induced draft , forced air cylinder

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Richard Orr
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induced draft , forced air cylinder

Post by Richard Orr » Tue Dec 15, 2009 3:33 am

I am designing my boiler and have been weighing out the options between forced and induced draft. I am going to run condensing so chuffing is out. I've read about design complexities involving induced systems and have been brainstorming simple ways to achieve this. I am imagining an air cylinder, much like a bicycle pump, hooked up in direct drive to my double acting single. If it were sized it right, the compressed air could counter balance the uneven forces of the downstroke. Then a tube could be led through the fire box, the air possibly getting an added pressure boost due to heat expansion and then exit through a venturi in the stack. I am wondering if this idea adds up mathmaticly to a plus. or weather the initial loss in compressing the air equaits to a negative. Any opinions?
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Maltelec
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Re: induced draft , forced air cylinder

Post by Maltelec » Tue Dec 15, 2009 7:34 am

I have given this a lot of thought and have come up with the conclusion that for a forced air system to work efficiently (i.e. you don't use 1/2 the power from the engine to drive the system) you want a big and slow fan.

The idea is that you don't need pressure so much as constant air flow. I have found a fan out of a household gas boiler which would be ideal for the job. It has a 12" diameter fan, one of these with many blades in parallel with its axis. This fan will blow air out from being spun slowly with your finger. There is no need to run it any faster than required, and I would imagine you only need 100 rpm with it.

I have also got my hands on some -5-0-5 cm of water gauges, which will be ideal for measuring the pressure within the boiler casing.
I've got the vehicle, just need the boat.
87gn@tahoe

Re: induced draft , forced air cylinder

Post by 87gn@tahoe » Tue Dec 15, 2009 7:40 pm

you could always try an exhaust turbine like the "big ships" used.. It would essentially be "free" energy, with little/no backpressure on the exhaust if sized correctly... You could make one, or use an automotive one like my father has (using the stock turbocharger from my '87 Buick Grand National).

I would suggest you make it an induced draught rather than forced, as my father originally had it set up as forced with the compressor pumping into the ashpan.. This caused combustion gasses to seep out aroung the firebox door (NOT GOOD for lungs!). Not only that, we were having to shut off the draught (flap over the turbo inlet bell) before adding anything to the fire.

He changed it so that the compressor side ran up a tube that entered the smoke hood and pointed straight up the stack. This creates a great draught, no more noxious fumes, and all of my arm hair remains after opening the firebox door.

A "fan" system wouldn't take too much power to operate (more than the exhaust turbine), though you would have to figure out some type of system that would "free wheel" while you ran in reverse, so that you wouldn't suck the fire(or smoke) out and dispense it into the boat. A bicycle rear sprocket could work.

wes
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Re: induced draft , forced air cylinder

Post by Edward » Tue Dec 15, 2009 8:41 pm

Dear Richard ,

One or two thoughts and opinions about your query .

First of all why do you want extra draught ?
If it is for raising steam more rapidly then an engine driven system is not going to help .
If it is for maintaining boiler pressure while running normally then I would say the boiler or firebox isn't big enough or your fuel doesn't have a high enough calorific value . Since you're at the design stage just make sure that the boiler/firebox/fuel system are up to your requirements.
If it is to get a bit more steam for flat out performance then some form of increased draught would fit the bill.

Mechanical methods as opposed to a simple steam blower in the base of the smokestack are probably an unneccessary complication . I write "probably" advisedly because some people enjoy the engineering challenge that they give .

A system that David Ayers developed for ELPENOR used exhaust steam to turn a simple turbine attached to a fan (centrifugal I think ) blowing air into the ash pan giving a forced draught . The draught was not very great , in fact it was quite weak but it was surprisingly effective and quiet . A nice touch was that in using exhaust steam it was utilising energy that would otherwise have been wasted . I don't think the fact that you're intending to condense would matter .

Historically A high level of forced draught could be a problem as on opening the fire hole door to add fuel the fire was blown out of the door into the boat. This was overcome either by having over pressure boiler rooms (Not really very practical on a small , probably open , steam launch) or a mechanical link between the door and blown air which diverted the air when the door was opened : Possible on a launch but an added complication .

I personally think the best system is a simple steam blower in the base of the stack . It is effective from about 30 psi upwards , is fairly quiet and doesn't use very much water . You would however need to take it into consideration if your'e going steaming on salt water when planning how much make up water you need to carry .

If you like the idea of the exhaust steam tubine driven fan there is a picture of it on ELPENORs' website .


