Modulating a stoker boiler

Steam

Hi all,

I was thinking of taking on a new project. I have a stoker boiler (Axeman Anderson 260). It has a 1/2 HP 120VAC motor that drives the auger, and reduction gear for the automatic ashing mechanism. Soon it will have a vaporstat, and I will be adding some proper venting to the mains. The system is single pipe steam.

I was thinking of a VFD to control the fan speed while operating, instead of it going full blast @ 1725 RPM. The auger and asher turn at a specific RPM, and work in unison with the fan. There are 2 pulleys off of one motor. One drives the fan, and the other drives a reduction gear that connects via chain to the auger, which is also connected to a cam that shakes the ashes off of the lobe that is part of the auger tube.


Thanks,
Steam

njtommy

How are you going to control the VFD? pressure?

Steam

Great question. Steam flow, and pressure transmitters, tied to temperature sensors? They would need to include return water temperature as well? Just a bunch of questions floating around. Thanks for your input

njtommy

Well I'm not sure how your steam boiler works, but will look it up.
I would assume you t-stat would take care of your stop start switch for the drive. Then you would have to find a pressure transducer that would handle you drive input IE 0-20ma, 4-20ma or 0-10vdc. Also you would have to figure out the minimal speed of the more most are around 20hz.
I like the Honeywell drives very easy to install and setup.

MarkS

I use a Dwyer MagneSense differential pressure transmitter to modulate the gas burner on my steamer. They have 4-20 ma and 0-10 vdc versions in ranges up to 25" W.C. (14 oz/sq in).

dwyer-inst.com/Product/Pressure/DifferentialPressure/Transmitters/SeriesMS

ratio

@MarkS, what kind of controls do you have? A plc, modulating gas valve, something homemade?

MarkS

The gas burner is a Midco LNB that modulates between 70 and 300 MBH. The controls are of my own design (I develop industrial process controls for a living). The modulating control for the burner is a PID velocity algorithm running in a small PLC, controlled on steam pressure using the aforementioned Dwyer MagneSense DP transmitter.

There used to be a lot of information here, photos, etc. on the system and its development over the last four or five years, but it was all deleted last spring in a freak behind-the-scenes accident with the forum software. There is, however, a YouTube video showing a heating cycle on the system: https://youtube.com/watch?v=NASFFSc9veg

RobG

Mark, how has the system been running, any problems? The older I get the more I forget. Did you run the boiler with a standard burner prior to installing the modulating burner? If so, how did it compare on gas usage? I vaguely remember the control panel you built but is that something Midco would provide with the burner? And lastly, not to discuss pricing, but would it be worth the upgrade for a residential or commercial boiler from a price standpoint?

MarkS

OP: sorry for the thread detour...

@RobG it ran with a single-stage oil burner from '09 to spring of '13. There were some settling-in issues when the Midco was first installed, not unexpected, all resolved and now it runs like a champ. Fuel savings last year (normalizing oil-vs-gas to btus) was a bit over 15.5%.

Midco provides a control cabinet for the burner itself; it's up to you to provide the 0-10 vdc modulating control signal, which is the part I designed and built. Since we were an oil-to-gas conversion it was a little easier to justify the extra expense. I think it would be harder to justify on a residential gas replacement unless the system were fairly large or the boiler were significantly oversized.

Steam

No problem Mark, the day is lost without learning something. That is interesting info. I have a gas boiler, but it is for backup.

How could the EcoSteam be adapted to my stoker? I would need a VFD to control the motor, and I guess that (along with my EcoBee thermostat) and controls could tie into such a system.

The feed, asher and fan all operate in unison. So the coal feed would be proportional to the fan speed only the feed is reduced down by a reduction gear.

hvacfreak2

That seems tricky to think about. It's almost as if you would need to use the analog signal for the fan vfd to " pulse " the auger on and off . At 10 vdc it would just be on , but at 5 vdc it would only be on for half that amount. I'm just banking on not being able to really modulate the auger motor due to the torque required to move the coal. I guess the common variable between the fan and auger would be a " heating demand " based on space / outdoor temperatures or something.

ChrisJ

I've had the non-modulating version of the EcoSteam since it was in it's beta stages and I'd never want to be without it.

