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burner pressure question

updated thu 18 sep 03

 

Craig Martell on wed 10 sep 03


Hi:

Wood and electric people hit delete now. :^)

I have a question about burner btu output. If a burner has a #37 orifice
that is rated for 390,000 btus per hour using a manifold pressure of 10psi,
at what guage pressure will this output value be achieved?

I'm firing my salt kiln with four Buzzer VNB-200 HP burners and the above
btu value is what they are rated at. I'm carrying a manifold pressure from
the tank of 10psi and when I apply a guage pressure of 3psi, they seem to
max out. I get a good temp rise but I can't go much over 3 psi on the
guages. I'm pulling as much secondary air as the kiln can handle too. The
guages are situated between the burner orifice and the control ball valve
that regulates the guage pressure. I'm just wondering how the guage
pressure and the manifold pressure relate to each other as an indicator of
btu output. Marc Ward says that guage pressure is just a "reference" and
that the manifold pressure is the important factor. What do some of you
sub-geniuses think?

thanks, Craig Martell Hopewell, Oregon

Michael Wendt on thu 11 sep 03


Craig,
Are you using a 500 gallon tank? If so, is there any way to measure the
level of the propane in the tank since that is the absolute way to determine
the BTU value directly. If you know how much propane is used per unit time
by direct measurement of the volume, and you know the BTU value of the
propane, voila!
It would be much easier with a smaller tank since they often show a line
where the propane is and you could simply compute the volume of the propane
in the tank at the level the liquid is at various times.
Regards,
Michael Wendt
wendtpot@lewiston.com

Kenneth D. Westfall on thu 11 sep 03


I believe you will find that it takes 10psi to produce 390,000 btu's. So
that means you have to have the valve completely open with no drop in
pressure which is unlikely to happen if your manifold has four burners and
is only at 10 psi. You would need to care say 15 to 20 psi manifold
pressure and 10psi gauge pressure. Your manifold also needs to be large
enough to carry the cubic feet of gas to run all four burners at once at 10
psi. Not knowing the particulars of your burners your using I'm only
giving a good guess. Ward's burner book rate a B5 Ransome for 462,400 btu
at 10 psi of propane, so I think im pretty close to being on the money.



Kenneth D. Westfall
Pine Hill Pottery
R.D. #2 Box 6AA
Harrisville, WV 26362
kenneth@pinehillpottery.com
http://www.pinehillpottery.com

steve harrison on thu 11 sep 03


Hi craig,
There should be a little drop in pressure from the manifold to the
burner head, due to friction, but 7psi sound like a lot unless there is
some long convoluted path in between.
I've had some strange experiences with guages lately, they vary a lot
straight out of the pack. try reversing them and see if they have the
same values. They just might not.

> I have a question about burner btu output.
> thanks, Craig Martell Hopewell, Oregon
>
Best wishes
Steve Harrison

Hot & Sticky Pty Ltd
5 Railway Pde
Balmoral Village
NSW 2571
Australia

http://ian.currie.to/sh/Steve_Harrisons_books.html

Craig Martell on thu 11 sep 03


Hello Amigos:

Thanks Kenneth and Steve for your responses to my post.

I don't think I was giving enough info of a specific nature in my first
post. My apologies for that.

I think the pressures are fine and my main area of confusion is how
delivery pressures to the manifold and gauge pressures between the valves
that regulate the burners and the orifice both relate to the btu output of
each burner.

I'm firing with propane and I'm pretty sure that pipe size is fine for the
delivery pressure. Two burners run off each manifold which is a 3/4 inch
black steel tee with flare adaptors to connect each burner. Actually, it's
one tee and one cross fitting but they are both 3/4 inch. I have a master
valve that conrols both manifolds and it has a guage. The guage on the
tank regulator and the guage at the control valve have been reading exactly
11psi and stay there at up to 5psi on the burner guages. I haven't tried
to exceed that pressure. (5psi) There's no need to do that. I don't need
390,000 btus per hr from the burners, only about 250,000. It's a 53 cubic
ft hardbrick kiln and I'm figuring the low value per cubic ft for input,
which is 16,000 btus per cubic ft per hour. I want to reach cone 10 in
about 12 hrs. I did that, right on time. The burners are rated at 280,000
btus/hr at 5psi and I never exceeded 2psi for the most part. Before I
forget, the burner guages are Ashcroft 0-15psi liquid filled jobs that are
very good quality and I think they are pretty accurate with a small margin
of error.

