Atmospheric or Blown? Advantages/Disadvantages?

[DerekC]

Sorry to ask this common, basic question, but what are the advantages/disadvantages of Atmospheric versus Blown Gas Forges?

[dablacksmith]

atmosferic do not require power for a blower ... but run higher pressures and seem to be harder to get running "just right" blower type forges are simpler to setup and run and dont require as high a pressure to work .ive played with bolth and if you have electricity to your shop i would go with blower .. but that is my opinion and i am sure others will have other opinions..

[DerekC]

Appreciate the reply.

Also, which type (all else equal) produces more heat/can reach a higher temperature, or are they basically equal in ability?

[DerekC]

Also, how hot can you get either type (maximum temperature achievable in a propane fired forge furnace?)

[larrynjr]

I've heard, but do not know for sure that the atmospheric do not get as hot as blown. High end temp depends partly on what you are using for refactory. Blown gas forge should be in the 2000+ F range.

[DerekC]

Thanks Larry.

[hill.josh]

So far as i know its possible to get a gas forge as hot as a charcoal forge with atmospheric.. never been able to do that but my gasser is not exactly well built either, I can get a yellow color from it though. Heres the link to the site I used to build mine. the burner is a little past midway down, under "homebuilt propane burners". I think if you put a little effort into this it would be more than sufficient.
melting metal in a home foundry, backyard metalcasting, metal casting

[Frosty]

There are a number of misconceptions, logical but still not accurate.

First is psi. PSI is only one side of the equation, what really counts is the volume of propane or other flamable gas introduced into the furnace chamber mixed with the correct amount of air. Volume, not pressure.

The amount of pressure through a specific diameter orifice will determine volume delivered.

A naturally aspirated burner (induction device to be more accurate) depends on a primary gas or liquid to induce movement in a secondary gas or liquid. The primary has to be moving at the correct velocity to induce the vacuum. (see Bernoulli) In a naturally aspirated burner it is important to provide the right ratio of fuel to air for a neutral burn.

This makes it a three sided exercise controlled by two factors, primary (propane) jet diameter and PSI. Too large a jet and you have to run too low a psi to induce the correct amount of air and mix them properly. Too small and your psi is too high and mixing isn't thorough AND much of the mixture gets blown right out the furnace door before it burns.

So, noting PSI slone is meaningless. 5psi through a 0.035 jet is in range for general forging if used in a 3/4" dia. naturally aspirated burner with a 7" long tube with a flare. 5psi. through a 4" diameter pipe is probably enough to melt a pickup truck.

Number two. Which gets hotter? A proper mixture of propane and air in a ratio of 1:17.5 will burn at around 3,200f. (according to Mike Porter)

How it's delivered to the furnace means nothing. What is important is that there be enough fuel air delivered per minute to heat the chamber efficiently. Too much and it'll be blown out of the furnace before it burns and is wasted. Too little and it won't get to it's potential heat.

I can say from personal experience my 1" "T" jet burner will melt steel well below it's max throttle. In another case a 3/4" Side Arm burner being used in an experimental chip bed forge melted 3,000f kiln shelf and the 3,000f refractory chips. If you'd like, I'll ask Mike who melted his chip bed forge and get pictures of the mess from him.

There are real differences though, not only between gun and naturally aspirated but between different types of naturally aspirated burners.

A naturally aspirated burner is much more finicky to build and tune. However, once tuned you have a one step throttle, adjust the pressure at the regulator and the BTU output (furnace temperature) goes up or down without having to fiddle with the air. They're also less expensive as you don't have a blower, motor nor electrical involved. You also don't need electric to run a naturally aspirated as the pressure of the propane is what drives the inducer.

A gun burner costs more to build and takes a little longer though it's much more tolerant so you spend less time measuring, jigging and generally getting everything as close to exact as possible. Unless you've made a few it'll probably take you less time to build a gun burner even considering mounting the blower, plumbing, nozzle, etc.

A gun is adjusted on the fly, meaning you light it, then adjust the gas and air till you have the correct fire. If you want more heat you have to turn up the gas and blower and balance them to the correct ratio.

A gun is also less sensitive to back pressure and winds in the shop. Naturally aspirated burners are notoriously sensitive to stray breezes. Of course some baffles will take care of it but that is another level of hassle.

Which is better depends on you. What you want to do, where you want to do it, how good you are at fabricating and adjusting precision tools, how much you have to spend in money and time.

Either will do the job. Either can achieve the max temp for a stoichiometric (sp?) air propane flame. Either will weld or heat your iron melter without problem.

Up to you.

Frosty

[looper567]

Frosty is dead on the money, excellent post!
Here is a photo of my 1 Burner at 13 PSI (.040 orifice), about 5 mins. after firing from a cold start. It's reading 2500.6 deg F, which is the temp limit on the thermocouple being used here. This thermocouple is, for lack of a better word, red-lined, the actual temp here may be higher. I Am currently looking for some K type thermocouples that will read above 2500 deg. F, so I can verify the actual top end temps. This atmospheric forge WILL melt steel...

[DerekC]

Thanks so much for the very complete and informative reply. Really explained it well.