Jump to content


Photo
- - - - -

48Cuf Gas Kiln Help


  • Please log in to reply
48 replies to this topic

#21 gianda

gianda

    Member

  • Members
  • PipPip
  • 17 posts

Posted 25 July 2013 - 04:48 AM

Hi Guys,

 

am in Italy at the moment and will be back in India on the 31st July. Then I'll get some pics and post them but here a couple more info on the burners.

 

They are made from off the shelf water/gas connection tubes and elbows I think, ( as I don't have them here with me) they are 1-1/4" dia. the design is much like the B/W Diagrean just below the colou picture. 

the only machined parts are the venturi funnel in ALU which has a dial/disc on a thread. The orifice can be removed and changed over and the burner head has the ring with the small holes on it, and the center is hollow.

 

As Ben says, I do feel that it is an air intake problem. and I feel it has to do with primary air, as the secondary has a 3-1/2" diameter ( straight hole). (The burner head is 2-1/4"  and rests  1-1/2" away from the port)

 

As far as primary I have tried to open the air to its max, but without success. And at this point I think that ( having tried both smaller and larger orifices) the problem may rest in the size of the burners tubes.

i.e. I think that the smaller orifice "sprays" a cone which is the right size for this tube, but that increasing the orifice size, the "spray" cone gets wider thus reducing the sucking action of the venturi.

As each burner ( and expecially venturi's) are specifically designed for a cerain operation condition ( BTU, pressure, orifice size) I feel that it may well be that the burner size is too small for the orifice I'm trying to use and that with a smaller orifice, which is right for the burner won't give the required BTU's. ( new burners are not a huge issue as being in India I can get them made for a very very reasonable price. )

 

If I only knew before that the guy who supplied them to me didn't have a clue I would have probably tried to make them on my won or purchased them online...

 

Till the pictures come bear with me!

 

 

P.S here some pics of pots and the open kiln with the 130 deg underfired pots...

 

If you want to see out web is www.italyindiadesign.com and FB is www.facebook.com/italyindiadesign

 

hear from you soon guys,

 

Gianda

 

 

 

 

 

 

1017605_446994462062678_318904792_n.jpg

1004445_458171470944977_165643523_n.jpg

1005182_458171474278310_645368784_n.jpg

946914_458171487611642_1877873077_n.jpg



#22 Wyndham

Wyndham

    Advanced Member

  • Members
  • PipPipPip
  • 376 posts
  • LocationSeagrove NC

Posted 25 July 2013 - 01:40 PM

I think you are on the right track

Wyndham



#23 wadar

wadar

    Member

  • Members
  • PipPip
  • 10 posts

Posted 29 July 2013 - 06:54 PM

I am anything but a kiln expert, but I do recall that the area of a circle is pi times the radius squared. that would give an area of 28,28 sq.inches for your 4 burner ports and I think you said your exit flue was around 28 sq. inches. If you consider that the volume of hot air coming out is greater than that of the cool air going in then I suspect that the problem is possibly that you cannot exhaust enough spent fuel. Making the burner ports bigger or the burner orifices larger will not fix the problem, if this is true.

Sure am curious to know how you eventually fix the problem, as I am creeping up on building a larger kiln myself.

Best of luck,

Bill



#24 gianda

gianda

    Member

  • Members
  • PipPip
  • 17 posts

Posted 13 August 2013 - 04:26 AM

Hello everybody!

 

sorry for the long wai in my reply, but spent some time welding the new stainless burners and did a firing this sunday!

 

So, started the firing ( Glaze) with the 2 old burners with a smaller orifice ( 1mm dia) and all went fine.

After 700C added the 2 new burners which have a pipe dia of 1"1/4 and an orifice of 1.8mm (enlarged the air ports)

 

In between power went off 2 times so couldn't monitor the temp. increase properly and it got stuck a couple of times.

 

Gas finished on the 2 small burners and all of a sudden temp. increased steadly and at a much faster rate, from 1160C to 1235C after which even the gas of the 2 larger burnrs finishes...

From 1000C all the firing was just slightly reducing ( and some rockwool in between fell on one of the burners also stalling the kiln for some time till I discovered the problem.)

 

From what I can say till now I do think that given the extra gas the kiln would have reached temperature, as the increase was steady and fast ( 1deg C every35-40 sec.) from 1160 to 1235.

