This article discusses a few options for homemade bulk steamers and pressure steamers. Typically this is used to sterilize bulk substrates suchas supplemented sawdust or fuel pellets. There are two configurations for steamers, horizontal or vertical. There are also a few options for heating your steamer as we will discuss.
There are limitless options for steamer vessels. Plastic or steel open top drums can be used. Some of the best vessels are stainless steel,including stainless steel drums and industrial storage tanks. Plastic open top drums are nice because they will not rust; only downside is they will not take much pressure if at all. Steel drums are better and can be slightly pressurized@ 2-5 psi, but will rust unless lined on the inside. A good few sources for drums is craigslist, eBay and bubbasbarrels.com.
"food grade" lined steel drums are cheap and typically readily available locally for $10-30
Open top Plastic drums are good for atmospheric steaming but can be harder to find, they can be purchased online used and shipped for $50-80
Stainless steel drums are excellent for pressurized steamers, they can be quite expensive at $400-900 ea
Some examples of stainless steel tanks found using a quick ebay search.
Many times pressure is not needed if you steam for a longer period. Typically with supplemented hardwood bags 10-14 hours with the core temp 200 F is sufficient for sterilization. A major factor with atmospheric steam sterilization is your elevation above sea-level. Sea level boiling is 212F, allowing for a shorter cycle, while at 3600' elevation boiling is 205F. With a pressurized steamer a shorter cycle is required, the more pressure the more you can shorten the cycle. One precaution with pressurized steamers is your likelihood of bursting bags, especially if you have a sudden pressure drop. Also you need to be sure your vessel is rated for such pressure.
Pressurizing the vessel should be done with precaution. Always perform a dry run without any substrate and as little water as possible to test the seals and connections. You should have at least two forms of pressure relief. Some primary means of pressure relief are a pressure cooker rocker and adjustable pressure relief valves. A few secondary relief methods would be a water column or an additional higher set pressure relief valve.
There are a few options for steam sources. The simplest method is simply a steel drum propped up on 3-4 rocks, or preferably fire bricks, then a wood fire or propane burner is used to heat the drum. A foot of water is placed at the bottom followed by a false bottom rack to hold the bags above the water level. To prevent bags from melting to the hot drum walls, line the drum with a canvas drop cloth or old grain sacks.
Wood false bottom to elevate substrate bags above water level
Gr0wers single drum steamer design packs all your needs into one drum. It consists of a stainless steel float valve and 1500w or larger hot water heater element. The float valve allows very little water to be placed in the drum, allowing for a lower false bottom and more substrate per load. The hot water heating element can be mounted via a 1" NPT locknut on the inside if using a steel drum, or by drilling and tapping a plastic drum with a 1 NPS tap. With 225 lbs of sub in a 55 gal drum and 1500w element this unit takes about 10-12 hours to reach 200F core temp so a 20-24 hour total run time is required. For a complete list of parts see here, https://kit.com/MyersMushrooms/drum-sterilizer
Float valve and heating element tapped and threaded into a plastic drum
Float valve and heating element in steel drum with locknut
How to monitor core temp. Take a filled sealed bag and flatten it out, place a temp probe in the center
Fold the bag over the probe sandwiching the probe in the center.
Place the probed bag on the top center to monitor core temps. Note how i have a ceramic plate under the exhaust vent to prevent bags from expanding and clogging the exhaust.
Move the timer/thermometer away from the drum to avoid heat and steam from damaging the unit.