Best wishes Edward .
Last edited by Edward on Wed Dec 16, 2009 11:07 pm, edited 1 time in total.
mcandrew1894
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Re: induced draft , forced air cylinder

Post by mcandrew1894 » Wed Dec 16, 2009 1:58 am

Edward speaks wisdom here...I, and many others, use a stack blower for two reasons...they work and their easy to make.

HIGHLY recommend them.....but make sure you have enough grate area to begin with.....

Dave
Richard Orr
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Re: induced draft , forced air cylinder

Post by Richard Orr » Wed Dec 16, 2009 2:09 am

Thank you all for your replys and opinions.
In answer to Edward, I am interested in forced or induced draft for the purposes of adding on extra steam at times and also the burning of damp fuel. I will be doing most of my steaming south of the border and am designing my boiler to run primarily on river cane, cahoon nuts and the occasional what-ever-else. I have not yet done specific measurements but my experiments with river cane is that it burns with great intensity even when cut green. I am attracted to how easy it is to harvest and how easy and iincrementaly it can be fed into the fire.
In reply to Maltech, the uneven forces I am refering to are the weight of the piston, piston rod, crosshead and half the weight of the connecting rod in addition to the extra area on the top of the piston. Those weights and forces are what I would balance the air cylinder to, minus that of the wet air pump.
Falling away from the technical vernacular and using a lot of psudo science, let me describe something I learned while starting my home fire in the morning. Rather than using a bellows to liven the coals, I have taken to using a 3/4 inch tube, pinched off at the end. At first I left the end open, blew through it and got zilch. After I pinched off the end, I blew through it and got Holy Moly riding in on wild horses (velosity). I found that I could back off the coals a good three feet before Holy Moly would at last fall off his horse and get trampled by zilch.
Now, if a fan is placed in front of the fire and put on high, the coals do indeed liven up over all but not with the localized intensity that is yeilded by the tube. So, if I think of my puny lung power and what it yeilds through a pinched off tube (velosity) as a ratio in relation to the fan and it's yeild (volume), it quickly becomes a David and goliath scenario and I conclude that as far as power yeilded per horse power, velosity has it all over volume.
So one of the designs I am thinking of is to run x amount of stainles tube along the fire box wall for pre-heating and then run x amount of tubing under the grate with perhaps 1/16 or 1/32 inch holes drilled every few inches. I am at least confident it would yeild more b.t.u's. than the amount I burned writing all this verbage.
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Re: induced draft , forced air cylinder

Post by Richard Orr » Wed Dec 16, 2009 2:53 am

Indeed, a design that is simple and proven to work is is probably the best way to go about it, but let me throw in this variable to see if changes the equation: The engine on my boat is 26 feet from where the stack of the boiler will be. How would an exaust turbine play out over that distance?
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Re: induced draft , forced air cylinder

Post by mcandrew1894 » Wed Dec 16, 2009 3:02 am

26'!.....yeash!

Well ....makes me want to put a stack blower in.....at the boiler.

You could remote the control for it...but 26' away...your going to have your hands full running the plant and firing the boiler.....

When we say stack blower, I mean live boiler steam stack blower...not engine exhaust stack blower...by the way.

Dave
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Re: induced draft , forced air cylinder

Post by Johnlanark » Wed Dec 16, 2009 5:59 pm

Crikey Richard that is quite a boat. Have I missed you telling us about it? John
87gn@tahoe

Re: induced draft , forced air cylinder

Post by 87gn@tahoe » Thu Dec 17, 2009 4:39 am

As far as the exhaust turbine over 26' goes... If you had the turbine and compressor (fan) immediately after the engine, then ran tubing from the compressor of a consistant diameter (and of same diameter as the compressor outlet) with the fewest number of bends possible for your application; I don't think you'd experience too much drop in velocity over the 26'.

On my father's boat:

A single turbocharger (for a 3.8litre v6, 20psi @ 5600rpm MAX) coupled to the exhaust of a 3"&5"x4" compound induces enough draught on a 40sqft VFT to have 5ft flame visible (in broad daylight) out the top of a 7ft funnel (about 10ft+ from grates to top of funnel) when not kept under control, without even trying (fireman was sleeping on the job :oops:). I am sure that much more spectacular "torch" displays could be accomplished if one were to intentionally attempt to scortch all of the paint off of the funnel. :shock:

The run from the compressor oulet to the outlet in the smokehood is approx. 7ft+, and is an immediate 180degree turn followed by a 90degree change in direction, a straight run of several feet, a 90, another straight run, a 90, a short straight, a 90, and a short straight ending with a venturi.

Now, the need for forced draught on a VFT is another discussion altogether.
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