Double bellows aren't working

[Ramsberg]

Hey Tim,

Exactly right!

This is one reason that I prefer bellows over cenrifugal fans. With a bellows the delivered volume of air is dependent on the rate at which one pumps the bellows, with a fan the delivered volume is dependen on the area of the discharge and the difference between the head pressure created by the fan and the resistance head produced by the delivery passage, discharge coeffecient and fuel stack.

I had a pile of ligh fuel "blast off" when I was using a fan. What happened was that I was experimenting with wood pellets(not the best) and when they char they just fall apart and were clogging the bottom blast forges discharge. There was virtually no air being delivered to the fire and I was cranking faster and faster, then all of a sudden the head pressure was enough to dislodge the fuel and the full blast of the air shot out, it must have shot the fuel two feet above the hearth! Scared the crap out of me! HA! After that I trusted fans less and less, bellows are much easier to control in my opinion. As an aside I had more clinker from those wood pellets then I had from good coal! Then there are the sparks that the pellet fuel produced. . .

Maybe this belongs in a new thread, but I wonder what the average firing rate of most blacksmiths is. There would be the hourly rate which would be the fuel consumed per hour and the firing rate which would be the fuel consumed per hour of blasting the fire with air.

It has been a while since I have had a shop set up, but I seem to remember going through roughly 20 - 40 lbs per hour, depending on the size of the work.

Caleb Ramsby

Caleb Ramsby

[Nakedanvil - Grant Sarver]

It is the volume of flow and the restriction to said flow that produces a pressure differential, either a negative pressure trying to fill a container or a positive pressure trying to empty it.


Caleb Ramsby

Flow doesn't create the pressure, compression does. The pressure in an cylinder engine is created after the valves close. It's created by the piston coming up and making the chamber smaller, just as squeezing a bellows makes the chamber smaller creating pressure.

So who burn 50 pounds of coal per hour?

[Ramsberg]

Grant,

I believe that I wildly overestimated the amount of fuel consumed per hour. From looking at a few posts on the subject it looks more like 2 - 5 lbs of coal per hour. I must have been studying locomotives a bit too much, they can burn well over 80 lbs per hour per sq ft of grate.

Otherwise I stand by what I said. The pressure that the bellows develops is dictated by the resistance to the air flow, the lesser the resistance the lesser the pressure.

I don't like to be argumentative, I like to be helpful when I can be.

Caleb Ramsby

[Ramsberg]

Grant,

To elaborate upon your analogy, take the head off of that engine and how much pressure is developed? Put the head back on and make a large hole in it, that will produce a little bit of pressure, the smaller the hole the greater the resistance to the air flow so the greater the pressure required to produce said flow. No hole and then the pressure developed is dependent upon the ratio of volumes and the polynomial coefficient of compression for that gas mixture in that engine.

Caleb Ramsby

[pkrankow]

A bellows is static pressure, simply take the weight applied to the top panel (including the weight of the panel) and divide by the area of the top panel and you get the pressure. Alternately apply the force on the bottom panel and divide by area to get the pressure on the bottom section.

Flow pressure is dynamic, and at the speed of sound everything get reversed! Below the speed of sound converging nozzles increase velocity and reduce pressure, diverging nozzles increase pressure and reduce velocity. Because the velocities involved are rather low flow pressure can be discounted and static pressure can be used instead.

Phil

[doc]

It ain't the area of the valves that creates the pressure, it's the area of the bellows.

I was taught that a pump creates flow but not pressure. Pressure is created when a restrition to that flow is introduced.
Since a bellows is nothing more than a pump it cannot create pressure only the size of the outlet will cause pressure to be produced.

Doc

[pkrankow]

I was taught that a pump creates flow but not pressure. Pressure is created when a restrition to that flow is introduced.
Since a bellows is nothing more than a pump it cannot create pressure only the size of the outlet will cause pressure to be produced.

Doc

If you inflate a chamber like a tire it has pressure.

A positive displacement system like a reciprocating piston pump has pressure in it. This is what bellows are.

A fan system creates flow. This is what a blower is. So you are correct for certain types of pumps.

