Propane kiln build: low pressure vs high pressure (and fixed vs adjustable)

[hanee]

I'm trying to put together an order for a propane kiln I'm building and have a few questions that have arisen from advice that's not lining up both on these forums and elsewhere.

First, because questions usually arise about the build. This will be a small 8-12cubic foot (maximum 30sqft internal surface area) fiber-kiln, probably initially will be tested out as an Ian Gregory style flatpack. The long term goal is building a similarly sized refractory lined fiber kiln for *clean* wood firing but I'm using propane and temporary fiber kiln designs to work out BTU needs/chamber-shape/etc... in all cases it's for mostly firing to Cone 010 - Cone 06 Oxidation for firing terracotta sculptures and some occasional Cone 6 Oxidation glaze firings for my wife whose just starting out in pottery. There is no intention to ever run a reducing environment or to ever run over Cone 6 (we're interested in being fuel efficient and clean burning with both LPG and eventually wood).

So, as for the questions:

At this scale and with the fiber approach, most people seem to be using a 0-30PSI adjustable regulator and something like an MR-750 venturi burner. Often people are using 'weed burners' as well with surprising success. All of this equipment is geared towards a Raku approach of very fast firing (which is not what I'm looking for) and bundled kits are sold for this setup. None the less, people *do* seem to be managing to bisque (Ian Gregory, Simon Leach, etc) using this exact set up.

Based on research on these forums and elsewhere it seems to me that whether using the MR-750 or similar (or building my own venturi burner), my BTU needs can also be easily served by low pressure systems. My maximum BTU/hr needed is estimated at around 100k.

I see two options that provide this capacity:

(A) 11" WC system with (2) MR-750s or similar with an extra large flexible hose (1/2 OD 3/8 ID) vs the usual smaller ones because as far as I can understand the smaller hose would be the limiting factor on flow and as far as I can tell 3/8 ID would be required to get up 100k BTU/hr at 11" WC with a 10ft (minimum length I'm comfortable with safety-wise) hose.

(B) ~5PSI system with (1) MR-750 or similar with standard issue 3/8 OD hose. With or without the regulator being adjustable.

The biggest thing that's tripping me up right now is that while the most popular view is something like:

"You need an adjustable regulator and high pressure to control the burner output and a dial to create a schedule" (these people are usually using portable raku-type setups)

I've also read on this forum something like:

"Pressure should be fixed because it has to do with orifice size, all control should happen with ball valve/needle-valve downstream of the regulator and NOT at the regulator" (these people are usually experienced in permanently plumbed kilns with larger diameter pipes and often with natural gas or residential propane where there's no control on pressure anyway).

Also some advice indicates that low pressure is intrinsically safer and preferable. But I can't wrap my head around that beyond leak-pressure on fittings. If, in either case, I need at the end of my burner 100k BTU/hr, it would seem it didn't matter if I got that via high pressure and low volume or via low volume and high pressure -- the same amount of cubic feet of gas is going to be coming out. Leaks aside, if there was no BASO, a flame blew out, and you had a danger of pooled propane, it would be the same total volume of propane in both cases and the same exact risk.

Also I've read some low pressure advocates saying that only low-pressure gives you the fine adjustability needed to bisque and yet I've read high pressure advocates saying high pressure is necessary to have fine control.

So at this stage, I've got to settle the low-pressure/high-pressure decision and whether the regulator is a point of control or can operate on fixed pressure.

Are these differences simply equivocal in my limited application or does one side truly have a better or 'more correct' argument?

Thanks in advance to any who have any insight to offer...

[neilestrick]

Raku systems are made to fire fast, and that's really their only concern. If you plan to glaze fire to a specific cone and use glazes, then you want far more control than a raku system will provide, especially if you plan to bisque fire in this kiln. Firing slow at the beginning can be very difficult in gas kilns. Personally, I prefer power burners since they can easily be sized for a wide range of outputs, and in smaller kilns you can just use one burner. They are a little more complicated to build than a Venturi system, but it's something you can build from hardware store parts fairly easily. Whichever way you go, you definitely need to have some sort of safety system, at bare minimum a Baso valve and pilot light on each burner. If you build a power burner, you can also easily (and should) add a solenoid valve on the gas line in addition to the Baso, a blower speed control, and even a hi-temp shutoff controller.

In regards to the high vs low pressure question, I have never used a high pressure system so I can't really help you there. I would definitely call Ward Burner, though. Nobody knows more about kiln burners than Ward.

