Burner for 80 gallon kettle

Any suggestions? Need help with btu estimates and burner manufacturers. I'm using propane because I have a 500 gallon tank outside. I have a homemade ss kettle and I'm going to install it permanently and brick it in. Vent up an old chimney flue and put in a fresh air vent.
I have a jet type burner and a banjo burner but it barely gets it to a boil. May have problems with the propane tank freezing up or too much heat escaping u the sides, thats why I'm bricking it in place.
Thanks
Bob

Bricking it in place is a great idea. Keep in mind that a chimney flue may not give you the right ventilation with propane. Propane produces a lot of CO2 and CO, rather than the hot gas mixture of a wood fire, and so sometimes needs to be vented horizontally. A fan assisted fresh air supply would be a good step in the right direction.

One potential problem with propane is the evaporation rate. Since propane is stored as a liquid and moved as a gas it has to evaporate. There are charts available that show the maximum evaporation rate for different styles and sizes of tanks, or you can calculate it yourself if you get the evaporation rate per square foot. For a horizontal tank you will need to assume the smallest area, which will be when it's full (80%). Note too that the evaporation rate varies with temperature, so you will want to get the rate at the coldest temperature you expect to use it at.

Further, you will have to account for the distance and size of piping based on the pressure. Most home setups have a low pressure regulator (11" water column) at the tank, but you may have to fit a high pressure regulator (2.5 PSI) at the tank and then a low pressure one at or near the burner if the piping is to small and/or long. This is similar in principle to electrical wiring - sometimes it's more efficient to run a higher voltage line which requires less amperage and therefore allows a smaller wire, and step it down as you reach the point of service.

I don't have any idea how many BTUs you need, but as an FYI we were advised by our gas company that we needed two 1,000 gallon tanks to support our 850,000 BTU burner, and we have a high pressure 1/2" line running from the tank to the building (around 200').

In theory... you should need somewhere around 200k BTU's if you're gonna do 70 gallons.

Get some mineral wool to insulate the kettle. 2" thickness would work nicely.

That is huge i would love to see some some pictures

Thanks for the info. The high-pressure regulator to low-pressure has been brought up before. I was thinking about 300,000 Btu.
I have a 600cfm variable speed vent fan that I can use if needed.
I was thinking of Kaowool insulation but its $7.95 a foot. I may use regular fiberglass. It will be protected from direct flame.
Photosof the incomplete system is on Picasa (I hope) at
http://picasaweb.google.com/bob.blu...?authkey=Gv1sRgCO3x5ILvhva8FA&feat=directlink