Jump to content


Photo
- - - - -

How Much Lpg?


  • Please log in to reply
9 replies to this topic

#1 46South

46South

    Newbie

  • Members
  • Pip
  • 5 posts
  • LocationRiverton, South Island, New Zealand

Posted 19 June 2010 - 05:08 PM

I am a beginner potter who has fired only electric kilns and I recently bought an old 9 cu.ft. fibre LPG kiln. Can any one advise me on the best size cylinders to get please?
The kiln has two burner ports at the front at the bottom of the door.

I believe the previous owner had two 12 kg cylinders but had trouble reaching top temperatures due to freezing of the gas. However, because I already have two 45kg cylinders installed on the property for my domestic use if I get more 45kg cylinders I have to get a number of certificates to comply with the local regulations. My question is; will the 45kg cylinders be a significant advantage and will they prevent freezing?

#2 ed keith

ed keith

    Newbie

  • Members
  • Pip
  • 1 posts

Posted 29 June 2010 - 08:20 AM

Hi there
As an experienced Raku potter and builder of kilns, I would suggest to try a cylinder between 12 and 45 kg. In Canada, the size is referred to as a "40 lb.er" There is much less freeze up problem and much greater mobility. Another 'work around' that could be employed is to still utilize, the smaller tanks but using an approved coupler with the tanks both opened completely and thus getting the same pressures needed with much less chance of 'freeze up'.

#3 JBaymore

JBaymore

    Moderator

  • Moderators
  • 2,962 posts
  • LocationWilton, NH USA

Posted 08 July 2010 - 01:55 PM

Hi.

The vaporization rate of propane from the liquid state (volume storage form) to the gaseous state (what you actually burn in most potter type burners) is mainly dependant on a couple of important factors.

The first is the outside air temerature in which the metal storage tank is located. Generally speaking, the higher the ambient air temperature, the larger the volume of propane that can be vaporized per hour from a given tank. So a given sized tank will be able to vaporize less gas on a cold day than on a hot day. One in the sun will work better than one in the shade. One exposed to a really cold wind will not work as well as one burried in the ground (as long as the ground is warmer than the wind temperature). And so on. You usually design a system around the lowest ambient air temperature in which you need to fire.

The second factor is what is called the "wetted area" of the tank itself. This is dependent on the exact geometry of the tank and the orientation of that tank*. It is the interior surface area of the metal walls in square inches/feet that is actually in contact with the liquid propane. The more contact area, the more heat energy can be absorbed from the ambient air into the store of gas. The standard tanks have a pretty well known factor for this for any given gas liquid volume (standardized).

It all actually gets a bit more subtle... with factors such as whether it is still ambient air or air with a wind, the relative humidity of that ambient air, and so on. For example whern the tank gets condensation or (worse) frosty on the outside due to high huumidity and a cold mass of gas, the heat transfer from the air to the gas decreases.

What is really driving all this is the latent heat energy in the liquid gas store that drives the evaporation of the gas off the surface of the tank. As the temperature of the liquid propane goes down, just like hot water versus cold water...... you get less evaporation in a unit of time off a given surface area.

A 9 cubic foot kiln is not going to draw all that much in the way of BTUs per hour. A fiber kiln, due to the lack of thermal mass of the kiln itself, makes that peak draw even less. You are likely looking at a peak draw of about 60,000 BTU/ hr. or so. And remember that is not a continuous withdrawal rate. You only reach this rate toward the end of the firing. The overall average is lower. But you typically use this for a "safety factor" for a well less than full tank. (Standard design principles utilize a 1/2 full tank in doing the calculations.)

Given that basic info above, and assuming you are using ASME type tanks, you likely need a storage of about 115 to 150 gallons. The former should cover temperatures down to a little above freezing, and the second should cover you down into the just below freezing ambient air range. If you live in a place that it gets to -20F ..... and you still want to fire.... the storage volume needed goes up fast Posted Image


best,

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

* Do not tip tanks on their side to an extent that there is a danger of withdrawing liquid propane into a system that is designed for the vaporous gas form. BAD thing. Liquid withdrawal burners exist.......... but they are not typically used by potters; provence of industry.

