Kiln firing variations

[hitchmss]

Hey everyone, so seeking some information regarding some variations in my firing times. Not new to gas firing; been firing gas for almost 15 years now, current kiln for past 6 years, doing about 40 firings a year, with my studio mate doing about 20 a year as well. Some info about the kiln; about 60 cu foot car kiln(short of 4' cube interior dimensions, before Arch), 2800 deg IFB's, 4.5" wall, 2300 deg-1.5" blanket on cold face, ITC 100HT coating on hot face, downdraft, 12"x8" x20' chimney, burner ports =flue opening, big Bertha burners on LPG, low pressure, using ball valves for gas flow, and low pressure psi gauge. Use the same firing schedule(what I think is faster, but not pushing it) for all my oxidation firings (soft cone 12), and reduction firings(cone 11) have slightly altered schedule (naturally). Question is; MOST all oxidation firings run almost 8 hrs on the nose from room to maturation, and redux are about the same length 8-8.5 hrs. However, (herein lies my question) is like on a day like today, loaded and fired kiln off in ox, using same schedule, and kiln shut off at 6 hrs mark. I get these rogue firings every so often and have tried to wrap my head around the "variables" which would play into this change in speed. For reference in the broad spectrum: been using the same kiln, and schedule for past 5 years, maybe 300 firings, and possibly a dozen or so rogues. A more finite reference is havijg done 6 firings in the past week: 5 ox, 1 redux, of the 5 ox firings, 3 were 8 hrs on the nose, 1 was 7.5, and today's was 6, redux was 8. The propane is the same fill-up, setup/schedule hasn't changed, load density is all about the same, with the shortest maybe being the most dense. The biggest variance is outdoor temp/weather; today was 30's, and windy (didn't check pressure), other days were warmer, but windy also. Could the drop in temp, increase the velocity in my stack enough to shave 2 hrs off firing? I understand that the denser my load the more the radiant heat, but also more mass to heat to that point (advancer shelves, so minimal furniture). I've been running on a 24 schedule (2 hr to load, 8 hr up, 12 hr down, 1 hr to unload), so when reloading there is some minimal heat in the brick still, maybe a 100 deg; cool enough to unload/handle brick with bare hands. What do you think? Kiln gods making sure I'm keeping eyes on my kilns? 

Here's my schedule;

Start 1/8 psi, damper 3 1/2" open

Hour later, 1/4 psi, damper same

Hour later 1 psi, damper same

Hour later, 2 psi, damper same

Hour later, 3 psi, damper same

Hour later, 4 psi, damper 4" open (damper open to maintain ox) 

Hour later, 5 psi, damper 4 3/8" open

Maintain until soft ^12

 

 

 

[neilestrick]

Firing times in gas kilns can definitely be affected by weather. We had one in college that fired an hour faster when it rained. The density and shape of the load can also affect things. It's aprt of the joy of firing with gas.

[hitchmss]

Thanks Neil, in college we had huge Johnson burners and weather never impacted our firings. Once going to natural draft 7 years ago I learned quickly how weather could impact a firing; sometimes the pressure and wind would cause initial ignition challenging unless all the doors in the studio were open. It is the only variable that I can think of which would impart a drastic difference. Figured I'd ask a group of more experienced than I to see if I'm overlooking so.ethjng obvious; oftentimes I am. 

[neilestrick]

The kiln I was referring to that was affected by weather had power burners. My old kiln, which also had power burners, was also affected sometimes. I would imagine that natural draft is more susceptible to the effects of wind, though, as it could change the draft a lot. I think there's something about barometric pressure that affects combustion in general.

[Magnolia Mud Research]

Yes, barometric pressure is an important combustion variable.  High barometer readings means that there is more molecules of oxygen per unit volume of air than in air with a low barometer reading.  The ratio of the two barometer readings gives the ratio of the number of molecules in the same volume. 

The temperature and the absolute humidity readings also effect the combustion.  When the absolute humidity is high (high relative humidity AND high ambient temperature) the air is diluted with water.  Said another way, you must "pump" more air into the kiln just to burn the fuel you are supplying the kiln.

The absolute altitude of the kiln is also an important variable. Moving a kiln from Miami to Denver will change the way it appears to fire. Regardless of power or natural draft burners. 

It's all summed up in PV = nRT  (a.k.a. Ideal Gas Equation)

LT

 

[neilestrick]

What's interesting is that if you tried to fire that kiln in college an hour faster on a normal day, it wouldn't work. It would fire unevenly and you'd be fighting it the whole time to try and keep it even. But on a rainy day, an hour faster and still even.

[Dick White]

I have heard (but with no scientific proof offered, as I learned of this during a BOGSAT (bunch of guys sitting around talking)) that on a rainy day the water molecules in the ambient humidity are cracked by the intense heat and release some additional energy, whether from the broken molecular bonds or from additional combustion from the now-free hydrogen. FWIW, YMMV.

 

dw