But as the kiln was always reducing with all 4 burners I guess another way to go about it is to use just two... as the 2 new burners can handle the requirement as it has shown...

 

As far as the exit flue goes, to decrease the reduction, I could enlarge slightly the outlet flue... ( from 900C upwards I had to keep the top peep hole open ( 2"x3" ) otherwhise either the gas pressure was too low even with the closed hole, or with more gas going in it would reduce....

With the open peephole, a 4" to 5" slow flame was coming out but temp. rising.

 

Total damage for this firing.

 

3 full commercial cylinders 19KG gas each

1-1/2 domestic Cylinder 14kg each

 

 

total gas used 78Kg of LPG (100US$ here in India). ANy idea if this sounds right for a 1280 firing in my kiln size (4'x3'x3')? I'm kind of convinced I should be using about 1/3 less gas then this...

 

Thanks guys!

 

Ps. here the pic of the old small burners with a tip dia just shy of 2" inner tube dia of 1"  . the new ones are nearly 2"1/2 inner tube dia 1'1/4

 

Attached Files



#25 Marcia Selsor

Marcia Selsor

    Professor Emerita, Montana State University-Billings

  • Members
  • PipPipPip
  • 3,753 posts
  • Locationwhere Texas, Matamoros, Rio Grande and Gulf of Mexico come together.

Posted 13 August 2013 - 06:43 AM

I can't say as I am not sure of the kiln insulation, but it does seem like a lot of gas after i did the conversion.
170 pounds of propane. Anyone out there firing a kiln this size with propane care to comment?

It sounds like your burners were putting out too much gas. see if you can use the larger ones alone and have them on a harder flame. can you add more air intake on them by turning a disc at the rear of the burner?


Marcia

#26 Wyndham

Wyndham

    Advanced Member

  • Members
  • PipPipPip
  • 376 posts
  • LocationSeagrove NC

Posted 13 August 2013 - 11:42 AM

If my numbers are close, that's about 43 gal of propane, where LP gas weighs 4.1 lb/gal. that might be a bit high but with some more firings, you might get it down 10 or 15%.

I pay about $1.60/gal and you about $2.20/gal just close not an exact amt.

Wyndham

 

My math might be way off, I'm not sure I have the numbers right



#27 Mart

Mart

    Advanced Member

  • Members
  • PipPipPip
  • 282 posts

Posted 13 August 2013 - 01:36 PM

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.

....

 

How did you arrive at 3 mm?

I searched around a bit and I found out that 3 mm (0.11811") is going to give you close to 105K BTU , so you are about 21 BTU's short per burner,  or 84 BTU in total.

 

 

EDIT: and if you are "reducing", you need to give it more air from intake.

Can you make the burner holes bigger, so more secondary air gets in?

 

PS! Do not take my word for it, I am not a pro nor even a noob :)



#28 gianda

gianda

    Member

  • Members
  • PipPip
  • 17 posts

Posted 14 August 2013 - 02:19 AM

Hi marcia,

 

the efficiency of the two single large burners stops at 3.5 psi  (I could fit a .2mm smaller orifice though and the increased velocity of the gas at the same pressure should be able to drag in more air. 

The larger burners primary air is regulated by a dial/disc and the secondary is pretty big at a square 3" in side size.

 

 

I can't say as I am not sure of the kiln insulation, but it does seem like a lot of gas after i did the conversion.
170 pounds of propane. Anyone out there firing a kiln this size with propane care to comment?

It sounds like your burners were putting out too much gas. see if you can use the larger ones alone and have them on a harder flame. can you add more air intake on them by turning a disc at the rear of the burner?


Marcia



#29 gianda

gianda

    Member

  • Members
  • PipPip
  • 17 posts

Posted 14 August 2013 - 02:37 AM

Hi Mart,

 

Having a look at the orifices the other day, I discovered the old ones in the small burners were 1.2mm dia.

The other issue is the pressure... the more pressure you put in the line, the more gas will come out, and with a LPG cylinder you can get up to 25 PSI.

But the burner size and design as well as pressure and orifice are all influential to a good flame with enough btu's.

 

Anyways... the big burners with a 1.8mm hole seamed to do well up to 3.5psi after which the flame quality deteriorated...

