On an atmospheric boiler

Tim McElwain

what effect does primary air at the burner opening have on the combustion analysis?



Does adjusting primary air change the analyzer readings?

nicholas bonham-carter

Does the burner KAIR about that?

Air is at different densities at different seasons, and this must make a difference with a mod-con making hot water in the summer, vs. heating using sub-zero outside air in the mountains.

Maybe HDE can explain.--NBC

JStar

Primary air

I'll typically be able to adjust the CO levels, but not the O2 levels by adjusting the primary shutters.

Steamhead

The reason for this

is that more primary air shortens the length of the flame, which makes it less likely to impinge (strike) colder things such as cast-iron. Chilling the flame in this manner causes incomplete combustion and CO.



The results are especially dramatic on oven and broiler burners in stoves.

Tim McElwain

We adjust primary air

in order to get the correct mix at the burner head of gas and air. The measurable amount called the ideal burning speed is typically a 10% gas 90% air mix. This will insure a soft blue flame not roaring and not lifting, with no yellow in the flame. With propane the correct mix is 96% air 4% gas. These numbers fall within the upper and lower flammability limits of the fuels 4% gas to 96% air and 14% gas to 86% air for natural gas and 2.1% gas to 9.6% gas for propane. When these same levels are found outside the designed combustion zone they become the upper and lower explosive limits.

Bob Harper

primary aeration effects

In addition to the points Timmie made, let me offer these additional concerns:

High primary aeration with a low manifold pressure may result in flashback

High primary aeration with a high manifold pressure may result in flame lift-off

Low primary aeration with moderate to high manifold pressure may result in yellow tipping.



The higher the primary aeration, the greater the velocity of air being entrained at the venturi and not just volume. Therefore, subtle increments can have a significant impact. This can be beneficial such as with LP where we need that turbo-boost to help shoot the fuel/ air mixture down the mixing tube towards the burner ports and create turbulence for better mixing.



One side negative of primary aeration is the propensity to cause poor combustion from the recirculation of combustion products, which is why we add in secondary air. This recirculation is mainly seen on a cold start, which can be seen on a combustion analyzer. Once secondary aeration is established, there is better flow up the stack and removal of combustion products. Too much secondary air can cool the flames resulting in poor combustion, which is one of the drawbacks of high excess air. Each fuel gas has its own specific properties in these regards, which is why it can be very difficult in some units to get them to behave when performing a fuel conversion.



With regards to flame speed as Timmie pointed out, the flame front moves faster at higher air/ fuel ratios. This is all very specifically calculated, engineered and tested with burner designs, venting, etc. One example of flame speed is on an Oxygen Depletion Sensor safety pilot of all ventfree appliances. As the ambient O2 drops to 18.5% the pilot flame appears to advance out past the tip of the thermocouple causing the pilot to dropout. This is because the flame is burning slower back towards the pilot burner thus creating the 'gap'. Now, if you have too much air, the flames can race back so fast that, depending upon burner design, port loading and esp. temperature of the burners, the flame front can run down through the burner causing flashback or extinction popping.A narrow mixer tube and smaller burner ports can increase primary aeration without flashback or extinction popping.



Lint can stop burner ports causing major changes in aeration and fuel/ air mixing so cleaning burners is very important to good combustion. Engineers have to figure what method they will use to eradicate lint from the burners such as incineration, filtration, re-directed airflows, etc.



It will be interesting to see the effects of high Wobbe Index fuels with time. The studies thus far indicate no major concerns but those were conducted by agencies with close ties to the fuel providers and government so I don't trust them one bit. I think you and I will inherit a mess of problems if LNG becomes more prevalent either straight in or mixed with existing stocks of pipeline NG for peak shaving during high demand periods.

Tim McElwain

Thanks Bob you just answered

the next five questions I was going to ask. Excellent information thanks.

Bob Harper

got carried away!

Sorry to steal your thunder Timmie. Please feel free to expand on any of these concepts. I'll chill and let some other people comment. You are da' man always. 

Tim McElwain

No thunder involved

please keep answering. This series of questions I have been asking was at the request of several folks who lurk here but do not post. They do however have some issues with determining proper air for combustion and in addition problems with venting both natural draft , fan assisted and high efficiency equipment. I find posting a question and getting answers is one of the ways to get folks involved. So please answer away and feel free to add anything you feel is important to the discussion.