Fume Coffin - Laser Cutter Exhaust Vent Filter

"The containment and removal of laser-generated air contaminants is critical, even for small compact laser cutters."
- Daniel Herrick and Robert Klein.
"Emerging Health and Safety Issues in Makerspaces"
Proceedings of the 1st International Symposium on Academic Makerspaces, 2016

~•~

Laser cutters produce a lot of nasty, smelly fumes and contaminants when cutting and engraving even common materials. Collectively these gaseous fumes and contaminants are referred to as laser-generated airborne contaminants -- LGACs.

Even if you limit yourself to only cutting plywood and acrylic the LGACs include toxins and carcinogens like:

• Benzene,
• Ethyl acrylate,
• Formaldehyde,
• PAHs.

Since the LGAC exhaust output from all laser cutters is effectively poisonous to humans, one must do something with the bad exhaust air in order to operate a laser cutter safely. There are three general solutions:

• Exhaust Air Scrubbing - employ an air filtration system to completely remove LGACs from the laser's exhaust output. The LGACs in the exhaust air are captured by the scrubber, and the processed clean air is released back into the work environment where it will be breathed by human operators.

• Venting - pump the exhaust air including LGACs through ducts and vent directly into the outdoor environment.

• Filtered Venting - partially or completely filter LGACs from the exhaust air before pumping the air to the outside.

The Fume Coffin performs this last type of solution -- it is designed to assist with filtering the exhaust vented from a 50 Watt CO2 laser cutter.

Airflow tests of the completed build indicate the net airflow of this system is 100 CFM. The measured value of 100 CFM meets the minimum exhaust airflow requirement of my particular laser cutter. But if you need more airflow you should consider modifying this Instructable with an upgraded fan with more airflow at high resistance pressure or adding a second fan to the airflow path to act as a booster.

Although it is similar in operation to some exhaust air scrubbers (such as this popular Instructable from which it was inspired), it is not intended to produce clean and breathable air from the exhaust of a laser cutter for recirculation into the work environment. (Why anyone would ever consider breathing the exhaust from a laser cutter is beyond me.)

The Fume Coffin materials cost about $350 including the consumables: a CanFilters brand Can 33 activated charcoal filter and pre-filter (about $105) and a Honeywell 24000-compatible HEPA cartridge (about $50).

Step 1: Design - CFMs, Filter Elements and a Fan

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:

• Phresh Filters,
• Can Filters.

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.

