How high temperature is enough for burning off carbon deposits on furnace heat exchanger?

cowdog

Carbon deposit is inevitable for oil furnaces.

Carbon deposit can be burned off by hot air combined with a red-hot heat exchanger.

How high temperature is hot enough for this burn off process?

Steel is definitely OK, can copper or cast aluminum heat exchangers endure this temperature?

archibald tuttle

hmmmmmmmmm . . . interested to see what this brings. my recommendation is convert to NG or propane before you get carbon deposits. I see what you are saying, but forgetting the possibility of melting copper or esp. aluminum, steel heat exchangers crack from expansion and contraction over time and have a short enough life already without this kind of treatment.

That said, if oil is the only choice, aside from physical cleaning of heat exchanger as provided access, I have never tried to burnoff deposits, as if the thing were a self cleaning oven . . . but maybe i'm just too conservative (my wife thinks so :-)

EdTheHeaterMan

So, this is interesting. there is a product that helps burn off carbon (soot) that has accumulated on heat exchanger surfaces. https://www.supplyhouse.com/Hercules-35427-Flip-Stick-Soot-Destroyer-85-grams. It is no longer available from SupplyHouse.com, however RE Michel still carries it. REM Part number 1M103. It is a chemical powder that you place in the combustion chamber of the oil burner and fire up the beast. The chemical(s) reacts with the carbon and burns it away at lower, normal heat exchanger operating temperatures.

This was the Lazy Mechanic way to "Vacuum Clean" an oil burner heat exchanger back in the day when I was learning the trade. There were also Liquid Chemicals that you would run thru the fuel pump that has similar properties. I was taught the proper way to use those chemicals and save time on lugging a Soot Vac in and out of the basement. It was all quite scientific. More often than not, the vacuum cleaner took less time with better results.

As far as just increasing the temperature of the heat exchanger, It will work, but do you really want to do that to a warm air furnace? I would be afraid of creating the beginnings of a cracked heat exchanger. If that happens, you don't need to remove the soot next year. You just remove the heater and install a new one.

EdTheHeaterMan

cowdog said:

Carbon deposit is inevitable for oil furnaces.

Not necessarily... Proper fuel/air mixture will produce a clean flame with No Soot, No Smoke and No Odor. It says so in this sales literature.

Carbon deposit can be burned off by hot air combined with a red-hot heat exchanger.

How high temperature is hot enough for this burn off process?

Steel is definitely OK, can copper or cast aluminum heat exchangers endure this temperature?

pecmsg

cowdog said:

Carbon deposit is inevitable for oil furnaces.

Says Who?

cowdog

pecmsg said:

Says Who?

If you use a catalyst converter and diesel particulate filter before the heat exchanger, maybe it's possible to deposit no carbon, but they are expensive and requires more powerful draft fan.

mattmia2

The clearences and insulation in the appliance and to the vent system are designed for the appliance operating within spec. There is a chance of starting a fire if you operate out of spec.

Jamie Hall

Reliably? 1000 F and up. Which for most fuels means direct impingement of the flame on the surface to be burned clean. Refractories can handle that. Most metals can't without at least warping. Better to adjust the burner so it doesn't soot in the first place, maybe?

pecmsg

cowdog said:

pecmsg said:

Says Who?

If you use a catalyst converter and diesel particulate filter before the heat exchanger, maybe it's possible to deposit no carbon, but they are expensive and requires more powerful draft fan.

Isn't that why you clean it every year?

mattmia2

ron said:

\
but having liquid water on the other side of a copper or aluminum alloy heat exchanger where the outside is operating at 1000°F I dunno... usually the heat exchanger walls are thin to facilitate heat transfer so at a 1000° with a ~0.020" wall thickness or whatever you would have to have some high gpm flow to prevent water flashing to steam and then all the water deposits that causes on the inside

You would not heat it to 1000 f if there is liquid water on the other side of it unless it is under very high pressure.

EdTheHeaterMan

So, @cowdog did you get the information you wanted? I have a real interesting question. Why do you need this information, and what are you going to do with it?

