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Kristin_Gail

Troubleshooting This Converted Kiln O' Mine: Taller Chimney?

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Trust me - if I could go back in time, I'd have never put the thing in!

 

I plan to add 4' of height to the chimney, plus the bag wall and regulator tweaking. Giver 'er another whirl. And another and another. If it comes to it, I can rebuild the chimney. But I'm going to call that my very last resort.

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Gail can you explain in detail how you built the chimney-Is the whole brick stack 9x9 inches inside or something else? and where is that something else located ?on the top or bottom of chimney? . Sorry if you have already explained this but so much has been covered.

 

Mark

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Mark - I read Nils Lou's book "The Art of Firing" just before building the chimney, and he suggests putting one course of bricks in the stack, a few feet above the flue, that reduces the size of the inside, in order to create turbulence.  I was under the impression that my chimney was way over-sized at the time, and thought this layer would possibly help, and if it turned out the chimney was far too wide, I could slip a smaller pipe down to sit on this layer if need be.

 

I didn't measure the interior dimensions of the hole when I built it, but it appears from photos to be about 6" x 6" or maybe a tad bigger.  http://kgspottery.com/MessesMostly/BigAnthony/43.jpg  It's about 2' up from the base of the exit flue. The rest of the chimney, both below and above this one layer, has a 9" x 9" space.

 

p.s. The second half of my name sounds funny when applied to me.  "Gail" alone is my mum!

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Ok I just reviewed both my Nils Lou books-I think its fine-You still may need more stack after you try again. I would not restrict it any more by putting a smaller pipe into it. 8 feet is just not very tall for a stack but you have burner/regualtor issues which need to be resolved 1st as well as the bag wall .

Mark

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I'm thinking that unless you want to change out the orifices, that you should look more at a low pressure (11" Water Column) HIGH VOLUME regulator.  Regulators have two ratings...... delivery pressure and total gas volume that they can pass in a unit of time (minute / hour).  The issue here is likely volume... not pressure (see my comments above).

 

The issue is,  we are dealing with a gas so the 2, volume and pressure are directly related.   The listed "high pressure" regulator means it CAN put out a high pressure but doesn't have to. (it depends on how large an orifice you adjust the screw on the adjustable regulator to)     The reality is,  given a specific orifice size,  flow rate of a gas through it is going to be limited by the upstream pressure.   (if the upstream pressure its between 1.7-1.9 times higher, you can consider the orifice "choked")

 

If you want to saturate the downstream burner orifice, which is sized to put out ~100K btu/hr at 11" WC, then you HAVE to have a larger upstream pressure.   You want that adjustable regulator because the mass flow (flow rate) through the burner orifice WILL change with thins like gas temp, pressure,  air temp etc.  

 

FYI a larger flow rate regulator, is a regulator with a larger orifice,  there is simply no other way to do it.  (so you might as well go with the adjustable regulator listed above)

 

Now that last part, gas temp is an issue.   

 

Propane is a liquid in the tank, and that requires it to vaporize to a gas in the tank before you can use it.  This process (a phase change) requires heat,  (a considerable amount actually)  the tank draws that heat from the environment it sits in.  

In short, a given tank size is rated to put out only a certain number of BTU/hr based on surface area of the tank and the liquid inside.  exceed that btu/hr and the tank will freeze up (you'll actually form frost on the tank on the lower portion that holds the liquid propane. that frost then acts as an insulator so your vaporization rate drops even quicker)  once the tank starts to freeze up,  the output of the tank drops as the cold liquid propane cant vaporize enough gas to keep up with your demand. eg the mass flow drops (aka btu/hr)

 

For instance,  a 100lb tank (the kind you have)  are rated to put out ~80,000 btu/hr at 60deg.  that number drops off quickly as the outside temp drops.   at 30 deg that is down to 52K btu/hr.  

 

SO even if you did fix your regulator issue,  If you plan to fire in late fall when its cold,  you may have issue with the tanks freezeing up.  They sell heating blankets for propane tanks to offset this issue, but they are expensive.  Alternatively you could switch to a larger tank that has more surface area.  (such as a 150 gallon (not pound) tank will put out ~214,000 btu/hr @40F)

 

(and yes when we were living in a house with a 500 gal propane tank when the temp hit -20F we were outside with a heating blanket on our propane tank so the furnace would get enough gas)

 

 

FYI people firing on natural gas don't have to worry about the above problem because the supply always comes in gas form (not liquid as propane does)

 

<---- mechanical engineer for a living

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Hi Marcia -  He set it up for propane - he knows I'm running on two 100-lb tanks, etc.  A few posts above yours, I shared Marc's response ("And the burner manufacturer's response:").  

 

schmism - I unfortunately cannot run on any larger tank than 100-lb.  Bigger than that, and it would have to be delivered, and they cannot deliver to my system.   The tanks did freeze, the first time, when the outside temps were hovering just above freezing.  But they did not the second time.

 

I have called more people than I care to remember, looking for a damn piece of culvert pipe to extend the chimney.  Once I finally find one, I will be trying this again, with that change (4-5' added to chimney height); higher bag wall; turn that dial in the regulator; and oh yes, by that time it will be threatening Spring here, so the air temp will be higher.

 

I had hoped to have done all this by now, but unfortunately the rest of life (what, there's life beyond that kiln?) has gotten in the way.  

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Just a thought about the gas... would a manifold between the tanks and burners help to prevent freezing of the tanks from too rapid depletion with associated drop in pressure. I built a forge using two venturi burners. The tanks were small and flow rate was high. Placing a large manifold between the tanks and burners helped to stabilize the tank temp, pressure, and flow rate.

 

Just a thought.

 

Jed

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Kristen,

 

It is all about a combination of vaporization rate............ which is impacted by surface area of liquid gas exposed to evaporate from combined with the volume of liquid storage, and the availability of heat energy to replenish heat energy used in the evaporation process. You can certainly improve that "draw" situation by adding a third yoked tank (increase both storage volume and evaporative surface area)..... but I do NOT think this is your problem at all.

 

You can only burn the gasseous propane as fast as it will evaporate off the surface of the tanks. That evaporation rate is determined by the square area of the liquid surface exposed, the temperature at any given time in the firing of the liquid gas store, and (technically) the partial pressure of propane in the open space in the tank (whcih relates to the draw rate).

 

YOU don't need to know all the technical stuff....... your propane supplier has handy little charts of vaopriozation rates for given sizes of tanks with the tank in an environment at a specific temperature. As long as your kiln's burners needs are LESS than the numbers you have.......... you'll have the BTU output to the burners to fire the kiln.

 

That rate is a function of the surface area of the particular tank design, and the thermal mass of the gas storage. To evaporate, it requires heat energy to change from liquid to gas state. That energy flow comes (typically) from the outside environment that the storage tanks are placed in. They absorb heat energy from the surrounding environment. This keeps the temperature of the liquid storage up, and allows evaporization to continue. If the heat energy flow into the tank liquid is not equal to the energy uised to evaporate the liquid to gas, the store of liquid cools off. As it cools, the evaporization rate decreases.

 

Non of this speaks to WHAT the burners and the kiln system DO with that gas's potential heat energy. THAT is where the issues lie.

 

best,

 

..................john

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