Step 2: Materials and Tools

Bill of Materials

• CAN 33 Activated Charcoal Filter (includes Pre-Filter). (Amazon)
  • cut pre-filter to 10-1/8" by 30-3/4" (258mm by 780mm).
• Can Filter 6" Steel Flange (includes weatherstrip and 3/4" sheet metal screws). (Amazon)
• Can Fan Max Fan 6". (Amazon)
  • Note: Testing of the build indicates that the Max Fan 6 is capable of producing about 100 CFM net airflow through the Fume Coffin system. If you need more than 100 CFM exhaust for your laser cutter you should consider upgrading the fan to a model with more airflow at 1.25 to 1.4 inch wg. resistance. Alternatively, it may be possible to increase net airflow by adding an additional fan as a booster, for example, between the laser cutter and the 4" intake port in the lid.
• Honeywell 24000-compatible replacement HEPA filter (by Global Filters). (Amazon)
• 1/2" (12mm) both-sides-smooth plywood. (need approx. 64"x48" (1626mm x 1220mm)). (lumber supply)
• DryerDock 4" Vent Hose Quick-Connect (HomeDepot) (Amazon)
• (3 ea.) #6 - 1/2" wood screws for affixing DryerDock into lid panel. (hardware store)
• Bottom for HEPA Filter - 14.25" x 14.25" - 1/4" (362mm x 362mm - 5mm) plywood. (lumber supply)
• 6"-to-4" galvanized duct reducer. (hardware store)
• (4 ea.) 12" (305mm) 1/4-20-threaded rod (usually comes in 12" or 36" lengths). (hardware store)
• (4 ea.) 1/4-20 wing nuts. (hardware store)
• (4 ea.) 1/4-20 cap-head nuts. (hardware store)
• (4 ea.) 1/4-20 lock-washers. (hardware store)
• (8 ea.) 1/4-20 washers. (hardware store)
• 242" (6150mm) 1x2 select grade wood for box frame (pine or similar). (lumber supply)
• (6 ea.) #10 - 1" stainless steel sheet metal screws (to mount Can 33). (hardware store)
• Approx 100 - #6 - 1-1/8" fine thread drywall screws for assembling box. (hardware store)
• (5 ea.) Toggle Latches (to hold lid tight). (Amazon)
• Assorted screws, washers, and bolts for Toggle Latches. (hardware store)
  • (10 ea.) #8 - 3/4" sheet metal screws to attach latch catches to edge of lid.
  • (4 ea.) #8 - 3/4" sheet metal screws to attach latch hooks to right side panel.
  • (6 ea.) #6 - 3/4" bolts, washers and nuts to attach latch hooks to front, back and left side panels.
• 100" (2500mm) teardrop profile silicone weatherstrip. (hardware store)
• 60" (1525mm) 3/8" foam press-on weatherstrip. (hardware store)
• 1/2" (12mm) staples (for staple gun). (hardware store)
• Silicone-latex sealant caulk (e.g. DAP Kwik Seal Plus). (hardware store)
• 1 qt Varathane Triple Thick clear satin interior polyurethane. (hardware store)
• Loctite or super glue. (hardware store)

Tools

• Track saw and guide suitable for precise cutting of 1/2 inch plywood sheet,
  • alternate: a table saw,
• Saw horses or breakdown table for cutting plywood sheet,
• Track saw or miter saw for cutting 1x2 box framing lumber,
  • alternate: a crosscut saw and a miter box,
• Electric drill and assorted drill bits,
• Fein MultiMaster tool or saber saw for cutting holes in 1/2 plywood,
• Laser cutter capable of cutting 14.25" round HEPA Bottom from 5mm plywood,
  • alternate: cut HEPA Bottom into a 14.25" square using a traditional saw,
• 2" synthetic bristle paintbrush for Varathane,
• Staple gun,
• Scissors or tin snips for cutting filter cloth,
• Hacksaw for cutting threaded rod,
• Nailset,
• Screwdriver,
• Caulk gun,
• Tape measure
• Rule,
• Square,
• Block plane,
• Eye protection,
• Dust mask,
• Ear protection,
• Protective gloves,
• Hand-held anemometer (for measuring airflow).

Step 3: CAD Model and Actual Parts

In order to figure out how I could fit these parts into a box, I took the rough dimensions of the parts to Fusion 360 and designed an enclosure to be made from 1/2-inch plywood and a frame of 1x2s.

Originally I had both the HEPA cylinder and the Can 33 cylinder oriented vertically and positioned side-by-side. But I reoriented the Can 33 filter so that the fan would attach at the end of the box. This allows the Fume Coffin to fit in the area available under my laser cutter on its cart.

The Fusion 360 project is fully parametric. Parameters for the constraining dimensions of major parts and materials can be readily adjusted.

Once I received the parts I updated the part dimensions with actual dimensions. Likewise, lumber dimensions were updated with the actual dimensions. For example, the plywood was actually 12.0 mm thick rather than the 12.5 mm (.453" inches) from Home Depot's website.

Dimensions of all the parts can be pulled from the following interactive viewer link.

Fusion 360 Project - Fume Filter

Also, the complete CAD model can be downloaded from the link as a Fusion 360 archive. From there you can make changes to the model or tweak the parameters. The CAD model can also be downloaded in many other formats including SketchUp.

(I've also included DXF sketches of the plan and elevation to facilitate recreating the model in another CAD program.)

