The priority of any roaster venting system should be to keep you, your machinery, and your roastery safe. Beyond safety, the system function should allow you the flexibility to use your airflow as a tool in your roasting to shape the character and quality of your coffee. Growing roasteries may also benefit from choosing modular systems that allow for reconfiguration and addition as machinery or layouts change.
When selecting a venting class/type, there are three major issues specific to roaster exhaust: heat, pressure, and residue. Systems must be rated to handle the heat of the exhaust, airtight to prevent the leaking of combustion gas in a pressurized system, and easily cleanable of flammable chaff dust and oil residue from its interior on a regular basis.
General Notes about Venting Coffee Roasters
When designing roastery venting systems in spaces/buildings containing combustible materials, we suggest employing a hybrid system that uses both single- and double-walled venting. With this strategy, single-wall venting is used immediately at the roaster and in the interior spaces until you approach any combustible interior finishes and/or the combustible materials of your building envelope. At this point we suggest transitioning the line to double-wall venting to travel next to, or penetrate through, walls/ceilings/structure to reach termination outdoors. This puts the double-wall venting where it is most useful: protecting your building components from the heat of your roaster exhaust. It also puts the easier-to-clean single-wall venting where it is most useful: closest to the source of the residue.
This option presupposes that your layout is designed, or your interior is accessible, such that a hot single-wall venting line is not a hazard inside your roastery. Roasters that are accessible to the public may want to keep non-trained persons from being able to come into contact with single-wall venting (along with a host of other things that are dangerous with industrial equipment).
Venting is commonly available in both single-wall and double-wall pipe. The primary safety concern in choosing one over the other is how hot the outside of the vent lines become during roasting. The greater the outside surface temperature of the vent lines, the greater clearance you will require to combustibles (interior finishes and building envelope construction) as well as increased precautions against someone in your roastery burning themselves. Double-wall pipe consists of two “walls”: an inner lining and out casing with an air space between which prevents much of the heat inside the pipe from passing through to the outside surface. The outside surface of the pipe stays cooler and requires less clearance to combustible building materials (wood studs, insulation, gypsum wall board, wood trims, exterior finishes) as it passes through wall and ceiling assemblies. Conversely, the outside surface of single-wall pipe can rise to nearly the temperature of the exhaust air it carries. Due to this potentially hot surface, building code can require six to seven times the clearance to combustibles than for double-wall pipe, resulting in larger openings in walls, ceilings, or roofs to allow single-wall pipe to pass to the exterior.
When not in proximity to combustible materials, single-wall pipe does have advantages over double-wall pipe in that its parts are smaller scale and more easily modified making your layout more adaptable and modular. Single-wall pipe is often easier to disassemble and handle while cleaning, which may increase the likelihood that regular cleanings occur thus increasing the overall safety of your roastery. From a cost standpoint, single-wall venting parts are not only less expensive, but also available in a wider range of sizes and materials, which can be further leveraged to lower costs.
Your local building and mechanical code will govern at your location. We suggest working with an HVAC professional when installing venting that interacts with your interior finishes and building envelope.
Venting manufacturers should provide the temperatures up to which their individual products can be safely used. For metal venting parts, these temperature ratings can vary widely depending on material (typically galvanized or stainless steel) and the metal gauge (commonly 20-22 gauge). Other common components in positive pressure venting systems include seals and o-rings that are typically offered in nitrile or silicone. These items will also have individual heat ratings. Due to degradation with prolonged heat exposure, these seals and o-rings may require periodic replacement. Check your manufacturer’s specifications
The exhaust of coffee roasters is typically pushed through vent lines to termination by fans that are pulling that air from the drum and cooling tray. Inside the venting, this type of airflow is called “positive pressure” because the pressure inside the vent line is greater than the atmospheric pressure outside the line. Because of this pressurized air, venting parts and connections must be airtight.
Conversely, much of the readily available venting (B-vent, L-vent, chimney pipe, “stove pipe”) is designed for negative pressure systems, which are not airtight. In these vent lines, hot air rises from the source and as it rises, it creates a vacuum behind it, pulling up more air from inside the vent below, but also pulling air into the vent through seams in the vent parts and connections. Using negative pressure venting to carry a pressurized airflow will result in leaking of exhaust air through those seams and connections that are not airtight.