I bring this up because as the Tech Support for all things HVAC at the supply company where I was employed for some time, I was asked some crazy questions. The one that comes to mind came from the FAA Technical Center in Atlantic County NJ (they operated out of Atlantic City Int. Airport). A scientist wanted to purchase a heat exchanger that would provide 2500°F air for some test they were performing, hoping that the HVAC industry has something they could just get "off the shelf". I never found out what the test was that they were working on, except that the 2500° air could not contain any byproducts of combustion. (hence, the heat exchanger)

I was unable to help him with his product, so I told him that normal commercial and residential HVAC heat exchangers only needed to provide air temperatures that people could survive in (below 120°F) with flame temperatures that did not exceed say 2500° from a really efficient fuel oil flame. Gas flame temperatures were even lower.

This scientist, if he actually needed to get such a heat exchanger, would have to spend lots of taxpayer dollars to have one invented for his experiment.

So @cowdog, are you working on a top secret way to save the planet? You can tell us here, we won't tell anyone else.

mattmia2

I suspect the chemical industry has what they were looking for.

cowdog

EdTheHeaterMan said:

So, @cowdog did you get the information you wanted? I have a real interesting question. Why do you need this information, and what are you going to do with it?

heat exchanger that would provide 2500°F air for some test they were performing.

In that temperature you don't use a heat exchanger. If NOx formation is not a concern, use arc heating between platinum hat electrodes.
If NOx is not allowed, use electroresistive heating with nichrome wires in quartz tube. You need many quartz tubes because heat conduction through quartz is low.

or separately heat nitrogen and the rest of air with arc, then mix the two hot gases at high temperature.

cowdog

EdTheHeaterMan said:

So, @cowdog did you get the information you wanted? I have a real interesting question. Why do you need this information, and what are you going to do with it?
.

For adding a "maintenance mode" to oil fired heat exchangers. Stop cold air flow and toast the heat exchanger with oxygen-rich flame until red hot.

EdTheHeaterMan

@cowdog, basically you want to make a self cleaning furnace. (not unlike a self cleaning oven that just burns off baked on food).

Is there a market for this product? Or you just making it for your own convenience?

STEVEusaPA

cowdog said:

Carbon deposit is inevitable for oil furnaces.

Carbon deposit can be burned off by hot air combined with a red-hot heat exchanger.

How high temperature is hot enough for this burn off process?

Steel is definitely OK, can copper or cast aluminum heat exchangers endure this temperature?

I disagree with pretty much this entire post.

Carbon deposits aren’t 'inevitable'. And they don’t need to be burned off.
Modern equipment, proper set up (correct draft, true zero smoke), proper combustion air, double filtration. Trouble free, clean, soot less operation.
Haven’t cleaned my boiler, or changed nozzle/fuel filters/pump strainer in 4 years.

EdTheHeaterMan

STEVEusaPA said:

I disagree with pretty much this entire post.

Carbon deposits aren’t 'inevitable'. And they don’t need to be burned off.
Modern equipment, proper set up (correct draft, true zero smoke), proper combustion air, double filtration. Trouble free, clean, soot less operation.
Haven’t cleaned my boiler, or changed nozzle/fuel filters/pump strainer in 4 years.

Bee nice Steve
He learned all these facts, I believe from his father from the 1950s when oil was dirty and gas was clean. He may still have that Armstrong Octopus Furnace with the Quiet May oil burner in the bottom.

He is reinventing the wheel for us. LOL

cowdog

EdTheHeaterMan said:

@cowdog, basically you want to make a self cleaning furnace. (not unlike a self cleaning oven that just burns off baked on food).

Is there a market for this product? Or you just making it for your own convenience?

I believe the market is on any fuel that would generate soot, such as oil, waste oil, wood, plastics, biomass etc. Sootcleaning, whether on heat exchanger or inside chimney, is a dirty job most homeowners and building maintenance technicians hate doing.

cowdog

EdTheHeaterMan said:

Bee nice Steve
He learned all these facts, I believe from his father from the 1950s when oil was dirty and gas was clean. He may still have that Armstrong Octopus Furnace with the Quiet May oil burner in the bottom.

He is reinventing the wheel for us. LOL

For the modern oil furnace, if we use waste motor oil in it (diluted to be consistent with diesel viscosity, and filtered against solids), can the heat exchanger be still free from soot deposit?

Can the same mechanism be used on wood burning furnaces and boilers?

ratio

The best solvent to remove soot is elbow grease. Apply liberally & rub it in thoroughly & the soot will just fall away.