Step 4: Making the Box Panels

Note: Building the airtight box and lid was the hardest part of this project.

Cut the Panels

First, cut the panels to their final dimensions using a tracksaw on a breakdown table. I used a Festool TS75 with a FS1900 (75") guiderail which can cut the width of a 4-foot sheet of plywood. Take care to compensate for the kerf of the saw blade cut.

Alternate:

If you don't have a tracksaw, you can use a table saw with supports for plywood cutting or a circular saw with some sort of guide.

You will need to create the following six panels:

• lid & bottom: 803.4mm x 462.7mm
• front & back: 803.4mm x 352.4mm
• right & left: 438.7mm x 352.4mm

(I try to do my wood cuts in mm to avoid accumulation of error from translating between systems. Also, my laser cutter and tracksaw are metric.)

Cut the Holes in the Right Side and Lid

I tried to cut the holes in the right-side panel and the lid using my laser cutter, but my laser cutter was incapable of cutting through the 12mm plywood.

• Right Projection_mm.dxf,
• Lid Projection_mm.dxf

If you have access to a laser cutter that can cut the 12mm panels, these DXF files are provided to be imported into a laser cutter control program such as LaserCut or RDWorks to cut the holes.

Alternate:

You will need to lay out the positions of the 4" duct hole and the four surrounding 1/4" rod holes in the lid. Use the following dimensions or you can get their positions and spacing from the Fusion 360 project viewer.

Find the duct-center on the lid panel:

• Find the left-right centerline by marking half the width at the left and right ends and connecting the marks with a line.
• Measure along the left-right centerline and mark 231.8mm from the left edge of the lid panel.

Make the following holes in lid panel:

• 50.8mm (2.0") radius hole at the duct-center on the lid panel.
  
• 4 ea. 6.4mm (0.25") diameter screw holes 131.3mm from the duct-center at 45, 135, 225, and 315 degrees.
  

Likewise, you will need to lay out the position of 6" hole for the Can 33 flange on the right side panel. There are six screw holes that surround the 6" hole (at 12, 2, 4, 6, 8 and 10 o'clock) and hold the flange and Can 33 in place. Use the following dimensions or use the Fusion 360 project viewer to get dimensions.

Find the center of the panel by drawing two diagonals from the panel corners.

Make the following holes in right side panel:

• 67.2mm (3.0") radius hole at the center of the panel.
  
• 6 ea. 3.2mm (1/8") diameter screw holes 131.4mm from center at 0, 60, 120, 180, 240, and 300 degrees.
  

I had to resort to drilling out the perimeter of the duct holes and using a Fein MultiMaster tool to cut out the holes. Fortunately, the duct holes can be a little rough because they will be covered by flanges.

Apply a Coat of Polyurethane

When cut to size and with duct holes in place prepare the outside of the panels with a coat of polyurethane. We will paint the inside of the box and lid in later steps.

I used a water-based Varathane Tripple Thick polyurethane.

Step 5: Building the Box

Cut Box Frame Pieces

Using a tracksaw or a miter saw cut the 1x2 pieces that will make up the box interior frame.

You will need the following pieces cut to the following lengths:

• 361.9mm:
  • bottom-left
  • bottom-right
  • top-left
  • top-right
• 779.4mm:
  
  • bottom-front
  • bottom-back
  • top-front
  • top-back
• 314.8mm:
  • right-front
  • right-back
  • left-front
  • left-back
• 76.2mm (4 pieces):
  • Can 33 supports

Assemble the Box and Frame

Using the drill and the drywall screws assemble the box starting with the bottom panel. Be careful to allow space for the side plywood panels as you build. Check your fit and measurements at each step.

Create and Install the Can 33 Supports

I cut a 21-degree bevel in the 1x2 pieces that make up the Can 33 supports. Then I carefully mounted these at the popper locations measuring from the front and right edges (167.5mm and 50.9mm) of the bottom panel. I fastened the supports with 2 drywall screws each from the underside of the box.