I'm now curious to print out a chart I've found on the net for different drill size and output of BTU's at different pressures and check with smaller size orifice at higher pressure.

It could well be that my burner is not able to generate enough venturi effect and is therefore not sucking in enough primary air to mix in the fuel.

Apart from that otherwise the kiln is quite responsive and I don't think there is a circulation problem... i.e. if the kiln is not reducing and I blast the gas though causing reduction, it takes less then 3 seconds for the unburned green gas flame to come out of the chiminey...

 

FOR your info, attached the PDF with orifice sizes which helped me to find out the burner tip size and orifice size...

 

 

 

 

 

 

 

 

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.

....

 

How did you arrive at 3 mm?

I searched around a bit and I found out that 3 mm (0.11811") is going to give you close to 105K BTU , so you are about 21 BTU's short per burner,  or 84 BTU in total.

 

 

EDIT: and if you are "reducing", you need to give it more air from intake.

Can you make the burner holes bigger, so more secondary air gets in?

 

PS! Do not take my word for it, I am not a pro nor even a noob :)

 

 

Attached Files



#30 Mart

Mart

    Advanced Member

  • Members
  • PipPipPip
  • 282 posts

Posted 14 August 2013 - 04:30 AM

Do you remember that Swiss guy Daniel Bernoulli, with a huge wig, who stated that for an inviscid flow, an increase in the speed of the fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid's potential energy.

Point is, that not only more gas comes out, but the gas comes out at faster speed. That is why at some pressure/nozzle orifice size it chokes and lots of gas is wasted.

1 kg of propane has energy 50.3 MJ (megajoules) or 47700 BTU and to burn 1 kg of propane you need 3630 g of O2.
Our atmosphere (air) has only 20.946% of O2 so you need about 17.3 kg of that stuff entering your burner/kiln in same time frame as propane.

If secondary air is not getting in , reduction happens (gas is wasted).

Now all we need is the nozzle_orifice/gas_pressure/burner_opening ratio. :)

Thankfully someone has done most of the hard work by calculating nozzle orifice/pressure to BTU output but we also need a kiln port?(opening for the burner) area calculation for the air.

I bet that this kiln book has it somewhere written down. If it has, can you please post it ?



#31 Mart

Mart

    Advanced Member

  • Members
  • PipPipPip
  • 282 posts

Posted 14 August 2013 - 12:43 PM

I guess that resolves the previous question :)

 

A rough rule-of-thumb ratio is one square inch of flue area to 8,000 BTU’s of maximum gas input

 

 

If area of flue opening (inside) is usually close to or equal to area of burner port opening .... 



#32 wadar

wadar

    Member

  • Members
  • PipPip
  • 10 posts

Posted 14 August 2013 - 05:49 PM

Hey there,

based on all i've read here and in "the kiln book" 3rd edition by F. Olson it seems like you're choking the kiln. you've gone from 27cf to 36 cf and 2 burners to 4 burners, respectively, so i would think that lack of fuel is not the problem. From page 201 of said book:

"Exit flues. There are 3 methods of determining exit flue sizes:

1. Add the total inlet flue areas (ports) to find the exit flue area.

2. A rule of thumb for exit flues is 2 1/2 sq. in. per cf kiln space.

3. I find that 7,000 btu burner input requires 1 sq. in. exit flue area, regardless of the burner system used A kiln requiring 1,000,000 btu burner input will need an exit flue of143 sq.in., or 12"x12".

It is always better to have flues on the large side than to have them small and choke the kiln. When the flues are too big they can be made smaller easily. Of the three methods, I use No. 1 and 3 and build the flue accordingly."

 

I also suspect that your inlet ports are too small, not allowing enough secondary air, but this is just a guess based on the size of the flue indicated by your required btu input for that size kiln.

