Jump to content
Sign in to follow this  
Biglou13

Venturi Vs Forced Air

Recommended Posts

one if the techs (in refernce to bailey gass kiln) said he prefers venturi to forced air.   he thinks that temperature is  more even throughout kin.  he was referencing botched biaque frirng,

 

ive just assumend forced air is more efficient but never thought about how even it would fire......

 

what is the more learned opinion?

Share this post


Link to post
Share on other sites

I have used both types of burners. Forced air is great on a salt kiln as she really blows the air around the kiln. I don't know that I would use forced air for a bisque-too hot, too soon. I always turned the blowers on after red heat, and a bisque is basically done after red heat. Just my opinion.

TJR.

Share this post


Link to post
Share on other sites

Lou,

 

Mark C will likely opine otherwise, but for a non-production operation, bisque firing in electric might be more economical than natural gas or propane. You can likely fire a couple of bisque loads for $20 to $30, depending on size of kiln. Using John B's estimate of 40 gal of propane @ $3.00/gal, that would be $120 for a propane load; even allowing for temperature difference of 33%, you are still looking at a cost in the $90 range. And, with electric, you don't really need to worry about evenness of firing for bisque. Lot of assumptions there, I admit.

 

For a production environment like Mark's, where he fires almost continuously, bisquing in his natural gas kiln makes more sense . . . more done at one time, but the volume is also there; less time loading/unloading; likely less physical impact . . . which counts if you fire as often as he does. The dollars for fuel are far offset by economies else where in his work.

 

Evenness has a lot to do about stacking and firing, not just the burners.

Share this post


Link to post
Share on other sites

It's not so much cost but about bisquing large pieces that don't fit in 1227 skutt that hopefully ultimately will be wood fired. Or for any larger/ sculptural work. Hence the bisque in larger car/ gas kiln with forced air burners?

Share this post


Link to post
Share on other sites

I will agree that at lower temps forced air could make for hot spots as that is the nature of forced air.Hot and fast.

Forced air is more fuel efficient as well I think.

When I started out I made a few forced air kilns with short stacks.

I was not a production guy from the start-its been a slow steady curve and really as a hobbyist it does not matter as much about most things.

The issue for me was power outages and constant monitoring and noise.At one point I gave away my burners and blowers and felt I 

 wised up and went with natural draft and have never looked back-kiln just purrs along-yes the chimneys are taller and they cost a bit more (no electricity charges)

I love the way a natural draft kiln fires-slow and steady-great for glazes-great for strew free firing-no matter what the weather.

They are not as loud and power outages are non issues- much less baby sitting. I recall getting to about cone 7 with forced air when the power when out in big storm-Had to start over the next day.

Blowers crapped out and smoked up a load .

As far as bending over into an electric for bisque - I do that a few times a year myself but as an older potter now it even has less appeal.

I will add I gave up Volkswagens after inheriting a beetle for decade  for a better car-Just saying you learn as you go-now Toyota's and Honda's seem to work better than Volkswagens did.

Mark

Share this post


Link to post
Share on other sites

great points

thanks for the lessons

 

I suppose if you had one kiln (maybe a car kiln)  in a perfect world  venturi for bisque,  then forced air for glaze. ward  does have a piggy back venturi burner!

 

does any one have numbers eg gas cost cost  comparison for forced air to modern venturi.   ( venturi is definitely going to cost you on bricks/chmney)

 

what about glaze comparisons venturi vs forced air, especially shinos

Share this post


Link to post
Share on other sites

what about glaze comparisons venturi vs forced air, especially shinos

I would think kiln design/type (updraft/downdraft/crossdraft, noborigama/anagama/train, etc) and length of firing (and glaze ingredients) would be more relevant than burner type to glaze results.

Share this post


Link to post
Share on other sites

Basic difference in my opinion is you use a lower stack (chimney) for forced air burner. Venturi rely on natural draft and need the high chimney for the pull of the draft. I have used both. Once you are familiar with the behavior , they are equal in my opinion.

 

Marcia

Share this post


Link to post
Share on other sites

Evenness has nothing to do with the type of burners. It has everything to do with kiln design. Either type of burner can fire a kiln evenly if the kiln is designed and built properly.

 

I prefer power burners because I can get away with having only one or two burners on a kiln, rather than 6 or 8, and I can build my own. I also think power burners are safer in that you have a dedicated flame sensor/thermocouple and pilot/ignition system for each burner, rather than relying on a pilot ring which may or may not be lighting every burner. Kilns with power burners do not need a chimney any taller than the kiln, so they easier to build. Take a look at all the threads here on the forum about kiln construction problems and they all use venturi burners and are struggling with gas pressure, burner quantity and flue height. With power burners, you can adjust the orifice size and blower size easily.

 

I never had any trouble bisque firing in my gas kiln. I used to bisque 50 pound planters in it without blowing them up. The key is to have a strong enough pilot that it can preheat the kiln to at least 200 degrees overnight, and you must have a blower control to slow down the blower speed so that the burner can be run at low pressure.

Share this post


Link to post
Share on other sites

 

They are not as loud and power outages are non issues- much less baby sitting. I recall getting to about cone 7 with forced air when the power when out in big storm-Had to start over the next day.

Blowers crapped out and smoked up a load .

 

Mark

 

If you're going to use power burners, you really need to hook up solenoids to the gas lines so that the gas shuts off if the power goes out. Otherwise you've got a very dangerous situation where pure gas is flowing, which often results in a very long flame licking up the side of the kiln. I saw a kiln shed catch fire once when  blower went out and the gas kept flowing.