Drill the Mounting Holes in the Flange

The right side panel should have 6 holes cut, or at least marked, where the sheet metal screws will go through the flange, through the side panel, and into the Can 33 filter.

If the holes were not cut through the plywood right side panel by the laser cutter, then now is a good time to drill them. Use a drill bit large enough to loose-fit a #10 screw such as a 13/64" bit.

Center the 6" flange over the right side panel. Mark the holes on the flange with a marker or nailset.

Drill holes through the flange using the 13/64" drill bit. Check that the holes in the flange are in line with the holes in the right side panel.

Apply Foam Weatherstrip to the inside of the flange. (Weatherstrip is included with the flange.)

Screw the flange into place temporarily with the six 1-inch sheet metal screws. (It may be loose. Don't worry. Use some tape to hold it in place while you work out the mounting for the Max Fan.)

Temporarily Mount the Max Fan onto the Flange

While we are here, place the Max Fan into the 6" flange. Position the Max Fan speed control at a convenient position toward the top edge of the side panel. (I positioned it at 45-degrees clockwise off of vertical.)

The flange is supplied with sheet metal screws.

Mark and drill four holes into the flange suitable to hold the Max Fan.

Drill four matching pilot holes into the Max Fan collar for the same screws.

Use the four sheet metal screws to mount the Max Fan onto the flange.

Mark the orientation of the flange with respect to the right side panel for future assembly.

Mark the screw locations and orientation of the Max Fan with respect to the flange for future assembly.

Unscrew the Max Fan and the flange for now, and set the fan, the flange, and the screws aside.

Complete the Box with a Coat of Polyurethane
Apply a coat of polyurethane to the inside and edges of the box.

The polyurethane will help make the box enclosure airtight.

Step 6: Building the Lid and the HEPA Filter

Assemble the Lid

Assemble the frame to the lid panel. Make sure there is enough room for the HEPA filter cartridge to fit inside the frame. Make sure the lid fits onto the lower box assembly - not too snug, but not so loose as to create gaps where air might leak. Leave space for the weatherstrip that will be attached to the lid panel lip.

Apply a Coat of Polyurethane

Apply another coat of polyurethane to the assembled lid as needed. The polyurethane will fill the gaps between the panels and the frame pieces and help with the airtight seal of the Fume Coffin.

Cut the HEPA Filter Assembly Bottom

This piece will go inside the Fume Coffin and serve as the bottom of the HEPA filter assembly sandwich.

I cut my HEPA Bottom panel from a piece of 5mm (1/4") Revolution Ply. I used the file HEPAbottom_mm.dxf to control the laser cutter.

Alternate:

If you don't have a laser cutter that can cut a 14.25" by 14.25" piece of plywood you could cut a 14.25" square piece of plywood with a traditional saw.

Remember to drill 1/4" holes in the HEPA Bottom piece for the threaded rods.

Apply a Coat of Polyurethane to the HEPA Bottom

Allow to dry.

Cut the Pre-Filter to Size and Insert It Inside HEPA Filter Cylinder

The Can 33 filter comes with a pre-filter that wraps the cylinder. We don't need this pre-filter around the activated charcoal. We want our pre-filter to come before the exhaust air hits the HEPA filter.

So we will cut the pre-filter to fit inside the HEPA filter cylinder.

Use scissors or tin snips to cut the pre-filter to 10-1/8" by 30-3/4" (258mm by 780mm).

Place the pre-filter inside the HEPA filter cartridge.

Addendum: Optional Improved Pre-Filter

I developed the following Instructable for an optional, improved pre-filter design that places the pre-filter inline ahead of the Fume Coffin in the exhaust airflow. It is easier to monitor and service.

Fume-Coffin Pre-Filter

Cut the Threaded Rod as Needed

We need four 12-inch lengths of 1/4-20 threaded rod. I purchased 36" lengths. So it was necessary to cut these to 12" with a hacksaw. Be careful not to damage the threads where the nuts must attach. If necessary deburr the ends of the threaded rods.

