An air filtration system performs two functions: it must create the minimum required airflow needed by the laser cutter, and it must be able to filter the air volume within that flow with some degree of effectiveness.

Every laser cutter has a specific minimum requirement for exhaust extraction that will typically be specified in CFM (cubic feet per minute) or m^3/h (cubic meters per hour). Often, this exhaust airflow requirement is met by an external fan that produces a negative pressure that sucks the fumes out of the laser cutter enclosure. Without the required exhaust airflow, fumes and contaminants will linger inside the laser cutter or leak through the air intakes directly back into the work environment.

A filter element may only be able to remove up to a specific maximum amount of contaminants from a volume of air that flows through it. Sometimes, the amount of contaminant that can be removed from a volume of air is a function of the amount of time that the air is in contact with the filter element. (This is the case with activated charcoal filters.) Increasing the airflow through a filter may actually reduce the filter's effectiveness because the air is moving faster and is in contact with the finite area of the filter material for less time.

Minimum airflow and maximum filtering capacity must be closely matched for effective filtration. But unlike an exhaust air scrubber which must always be able to remove 100% of the contaminants in a single-pass of the air through the filter system, a venting filter has some flexibility. It is desirable for a venting filter to remove most of the contaminants from an airflow, but since it is assistive we can get away with less than 100% effectiveness, and we can probably get away with some level of degradation of effectiveness before we need to replace the filter elements. As long as the minimum airflow requirement is met, the filter system is still helpful even if the filter elements are not 100% effective.

Design Goals for the Fume Coffin

Provide effective assistive filtration for application in a laser cutter venting system.

Keep the cost of consumable filter materials to a minimum.

Make the system modular for design flexibility.

Operational monitoring and maintenance should be as easy as possible.

Existing Designs

There is very little information available on how to build a venting exhaust system. And there are very few commercially available venting filter systems targeting the small CO2 laser cutter market. Most commercially available exhaust filtering systems are exhaust air scrubbers designed to provide filtering systems where venting is not an option.

I took the previously mentioned Instructable exhaust air scrubber (IEAS) as a starting point for an effective and successful filter system for laser cutter exhaust. The IEAS system filters laser cutter exhaust with the following major elements in this order:

a fan to drive the system,

a pre-filter for catching dust and chips,

a HEPA filter element,

an activated charcoal filter element,

a post-filter to capture charcoal dust.

I figured I might be able to convert the design into a venting filter system by enclosing an IEAS filter system in an airtight box with a single exhaust duct.

The Fume Coffin did not need a post-filter since the exhaust would not be venting directly into the workspace, and charcoal dust in outdoor vented exhaust is not a major concern.

I also considered ways to drive the system with negative pressure from a fan at the cleaner exhaust-end of the airflow path.

The Fume Coffin's design ultimately ended up taking a different form from the IEAS, but it has similar major elements -- a pre-filter, a HEPA filter, an activated charcoal filter, a powerful fan to drive the system.

HEPA Filters



The HEPA filter is one of the major filter elements in the Fume Coffin filter system.

A HEPA filter captures sub-micron aerosol particulates.

An excellent source of HEPA filtering is replacement filter cartridges for the many types of commercial HEPA filter systems. The IEAS uses a Honeywell replacement cartridge. The replacement cartridges for 14" diameter Honeywell-style filters are very good values because of their large cylindrical surface area.

But Honeywell has discontinued their HEPA filter systems that use these 14" cylindrical cartridges. The company that makes Honeywell air filters, kaz.com, no longer makes replacement filters for all the 14" cylindrical models. Instead, they make a single 3" high by 14" diameter filter ring that when stacked in multiples fits most of their discontinued filter systems. I too could stack multiple 3" high rings to create my HEPA filter element, but more parts would bring more mechanical issues and more opportunities for air leaks.

It turns out there are numerous 3rd-parties who make HEPA replacement cartridges for many of the discontinued Honeywell HEPA systems. It is likely they will remain available for many years while those formerly popular filter systems are still operational. The 10" high by 14" diameter model 24000 cartridge is a good value. (It provides 290 sq in of interior surface area for under $50.)

The 24000 filter cartridge can be easily built into a complete HEPA filter element by sealing the bottom and top of the cylinder and adding a duct port into the interior area.

The Pre-Filter

A pre-filter to catch dust particles and prevent the HEPA filter from becoming prematurely clogged can be added by lining the interior area of the 24000 cartridge with air-conditioner filter cloth (such as 3M Filtrete A/C Filter) cut to size.

Addendum for New Pre-Filter

After building the Fume Coffin I designed and built this optional improved Pre-Filter for it.