 

Again, good luck with it,

Bill



#33 wadar

wadar

    Member

  • Members
  • PipPip
  • 10 posts

Posted 14 August 2013 - 06:27 PM

on the previous page,

"...if the kiln is under constant reduction and will not climb in temperature easily, the portholes can be widened to provide more secondary air for combustion. at the same time make sure that the exit flue is not too small. However, most of the time the burners will have oversixed orifices, thus throwing off the air/gas ratio for proper combustion.To solve this problem, replace the orifices with smaller ones. (Normal orifices for low pressure burners are between ..... 38 and 42 for propane"

 



#34 gianda

gianda

    Member

  • Members
  • PipPip
  • 17 posts

Posted 15 August 2013 - 12:30 AM

If I go by this  and consider the kiln to require 12K BTU per CUF   it will come to 36x12/8=54 SQ INCH   my flue size now is 7"x4.5"= 31.5  SQ INCH

I don't think an increase in flue size by  40% more or less is the solution...  especially if the burners are homemade ... I consider it to be more of a orifice size/ air inlet problem.

But this said, I am planning to remove 2 bricks and add 2 1/2 bricks to the flue gate, thus increasing the flue size by 13.5sq" ... to 45SQ"  and block off part of it to make it roughly 37-38 sq".

 

 

 

 

 

A rough rule-of-thumb ratio is one square inch of flue area to 8,000 BTU’s of maximum gas input

 

 

If area of flue opening (inside) is usually close to or equal to area of burner port opening .... 



#35 gianda

gianda

    Member

  • Members
  • PipPip
  • 17 posts

Posted 15 August 2013 - 12:40 AM

For gas kilns in The Olsen book it states that the exit flue port should be equal to the area of the inlet ports sizes.

 

On rule of thumb NB 2 it would come out to 90Sq".... 3 times more then what i have now...

 

By rule 3 it should come to 63 sq "  ( double)

 

An by rule 1 which is on the book ( 2 large square ports at 3"x3" + 2 round ports at 2"1/2 dia  =   34sq")

 

 

 

 

 

Hey there,

based on all i've read here and in "the kiln book" 3rd edition by F. Olson it seems like you're choking the kiln. you've gone from 27cf to 36 cf and 2 burners to 4 burners, respectively, so i would think that lack of fuel is not the problem. From page 201 of said book:

"Exit flues. There are 3 methods of determining exit flue sizes:

1. Add the total inlet flue areas (ports) to find the exit flue area.

2. A rule of thumb for exit flues is 2 1/2 sq. in. per cf kiln space.

3. I find that 7,000 btu burner input requires 1 sq. in. exit flue area, regardless of the burner system used A kiln requiring 1,000,000 btu burner input will need an exit flue of143 sq.in., or 12"x12".

It is always better to have flues on the large side than to have them small and choke the kiln. When the flues are too big they can be made smaller easily. Of the three methods, I use No. 1 and 3 and build the flue accordingly."

 

I also suspect that your inlet ports are too small, not allowing enough secondary air, but this is just a guess based on the size of the flue indicated by your required btu input for that size kiln.

 

Again, good luck with it,

Bill



#36 gianda

gianda

    Member

  • Members
  • PipPip
  • 17 posts

Posted 15 August 2013 - 12:58 AM

In bernoulli's principle the talk about "potential energy" is in terms of pressure not BTU's....

 

If the PSI on the pressure regulator at the cylinder is the same both for a, lets say, 1.6mm diameter hole or a 1.8 dia hole, the gas quantity coming out of the orifices might be the same, only the speed at which the gas travels will be different.

if the gas travels faster it should create more venturi effect ...

As far as secondary air, the inlets are quite big,,,

 

If you do a test with the burners out of the kiln it is quite easy to determine the flame quality and to see if enough primary air is there or not...

This is mostly due to burner design....  i.e. I can have a perfect burner design, but if the orifice is way over or under sized, it will work at a % of its real efficiency.

 

I will have to do an empirical test...  and go down from 1.8mm dia at 3.5psi to a 1.6 and maybe 1.4 at a slightly increased pressure and see if there is too much turbolence or if the flame burns better...

 

Do you remember that Swiss guy Daniel Bernoulli, with a huge wig, who stated that for an inviscid flow, an increase in the speed of the fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid's potential energy.

Point is, that not only more gas comes out, but the gas comes out at faster speed. That is why at some pressure/nozzle orifice size it chokes and lots of gas is wasted.

1 kg of propane has energy 50.3 MJ (megajoules) or 47700 BTU and to burn 1 kg of propane you need 3630 g of O2.
Our atmosphere (air) has only 20.946% of O2 so you need about 17.3 kg of that stuff entering your burner/kiln in same time frame as propane.

