Air/Gas Ratio Control

As its a ratio of mass flow, the temperature difference is minimal as the temperature must be in kelvin absolute units...

So the ambient temperature is varying from 263 to 298 K which is only a 12% change approximately//..

Is that small difference going to make that much difference? Consider also that the incoming gas pressure is related directly to ambient pressure, with a storm front approaching and the ambient pressure dropping to less than 960 milibar, or a warm weather front approaching with the ambient pressure rising to over 1030 millibar - a change of approx 7%.

12% doesn't sound much of a change when the weather is changing by 7%!!

John.

You can estimate your variability in air density based on different atmospheric conditions. That will tell you a range in how much "off" you might be.

You may be able to find tables that relate air density to temperature and barometric pressure. If you can't find the charts, the equations are easy to handle for an engineer.

Why not put an inexpensive excess O₂ monitor in the exhaust and run it at, say 2%? Then you could adjust the A/F ratio as often as you feel like it. You'd save money on fuel and minimize pollution.

I agree on that. If you have a forced draught you can use the O2 content to control speed, louvres, etc.

If you don't you will have to train your operators/firemen to adjust yor furnace to the O2 levels.

In both cases you have to look at all temperatures involved : stack, skin, flame pattern, impingment, etc. Iam certain "HEATER SPECIALISTS" can give a lot more of reasons to visualy inspect and trim burner. This is independant of the A/F ration wich in many case determines the "Energy Efficienty"

Hope this doesn't sound too conservative as today the control system mastes and the outside control is at a low appreciation.

Siegfried

I mentioned that: " Try putting in an Oxygen Trim if it is not expensive "

Yes, you did. I missed your posting when I did mine. A "good answer" to you!

No I thought Mine was different than your or not ? With technology things do change. Just to clarify.

Yeah, I'm used to automatic ones, but many are manual.

Yeah but mind you these O2 Trims are only good with the New Burners or where the O2 level is within 1.5% to 2.0%. On older Boilers not viable because of low turn-down ratio. Manual indicator & control then his option.

"do these temperatures difference have a big impact on our gas consumption?"

Yes that is true, and for this reason, sometimes in certain process in oil refining, we have to use a well designed air or gas preheater.

Gas consumption efficiency depends on the efficient used of fuel that provides the optimum amount of heat generated which can be determined from the refuse and in flue gas. Inefficies are usually caused by the loss of combustibles charged and the incomplete or partial combustion of the combustibles burned. In the case of complete combustion, an amount of air is needed and a sufficient heat is required to oxidize the fuel. Therefore, the difference of the ambient temperature can influence the impact of gas consumption. Excess air also promotes an efficient fuel consumption but a greater amount can lower the flame temperature which affect negatively the heat transfer efficiency.

edbesas

Willam - Go to The Engineering Tool Box. Click "Combustion" extreme left. Click "Combustion Efficiency & Excess Air". You will find the chart and answer to your question.

Thanks for your replies.
I will first call the company maintaining our burners to know the air/gas ratio, the weird thing is that they don't write it down on their report....

DEAR FRIEND,

the Answer for your question is YES.

Secondly Temp. of -10 Deg.C to 25 Deg.C will have an impact on the RELATIVE HUMIDITY which is to be accounted for.

Thanks,

RAJESWARI.