Hot & Cold Running Oxygen

August 20, 2006 8:09 AM

When your [furnace/water heater/sauna/time machine] burns natural gas, should you feed it hot or cold air?

What's more efficient to promote combustion: cold air (higher density) or warm air (less energy used to increase temperature of gas/air mixture)?

My guess is that the higher density of cold air has a bigger positive effect than the heat stolen to warm the mixture up to the combustion temperature, so cold is better. But I don't know how to calculate this. Can any MePhysicists help? (No, this is not a homework question.)

posted by spacewrench to science & nature (8 comments total)

All the ones I have seen and used use ambient-temperature air.
posted by markmillard at 8:13 AM on August 20, 2006

The limiting factor is the gas, not the air. There's a certain amount of energy in it, and essentially 100% of it will burn. If the air is warm and less dense, it will just suck in a higher volume. The increased flow rate of the warmer air would partially offset the lower temperature of the cold air.

The net effect of cold air might be _very_ slightly negative. The loss of heat from cold air in contact with the device and the pipes would be enormously more important.

All other things being equal, if you could somehow isolate just warm or cold air to the gas intake, I'd be surprised if you saw even a 0.1% energy bill difference. That said, warmer air is probably just the tiniest bit better.
posted by Malor at 8:19 AM on August 20, 2006

Any difference in combustion is too trivial to care about, although perhaps not too trivial for the engineering geeks to argue over. A real issue is whether you are sucking air out of your living space and sending it up the chimney. If you spent a bunch of money heating or cooling that air to make it comfortable you might want to keep it in the living space and use outside air for your combustion. People do it for fireplaces, but not usually for anything else as far as I know.
posted by caddis at 8:33 AM on August 20, 2006

My speculation is that the other answers are slightly understating the advantages of warmer air.

Flames lose a lot of their energy to radiation, especially radiation in the infrared region. Radiative losses go at something like the 4th power of the difference between the source and the environment, so any decrease in the temperature of the environment would seem likely to have a perhaps surprisingly large effect on losses from combustion. Malors answer, "the loss of heat from cold air in contact with the device and the pipes would be enormously more important" essentially captures what I'm saying here, I guess, because that's the environment the flame would be radiating into.

There is another issue of cold reactants in play here, that of the temperature of the gas itself as it emerges from the pipe. The gas in the pipe is under pressure, and as it comes out of the pipe it will expand adiabatically into the much lower pressure of the atmosphere, which will have a profound cooling effect upon it. I bet that temperature drop is greater than the average difference between warm and cold air available for combustion in most places in the US.
posted by jamjam at 12:03 PM on August 20, 2006

Here's some info on this question in an industrial setting. Basically they say with highter natural gas prices it's worthwhile using the waste heat that goes up the chimney to preheat the air used in the combustion. With a little ingenuity this could be done in a home setting, by bringing the air supply in by way of the exhaust flue and installing some kind of heat exchanger.
posted by beagle at 12:26 PM on August 20, 2006

People do it for fireplaces, but not usually for anything else as far as I know.

Most gas stoves are installed these days with outside air intakes for better efficiency -- avoids having the make up for the lost warm air out of the living space.
posted by beagle at 12:28 PM on August 20, 2006

When I worked at a power plant, there was a system in place to cool the intake air for the gas turbine. This was supposed to result in several percentage points increased output. (Which could be several thousands of dollars an hour on a hot summer day.) Here's a link to something similar.

But, this was for a turbine that eats huge amounts of air, (in fact, if I remember right, about half the power developed in the turbine stage was used by the compressor stage to feed itself more air) and probably doesn't apply to heaters for your home.
posted by TheOnlyCoolTim at 2:59 PM on August 20, 2006

Unlike an industrial turbine, a domestic combustion stove's performance is not terribly dependent on incoming air density. Your furnace is always going to suck more air than it needs for complete combustion, just to make sure combustion is complete; domestic furnaces are designed to minimize carbon monoxide emissions.

But focusing on raw combustion efficiency, in a home furnace setting, is trying to optimize the wrong thing. What you actually want to focus on is minimizing the total amount of heat leaving your living space.

For a combustion stove, that means you want to capture as much of the combustion heat as possible and retain it inside your house.

Assuming complete combustion, the way to do this is to minimize the temperature of the flue gases that leave the building.

You can do this with a contra-flow heat exchanger fitted to the flue: cold outside air is drawn inward through the exchanger, cooling the flue gases travelling outward, so that the coldest outside air cools the already-coolest part of the flue. This results in maximum heat transfer from outgoing flue gases to incoming air.

If you can reduce the temperature of your outgoing flue gases to something very close to the outside air temperature, and transfer the recovered heat to the inside of your building, the question of where your furnace is drawing combustion air from becomes moot.

I've long thought that it should be possible to design a cheap contra-flow heat exchanger for air-conditioned buildings, too. This would allow fresh air to be pumped through the building without losing cold to the outside.
posted by flabdablet at 8:37 PM on August 20, 2006