Frequently Asked Questions about Venting a Mill City Roaster
For roasters that are 3kg and smaller, the heat rating required in your stack is a minimum of 500°F. Search for Double-wall, Special Gas Vent. Special Gas Vent is a common name for positive pressure venting that typically has a heat rating of about 550° and a clearance to combustibles of 1″-3″. When you find one, read the specifications carefully and verify these four things: 1) positive pressure 2) double-wall 3) has a heat rating of 500° or higher, and 4) note the clearance to combustibles. If it happens to mention that the venting is appropriate for Category III ducting, you’re on the right track. (Cat. III venting is not required but has very similar specs.)
For roasters that are 6kg and larger, the heat rating required in your stack is a minimum of 1000°F. Search for Double-wall, Pressurized Industrial Chimny -or- Factory Built Chimney. These are common name for positive pressure venting that typically has a heat rating of about 1000° and a clearance to combustibles of 1″. When you find one, read the specifications carefully and verify these four things: 1) positive pressure 2) double-wall 3) has a heat rating of 1000° or higher, and 4) note the clearance to combustibles. If it happens to mention that the venting is appropriate for Category III ducting, you’re on the right track (Cat. III venting is not required but has very similar specs.)
We are not able to address site-specific code requirements, but common things to be aware of are clearances to combustibles in your interior, building envelope and exterior; distances between your exhaust termination and exterior doors, windows and overhangs; distances of exhaust termination above roof structures and beyond exterior walls, maintaining minimum path widths for egress; protecting the general public from your equipment, venting, and exhaust; proximity to electric panels, gas meters, and fire suppression units.
Please be advised that your local code official may defer to the venting manufacturer’s documentation regarding heat ratings and suggested clearances to combustibles. When choosing a venting manufacturer, you may want to choose one that has that information readily available, or it could delay your project.
Yes. Though drywall or gypsum wallboard (GWB) is considered non-combustible per building code as it performs as a component in fire-rated wall assemblies, per the mechanical code it is considered to be combustible. Mechanical code considers performance during exposure to a constant high-heat source, which GWB fails as its papery surface is combustible. Also, the implementation of GWB in preventing fire spread is largely due to the high moisture content of the gypsum core. If you place a radiant heat source (venting or chaff collector) too close to drywall, the radiant heat will cause that moisture to evaporate, and prolonged exposure will degrade the fire rating due to calcination (removal of water) of the gypsum.
We do not recommend using flex duct. Roasting exhaust carries chaff dust and oil residue that can quickly build up on the rib structure of flex duct. Because of the difficulty in keeping it clean, flex duct often ends up being a temporary solution, which can be quite costly when properly rated to carry the average exhaust temperatures.
Use venting parts and adapters available to get as close to the size of your inlets and outlets without reducing. If you are able to achieve a “glove-like” fit using your venting parts, we suggest using a high-temperature rated silicone sealer such as Permatex to create a fully sealed, airtight connection. For those connections where gaps are too great for a paste/caulk-type sealer, we recommend wrapping the inlets/outlets of your roaster/fan/chaff collector with properly rated foil tape to increase the diameter of your inlets/outlets until it creates a slip fit with your venting.
Depending on your venting layout, you may need a structural connection to support the venting where it meets the inlets/outlets. You can use self-tapping screws to physically connect your venting to your machinery, but it is not ideal. With any connection type, remember that you will need to remove the venting often to clean out all venting lines and fans. Design your connections and layout accordingly.
Yes, you can. Keeping all clearances to combustibles and considerations of grade, overhangs, openings, etc. on the exterior walls, you can terminate through an exterior wall. We suggest extending a minimum of 12” (or as required by applicable code, whichever is greater), using a 30 degree elbow to turn your horizontal run down slightly (preventing rain and snow from entering the pipe) and to cap it with a wired bird screen cap (no tighter than 1/4” or 3/8” wire spacing).
There could be. It’s more prevalent on those terminations that are made through exterior walls. Extended your termination beyond the exterior surface by 12” or more and point away to minimize.
In general, we do not suggest reducing the diameter of your vent lines to be smaller than the outlets to which they connect.
Frequently Asked Questions about MCR Venting Kits
Transition adapters are available to clamp at the end of our kits to transition to other ducting types.
The kits require you to use a very specific layout for your components (roaster, chaff collector, and cooling fan). Layout drawings are available for each model to ensure the kit will work in your space. You should consider the footprint and the height of the kit and verify both will work with your space.