So what I'm still wondering is if the pressure applied at the orifice needs
to equal the recommended line pressure for a given btu output. I'm
thinking from the performance of my kiln and burners that it doesn't. The
only way to tell for sure is to put a flow meter on one of the burners and
measure the cubic ft/hr of gas flow. I don't want to do that though. Flow
meters cost a lot of dough.

Am I making any sense? Craig Martell Hopewell, Oregon

Kenneth D. Westfall on fri 12 sep 03


For a burner to produce a giving amount of btu it need three things
air, fuel, pressure. If you need 250,000 btu from each burner then you
need two things pipes that will carry enough cubic feet of gas to produce
250,000 btu at what ever pressure is needed for the burner to produce that
number of btu's. You just need to know that it take 5 psi for a burner to
produce 250,000 btu of heat. The numbers I have use are for natural gas @
1/2 psi so they don't use them for propane. (too lazy to look up the
numbers for propane).
Four burners at 250,000 btu equals 1,000,000 btu's
1,000,000 btu's divided by 1400 btu's (amount of btu's in natural gas per
cubic foot) equals 714.28 cubic feet of gas
714.28 divided by 2 (cause you said you had two manifolds) equals 357 cubic
feet of gas per manifold.
A 3/4" pipe is rated to carry 360 cubic feet per hour up to 10' in length.
Subtract 2.2 feet from 10 feet for the tee and your left with 7.8 feet of pipe.
So if your pipe for each manifold is less then 7.8 feet your under the
maximum capacity for 3/4" pipe and it should be fine but I like to have
more than enough and would have made the manifold out of 1" then reduced to
3/4" from the tee to the burner. You would need larger pipe coming from the
supply to feed the two manifolds.
Now because your talking high pressure propane all the pipes can be reduced
but you would need to check out the maximum delivery cubic feet per hour @
5 psi for 3/4" pipe to know for sure if your pipe is sized correctly.
If you need each burner to produce 250,000 btu and the company says
at 5psi they put out 250,000 then you need 5 psi at the orific to get
250,0000 btu out of the burner. You also need the pipe to be of correct
size to keep more then the 5 psi. of pressure for each burner or have
higher manifold pressure to keep each burner supplied with propane at 5
psi. If you fired your kiln off in 12 hours as you wanted on only 2 psi
then I would say you don't need 250,000 btu's times four burner. You may
get better burner performance by changing the orifice to a smaller size and
using the 5 psi of gas.

At 07:34 PM 09/11/2003 -0700, you wrote:
>Hello Amigos:
>
>Thanks Kenneth and Steve for your responses to my post.
>
>I don't think I was giving enough info of a specific nature in my first
>post. My apologies for that.
>
>I think the pressures are fine and my main area of confusion is how
>delivery pressures to the manifold and gauge pressures between the valves
>that regulate the burners and the orifice both relate to the btu output of
>each burner.
>
>I'm firing with propane and I'm pretty sure that pipe size is fine for the
>delivery pressure. Two burners run off each manifold which is a 3/4 inch
>black steel tee with flare adaptors to connect each burner. Actually, it's
>one tee and one cross fitting but they are both 3/4 inch. I have a master
>valve that conrols both manifolds and it has a guage. The guage on the
>tank regulator and the guage at the control valve have been reading exactly
>11psi and stay there at up to 5psi on the burner guages. I haven't tried
>to exceed that pressure. (5psi) There's no need to do that. I don't need
>390,000 btus per hr from the burners, only about 250,000. It's a 53 cubic
>ft hardbrick kiln and I'm figuring the low value per cubic ft for input,
>which is 16,000 btus per cubic ft per hour. I want to reach cone 10 in
>about 12 hrs. I did that, right on time. The burners are rated at 280,000
>btus/hr at 5psi and I never exceeded 2psi for the most part. Before I
>forget, the burner guages are Ashcroft 0-15psi liquid filled jobs that are
>very good quality and I think they are pretty accurate with a small margin
>of error.
>
>So what I'm still wondering is if the pressure applied at the orifice needs
>to equal the recommended line pressure for a given btu output. I'm
>thinking from the performance of my kiln and burners that it doesn't. The
>only way to tell for sure is to put a flow meter on one of the burners and
>measure the cubic ft/hr of gas flow. I don't want to do that though. Flow
>meters cost a lot of dough.
>
>Am I making any sense? Craig Martell Hopewell, Oregon
>
>______________________________________________________________________________
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>
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>
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>melpots@pclink.com.

