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48Cuf Gas Kiln Help


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#1 gianda

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Posted 15 July 2013 - 01:10 AM

Hi to everybody!

Am a newbie on this forum and have a problem with my self-built gas kiln. (following "The Kiln book")

 

Here the specs of the kiln:

 

48CuF downdraft  Gas Kiln (LPG) with 4 L shaped burners.

 

Sides in lightweight bricks with rock wool lining and roof in full rockwool 6" thick.

Chiminey in high temp bricks.

4 ports 3" dia and a 28Sq.in outlet.

We Heat the cylinders in a barrel with hot water.

 

We have been firing the kiln successfully and with low gas requirements for bisque at 850-900deg C.

 

(Before enlarging the burner ports from 2 to 3" dia we had heavy reduction in the kiln at temps above 900C)

For a cone 9 firing we have had problems . We managed to reach 1150C degrees,  with a stable climb but at this temperature the kiln gets stuck.

 

We slowly try to increase gas pressure but the temperature doesn't rise past 1150C and there is a slight reduction in the kiln.

 

The only problem I can think of at this stage after repeat firings gone wrong and tweaking is that the nipple/pin hole size is too small. (J size) and therefore when the pressure is increased, the velocity of the gas is too great for this size, therfore reducing the air intake (unforced) resulting in reduction.

 

We have virtually tried everything to solve this problem and wasted quite a number of cylinders ... any help would be greatly appreciated!!!

 

Thanks,

Gianda

 

 

 



#2 Mark C.

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Posted 15 July 2013 - 01:33 AM

My car kiln is only 35 stackable and the burner inlets are way bigger as well as the flue size is bigger (nat. gas)

My guess is your flue is to small or chimney is to short for draw or both-thats just a guess. As you are reducing above 1150 I think its to much fuel for it to exit well. Can you enlarge the flue?Or enlarge the burner inlets?

The reason its reducing is it cannot get out or draw enough to be neutral (hope this is explained clearly)

I always make the flue larger and block it down with bricks to fine tune it.

How tall is the stack? and is it tight ((for good draw) with a good fairly leak free damper?

Mark


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#3 gianda

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Posted 15 July 2013 - 02:38 AM

Hi Mark,

 

we had a 3'x3'x3' kiln which we used with the same burners without problems. they are pipe L shaped venturi burners with a 2" head. ( old kiln 27cuf, new one 48) we need to input obviously more gas then the old kiln..

 

Inlets were also 3" and the flue exit to the chiminey has been calculated on the fact that the flue outlet must equal or be greater then the inlet.

Therfore 1-1/2" x Pi x 4 butners is approx 19 sq." our flue exit port is approx. 30sq"  adjustable to 21sq." .

But either way it still reduces at higher temps ( above 1100C)

 

Chiminey hight is the same as the older kiln so I wouldn't think that to be the problem.

Reducing reason is clear... too much gas to be a neutral firing, but the reasons can be different...

Primary air, secondary air, burner port size, orefice size, circulation problems....

I think it could be the orefice size being to small and with increased pressure 14+ the gas jet too fas, creating less suction of primary air and combustion problems.

 

The reduction though is not extreme... only a small 5"-6" flame coming out of top peephole and not out of the chiminey at all.

The chiminey has an area of 60sq" is 5' tall no dumper.

 

thanks.



#4 Marcia Selsor

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Posted 15 July 2013 - 07:20 AM

Is this natural gas or LP?

Orifice sizes are different for the two fuels.Your gas company might have an orifice size chart.

 

Marcia



#5 Mark C.

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Posted 15 July 2013 - 09:45 AM

OK I'm with the orfice size not being right.As Marcia says check with your gas company or call Mark Ward at Wardburners.com to get the orfice dialed in. Without a damper in chimmey how are you controlling reduction?

Mark


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#6 Marcia Selsor

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Posted 15 July 2013 - 12:03 PM

No damper? what Mark says. how do you reduce without a damper. And, are you sure the chimney is ok for the larger kiln. Have you checked the draft using a torch of newspaper?

 

Marcia



#7 gianda

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Posted 17 July 2013 - 02:33 AM

For reduction we normally, during and at the end of the firing after soaking block partially the chiminey with bricks at the top and lower the primary air intake.