Attach the End-Cap Nuts to the Threaded Rods

Attach an end-cap nut to each 12" threaded rod. I applied a drop of Loctite (or super glue) to the inside of the end-cap nuts. We don't want these nuts to come loose.

Assemble the HEPA Filter

Place the lid upside down.

Place the HEPA filter cartridge into the frame of the lid. It should be snug on the left, front and back. The rubber gasket of the filter cartridge should make a solid fit against the inner surface of the lid. If it does not, apply a bead of silicone caulk to the filter cartridge gasket where it comes in contact with the lid. (Any time you replace the HEPA filter cartridge you will need to check and recreate this seal as necessary.)

Add a washer and pass each threaded rod up through the top face of the lid into the center of the HEPA cartridge cylinder.

The rods must run on the inside of the pre-filter material.

Place the bottom 1/4 plywood piece on top of the sandwich.

Using two fingers guide each rod through the bottom piece, and while holding the rod in place from the end add a washer, a lock washer and then a wingnut.

When assembled all four rods should run from the top face, outside of the lid into the center of the HEPA filter cylinder and out through the bottom piece of the filter assembly.

Shift and adjust the assembly as needed to make the 4 wingnuts square with the lid. Observe through the 4" duct hole in the top to make sure the pre-filter material is in place between the rods and the HEPA filter inner surface.

Tighten the wingnuts, but don't crank them down too tight. The bottom, filter cartridge, and lid should create an airtight assembly together. If necessary, add some silicone caulk to the top and bottom gaskets of the filter cartridge to create a tight seal.

Install the DryerDock Quick-Connect

Apply a bead of silicone caulk to the underside of the DryerDock piece with the screw holes.

Use the wood screws to affix the DryerDock quick-connect port to the top of the lid.

Attach Teardrop Silicone Weatherstrip to Lid

Cut the teardrop-shaped weatherstrip to fit the lip of the lid panel.

Remove tape and apply weatherstrip in place. Secure weatherstrip with a few 1/2" staples.

Check that Lid Fits

It should be snug, but easy to remove, and it must create an airtight seal.

Step 7: Mounting the Activated Charcoal Filter

Drill the Mounting Holes into the Can 33 Filter

Place the Can 33 filter onto the supports in the lower box.

Press the Can 33 opening up against the right side panel aligned with the 6" hole.

Mark the locations where the screw holes in the right side panel will hit the end of the Can 33 cylinder.

Drill pilot holes into the Can 33 cylinder using a 5/32 drill bit. Check that the holes in the flange, side panel and Can 33 are aligned.

Apply foam weatherstrip onto the end of the Can 33 cylinder to assure a good air seal.

Mount the Can 33 in Place

Using the six stainless sheet metal screws mount the Can 33 in place by screwing through the flange, through the right side panel and into the Can 33 pilot holes. (You may need to pre-screw the screws into the Can 33 and unscrews them before your try to assemble all the pieces.)

Remount the Max Fan onto the Flange

Reattach the Max Fan to the flange using the 4 sheet metal screws and the holes we pre-drilled in Step 5.

Step 8: Installing the Toggle Latches

The Fume Coffin is almost done!

Attach the Toggle Latch Hardware

The toggle latches hold the lid tight and secure.

There are five toggle latches: one for each panel except the right side panel which gets two. (The right side panel is obstructed by the flange and the fan so it gets two latches for symmetry.)

Caution: Mounting and adjusting the toggle latches is a bit tricky. You might want to experiment with one on another couple pieces of wood to get familiar with how the mechanism works before you try to mount and use them on your Fume Coffinpanels.

The latch catches and latch hooks have to be mounted in the same plane. That leaves few options for locating the latch hooks except onto the edges of the plywood lid panel. I chose to use fairly hefty #8 - 3/4" screws to mount the latch hooks. I used a variety of screws and bolts to mount the latch catches to the panels. Where there is framing behind the latch, on the right side panel, I used #8 - 3/4" screws. On the other panels, I used #6 bolts, with lock washers, and nuts on the inside.