Steam

HVAC freak. Thanks for the reply. The feed is proportional to the fan speed. I believe the reduction is like 50:1. If there is a call for heat, and even though the fan might be running at a slower RPM (meaning less fuel), the amount of coal that will be fed will be slightly in excess to the fuel being used.

Even if I utilized a separate motor for the auger/asher, any excess coal goes back down the center of the auger feed tube. I can never overfill it. So if the asher clicks over and the auger runs, it will ash and feed.

ChrisJ

Steam said:

HVAC freak. Thanks for the reply. The feed is proportional to the fan speed. I believe the reduction is like 50:1. If there is a call for heat, and even though the fan might be running at a slower RPM (meaning less fuel), the amount of coal that will be fed will be slightly in excess to the fuel being used.

Even if I utilized a separate motor for the auger/asher, any excess coal goes back down the center of the auger feed tube. I can never overfill it. So if the asher clicks over and the auger runs, it will ash and feed.

Any chance you can post a youtube video of this in action?

hvacfreak2

Ok , I got it. So with that out of the way it's not as difficult. So as far as control , the " heat demand " variable , it sounds like there are plenty of options ( the EcoSteam or a programmable controller previously mentioned ). As far as a programmable single phase drive I don't know of one. I know they do exist , I have seen them on draft venting systems lately.

Steam

ChrisJ said:

Any chance you can post a youtube video of this in action?

I'll see if I can find a youtube video of a similar boiler running.

Steam

https://www.youtube.com/watch?v=aXRrZGkx4m0

That is of an Axeman Anderson 130. Same setup as far as operation (though they might have

Here is another one.

https://www.youtube.com/watch?v=emwHCuZ0thk

Steam

hvacfreak2 said:

Ok , I got it. So with that out of the way it's not as difficult. So as far as control , the " heat demand " variable , it sounds like there are plenty of options ( the EcoSteam or a programmable controller previously mentioned ). As far as a programmable single phase drive I don't know of one. I know they do exist , I have seen them on draft venting systems lately.

Would an Invertek optidrive work?

http://www.invertekdrives.com/variable-speed-drives/optidrive-e2-single-phase/

hvacfreak2

Those drives look like they will. I didn't read into the motor requirements for those , that is the next unknown. After seeing those I'm guessing someone makes an " inverter duty " single phase motor.

SWEI

There are other single phase drive options as well, but if you're replacing the motor anyway, why not just buy a 3Ø motor? The drives are cheaper and better and the motor is happier (and often less expensive, given the lack of a starter winding and cap, etc.)

Steam

So, If I wire in a new 3 phase motor with the VFD, I would have to separate it from the 120V controls. There is a L8124C @ 120V in there with 2 24V TT terminals for thermostat, plus the pressure trol.

The motor requirements as it stands now is 1/2 HP 1725 RPM (farm duty w/capacitor). Could the HP even be reduced given the VFD, or should I just keep those same HP requirements?

SWEI

Depends on the VFD. This should work for you. I would stick with the same motor rating unless you are very sure of the load.

I assume the pressuretrol is just a safety high-limit? In that case, it could interrupt the power to the VFD. If it's actually an operating control, I would use the low voltage start/stop contacts on the VFD.

Steam

OK, so how would this tie into the other controllers, IE the ones I would need for the system? If this is compatible with EcoSteam (which I don't see why it wouldn't be) and it would work with my smart thermostat, I would definitely be interested in integrating it with my steam system. If successful, there would be a lot of other individuals interested that have similar setups.

All this help is very much appreciated. Thanks

SWEI

The VFD will work with 0-10V or 4-20mA analog signals, so whatever he's doing for the Midco LNB will be fine.

So-called smart thermostats are actually pretty dumb from a controls perspective since they only have an on/off output. The EcoSteam is far more sophisticated -- on multiple levels.

MarkS

With the Midco gas burner, there is a fairly short time (we're talking 10-15 seconds) between changing the output (burner firing rate) and the system response to that change (steam pressure rise or fall).

On the coal burner, I'd expect the lag between output change (coal feed rate) and input response (assuming pressure change) would be much greater, and a PID algorithm would be difficult to tune. So much so, that I wonder if perhaps some form of step control would be more appropriate than a PID controller in this case. This is a bit outside of my wheelhouse so I'm not stating any of this as fact or even expert opinion.

But I do like a challenge and would be interested in working with you on your project. Send me a PM with contact details either through the forum or the EcoSteam link in my signature.