another newbie

[timgunn1962]

As I'm sure you are coming to expect by now, things are not as simple as they seemed back when you knew absolutely nothing.

The blue flame outside the forge is, as you say, partially-burned fuel (more-or-less) finishing its burn as it encounter the additional Oxygen it needs. However, the "to no good purpose" is very wrong.

Viewed solely from the standpoint of heat release, it does indeed serve no purpose. However, as smiths, we are not concerned solely with heat transfer. There are several other factors which we need to consider.

The main one is the forge atmosphere. If the forge atmosphere is Oxidizing, the unburned Oxygen will combine with the steel and we will lose a significant amount of it to scale. We therefore want a reducing atmosphere: one in which there is a substantial concentration of Carbon Monoxide (CO). The CO "wants" to react with Oxygen to form Carbon Dioxide (CO2). It wants to do this so much that it will tend to remove the Oxygen atoms from Iron Oxide, reducing the Iron Oxide to Iron. It is generally true to say that we want to increase the level of CO in the forge to maximize the amount of steel retained.

Against this, we need to achieve the forge temperature needed for the job in hand.

The forge temperature depends on many things. Most of them are fixed during the design and construction of the forge and so are outside our control once we are actually using the forge.

The 2 remaining factors are the flame temperature at which the gas burns and the amount of gas/air mixture being fed to the forge.

The maximum flame temperature occurs near to the stoichiometric air:fuel ratio. This is the mixture at which all of the fuel gas burns with all of the Oxygen from the air, leaving no unburned fuel and no unburned Oxygen. in a (purely theoretical) perfectly insulated forge, this would give a flame temperature of around 1980 degC, 3596 degF. 

If we change the mixture, the flame temperature reduces. It does this either side of the stoichiometric ratio, with the flame temperature reducing as we get further from the stoichiometric ratio. We don't want to worry ourselves unduly over the temperature reduction on the "lean" side (fuel-lean: combustion with excess air) because this would give an Oxidizing atmosphere with the problems that entails and we're not going there. Instead, we want a "rich" mixture (fuel rich: combustion with excess fuel) to retain as much as possible of the workpiece.

Unfortunately combustion Chemistry is quite a lot more complex than we'd like and there is not a simple point, just slightly richer than stoichiometric, at which the forge atmosphere stops attacking our steel. Instead, there is a sliding scale over which scaling becomes less pronounced AND over which the flame temperature reduces. 

We therefore need to find the sweet-spot where the flame temperature is high enough to get the job done, but the atmosphere is also sufficiently reducing to keep the workpiece sufficiently scale-free to get the job done.

The sweet-spot can be quite wide and there are lots of other variables that will impact on this. They include the material being worked, the skill and speed of the smith, the nature of the task and many more.

We also have control over the amount of gas/air mixture being burned. In Naturally-Aspirated burners, this is varied by adjusting the gas feed pressure, since the air:fuel ratio of a given burner is pretty much constant over a fairly wide range of gas pressures. We cannot get the forge temperature to exceed the theoretical flame temperature for the mixture, however much gas we put in,  but at high gas flows, the forge temperature will get closer to the theoretical flame temperature than it will at low flows.

If we have a Naturally-Aspirated burner with a choke, the choke provides a means of varying the air:fuel ratio on-the-fly. In most cases, choked burners are initially tuned with the choke fully open and they are treated just like unchoked N.A. burners during the tuning process. In broad terms, the gas jet is adjusted to get the richest mixture that provides a high enough temperature for the hottest task intended. The adjustment may be changing the jet diameter, changing the axial position of the jet, or a combination of the two. Once the hot setting has been established, running at lower temperature can be achieved by reducing the gas pressure. On choked burners, there is also the facility to richen the mixture by closing the choke to get a lower temperature in conjunction with a more reducing atmosphere.

For many (most?) smiths, an unchoked NA burner seems to be quite sufficient. When tuned for welding temperature at high pressure, the pressure adjustment alone seems to allow forging temperatures to be achieved simply by reducing pressure. 

For knifemaking, the added complication of decarburization of the steel (Carbon Dioxide reacting with the Carbon from the steel at its surface to produce Carbon Monoxide) can make a choked burner sufficiently advantageous to justify the additional complexity. A very finely-adjustable choke can even allow operation with a flame temperature down in the Heat-Treating temperature range. This allows relatively long soak times and therefore allows steels like O1 and 52100 to be treated to more-or-less their full potential without an electric HT oven.

Personally, I'd see what your current setup will do first. If you then find you need to go hotter, try a smaller jet.

A smaller jet will get you closer to the maximum flame temperature. However, it will mean that there is less gas being burned. Whether or not it will get you a higher forge temperature will depend on whether it is the flame temperature or the heat input that is restricting your temperature at present. Looking at the amount of DB your forge has, my guess would be that the flame temperature is what is limiting the temperature, in which case a smaller jet should get things hotter.

Note that some of the CO burns to CO2 in the DB, but not all of it. Adjusting the jet size/position to reduce gas consumption and CO production is certainly not a bad thing, but you cannot realistically expect to reach zero CO release and will always need to take safety precautions.

Against the benefits of reducing gas consumption and CO production must be weighed the potential costs of running a less reducing forge atmosphere. This may well include increased time and materials for cleaning up more heavily-scaled work. Halving your gas costs in exchange for a doubling of clean-up costs might not be a bargain. 

Most apparently simple things turn out to be quite complex once you start to understand them.

 

 

 

 

 

[MotoMike]

Tim

Thanks very much for this thoughtful response that addresses my questions in a very thorough and understandable way.  You are exactly right about the complexities becoming more obvious as I peel away the layers.  But I am a nerd at heart and I am, so far, enjoying every aspect of it as I find a new riddle around every corner.   I of course know that CO is a product of combustion no matter how efficient and will be safety conscious.  probably one of my favorite sayings is more true here than anywhere else I've applied it.  "I know enough to be dangerous"

Thanks again

Mike

[Irondragon Forge ClayWorks]

"I know enough to be dangerous" ... Just memorialized in Gems & Pearls... great saying.

[Mikey98118]

On 11/28/2017 at 6:04 PM, MotoMike said:

Mikey, thanks for those comments.  I intend use it in an open garage door, but I am interested in what you suggest to improve it.  can you direct me to a link that would discuss what you are talking about.

Thomas has answered the question wondrous well on this thread; unfortunately, his very well thought-out remarks can only serve as a summary, because the subject starts simply in theory, and turns complicated in practice  I would suggest putting it in your shop notes and rereading it a couple hundred times (as an introduction).

[MotoMike]

CO detector mounted, tested  and operating in the garage.  

 

[MotoMike]

front porch built and installed.