Marc R Keith, Malo, Washington, USA
The following pages detail the wood fired steam generator pictured above which I designed and built for use on our mushroom farm. The boiler provides steam to our autoclaves for sterilization of grain spawn and supplemented sawdust mushroom substrates, while also supplying steam to a hydronic heat exchanger in our mushroom house to warm the incoming air. The boiler further supplies steam to copper tubing embedded in the concrete slab of the grow room which provides thermal mass to ameliorate the wild temperature swings of our northern mountain climate.
The boiler has been in operation for over a year now, so we've had an opportunity to work the bugs out, while learning to use it properly. On these pages, I'm going to take you through the full construction, troubleshooting, and operation of the steam generator, including the mechanical and electrical control systems.
Where appropriate, sources and actual part numbers of the various components used are given to make it easier for those of you considering a similar project. Steam generators such as this are not limited to mushroom farms. Homes and businesses throughout the world use hydronic heating, and a boiler such as this can be built for a fraction the cost of purchasing and installing a manufactured unit.
Obviously, before embarking on a project such as this, the individual must be certain of his mechanical, electrical, and construction skills, local codes and regulations, plus suitability of the boiler for his intended purpose. Obviously, these pages are for information only and not to be construed as a step by step instruction manual.
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Our wood fired boiler design uses a controllable air intake shutter which is operated by an electrical solenoid. This solenoid is energized by a control relay, which in turn is operated by a pressure switch. When a drop in pressure is detected, the contacts on the pressure switch close, energizing the coil of the control relay, opening up the intake air shutter. The pressure switch opens on a rise in steam pressure, de-energizing the solenoid valve, causing the air intake shutter to be pulled to its idle position by a mechanical spring.
This system has proved effective in maintaining pressure to within 2/10 of 1 psi. When the pressure switch is set to our operating pressure of 1 psi, the contacts close at .9 psi and open at 1.1 psi. A second pressure switch with contacts which close on rise is set to 1.6 psi to open a steam solenoid to dump pressure should it rise to that point. For safety, an additional mechanical pressure relief is installed to open up at 5 psi.
On the following pages, you'll see dozens pictures taken at every step during construction along with detailed descriptions of how and why each step was performed. You'll get safety advice, the benefit of 40 years construction and engineering experience, plus the wiring diagram and part numbers of everything you'll need to lay out and complete a similar project of your own.