 

As for the Fuel we use LPG. We are based out of india, and the gas provider is probably the last person who is able to help us.

Also the guy who made the burners for us is postponing his visit to check them out and not showing any interest in it at all.

 

As far as it goes, the burners are identical to the ones fitted to our old 27cuf kiln. same nipples/orifices and all.

Afther telling him that our new kiln was 36cuf he said the nipples/orifices are fine being the same as for the old kiln.

 

I made a couple of calculations and estimated as per the Kiln book ref. the BTU imput required to be:

 

14K BTU per cuf. or kiln X 36cuf= 504KBTu

Now if we divide this number by 4 burners we will get a result of 126KBtu per burner.

This should give an orifice size of 3mm diameter.

 

The ones we have now are nearly half this size. lets call it 2mm.

I'm not sure if the tube burners we currently have are big enough to support this size orifice and take in enough primary air. ( the burners have an aluminum cone shaped primary air controller and fixing for the orifice, the body of the burner is made of normal 1" pipe and the burner head is 1-3/4" dia.)

 

The top burner in the pic looks more or less like ours... only our one has a conical primary air fitting and one single orifice and is much more rustic, being built with off the shelf pipes.

 

33.jpg

 

Morelike this one, just with a L shape:

burner_b.jpg

 

Ps.thanks for the nputs



#8 Marcia Selsor

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Posted 17 July 2013 - 07:34 AM

maybe take them to a machine shop and get the orifice drilled to the size you need. Check to make sure your gas lines will be able to supply that much gas.

Marcia



#9 gianda

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Posted 18 July 2013 - 01:43 AM

Line is fine being a LPG cylinder pressure is not an issue...

 

the Orifice size also I can drill myself... or buy ready ones here in New Delhi...

The info I needed though is more in relation to the calculations I made about the BUT's needed and orifice size and if the burners we have are able to support such an increase in orifice size...

Otherwhise, I'll have to make new burners...

 thanks again!



#10 Wyndham

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Posted 18 July 2013 - 09:38 AM

Do you have a gas regulator between the tank and the burner or are you adjusting gas flow with the tanks valve without a regulator?

If you have a regulator, what is the pressure gauge  reading scale, in water column inches or in pounds of pressure

Wyndham



#11 gianda

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Posted 21 July 2013 - 11:50 AM

Hi Wyndham

in PSI we go from 3PSI at the start of the firing to 11-12PSI towards the end. we do have two lines, each with 1 pressure regulator for 2 burners. (Total 4 burners)

Anything above 12-13 PSI gives reduction problems. and as said before we get stuck at 1120-1150 deg Centigrade.

 

Thanks,

Gianda



#12 Wyndham

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Posted 21 July 2013 - 01:27 PM

I think part of the problem is the dia of the tubes on the burners. I fire with venturi burners that create more draw through the burner than just a tube burner.

The solution might be as simple is adding 2 more burners like you have, one on each side. I think that would increase the primary air by about 1/3 as well as more secondary air. Adding an extra burner on each side would even out the heating. Increace the exit flue as well

I have a 50 cu ft stacking kiln that uses 6 raku venturi(705) burners that at peak gas input us .75 lb propane gas pressure at each burner resulting in only 3lb. I also use a 3/4 in gas line coming in as not to starve the burners for fuel.

 

If you can rebuild the chimney and put the damper about2ft up the stack and use a kiln shelf as the damper, you will create a venturi effect above the kiln shelf damper that will aid in the draw as well as regulate the back pressure in the kiln. This will allow you to control the amt of reduction and when you reduce and even out the heat in the kiln.

You should not need that high a gas pressure to fire the kiln.

 

You may also try getting larger tubes for the current burners and enlaging the exit flue opening. It realy looks that you need more air to combust the available fuel.

 

Hope this helps

Wyndham



#13 gianda

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Posted 23 July 2013 - 02:41 AM

Thanks Wyndham,

 

this helps quite a bit.  What are the sizes/diameter of your 6 orifices? All the same or different sizes so to be able to compare with my calculations before increasing sizes of burners and orifice.

Before rebuilding the chiminey I think I'll try with the burners... much cheaper and less time consuming if it works.