When mounting the toggle latch hardware first mount the hook to the lid edge with the screws. (Pre-drill pilot holes.) Keep the latch catches screwed out such that the end of the catch bolt is flush with the threaded pivot. Position the latch catch with the catch and hook "comfortably" clasping, and mark the two catch mounting hole locations. Mount the latch catch with screws or bolts as appropriate. Now tighten the latch catch by rotating the catch bolt such that the closed and clasped latch pulls the lid into a snug fit. But do not over-tighten. It would be easy to pull the latch hooks out of the lid with too much leverage.

Step 9: Installation, Testing and Monitoring Performance

You will notice the Fume Coffin with the Can 33 mounted inside is now heavy. When new it's easier to move the Fume Coffin in two pieces: the lower box with the Can 33 and the lid with the HEPA filter.

Keep in mind, however, that once you start using a Fume Coffin the interior will become contaminated with concentrated toxic chemicals. Precautions need to be observed whenever opening the Fume Coffin. Use protective gloves, eye protection, and a respiration mask when working in or servicing a Fume Coffin that has been in use.

Testing

I have tried the Fume Coffin as built here, and empirically it seems to perform adequately. There is little or no observable smoke or smell at the exhaust.

Update [6/20/2017]:

After repairing the hand-held anemometer (an older Kestrel 3000), I was able to test air flow through the entire filter system and from various points in the system by disconnecting pieces starting with the duct leading from the laser cutter and working downstream toward the fan. (see table and chart)

The net result is that the system pulls about 100 CFM from the laser cutter and through the entire filter system. This falls short of my hopes, but according to Voccell, the manufacturer of my laser cutter, 100 CFM is sufficient exhaust airflow.

What's Happening to the Airflow?

The fan can move 237 CFM with no filter system or laser cutter attached. This is pretty close to spec.

But the ducting and the various filter elements present resistance to airflow.

The loss in airflow (see chart):

• laser cutter and duct 1 = 54 CFM
• pre-filter = 6 CFM
• HEPA filter = 53 CFM
• activated charcoal = 28 CFM

There was a little bit of room for improvement by optimizing the 4" duct between the laser cutter and the Fume Coffin by using shorter ducting with fewer curves. I also shortened the post-fan duct to the window vent. The net result was an improvement of about 4%. So every little bit helps.

Performance of the filter system over time is critical. The filter system is brand new with brand new filter media. I will endeavor to test the effectiveness and efficiency of the system as it gets broken in.

Noise

I tested the noise from the Fume Coffin filter system and its fan with a hand-held sound pressure meter.

At 1 meter from the unit I measured 59 dB (A-weighted) with the fan running at high speed. At low speed the noise was 56 dB.

For comparison, the CW-5000 chiller produces 60 dB when the compressor kicks in, and the air compressor for the air assist function of the laser cutter produces 62 dB.

Power

I measured current draw of the fan at high speed with a Kill-A-Watt meter at 0.57 Amps which equals about 68 Watts. (At low speed the current draw was 0.40 Amps.)

I was curious if I could use the current to indirectly measure the amount of air obstruction of the system. Obstructing either the input or output airflow resulted in a lower current of 0.50 Amps. So the relationship between current and airflow resistance is inverse and thus complicated. On the other hand, it appears that airflow obstruction does not overdrive the fan.

Monitoring Performance

One lingering problem with a system like this, that is designed to eventually expend its filter materials, is that it's very difficult to know when those filter materials are "used up".

Obviously, if the filter system is not working in that it's not capturing fumes or smell coming from the laser cutter exhaust then one or more of the filter elements is spent. But since this is a venting filter, you might only notice that at the outdoor vent duct.

Some filters tend to restrict airflow when they get full. One way to measure the operational efficiency of the filter system is to compare the airflow through the system against a baseline measurement of the system when the system is new and all the filter elements are known to be unobstructed.