Fume Coffin Pre-Filter Addendum

Activated Charcoal Filters

The activated charcoal filter is another main filter element in the Fume Coffin system.

The activated charcoal filter element adsorbs fumes and gases.

Because the activated charcoal filter works on gases you want to position it after you have removed dust and aerosol particulates from exhaust airflow with a pre-filter and a HEPA filter respectively.

The IEAS employs an activated charcoal filter made from about 20 lbs of hand-packed bulk activated charcoal from coconut shells.

Comments in the IEAS discussion led me to investigate the many pre-fab "can-style" activated charcoal filter systems that are popular in recirculating air filtration systems used in cannabis grow houses. These filters have the advantages of being readily available, factory-packed and modular. Factory packing should produce more efficient filtering because it reduces the likelihood of preferred air-channels developing through the charcoal bed. (Also, hand-packing a filter with bulk activated charcoal is very messy. And safe handling and disposal of contaminated activated charcoal can be problematic.) Many can-style filter models are available with sufficient amounts of charcoal for effective LGAC filtering, and because they are factory-packed with high-performance coal they should be more efficient per pound of charcoal than bulk activated charcoal.

Here are two can-style filter product lines:

The Can 33 filter from Can Filters is a good solution for the activated charcoal element. It has 16.5 lbs of activated charcoal in a 2" cylindrical bed. The activated charcoal is made from bituminous coal. The Can 33 supports a 200 CFM (350 m^3/h) "exhaust airflow". ("Exhaust airflow" refers to the application of the filter for single-pass 99.9% filtration. Single-pass 99.9% filtration requires the contaminated air remain in contact with the charcoal bed for 0.1 sec.) The Can 33 is a 12" diameter by 13" high cylinder. And it is often available from Amazon for a discounted price.

Filter Fans

My 50 Watt laser cutter came with a China-sourced squirrel-cage-type exhaust fan that runs at about 115 CFM as measured by a hand-held anemometer. This fan is intended for direct venting to the outdoors. It also uses a lot of power.

There are many fans available for exhaust venting and filters. But it's often hard to find much detailed information about most of them.

Can Filters sells a range of fans for use with their filters. Their Max Fan is a German-made fan with three speeds. It is distinctive in its consistent performance even under the high static pressure (1.25 to 1.4 inches wg). It also has a 5-year warranty.

Note: Testing of the final build reveals that the Max Fan 6 produces 100 CFM net exhaust airflow with the Fume Coffin filter system and drawing from my laser cutter. If your laser cutter requires more than 100 CFM exhaust airflow you should consider a more powerful fan or multiple fans in the airflow path. A more powerful fan will most likely have to be an 8-inch fan. It is critical that any fan you choose specifies that it can create your laser cutter's required exhaust airflow at 1.25 to 1.4 inches wg of pressure. (Often the numbers you see in the advertising for fans indicate airflow with little or no resistance pressure. These specifications for low-pressure airflow can't be used to predict the performance of the fan driving a laser cutter exhaust and filter system.)

Put all the Things in a Box

I decided to put a Can 33 activated charcoal filter and a Honeywell 24000 HEPA filter cartridge into a sealed box and drive the system with a Max Fan attached to the exhaust flange of the Can 33.

The exhaust airflow encounters the major elements of my filter system in this order:

pre-filter - A/C filter cloth lining inside of HEPA cartridge,

HEPA filter - made from a Honeywell 24000 HEPA replacement cartridge,

activated charcoal - using a pre-fab Can 33 activated charcoal filter cylinder,

fan

The Honeywell 24000 would be held in place and sealed by sandwiching the filter cartridge between a circular 1/4" plywood panel on the bottom and the Fume Coffin lid on the top using 12" threaded rods running through the cylinder.

The Can 33 activated charcoal filter element would be mounted inside the Fume Coffin box with its interior ported to the outside of the box.

The Max Fan would be attached to the out-port of the Can 33 filter element and drive the entire system by creating negative pressure inside the Can 33.

The fan sucks air from the interior of the activated charcoal filter cylinder. Thence a negative pressure vacuum is created in the open interior area of the sealed Fume Coffin box. Air is drawn through the pre-filter and HEPA filter from the interior of the HEPA cartridge cylinder. And the 4" duct in-port into the HEPA cylinder leads back to the laser cutter exhaust.

Driving the system with negative pressure should discourage contaminated air leaks into the work environment.

Because the fan is located at the clean end of the filter system, the fan blades and motor will have less contact with contaminated exhaust, and therefore the fan will be less likely to become fouled over time.