 

thanks.Gianda



#14 Ben

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Posted 23 July 2013 - 08:11 AM

Hi Wyndham

in PSI we go from 3PSI at the start of the firing to 11-12PSI towards the end. we do have two lines, each with 1 pressure regulator for 2 burners. (Total 4 burners)

Anything above 12-13 PSI gives reduction problems. and as said before we get stuck at 1120-1150 deg Centigrade.

 

Thanks,

Gianda

You are stalling at 1150C. At that point you need more heat in the kiln. (I know I'm oversimplifying the obvious).

You have to be able to burn the correct amount of fuel per hour, at the correct air/fuel mixture to burn the fuel properly to get heat rise.

Options to get more heat in a kiln:

1. Don't let as much out. If the exit flue is too big you'll let out too much heat. This doesn't seem to be your problem. But remember, the exit flue affects your combustion mixture (oxidation vs reduction)

2. Put more heat in.

Usually if you burn more fuel in the kiln per hour you'll add more btus so turning up the fuel supply should help. The suggestions to open the orifices are an effort to get more fuel in the kiln BUT you should always be able to burn that fuel in an oxidation atmosphere. If not, the amount of fuel burned in the kiln will go down. The more reducing the flame, the less of the fuel is getting burned so turning up the gas is not going to help unless you can supply more primary or secondary air.

 

I'm assuming that these are venturi burners and not forced air burners. If they are venturi burners the burners ability to draw primary air is limited by the design of the burner. That's why when you increase the pressure the reduction increases as well. The burners either can't draw enough primary air or the burner ports can't allow in enough secondary air.

 

For a 48 cu ft kiln I calculate that for hard fire brick you need about 250000 btu x 4 burners and with insulating brick about 180000 x 4 burners. 

For a 48 cu ft kiln I calculate that for hard fire brick you need about 160000 btu x 6 burners and with insulating brick about 120000 x 6 burners. 

 

 

For a 36 cu ft kiln I calculate that for hard fire brick you need about 180000 btu x 4 burners and with insulating brick about 135000 x 4 burners. 

For a 36 cu ft kiln I calculate that for hard fire brick you need about 120000 btu x 6 burners and with insulating brick about  90000 x 6 burners. 

 

What type of brick is your kiln made from?

What are the interior dimensions again? I'm not sure if it is 36 or 48 cu ft.

 

If we assume your burners to be 126000 btu, I think you need more burners either way you go about it.

 

Either way, when you turn up the pressure you are putting more gas in the kiln but the kiln stalls. It either needs more air or more gas or both.

To maintain oxidation or nuetral atmosphere with your burners you are limited in how much you can turn them up. If you want more heat but can't turn the burners up higher without increasing reduction (which decreases efficiancy), you need more burners.

 

Can you add 2 burners to your kiln?

If so, you will have plenty of heat available and will be able to get that heat at the air/fuel mixture you want to control the atmosphere.

 

Also, can you modify the burners to add forced air?

 

Hope this helps and is not to disorganized,

Ben



#15 Wyndham

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Posted 23 July 2013 - 08:44 AM

I don't have the orific size handy but they are set for natural gas on a raku burner, which is a bit bigger than propane.

 

My question is, are the burners tube type or venturi. the pictures look like tubes?

But in any event  the extra burners and/or larger tubes and larger secondary air should be be considered.

 

Here's a link to the MR750's I'm using

http://www.wardburne...uriburners.html

 

Wyndham



#16 gianda

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Posted 24 July 2013 - 05:48 AM

Wyndham,

Thanks for the link, just one more question,

 

whith a simple ruler/scale could you measure the orifice size for me.... Some rough measure, no need to go down to .1 mm and again how many burners you have for how many cuf. and pressure u use.

 thanks!

 

gianda



#17 gianda

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Posted 24 July 2013 - 06:24 AM

Hi Ben,

 

Thanks for the long and clear reply,

 

I think that we need as you said to put in more heat. and considering the increased pressure which we try to push in towards the end and the resulting stalling in temp, I could deduce that there is not enough air going in or that the combination of the size of the venturi and the orifice is not right.

 

As pointed out by Wyndham here is the venturi burners website, but they don't state what size orifice comes with the burners.