Kenneth D. Westfall
Pine Hill Pottery
R.D. #2 Box 6AA
Harrisville, WV 26362
kenneth@pinehillpottery.com
http://www.pinehillpottery.com

Louis Katz on sun 14 sep 03


Hi Craig,
When you have run out of air your burners are at their maximum BTU's
You can forget what the manufacturer says once you hook them up to your
kiln.
Burners are rated contingent on:
1. BTU value of gas
2. Delivery at a certain pressure
3. percent air entrainment (primary). Usually this is somewhere around
70%. I think its designed to keep from developing soot but to maximize
use of secondary air. If you are a manufacturer you want your 2"
burners to be rated as high as possible no?
4.negative pressure at the burner port caused by the chimney.

To get more entrainment you can decrease the orifice size and increase
the pressure. This works up to a point but there is probably a limit to
this. You can also increase the relative suck at the burner ports by
opening the damper, and you can make the burner ports a more efficient
shape.

I like to buy burners with adjustable orifices. I fire the first load
by opening the orifices to determine the minimum orifice size I can
fire with. This gives me the most possible primary air entrainment.
After firing this way once. I lock down the orifices and fire using a
valve.
Good Luck, especially if you were mean to a little brother!

Louis

Craig Martell on sun 14 sep 03


Michael suggested:
>Can you install a "Tee" with two ball valves to allow switched use of a
>smaller tank for a timed interval, say 10-20 minutes?

Hello Michael:

Excellent idea, thanks. Actually, all four burners are connected with
flexible propane hoses to flare adaptors. I can hook one up to a five
gallon tank and fire it for a while and figure out the btu output. I have
the flex hoses because I disconnect the burners after I shut the kiln down
and take them inside for cleaning and maintainance. Salt firing is hell on
wheels with anything metal, as you well know.

thanks, Craig Martell Hopewell, Oregon

Craig Martell on sun 14 sep 03


Kenneth sez:
>For a burner to produce a giving amount of btu it need three things
>air, fuel, pressure.

Hello Kenneth:

Thanks for all the info in your last post.

I think the gas delivery is ok. I'm getting way more than I need at
pressures above, say, 4psi. It's the air that I'm lacking when I pump the
pressure up. Everything about this kiln checks out though. The inlet and
exit port sizes are equal in square inches and the inlet ports have
adequate clearance around the nozzles for good intake of secondary
air. The stack is 16 ft high, about a foot more than I actually need.

For that kiln to fire from a cold start to cone 10 in 12 hrs, I have to be
fairly close to the 250,000 btu per burner mark. I'd just like to know how
close I actually am. I think I'll need to use a flow meter to find out for
sure.

regards, Craig Martell Hopewell, Oregon

Craig Martell on sun 14 sep 03


Michael asked:
>Are you using a 500 gallon tank? If so, is there any way to measure the
>level of the propane in the tank since that is the absolute way to determine
>the BTU value directly.

Hi Michael:

Yes, I have a 500 gal tank. The only thing I have is the percentage dial
on the tank that indicates how full it is. The propane company has told me
that these guages are an estimate only and don't really tell much. I think
that I would have to put a flow meter on one of the burners to determine
the amount of gas and btus. I'm thining of asking my supplier if they have
one I could use. Doubtful, but worth asking about.

Thanks for your suggestion, Craig Martell Hopewell, Oregon

Michael Wendt on sun 14 sep 03


Craig,
Can you install a "Tee" with two ball valves to allow switched use of a
smaller tank for a timed interval, say 10-20 minutes? Place the full 5
gallon tank on a scale and run with it just long enough to ascertain the per
minute usage of the burners (provided you can still maintain the same
pressure). The main reason we fire with 500 gallon tanks is to avoid freeze
ups as you no doubt know, but a small tank can give the same delivery rate
for a short time and give you the needed information for the cost of a
little copper tubing, some valves and a "Tee".
Moreover, the information gleaned will be empircically correct, not a
calculation or a near guess.
Just a thought,
Regards,
Michael Wendt
wendtpot@lewiston.com

Craig Martell on tue 16 sep 03


Louis sez:
>When you have run out of air your burners are at their maximum BTU's
>You can forget what the manufacturer says once you hook them up to your
>kiln.

Thanks for the info Louis. I was sort of skeptical about a 2" venturi
being able to efficiently crank out 390,000 btus. They are a lot better
than other burners I've used though and the kiln does fire very well
now. A 12 hr firing to cone 10 in a hard brick kiln is ok with me and the
pots looked just fine. I was just trying to understand the difference
between delivery pressure and guage pressure in determining the actual btu
output.

regards, Craig Martell Hopewell, Oregon