[hanee]

Thank you for the quick reply @neilestrick. With regards to power-burners, unfortunately I'm off-grid with a very minimal solar system. So venturis it must needs be.

I do tends towards grogged bodies and know how to ensure my work is dry. Between that and possibly candling with a lower power Bunsen burner or pilot, I could probably stand a little more thermal shock than average, but still definitely not looking to fire at Raku speeds.

The MR-750 is listed as going down to 17kbtu in several places. Not quite sure if that'll be slow enough. Worst case I could just waste the heat by opening the kiln a bit too much or throwing in a little thermal mass, but I'd rather buy or build the right burner or burner combo to begin with. One reason to build my own Venturis is that I could swap between a lower BTU geometry/orifice and a higher one half-way through the firing if needed. I can't afford one of the low-BTU Ransome's, let alone two.

Hopefully someone chimes in on the pressure questions (otherwise I will indeed try contacting Ward Burners).

(And yes, the BASO is part of the plan. But thanks for the reminder!)

[Bill Kielb]

8 hours ago, hanee said:

So at this stage, I've got to settle the low-pressure/high-pressure decision and whether the regulator is a point of control or can operate on fixed pressure.

Are these differences simply equivocal in my limited application or does one side truly have a better or 'more correct' argument?

 

If you look at orifice tables they are generally in inches so most burners are designed for inches of wc because natural gas has a bunch of energy and even with a really small pressure one can achieve lots of btu with a tiny orifice. At 1/8” diameter I can get 100,000 btu with only 9” of. Pressure. (About 1/3 of a psi) That same orifice at 2 PSI will be 144,000 btu.

Propane tanks are relatively high pressure so they can hold a reasonable amount of liquid propane in a small bottle and operate in low temperatures.. My advice: follow the design tables that we have used for the last hundred years. Will it improve adjustability, definitely, just use a metering hand valve to control your burner like most kilns. When you adjust the hand valve you are essentially dropping the pressure across it from zero to 1” to 2” etc…. That’s WAY harder to do with 30 psi on one side of it than with 11”.  

BTW you need a valve so you can adjust the kiln output as needed. In the beginning, small flame gradual warming, at the end more heating. This is just firing, a kiln doesn’t cycle on and off maintaining a temperature as if it were an oven. People hand fire it according to their schedule. It never starts out full on and just roars through the firing. Set your regulator at 11” and adjust downstream with the hand valve. This way as tank temperature and pressure changes you will always have a steady 11” to your kiln and your last hand setting will remain consistent.

Are burners in general sized for inches of water, YES - Stoves, heaters, grills, dryers, fire places, kilns, Venturi burners, basically gas appliances.  Can you pipe high pressure propane - YES. Is it more dangerous than low pressure if there is a leak - YES, without a doubt. If you run low pressure do you need a larger diameter hose? - yep, follow the tables.

Can you cram PSI through burners sized for inches? Yep, the head loss (friction loss) goes up enough so the burner puts out more but the head loss becomes really high.

Are orifices designed to operate in inches that way for a reason Yep, always have been. Do most propane appliances have a regulator built into them? YEP PSI to inches of water column.

Are inches of pressure misunderstood by many - in my experience, YES

Call Ward for sure, he is used to this - once inches of water column is understood, generally the mystery goes away.

here is a very generic video you might find interesting  https://www.youtube.com/watch?v=T-QhRrQA21g

Orifice table (inches of water Column)

 

Finally,  for CSST it looks like 3/8” inside diameter at 10 feet is fine for 100000 btu but check with your hose provider based on 11” of water column what diameter hose

Edited November 7, 2022 by Bill Kielb

[Kelly in AK]

Since you're gathering information, I'll throw something out. My small kiln. It's unsafe because no automatic shutoff system, so don't do what I do.

It works very well though. Fires evenly (after fussing with the bagwall a bit), uses very little fuel. Cone 6 is no problem. It's certainly the most bang for the buck of any kiln I've built. Two layers of 1" fiberboard. Kiln shelves with insulation on top are the lid/roof. The burners and adjustable regulators are for propane cookers.

For bisque I candle it with a burner from a plumber's torch that's adapted to fit on a big tank. A pyrometer is necessary to keep from blowing things up. 

Weed burners I've used are awful. Super loud, tricky to adjust, and no less expensive than this setup.

The fiberboard shrunk when fired, I had to reset the inner panels after the first few firings. It's also rather delicate.

Edited November 8, 2022 by Kelly in AK