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Boiler pressure switch

Posted by ReubenT 
Boiler pressure switch
March 10, 2016 07:50PM
Anybody try using an air compressor switch on a boiler to control a draft blower? solid fuel of course. They're $10 ea in the surplus catalog. Various models, some adjustable, some not, up to 200 PSI. Most are either 125 or 175 top end. A vertical trapped air pipe just inside the drain valve might keep the heat and water away from it. I just ordered a 140-175 adjustable and 160-200 fixed. I'd prefer a tighter spacing on the differential, but we'll see how it works. May be able to raise the turn on pressure on the adjustable one.

That would automate one thing, at least when not exhausting the steam up the stack for draft. Which I won't be doing at least part time. Boiler feed automation can be had by using the LED level indicators to switch a pump. At least in stationary running. While mobile I can switch it over to manual to prevent rapid cycling. Then all I'd need is a hopper load of chips and a feed screw, and I'd have an engine that could be left to run unattended for some hours at a time.
Re: Boiler pressure switch
March 11, 2016 09:31AM
I use Mercoid pressure switches. You can find them very reasonable on E Bay
Look for the ones with snap switches not mercury switches.
You should select a Pressure switches with the highest rating closest to what you are going to use to have the least differential.
Unlike a pressure gauge if you’re running 150 PSI you should have a gauge for 300 PSI. Pressure gauges are most accurate in mid-range.
Re: Boiler pressure switch
March 11, 2016 09:36AM
The boiler on my steam scooter has an electronic pressure sensor ($20 on ebay) that goes to my arduino that controls the draft blower on the pellet burner. It works perfectly, and I'll be releasing all the code and the full guide on it this summer hopefully. The blower is PWM modulated, so as the pressure approaches the set point, the blower fan slows down, until it stops completely at the set point. Now that I've got everything tuned, it does a good job of maintaining pressure, with the blower fan just doing a quiet whirr for a few seconds every few minutes, without turning on full blast and making alot of smoke just to raise another 10 psi.. This also removes any need for set point hysteresis.

I've also automated the feed water on my boiler. It uses a spark plug water sensor to read the water level halfway through the monotube coil stack. I eliminated rapid cycling by only checking the water level every 15 seconds or so. If the water sensor is off, it turns on the pump until the sensor turns back on.

Here is a video of it in action
This was before I got the feedback loop set up right, so it would pop the relief valve. Now it comes up to pressure and maintains it perfectly*.

Once I get the engine working this spring, I plan on releasing all of the information about it in an instructable. I will also be converting it to propane, for a number of reasons.
Re: Boiler pressure switch
March 12, 2016 09:15AM
I'm very interested, especially in your control system. I suspect that it would have stability problems dealing with a high pressure monotube. Some similar boilers were built by the late John Wetz but with mechanical controls. I would think that electronics could do the same things better today. I spent a lot of time on this forum researching how John's controls worked. Once you understand it I think a similar system could be implemented with an Arduino. This forum also has a lot of information on the control of high pressure monotubes and Lamont boilers. I suspect your system would work well on a Lamont boiler.

Keep experimenting.

Lohring Miller
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