Tell me about preheating primary intake

[jrgpots]

As I was building the kiln,  I pushed the chimney away from the back of my kiln by 9 inchs. So I have a 9" gap.   I was thinking about making an air manifold  in the gap to warm up the primary air.  I have a 9 x 24 steel tank that fits between the back kiln wall and the chimney.  Air would go into the tank, be heated by the kiln and chimney, then piped to the burners.  I am currently planning on using old venturi burners as I have mentioned in a previous thread, but am afraid they won't work.

If they don't work out for me, I will get power burners.  So I was wondering if this idea to preheat the air would work?

 

Jed

[Bill Kielb]

1 hour ago, jrgpots said:

So I was wondering if this idea to preheat the air would work?

Venturi burners would be tough and I suspect you will be providing some preheated air as primary and secondary air as it will be hard to segregate the two. So preheating air saves energy, mainly. Scrubbing some heat off your chimney also potentially reduces its draw. I am curious:

• what are you trying to achieve? Maybe just reclaim some heat is the answer.
• How much preheat do you feel would be successful, IE, 10,20,30 degree warmup or differential. That is to say 40 degree air in, 60 degree air out to the burners.
• If you are not sure of the above, given the burner size,  how much percentage savings (or increase in power) is worth pursuing?

Just curious of your thoughts, which I will preface with, usually when we pursue extracting meaningful (Numeric) heat it requires a heat exchanger embedded in the flue gasses.

[jrgpots]

I would not do an exchanger through the chimney. That is above my DIY level.  But, if I could salvage  some of the heat from a more passive system, it would improve effeciency.   I converted a smaller kiln to gas and struggled to get it to temp.  It kept stalling out.  So this time around,,

1. I have changed to natural gas.------ I had problems with decreasing vapor pressure from large drop in tank temp from too small of tank.

2. I have 9" walls  and better draw from the chimney.

I just am afraid of making another dudd.  So I overthink it.    I was hoping for a 100 degree differential.  To do this I may have to stuff the tank full of steel wool to increase surface area and thus heat exchange.   So what would you define as "meaningful" heat exchange?

 

Jed

[Mark C.]

I would scrap that idea and stick to basics-let the burners heat whatever air you have-not worth the effort I think.In the grand scheme a 100 dregrees in a high fire or midfire kiln is meaningless.

I think you are overthinking it.Just try the burners and see what happens.

Jed I looked up the article from Nils Lou on the flat top (in studio potter Vol.6 no 1-1977)(yes I wasa subscriber then as I still am) the burners are from him I bet they are FL-1 liquid propane (Lou is dead now) burners-no blowers. I recall these as high output propane burners . I do not think they will work near as well on natural gas. as it less BTUs. Your kiln is so much smaller than a regular Minnesota flat top they could work-just get as much pressure as you can to them and it may take a few tries with orfice sizing.I would try them a few times with orfice changes before scrapping that idea.

[Hulk]

Warmed air is less dense, hence less O2 per unit volume. Supercharge it!

[liambesaw]

Powered burners for something like this, I believe the hot air will mess with the venturis.

There's a website out there documenting a kiln build using mullite tubes in the chimney to direct hot air through the powered burners and the exhaust from the chimney is basically "warm".  Great use of energy, and the same idea as a heat exchanger in a home

[Mark C.]

I have used the stack chimney air with a copper heat exchanger to heat a hot tub during firing-worked great until someone shut off the pump than bam blew the hoses. I found it was not worth it as water and pottery are a poor mix during firing.

Now a low tec building heat source with a blower may work great but its only when you are firing and thats ashort time in the overall time frame for all that work.

[Bill Kielb]

2 hours ago, jrgpots said:

So what would you define as "meaningful" heat exchange?

Meaningful for me is some measure of improvement so 10-20% makes me real interested. Which generally brings me to think how much is available to be had which then leads me to think in quantities just to see if my hunch is correct. So ......... 100000 Btuh and for natural gas  running let’s say 10:1 air fuel ratio. For natural gas at about 1000 btu per cubic feet of which about 1/2 will be primary air and 1/2 will be secondary air We ought to be able to get a sense of how much energy we could get from X number of cubic feet of air heated X number of degrees (sensible heat) 

A long process I know, which is why I asked what is success for you? 1%,5%,10%? Maybe any percent? Once we know some number we will need to translate that into heat transfer possibilities and see if any of this is remotely doable.

My initial thought is it will be less than 1% without a decent internal exchanger, but that is just a guess based on current boiler and stack waste heat reclamation.  If I get some spare time, post the size of your burners and I will work up some approximations based on enthalpy and we will see.

[neilestrick]

If everything else is done correctly, this is unnecessary. It should not be needed to make it work, but rather improve efficiency of an already functional system.  If you don't know whether or not the basic system works, it's going to be much more complicated to diagnose problems since you'll have more varaibles.

If go forward with it, you'll need power burners because they will pull the  preheated air through the burner. I just don't see how you could do it with a venturi setup. Personally, I prefer power burners because you can use fewer burners, they're easy to adjust, and you don't have to rely on the chimney nearly so much. Plus they're easy to build.

 

[jrgpots]

I'm worrying about heat input. . .That's a bit ironic because today's temp is 108 degrees at the kiln before heat from a burner is ever applied to it.. 

 

Jed 

[Mark C.]

My worry is more about BTU in natural gas vs Propane. Those burners where made for propane (like in Nils Lou 1977 article on building a minasota flattop) they kicked ass with propane .

Not so much with lesser fuel. Orfice size and more presure will be critical.

108 degree air is preheated so forget the more complicated system

[jrgpots]

21 minutes ago, Mark C. said:

My worry is more about BTU in natural gas vs Propane. Those burners where made for propane (like in Nils Lou 1977 article on building a minasota flattop) they kicked ass with propane .

Not so much with lesser fuel. Orfice size and more presure will be critical...

 

From my calculations, at 2# pressure and the current orifice, (drill bit size 52 x 3 holes) the natural gas should produce about 117,000 BTU for each burner.  I hope that 2# is sufficient to push the burners.   The numbers are right.   I hope the burners will work at this pressure.   Next step is to test them.

 

Jed

[Bill Kielb]

3 hours ago, Mark C. said:

My worry is more about BTU in natural gas vs Propane. 

Those burners where made for propane (like in Nils Lou 1977 article on building a minasota flattop) they kicked ass with propane .

Not so much with lesser fuel. Orfice size and more presure will be critical.

@jrgpots
Propane is double the btu of natural gas. Propane is approx. 2000 btu per cu ft, natural gas is about 1000 btu per cu ft. Whatever fuel you are using use the right orifice and matching pressure. Generally in inches which is way less than PSI. Natural gas orifices are Generally bigger in diameter for the same BTU Than propane.  PickIng them from a table and operating them at the prescribed pressure is easy but often Is misunderstood which causes a bunch of needless confusion.  A 100000 btuh propane burner is in essence the same as a 100000 btuh natural gas burner from a  kiln thermodynamic point of view.

[Bill Kielb]

13 minutes ago, jrgpots said:

From my calculations, at 2# pressure and the current orifice, (drill bit size 52 x 3 holes) the natural gas should produce about 117,000 BTU for each burner.  I hope that 2# is sufficient to push the burners.   The numbers are right.   I hope the burners will work at this pressure.   Next step is to test them.

 

That doesn’t sound correct, use a table for appropriate orifice with corresponding pressure. Btu increase exponentially for pressure changes so raising pressure by just 10% will increase output by over 20%. These things are not linear or easy to calculate. Tables have been around for decades. Calculations are very difficult to get right.