PS: The above is based upon "code". If you are dealing with governmental type people in all this or insurance companies... this is the kind of thing thay will expect. You'll find that potters often go to gfreat controtions to make too small storage tanks work.... such as spraying with warm water, sitting small tanks in warm water baths, and so on.
John Baymore
Immediate Past President; Potters Council
Professor of Ceramics; New Hampshire Insitute of Art

http://www.JohnBaymore.com

#4 Mudlark

Mudlark

    Advanced Member

  • Members
  • PipPipPip
  • 44 posts
  • LocationWoodend Australia

Posted 08 July 2010 - 07:46 PM

I am a beginner potter who has fired only electric kilns and I recently bought an old 9 cu.ft. fibre LPG kiln. Can any one advise me on the best size cylinders to get please?
The kiln has two burner ports at the front at the bottom of the door.

I believe the previous owner had two 12 kg cylinders but had trouble reaching top temperatures due to freezing of the gas. However, because I already have two 45kg cylinders installed on the property for my domestic use if I get more 45kg cylinders I have to get a number of certificates to comply with the local regulations. My question is; will the 45kg cylinders be a significant advantage and will they prevent freezing?



#5 Mudlark

Mudlark

    Advanced Member

  • Members
  • PipPipPip
  • 44 posts
  • LocationWoodend Australia

Posted 08 July 2010 - 08:52 PM

If you decide to use the domestic 45 Kg cylinders you will need to consider the distance between them and the kiln, assuming that they are to remain in-situ. The flow rate of the gas can be restricted if the plumbing between the cylinders and kiln is not of sufficient diameter to compensate for the distance travelled by the gas. I run a 35 cuft fibre kiln and had no trouble with maintaining working pressure and flow rate when it was originally installed with the cylinders a couple of meters away. We moved house and due to access requirements for the gas delivery truck had to position the cylinders near the front of the property whilst the kiln was behind the house. I could not draw enough gas (volume) or maintain working pressure with the half inch dia plumbing insisted upon by the plumber. The max working pressure is 50kpa and with all burners at full the flow pressure dropped off to below 10kpa, resulting in a stalled kiln unable to get beyond 1000 deg C. All of this was eventually solved by using a one inch dia pipe from the cylinders above ground and a one and a half inch dia plastic pipe below ground terminating in a very short length of half inch dia pipe at the kiln connection point. Two 45kg cylinders should be more than adequate for a 9 cuft fibre kiln. You are in for a very interesting experience, fibre kilns are different to any other type. I suggest you keep a detailed diary of your first few firings so that you can make educated decisions along the way if things do not turn out as you intended.
Good Luck,
Mudlark.

#6 Marcia Selsor

Marcia Selsor

    Professor Emerita, Montana State University-Billings

  • Members
  • PipPipPip
  • 3,925 posts
  • Locationwhere Texas, Matamoros, Rio Grande and Gulf of Mexico come together.

Posted 09 July 2010 - 06:30 AM

Hi.

The vaporization rate of propane from the liquid state (volume storage form) to the gaseous state (what you actually burn in most potter type burners) is mainly dependant on a couple of important factors.

The first is the outside air temerature in which the metal storage tank is located. Generally speaking, the higher the ambient air temperature, the larger the volume of propane that can be vaporized per hour from a given tank. So a given sized tank will be able to vaporize less gas on a cold day than on a hot day. One in the sun will work better than one in the shade. One exposed to a really cold wind will not work as well as one burried in the ground (as long as the ground is warmer than the wind temperature). And so on. You usually design a system around the lowest ambient air temperature in which you need to fire.

The second factor is what is called the "wetted area" of the tank itself. This is dependent on the exact geometry of the tank and the orientation of that tank*. It is the interior surface area of the metal walls in square inches/feet that is actually in contact with the liquid propane. The more contact area, the more heat energy can be absorbed from the ambient air into the store of gas. The standard tanks have a pretty well known factor for this for any given gas liquid volume (standardized).

It all actually gets a bit more subtle... with factors such as whether it is still ambient air or air with a wind, the relative humidity of that ambient air, and so on. For example whern the tank gets condensation or (worse) frosty on the outside due to high huumidity and a cold mass of gas, the heat transfer from the air to the gas decreases.

What is really driving all this is the latent heat energy in the liquid gas store that drives the evaporation of the gas off the surface of the tank. As the temperature of the liquid propane goes down, just like hot water versus cold water...... you get less evaporation in a unit of time off a given surface area.

A 9 cubic foot kiln is not going to draw all that much in the way of BTUs per hour. A fiber kiln, due to the lack of thermal mass of the kiln itself, makes that peak draw even less. You are likely looking at a peak draw of about 60,000 BTU/ hr. or so. And remember that is not a continuous withdrawal rate. You only reach this rate toward the end of the firing. The overall average is lower. But you typically use this for a "safety factor" for a well less than full tank. (Standard design principles utilize a 1/2 full tank in doing the calculations.)