An easy way to measure the system's baseline airflow is to disconnect the duct 1 hose from the laser cutter and measure the airflow from the 4" post-fan exhaust duct at a standard place. (On my system, there is another DryerDock where the 4" exhaust duct from the Fume Coffin attaches to the window vent.)

I measure the airflow in feet per minute (FPM) using a Kestrel 3000 hand-held anemometer centered in the 4" exhaust output duct.

FPM from a 4" duct can be converted to CFM by multiplying the FPM by the area of the duct in square feet.

CFM = FPM * pi * r^2 (where r is the duct radius in feet). 

CFM = FPM * 0.0873

On my system, this system baseline measurement is 150 CFM with the fan set to speed 3.

(Note: the system baseline measurement ignores the airflow loss from the laser cutter and duct 1. Presumably, these won't vary.)

But a single system-wide measurement will not tell us what part of the system is restricting airflow and presumably spent. It is very likely that the pre-filter, HEPA filter, and activated charcoal will wear out at different rates for different uses.

Fortunately, the pre-filter can be visually examined by looking into the 4" duct hole. If you use white filter material it should be obvious when it's getting dirty.

Also the pre-filter uses cheap filter material, and it may make sense to just replace it on a regular schedule.

We can easily measure the airflow loss from the combined HEPA and pre-filter elements, by opening the Fume Coffin lid so that air can easily flow in and again measuring the flow at the exhaust just as we did for the system baseline measurement.

The baseline airflow measurement before the activated charcoal filter on my system is 209 CFM. Thus the baseline difference attributable to the HEPA and pre-filter is the difference between 209 and 150 = 59 CFMs.

I can compare the measurements at any future time against that value. Keep in mind that value represents the combined performance of the HEPA AND pre-filter. So comparisons should only be made with a new clean pre-filter.

And finally, the activated charcoal system performance is just the difference between the current measurement with the lid open and the baseline measurement with the lid open.

Once we have developed some history, we should be able to predict the lifecycle for each filter element. At that point, it will be easiest to just use a calendar or hours-of-use gauge to predict when each filter element is spent.

Step 10: Maintenance - Replacing the Filters

Caution: Remember dirty filters and the inside of the Fume Coffin box are contaminated by concentrated hazardous chemicals. Use protective gloves, eye protection, and a respiration mask when working in or servicing a Fume Coffin that has been in use. Always bag and dispose of the spent filter material appropriately.

Replacing the Pre-Filter

Cut a new piece of air conditioner filter material to size: 10-1/8" by 30-3/4" (258mm by 780mm).

Open the Fume Coffin, remove the lid, and remove the HEPA filter assembly from the lid.

Remove the dirty pre-filter material. Bag and dispose of the spent filter material appropriately.

Replace the pre-filter and reassemble the HEPA filter assembly as in Step 6.

Addendum: Optional Improved Pre-Filter
I developed the following Instructable for an optional, improved pre-filter design that places the pre-filter in line in front of the Fume Coffin. It is easier to monitor and service.

Fume Coffin Pre-Filter

Replacing the HEPA Filter Cartridge

Open the Fume Coffin, remove the lid, and remove the HEPA filter assembly from the lid.

It makes sense to replace the pre-filter whenever replacing the HEPA filter cartridge.

Remove the pre-filter and the HEPA filter cartridge. Bag and dispose of the spent filter material appropriately.

Replace HEPA filter cartridge and pre-filter with new filters and reassemble the HEPA filter assembly as in Step 6.

Replacing the Can 33 Filter

Open the Fume Coffin and remove the lid.

Remove the Max Fan by removing the 4 screws that hold it in place. Set them aside.

Unscrew the six screws holding the Can 33 and flange in place. Set the flange screws aside.

Remove the spent Can 33 filter and dispose of it appropriately.

Follow the instructions in Step 7 to mark and mount the new Can 33 filter.