Phil

[Tim McCoy]

For a detailed exploration of the theories covering both gas and fluid flow see the discussion on Bernoulli's Theorum at the follwing link:

http://en.wikipedia.org/wiki/Bernoulli_theorem

... Discusses both gas and non-compressible fluids. Fans move air and create flow while straight ducts, convergent ducts, divergent ducts (as exist in a bellows) all have different effects on gasses and cause either increases in pressure resulting in slower flows or decreases in pressure resuliting in faster flow or in straight pipe pressure can remain constant but rate of slow can decrease because of resistance along the wall of the pipe ... Bernoulli's theories explain most of what we see with fluid used in hydraulic pumps, air flow in jet engines and pressure nozzles and the like.

A bellows acts as a "jet" by accelerating a gas (our oxygen) through a nozzle (created by the hole that is between the actual bellows and the nozzle that feeds the air into the burning fuel). An internal combustion engine does compress the gasses and fluids fed into the burning chamber (cylinder) but some of the resulting forces are explained by other theories ... who'da thunk that burning some coal could get so complicated?
:blink:

[pkrankow]

I guess part of the added confusion is we are not looking at the system as a whole but a simple sub component of a part of the system.
Phil

[doc]

If you inflate a chamber like a tire it has pressure.

A positive displacement system like a reciprocating piston pump has pressure in it. This is what bellows are.

A fan system creates flow. This is what a blower is. So you are correct for certain types of pumps.

Phil

A piston pump that displaces one gallon of water in one stroke will produce a lower pressure when it is disharged thru a 1" pipe than when it is discharged thru a 1/16th" pipe.
The pump doesn't create the pressure the restiction does. The pressure will continue to increase as the size of the opening is decreased until either the internal leakage in the pump is to great to equal the pressure or the driving force ie; motor/engine can not over come the load. These factors are true no matter what type of pump is being operated.

Doc

[doc]

If you inflate a chamber like a tire it has pressure.

A positive displacement system like a reciprocating piston pump has pressure in it. This is what bellows are.

A fan system creates flow. This is what a blower is. So you are correct for certain types of pumps.

Phil

Phil I forgot to add that the pressure in the tire is stored energy equal to the elastic force of the rubber trying to relaxe to it's original none expanded size.

[basher]

I have 2 sets og these to restore so all of this is interesting . they really are huge things weighing in at near 200lb .
the double bellows are a great piece of folk design . mine have 3 boards and extra rib spacers between them .
gonna need a lot of leather to cover them back up .

[Ramsberg]

I found a reference to the supply pressure of air to a side blast forge in Forgecraft, here is a link to the full book scanned by Google:

http://books.google.com/books?id=gYJIAAAAMAAJ&printsec=frontcover&dq=forgecraft&hl=en&ei=rt2LTIKKNYOinAeW4_CSCw&sa=X&oi=book_result&ct=result&resnum=1&ved=0CCUQ6AEwAA#v=onepage&q&f=false

It says that the minumim blast pressure for an 8" deep fuel bed is 3 ounces per square inch, that equals 5 1/5" of water column. It also says that two or three pounds per sq in is a bit too much, as it will oxidize the fuel.

It has been a while but I recall that air passes through charcoal much easier then coal, deeper beds for heating larger stock require more air pressure to get it through then shallow beds.

I have seen guys pump the top bellows full and keep on pumping to get the fire really going, never went that far myself, that must have been a pretty good pressure developed!

Back to Rockstars bellows, 4' long by 2' wide with a taper, an estimate is that it would have 3 1/2 to 5 1/2 sq ft of head area, for a 5" water column of air pressure that would be 26 lbs per sq ft or 91 lbs to 143 lbs of force on the head. A thick head board with the weight near the big end giving i leverage would be very close to that.

Does anyone reading this have any idea how much force they have on the top head of thier bellows and its area?

Caleb Ramsby

[Conrad Hodson]

Thank you all for the insights. I only have one 2-1/2" inlet on the bottom and one on the middle board. It sounds like I need to add another one to each board. I was concerned that if the hole was too large it wouldn't work properly but I can't think of a reason why. I'll add the inlets this weekend and post my results. Thank you all for your help!

I built a bellows about the size of yours two years ago. The valves are 3x5 inches and none too large. My valve flaps are thin cedar and close onto felt that surrounds each hole.

One innovation I added: I built each whole valve assembly into a piece of board, and then set those into the leaves of the bellows. The joints are sealed with more felt and the valve boards are held in by turnbuttons.

This way, if the valve ever gets out of order, or the bellows leather needs restitching, I can just remove the whole valve assembly for repair, or for giving me a comfortably large access hole into each chamber.

Conrad Hodson