. The key is the fan curve, a sloping line that tells a lot to those who know how to read it.