Jamie Hall

It might help to remember exactly what "soot" is. Simply and briefly, it is more or less pure carbon. In the combustion chain of any carbon containing fuel -- and it doesn't matter whether it is natural gas or Bunker C or wood chips or cow plops, there is at least some carbon. The chain involves vapourizing the fuel (if it isn't already) and then heating it to the point where the hydrogen is freed from the carbon, and then combining the hydrogen and carbon with oxygen to form carbon dioxide and water. The hydrogen-oxygen reaction takes place more readily, so in the absence of enough oxygen, either carbon monoxide will form instead of carbon dioxide, or some of the carbon simply won't be oxidized at all.

That's a bit simplified.

However, if the air to fuel ratio is too rich -- too much fuel in relation to air -- whether locally within a flame or as an overall ratio, the end result is some unburned carbon. This unburned carbon will condense on any cooler surface or, if it makes it into more general air but at too low a temperature, into grey to black "smoke" (fine particulates of carbon).

The problem of getting an even mixture of air to fuel is much simpler with liquid or gaseous fuels, of course. With uniform solid fuels, such as coal, it's more or less manageable (although coal has other problems in the form of incombustible, mostly silicate, contaminants which appear as fine particulates -- fly ash). With non-homogeneous materials, such as wood, it becomes exceedingly difficult and, to get clean exhaust, pretty much requires either afterburners or catalytic devices, which run at extremely high temperatures.

So to go back to @cowdog 's question up there -- having gone most of the way around the barn -- yes, if the waste oil is really clean and you set the combustion correctly, using the correct nozzles and spray pattern, and avoid impingement (which cools the combustion prematurely) you can get soot free combustion. If you were actually designing the thing from scratch, you would add secondary air injection with a lot of turbulence so you had a slightly rich core flame surrounded by a significantly lean but very high temperature flame.

Now even there there is a caution: you will get soot on ignition, when the flame is not up to temperature and, of necessity, the fuel/air mixture may be slightly rich (think the choke operation on a gasoline engine, if you are old enough to remember chokes). It should be minor and transient.

There is a secondary problem: flame temperatures high enough to ensure complete combusion of all the carbon are also high enough to cause combustion of some of the nitrogen in the air, to form various nitrogen oxides -- so either use catalytic reduction to get rid of the nitrogen oxides, or you try to keep the flame temperature "just right". The latter approach works rather poorly...

Going little further, modern internal combustion engines manage to be essentially soot free, but they do this with direct fuel injection of very carefully and instantaneously controlled amounts of fuel, at least two air mass flow and oxygen sensing feedback loops, and catalytic converters to remove nitrogen oxides and any excess unburned carbon.

HVACNUT

archibald tuttle said:

hmmmmmmmmm . . . interested to see what this brings. my recommendation is convert to NG or propane before you get carbon deposits. 

Carbon deposits, or soot is fair warning that things aren't hunky dorey. With fuel oil you get the soot, and then the CO. With gas you get the CO, then get the soot, after the canary dies.

EdTheHeaterMan

cowdog said:

EdTheHeaterMan said:

@cowdog, basically you want to make a self cleaning furnace. (not unlike a self cleaning oven that just burns off baked on food).

Is there a market for this product? Or you just making it for your own convenience?

I believe the market is on any fuel that would generate soot, such as oil, waste oil, wood, plastics, biomass etc. Sootcleaning, whether on heat exchanger or inside chimney, is a dirty job most homeowners and building maintenance technicians hate doing.

You believe there is a market, but you have not done the market research then. The cost of your invention or device might be a little cost prohibitive. Does this self cleaning invention work as a retrofit to existing equipment? Does this self cleaning mode only work for new equipment as a factory installed option?

Your concept has merit. Your design ideas may have some practical applications. I just find the implementation may just cost too much. I certainly know that the amount of oil heating equipment installations is declining by all measures I know of. Is there anyone willing to foot the bill for this experiment?


As a side note... In reference to cleaning carbon off of unwanted places. I own a 1923 Ford Model T. I had some problems learning how to get the fuel mixture correct when I first acquired it. This left the spark plugs very dirty with copious amounts of carbon buildup. My Mapp Gas torch was the easiest way to get the spark plugs back to the pristine carbon free state that made them operate properly. Just burned that stuff off in seconds.

MikeAmann

EdTheHeaterMan said:

Be nice Steve.
He learned all these facts, I believe from his father from the 1950s when oil was dirty and gas was clean. He may still have that Armstrong Octopus Furnace with the Quiet May oil burner in the bottom.

He is reinventing the wheel for us. LOL

Then I will make sure that I don't give him any bad ideas.