""Here's a link to the MR750's I'm using

http://www.wardburne...uriburners.html""

On the other hand it is not really an option to get two more burners which are not reaaly effective... I'd rather gather some good info and ( as I'm in India, ) get 4 new ones made from scratch for cheap rather then having to break through the floor of the kiln.

 

At the moment we're running 4 pieces of  2" burner head but I fear that the venturi burner size is small for the orifice size we need using only 4 burners.

 

 

200_siloutte_measure.jpg  
  diam. in inches Btu/NG Btu/LP   Model A B C 7"wc 11"wc 1 psi 2 psi 5 psi 10 psi 20 psi Price B-1 8 1.38 1.85 5,500 5,900 9,700 13,900 21,600 30,700 43,000 $145.00 B-2 11 2.25 2.75 13,900 9,000 14,800 20,900 33,200 46,700 66,400 $199.00 B-3 12 3.25 2.75 41,600 34,200 55,300 77,500 123,000 175,900 246,000 $215.00 B-4 17 3.75 3.5 67,800 55,300 89,800 126,700 200,500 286,600 403,400 $229.00 B-5 21 5 4.12 81,600 89,800 145,200 205,400 324,700 462,400 649,400 $375.00

 

LOOKING at the above table I see that the 2.25" B-2 burner they offer only goes to 66,400BTU at 20PSI, whilst the B-3 3.25" goes at 5PSI at 123BTU...

Shame they don't specify orifice size, as 4 of these burners would do the job.

 

By the orifice table size, It comes out that I should use a 3mm dia hole... I just need to double check with someone who has venturi burners and similar size kiln...

 

Our kiln is IS-8 high alumina briks sides lined with 1" of rockwool, top is full Rockwool, chiminey is IS-8 bricks and door is IS-8 bricks on the inside, 1 layer of 1" rockwool and insulation firebricks on the outside.

 

 

 

Thanks for any help!

Gianda



#18 Ben

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Posted 24 July 2013 - 07:03 AM

I don't think that drilling out the orifice is the solution and here is my reasoning.

 

The kiln needs to be able to get to top temp in oxidation if it ever hopes to get there in reduction since reduction is a less efficient use of fuel. Your kiln can't maintain oxidation and gain temp. I'm not certain why but I suspect the burners are simply not that great at mixing in combustion air at higher pressures.

As it is I think the kiln seems to be stalling due to lack of air, not lack of fuel.

You say you have no damper and yet the kiln is reducing. Most high fire kilns use the damper to limit secondary air intake to make a reduction atmosphere.

You don't have a damper and get reduction anyway so I think you are suffering from either too little primary air being drawn in and mixed with fuel by the burner or too little secondary air coming in through the burner ports due to port size or exit flue size.

 

 

My reasoning could be wrong and your suggestion that the burners become less efficient at mixing in air as the pressure goes up may be correct. If that is so, let me ask, Do your burners have a removable part that has the orifice drilled in it? If your burners have a screw in part with the orifice hole drilled in, I would buy some extras and drill one set larger and install them in the burners. If the new parts work it should be a cheap fix.

And while we are talking about the burner parts, does each burner have its own primary air adjustment? Maybe the primary air inlet is too small??

 

Maybe you can do a test for us. When it is dark and the kiln is empty with the door open light the burners and tell us what color the flames are as the pressure increases. It should have little to no yellow. The flames should be blue all the way to their tips.

 

 

Another thought. The best thing for measuring and drilling orifice size is a letter drill set. If you can borrow one it is best since you may not use it more than once. Wish you were here. We could use mine.

 

Please test fire the burners and let us know the color of the flames. That will tell us a lot.



#19 Wyndham

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Posted 24 July 2013 - 10:10 AM

Ben has a good point on the primary air intake. Hopefully it is adisk on a thrededintake tube.If so back it out all the way to get the max primary air.

 

The key is going to be getting the right fuel/air mix.

 

Does your burner have, as in the first picture, severalgas nozzels or only one?

if it is several gas nozzels than this may explain why you have so much fuel, hard to understand as I'm not familar with this type of burner

Wyndham



#20 Ben

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Posted 24 July 2013 - 11:25 AM

gianda, can you post pictures of the burner and the kiln?






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