Propane / forced air: Less gas => temperature goes UP?!

This afternoon I melted cast iron, it takes normally half an hour to melt 0.7kg of it to heat it to 1400 C and furnace chamber 1450 C. But now ... after 45 minutes the temp of the chamber did not rise above 1390 C and the metal was only partially molten: too cold. Upping gas pressure to 3 bar (45 psi) and max forced air did not help.
Then I did the unexpected: lowering gas to 2 bar (30 psi) increased the chamber temp to 1500 C within ten more minutes and the iron did melt and was blindingly hot, I needed my dark goggles to see anything. So I wasted half an hour of gas burning ...
Maybe gas / air ratio ? Too much gas cools down the mixture ?

Any experience on this ?

 

From what I can see, furnaces have a fixed volume combustion chamber, trying to burn more fuel results in it burning outside the furnace chamber. This is more noticeable with an oil fuel furnace as the fuel has to vaporize before burning. I ran an A8 crucible, small enough to double the combustion volume compared to the tight squeeze A25 size and I was able to comfortably melt iron with diesel fuel compared to making the iron soft and mushy with the A25, reaching only 1350 deg C (K type melted). My new furnace roughly doubles the furnace chamber volume with 1.37 times more chamber surface area. So double the burn rate with only 37% more chamber size that sets the heat losses. Furnace weight from refractory is about the same as the old furnace, so similar heating times for the refractory.



Supercharging with a higher pressure centrifugal blower would work too, but needs an airtight furnace, otherwise you'd be blowing fuel out the cracks and not increasing the chamber pressure.

 

@Mark, That is what I already suspected. The hottest part was probably outside the furnace chamber as I saw flames playing off the vent hole. Loweing the pressure made the flames outside much smaller and the temperature inside higher.
BTW, that is the reason I use type S thermocouples, they can measure up till 1600 C.

 

When I woke up this morning I flashed back on my dreams. I was casting a lemon juicer in iron. That had to come from reading this post yesterday...

 

At least with a piston engine, burning on the lean side of the stoichiometric ratio means hotter combustion. Too lean = melted pistons
Running rich keeps it cool and is considered "safe"

So I think that's a bit of a factor, too

Edit: Can you describe your S-type thermocouple setup? I need a pyrometer that I can use with bronze or iron

 

It is an old one but it still works great (right one below with correct temp), I have used it several times.
The other one is shorter and shows sometimes too high temp, but only at room temperature. At 'metal temps' both are correct.
Here some T/C s on Amazon.

https://www.amazon.com/s?k=type+s+t...e+s+thermocouple+0.5mm,aps,164&ref=nb_sb_noss

 

The thermocouple on the right with the pink sheath is similar to the S type I bought online which has a 1300 degree rating: I assumed either the ceramic sheath or whatever wire was attached to the platinum junction wasn't able to handle the temperatures.

 

Weird. Mine is heated several times to 1400+ degrees and sometimes (last Sunday as well) to 1510 C. But I don't let get it hotter (and I don't need it).
The white piece of Kaowool is because sometimes a bit heat gases leak out of the thermocouple orifice in the furnace wall.
I have a pyrometer as well (infrared gun type) which measures up to 1800 C, but pointing at the heat source is difficult because the red laser dot gets invisible on bright yellow hot objects of 1300+ degrees. And I put an almuminum heat shield in front of it as the radiant heat can melt the plastic housing.

 

It seems weird to make a S type thermocouple and limit the use to 1300 degrees: I assumed that it was a ceramic temperature limit or there was only a tiny bit of platinum rhodium at the junction and lower temp wires used.

 

I'll get hold of the same temperature display and suitable S type extension wire and try my S type out based on your experiences.

 

Ok so I am getting ready to order a type S thermocouple probe and a digital meter to go with it, I am seeing that the wire used seems specific to the thermocouple type. Should I order the $$ proper stuff (30yd roll that I will only need a few feet of) or is copper wire acceptable?

 

I just went through this exercise myself. Ali Express had S type nickel/copper extension wire, 5 metres for $15. If you search for "Thermocouple extension wire" you'll get plenty of hits.

The display unit I bought was faulty: pressing buttons would short the battery lugs to the circuit board copper ground plane, so I had to bend the solder lugs down.

 

I have just standard copper cables as extension and it works flawlessly.

 

Thanks Mark and metallab. I ordered supplies and went with fiberglass insulated copper wire for now for expediency, I will have a look at aliexpress offerings for extension cable.
I appreciate the heads up on the shorting issue with the display unit - will inspect when it arrives

 

This is the unit I bought with the battery short fault: the unit turned off and the three AAA cells got quite hot when the "type" button was pressed. Bending the terminal lugs away from the board fixed it.

 

Thanks for the picture Mark, I think mine is the same. If it goes scooters I will have something to check.

 

I was going to say something along these lines ^^

Most efficient cruising for a petrol engine is "lean of peak" - as you lean the mixture out the temperature rises to a point (peak) and then starts to lower

Adding more fuel, or running rich, cools the cylinders/valves/pistons by taking heat out the exhaust with the unburned fuel - the backfires you typically in V8 racing is fuel burning in the muffler, not the cylinder.

 

Ok so after an issue with Amazon (package with thermocouple arrived empty?? wtf) I managed to find time to put my S-type pyroboy together


I modeled it after my K-type thermoboy, came out decent for a thrown together build. Holding onto the thermocouple itself was challenging it's a weird shape, and the probe in mine was glued in at a ridiculous angle
Mark - my meter has had no issues with the batteries shorting thus far

Temp readings seemed a bit wonky near room temp/freezing and closer near boiling water, and in a flame or furnace exhaust seemed reasonable
I tested it out for the first time on some bronze yesterday and after about 6 seconds the tip broke off into the melt and killed the probe
I'm sure these units aren't rated for immersion in metal but I expected it to last a bit longer, do these require a generous pre-heat?

 

dont mess around with Cheap POOH, go buy one of these will probably last you a lifetime if handled correctly
https://mifco.com/shop/pyrometers/004062-12-reinforced-tip-thermocouple/

watch to the end for instructions



V/r HT1