If secondary air is not getting in , reduction happens (gas is wasted).

Now all we need is the nozzle_orifice/gas_pressure/burner_opening ratio. :)

Thankfully someone has done most of the hard work by calculating nozzle orifice/pressure to BTU output but we also need a kiln port?(opening for the burner) area calculation for the air.

I bet that this kiln book has it somewhere written down. If it has, can you please post it ?



#37 Mart

Mart

    Advanced Member

  • Members
  • PipPipPip
  • 282 posts

Posted 15 August 2013 - 05:34 AM

What made you think I was talking about BTU's, when I mentioned Bernoulli's principle? :)



#38 wadar

wadar

    Member

  • Members
  • PipPip
  • 10 posts

Posted 15 August 2013 - 09:13 AM

 

For gas kilns in The Olsen book it states that the exit flue port should be equal to the area of the inlet ports sizes.

 

On rule of thumb NB 2 it would come out to 90Sq".... 3 times more then what i have now...

 

By rule 3 it should come to 63 sq "  ( double)

 

An by rule 1 which is on the book ( 2 large square ports at 3"x3" + 2 round ports at 2"1/2 dia  =   34sq")

 

 

Gianda,

2 ports at 3"x 3"= 18 sq in + 2 round ports at 2 1/2" d. (Pi(3.14) times Radius squared) 1.25"X 1.25"=4.9" sq in" so 9.8" + 18" =27.8" should be your exit flue size. That indicates me ( kinda working backwards here) that it is likely that your inlet ports are too small. Have you checked the table in the book that give recommended sizes of inlet ports for a given size burner nozzle? If not, do so , and then adjust your inlet and exit flues accordingly.

You should be able to fire to temp easily in oxidation, if you can't then it's probably  the kiln being choked at inlets or exits or both. I sure am curious to see what the problem actually turns out to be. Please let us know when you figure it out. Bill

 

 

Hey there,

based on all i've read here and in "the kiln book" 3rd edition by F. Olson it seems like you're choking the kiln. you've gone from 27cf to 36 cf and 2 burners to 4 burners, respectively, so i would think that lack of fuel is not the problem. From page 201 of said book:

"Exit flues. There are 3 methods of determining exit flue sizes:

1. Add the total inlet flue areas (ports) to find the exit flue area.

2. A rule of thumb for exit flues is 2 1/2 sq. in. per cf kiln space.

3. I find that 7,000 btu burner input requires 1 sq. in. exit flue area, regardless of the burner system used A kiln requiring 1,000,000 btu burner input will need an exit flue of143 sq.in., or 12"x12".

It is always better to have flues on the large side than to have them small and choke the kiln. When the flues are too big they can be made smaller easily. Of the three methods, I use No. 1 and 3 and build the flue accordingly."

 

I also suspect that your inlet ports are too small, not allowing enough secondary air, but this is just a guess based on the size of the flue indicated by your required btu input for that size kiln.

 

Again, good luck with it,

Bill

 



#39 Mart

Mart

    Advanced Member

  • Members
  • PipPipPip
  • 282 posts

Posted 15 August 2013 - 11:24 AM

Looks like "Eeny, meeny, miny, moe" wil give the final answer to a young kiln builder :)

Fantastic indeed

 

BTW, it is EXTREMELY annoying when you "top quote" ;)

 

Please use:

 

quoted text

 

 

Your answer

 

----

 

and not:

 

Your answer

 

quoted text

 

 

 

Cheers!



#40 wadar

wadar

    Member

  • Members
  • PipPip
  • 10 posts

Posted 15 August 2013 - 04:54 PM

Looks like "Eeny, meeny, miny, moe" wil give the final answer to a young kiln builder :)

Fantastic indeed

 

BTW, it is EXTREMELY annoying when you "top quote" ;)

 

Please use:

 

quoted text

 

 

Your answer

 

----

 

and not:

 

Your answer

 

quoted text

 

 

 

Cheers!

Didn't know there was a rule. Seen others do it, so I figured I'd just keep my reply next to what i was responding to. Sorry to have annoyed you so.

Hope you have a good evening even though.






0 user(s) are reading this topic

0 members, 0 guests, 0 anonymous users