Share this post


Link to post
Share on other sites

I was working in a university pottery studio when they switched from two forced air pipe burners to two venturi burners. We only did cone 10 reduction glaze firings in the sprung-arch hard-firebrick downdraft gas kiln (bisques were all electric). I believe the kiln chamber was about 40-50 cubic feet. It was absolutely wonderful to no longer have to worry about power outages and back burning.  BUT our firings were long...ridiculously long...up to 24 hours versus 12-14 hours prior to the burner change. Long story short, our venturi burners were too small. Changed them out for two larger venturi burners and everything was back to normal. We did not have to add any burners. So, if you decide to make the change, be sure to cover that topic.

 

I fire all electric now and can tell you that my firings (slow ramp up, no down firing), whether cone 06 bisque or cone 6 glaze, run between $10-15 each.

 

There is a part of me that wistfully remembers gas firing....

Share this post


Link to post
Share on other sites

People 'worry' about power outages with their power burners, but the subject never comes up when talking about electric kilns which are just as dependent on electricity. Like I said before, if you have the proper safety systems on your kiln, there is no danger of fire with a power outage. Yes, it's inconvenient for a short time. And if it's a long term outage, then you've probably got bigger problems than your kiln not firing. We have very dependable electrical service in this country. It seems to me that it's not something we really need to fret over.

Share this post


Link to post
Share on other sites

Evenness has nothing to do with the type of burners. It has everything to do with kiln design. Either type of burner can fire a kiln evenly if the kiln is designed and built properly.

 

 

I never had any trouble bisque firing in my gas kiln. I used to bisque 50 pound planters in it without blowing them up. The key is to have a strong enough pilot that it can preheat the kiln to at least 200 degrees overnight, and you must have a blower control to slow down the blower speed so that the burner can be run at low pressure.

school is using bailey car kiln with which I have no experience

im asumming bailey is well designed kiln

which leads to next assumption  "user error" 

ill do more research!!!!!

Share this post


Link to post
Share on other sites

Electric kilns dependent on electricity? Silly Neil. Doesn't everyone have a giant hampster wheel, attached to a generator to power their kiln, in case of power outages?

Careful, Guinea might be reading this!

TJR.

Share this post


Link to post
Share on other sites

Electric kilns dependent on electricity? Silly Neil. Doesn't everyone have a giant hampster wheel, attached to a generator to power their kiln, in case of power outages?

I have guinea pigs in mine...maybe that's why I fire so slowly. Guineas are lazy-butts. ^_^

Share this post


Link to post
Share on other sites

We use just about the cheapest commericially available burners on the market. Two big berthas ($140) power our ~60 cu./ft downdraft, sprung arch, soft brick kiln. We only have one layer of brick, with two layers of 1" kaowool for our "cold" face. We fire from room to ^12 in 7-8 hrs, no preheat, just going slowly until color. It costs us about $48 a firing using propane. We've calculated this cost in a very rudimentary way using the fill gauge from the propane tank to estimate our usage. We may be off here or there. My olypmic 2527 oval holds about 12 cu/ft, and costs me about $10 to fire a bisque load. So, gas kiln is about 5 times more costly, but also has about 5 times more volume too. When its all said and done though it costs maybe 1/30th, 1/40th? of what the kiln can produce in finshed wares.

Share this post


Link to post
Share on other sites

Oh, to answer your question; I do prefer an updraft vs a downdraft kiln. I find an updraft is easier to maintain consistent temperature, but can be a little more finicky to losing atmosphere control, whereas the downdraft is the opposite; just my opinion. We still have a big whoosh when firing the venturis, so I dont think sound wise I would care if there was a few extra dB from blowers. I agree with above comments, I dont think there is much difference in even heating when comparing burner type; more dependent on kiln design and firing methods. Biggest different I see? Cost, A few hundred or few thousand for equivalent heating capacity (1,000k btus) looking at Wards/Johnsons to Big Berthas. Big Berthas have less control (no air control, atmosphere is controlled via damper), but they have pretty much nothing to fail on them either. We installed baso valves with flame sensing rods, along wiith two ball valve shut offs for safety.

Share this post


Link to post
Share on other sites

Lou,

My natural gas bill for all of 2014 which includes studio heat was $ 3,910.00.

This gas meter serves only my kiln and studio-not the house.

As I just finished with tax prep-that was the column total for 2014.

 

This is for 25 car kiln loads(35 cubic loading space) (both bisque and glaze fires) so thats 50 fires total on this kiln as well as 18 small 12 cubic updraft glaze only fires

(no bisques)

I will also try to remember to log a fire for BTUs for you.

This bill seems very reasonable to me.All that fun for so little $$.

1,091 cubic feet of pots divided by the dollars minus studio heat(less than 5%of bill).Ok what's that per cubic foot of pots?

Mark

 

PS I have a few electric bisques in this to further complicate the math as well.

I think my 1227 is 7 cubic feet and there are 10 fires for 2014 about double for my normal year with electrics

since my electric meter is for the whole property that caculation is more complex.

Share this post


Link to post
Share on other sites

I fired a Bailey downdraft at Corpus Christi Art Center when I did a ^6 reduction workshop. The person who bought it and donated it had died. I started loading it but noticed the target bricks were missing. I looked around on a shelf and they were still wrapped in paper. I put them where I thought they should go. Loaded up and fired it. Came out perfectly even. Bailey equipment is designed to the max!. You won't have any trouble. 

Marcia

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this  

×

Important Information

By using this site, you agree to our Terms of Use.