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 $400-600 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. One advantage of atmospheric steaming is you can pre seal the bags greatly reducing any chance of contamination durring cool down and pre spawn storage. A major factor with atmospheric steam sterilization is your elevation above sea-level. Sea level boiling is212F, 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. If You are using a drum you should attempt to find the UN or pressure rating. The UN rating includes the rated max pressure in kilopascals (kPa). A UN rating reads as, 1A1/Y1.8/100/12/USA/NC1 the 100 is how much pressure the drum is rated for, 100 kPa=14 psi. This doesn't mean you can run the drum at 14 psi, you always want a safety window.Somewhere between 3-8 psi would be safe for this drum allowing a large window in the event of over pressurizing. 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
A more advanced option is a sauna steamer, boiler, or DIY boiler supplying steam to a vessel. Typically a DIY copper steam coil with holes is placed at the bottom of the vessel to evenly distribute steam across the bottom. One advantage of this configuration is the entire capacity of the vessel can be used for substrate. A sauna steamer is almost plug and play, just hook up the hoses and power supply and your steaming. Some people make their own boilers using beer sanke kegs. The keg stem can be removed fairly easily leaving you with a 2 inch tri clover fitting to attach a tri clover to NPT fittings onto, these adapters are readily available as kits from brewing hardware websites. Simply fill the keg with water and place onto a turkey burner. You Can also install an electric element by drilling a hole and welding a 1inch NPS fitting to mount the element.
Horizontal steamers are quite common typical configuration is a horizontal drum or two with shelving installed inside. This allows for a slightly more ergonomic loading and unloading of the drums at the cost of less space efficiency. There is typically a vertical drum or keg as the steam source placed below the drums. Steam enters the drums at the bottom at one end then vents out the top of the drum at the other end, this causes a cross flow of steam. The top drums should be slightly tilted to allow condensate to return via the steam supply lines or a separate condensate return line.
Double barrel steamer with shelving. The bottom drum/boiler is gas fired in this example.
Adjustable pressure relief valves in the rear tops.
A more recent steamer design is a horizontal all in one electric steamer. 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 1 NPS. This can be pressurized but be sure the supply water is slightly restricted as when the float valve actuates water temp will drop and reduce pressure slightly.Restricting the water flow will slow this temperature drop. With 225 lbs of sub this unit takes about 10-12 hours to reach core temp so a 24 hour total run time is required.
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.