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View Full Version : How come a gas fireplace burns yellow but my gas stove burns blue?


Duck Duck Goose
12-20-2000, 09:10 AM
I know how they work--the flames come up past a couple of cement logs or something. Is there something impregnated in the cement that gives that picturesque yellow flame?

Also, some logs are evidently painted. http://hearth.com/stoveworks/vclogs.html How do they keep the paint from burning off?

Sorry if this is a dumb question.

zen101
12-20-2000, 09:18 AM
Who is afraid of Duck Duck Goose? Pull your head out of the stove woman!

JeffB
12-20-2000, 09:19 AM
No real answers, just some speculation. The different color of the flames could be caused by different temperatures. Those blue flames on the stove are much hotter than typical fireplace flames (whether gas or real wood logs). The logs could also have chemicals to make the flames that color. I remember as a kid having stuff to put in our fireplace to make the flames different colors.

As to the "painted" logs -- the link references a "six step painting process", which could include some type of baking like with enamel. In fact, they probably use enamel paint.

Pnuk Guy
12-20-2000, 09:21 AM
The yellow flame is due to the flame not burning efficiently and therefore producing carbon.

I reckon its because you gas stove hs a good Oxygen supply and so burns very efficiently, (i.e. blue)

In your 'log' fire, there is not as much available oxygen as the 'logs' cover it up, and so the flame is yellower.

I don't know about the paint... a wonder of modern technology, i suppose. :)

Pnuk Guy
-----------------------------------------------------------
'Outside of a dog, a book is man's best friend. Inside of a
dog, it's too dark to read.' - Groucho Marx

Philster
12-20-2000, 09:25 AM
The stove is designe to burn "hot", because after all, it's to cook with. Also, hot water tanks, grills and heaters, when running correctly, should burn with blue flames.

Blue is indicative of a good burn, focusing energy in one spot for the purpose of cooking/heating a specific area. Some chemist will get on hear and tell you more about that, but in the home, blue flames are a sign of a good hot burn.

Gas fireplaces are more ornamental than anything else. Sure you might scavenge some heat, but they are decorations primarily. The burner isn't designed to cook anything, but rather be bright enough and big enough to have an ornamental effect. Relative to the paint and the ceramic logs, the fire isn't all that hot.

Why doesn't the paint on the logs burn? Why doesn't the paint on an engine burn? They are both designed to handle the heat. Most paints like this are already dark in color and are usually pretty "flat" in color.

ticker
12-20-2000, 09:34 AM
Some fires work by having the flame heat a (usually) ceramic material which then radiates heat. In mine the flames start off blue but turn red/yellow when the ceramic starts to glow red hot. Perhaps the effect is to do with the radiated light passing through the flames.

Duck Duck Goose
12-20-2000, 09:39 AM
Um--engines are painted?

Saint Zero
12-20-2000, 09:44 AM
A: Yeah, engines are painted. Blue and Orange, among other colors. Ford is mostly blue, and Chevy is/was mostly orange.

B. For the record, my fireplace and stove are both gas, and both are burning blue right now. You will get orange/yellow streaks everyone once in awhile, but I've always attributed that to dust or lint being kicked up into it. Could always have it checked if you get paranoid. :)

muttrox
12-20-2000, 09:44 AM
Also note that if multiple colors where present, the yellow would be higher, "on top" of the blue. In a gas stove, you can easily see the "center" of the flame. In a gas firelpace, you can't, as it's hidden by those very logs and whatever stones and such you keep in the fireplace.

Experiment: Remove the logs (wait until they have cooled off!), clean off your grill and such and see if you can spot any blue.

I don't know the answer, but it seems like a good shot.

Triskadecamus
12-20-2000, 10:53 AM
Gas burners are designed with mixing valves which take in air, and mix it with the gas before it reaches the combustion location. As it exits into the flame, it is already mixed with enough air to burn completely, and produces a very hot flame. That turns out to be a blue flame, given the temperature of burning methane. If your gas stove, or water heater is not burning blue, it needs cleaning, and it needs it soon. Incomplete combustion in a non vented location is dangerous.

Your gas logs are designed differently, to achieve a different result. The gas is not mixed and is released at the ignition point and instead is allowed to mix with air while it burns. This produces a cooler, and less complete burn, which allows some of the carbon in the methane to remain unburned. That carbon absorbs heat, and then releases it as radiant energy in a characteristic frequency, which is yellow. Since the carbon in wood also releases that same frequency, this is the desired effect of a properly designed fireplace log. Because of the risk of incomplete combustion mentioned above, the fireplace must be vented well.

Tris

handy
12-20-2000, 10:53 AM
Easy. Turn you gas stove way up, you see yellow.

Nbr 2. High temperature paint.

Podkayne
12-20-2000, 02:10 PM
Just a word of caution: a flame is not a blackbody. Particles of soot (made mostly of carbon) radiate the light that gives the flame its color. The spectrum looks like a continuum, but what you're really seeing is very closely spaced line emission. The peak wavelength does decrease with temperature (so bluer flames are indeed hotter) but the colors do not correspond to a blackbody curve. E.g. Candles burn yellow, corresponding to a blackbody temperature of 6000K, but really the temperature of the flame is only 700 F.

This is cribbed shamelessly from: Sten Odenwald's Ask an Astronomer (http://www.itss.raytheon.com/cafe/qadir/q490.html).

Duck Duck Goose
12-21-2000, 10:25 AM
Thank you, everybody, but especially Tris. (You're not a fireplace salesman IRL, are you? :D )

Does this mean that Saint Zero's fireplace has something wrong with it, if it's burning blue? All the magazine illustrations of gas logs I've ever seen show them burning yellow. Or does he just have a different model? Or do the manufacturers know that consumers want a cozy yellow flame, so that's how they show it in the ads?

wooba
12-21-2000, 10:32 AM
I'm no expert, but I would speculate it's got something to do with the amount of oxygen the flame can get.

Remember the Bunsen Burner (sp?) back in science class.
Start it with the shutter at the base closed and you get a large yellow flame you can put your hand through.. open the shutter and you get a small blue (and very hot) flame.