Given that basic info above, and assuming you are using ASME type tanks, you likely need a storage of about 115 to 150 gallons. The former should cover temperatures down to a little above freezing, and the second should cover you down into the just below freezing ambient air range. If you live in a place that it gets to -20F ..... and you still want to fire.... the storage volume needed goes up fast Posted Image


best,

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

* Do not tip tanks on their side to an extent that there is a danger of withdrawing liquid propane into a system that is designed for the vaporous gas form. BAD thing. Liquid withdrawal burners exist.......... but they are not typically used by potters; provence of industry.

PS: The above is based upon "code". If you are dealing with governmental type people in all this or insurance companies... this is the kind of thing thay will expect. You'll find that potters often go to gfreat controtions to make too small storage tanks work.... such as spraying with warm water, sitting small tanks in warm water baths, and so on.

John's points are all excellent. I use 4-30 gallon tanks plumbed in pairs to fire my 15 cu. ft. raku kiln. It is 2" fiber and I can fire several different sessions of 5-8 batches without refilling. You would probably need more gas because you'd have a longer firing. However, for small vertical tanks, your evaporation surface stays fairly constant as the gas is depleted. If the burners use the gas faster than it is evaporating from that surface, you get ice buildup. That is why it is good to pull vapors from two tanks simultaneously for one burner. My tanks don't freeze. I had three tanks for 2 burners in Montana. Here in steamy south Texas, I have four tanks for 2 burners with 2 tanks on each burner. I have a regulator set for 3-5 pounds pressure at maximum. Follow your BTU estimated need, use the right size orifice for LPG to obtain that BTU output needed at max. temp.
There are charts. Your gas company should have them. An old extimate for hard brick was 120,000 btu's/cu. ft. Insulated brick was 30,000 btu's/ cu. ft.
I would have to research what fiber requirements are. Do your homework and it should be fine.



#7 JBaymore

JBaymore

    Moderator

  • Moderators
  • 2,962 posts
  • LocationWilton, NH USA

Posted 09 July 2010 - 08:49 AM

Marcia (+ all),

For calculating input for my kiln design work, for hard refractrories I use 15,000 BTU /hr. /c.f., for (standard) insulating reftractories I use 10,000 BTU/ hr. / c.f., and for RCF (fiber) refractories 7,000 BTU / hr. / c.f.. Obviously, blending those types of materials in any significant way results in intermediary values.

The piping requirements between the gas storage and the kiln are, of course, another whole little technical consideration, as is mentioned above by mudlark. This is true no matter what volume the gas storage is.

best,

.............john
John Baymore
Immediate Past President; Potters Council
Professor of Ceramics; New Hampshire Insitute of Art

http://www.JohnBaymore.com

#8 Marcia Selsor

Marcia Selsor

    Professor Emerita, Montana State University-Billings

  • Members
  • PipPipPip
  • 3,925 posts
  • Locationwhere Texas, Matamoros, Rio Grande and Gulf of Mexico come together.

Posted 09 July 2010 - 09:18 AM

Marcia (+ all),

For calculating input for my kiln design work, for hard refractrories I use 15,000 BTU /hr. /c.f., for (standard) insulating reftractories I use 10,000 BTU/ hr. / c.f., and for RCF (fiber) refractories 7,000 BTU / hr. / c.f.. Obviously, blending those types of materials in any significant way results in intermediary values.

The piping requirements between the gas storage and the kiln are, of course, another whole little technical consideration, as is mentioned above by mudlark. This is true no matter what volume the gas storage is.

best,

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

Thanks for the fiber and more accurate brick figures. My info was from the ancient Soldner Kiln book I believe. And yes...piping is a whole 'nother ball park.
marcia



#9 46South

46South

    Newbie

  • Members
  • Pip
  • 5 posts
  • LocationRiverton, South Island, New Zealand

Posted 11 July 2010 - 04:01 PM

Thank you so much to all of you who gave advice. I have learned a lot and feel confident now to approach a gas fitter, armed with the information.
I think the problem the previous owner had may have been the very small diameter 'gooseneck' pipes between the cylinders. Thanks again 46south

#10 OOF!

OOF!

    Member

  • Members
  • PipPip
  • 19 posts

Posted 30 July 2010 - 10:36 AM

Although they are based in Tennessee, USA, Ward burner Systems has a good bit of technical help in the "Technical info" section of their web site (as you might expect.) It might be of some use to someone living outside the US.

wardburnersystems
"Bye!"
-OOF!




0 user(s) are reading this topic

0 members, 0 guests, 0 anonymous users