Combustion efficiency

realolman

what is the top efficiency possible from a cast iron oil boiler as calculated by a combustion analyzer.

I have heard some high numbers being thrown around that I dont believe.

Unknown

Crown Scotch Marine,,,

their "Freeport", claims the CT-3 model with .75 GPH nozzle and a Riello burner will produce 87.6%. That's the highest I know of. There may be others I don't know about that will go higher or at least match this.

Brad White_200

Name your limit

Technically, a small burner in an oversized chamber can get into the 90's but no one is saying the boiler will last long nor is it efficient to run it as such... The retention time and extraction knock down those flue temperatures.


But for rated equipment, properly matched, 87% seems to be the benchmark, maybe a few decimals above.

Robert O'Brien

I

routinely get readings in the 89-90 range(with zero smoke),which then have to get knocked down because of stack temp and insufficient excess air. A Vitorond or 115 DV unit can easily run at 89+ if you don't mind under 10% EA!

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Brad White_200

Good point Robert

on the EA factor. I was thinking along the lines of the better 3-pass boilers but conventional.

Data, actual reading data, on some of the "no low water return limit" biferral and eutectic CI boilers is of interest to me. We normally see those cataloged in the 86-87% ranges (but above the AFUE 300 MBH limit), yet "conventional 3-pass" cast iron has similar numbers.

Thanks for sharing and prodding my brain. "I forget to remember what I do not yet know..."

Brad

Jim Davis_3

Any appliance can be misfired to produce a high false efficiency calculation. Unless you condense no oil appliance can operate much above 75% real efficiency. gas is limited to 72% if it is non-condensing.

Combustion efficiency calculations have been poor indicators of actual equipment performance since they were first developed. Maybe 1% of the time a piece of equipment might run near the actual calculations but more by accident than on purpose.

Unknown

How ,,,

is a product like the CT-3 achieving these numbers then? Are we setting them or the flue up for early failure?

Because I'm new to the electronic analyzer, having only had the standard bacharach kit for many years the EA figure is new to me. What is the accpetable levels for EA? I've been going on the information that 300 net stack temp is minimum. Is that a good number?

I've never set up the CT-3 riello combo so I don't know what kind of ST they produce. I do recall the CT-4 being down near the 300* mark when I set it up though. I don't service that one, I just installed it so I have no idea how it's doing today-10 years later.

Jim Davis_3

If the CT is oil it should never run under 400 degrees. Oil flames are 1000 degrees hotter than gas flames so it is expected that they should be at least 100 degrees higher than any gas boiler of similar design. Underfiring will create low flue temperatures and high calculated efficiencies. Actual performace will be much less. Because all equipment efficiencies are based on bogus combustion efficiency calculations and not performance we end up with high fictitious efficiencies.

Unknown

Jim is correct

however as you all know everyone wants to talk the efficiency numbers. I often post here concerning for example combustion eff numbers on conversion burners as 80 to 82% which is what the analyzer will give you AND THAT IS WHAT YOU ALL WANT TO TALK ABOUT. What we should be doing is firing the equipment to the maximum firing rate and watching figures like O2, Flue Temp and CO which will follow based on how much excess air we have. Excess air is necessary for safe burning, however excessive excess air will reduce efficiency. Your readings will tell you that but the analyzer eff reading is not one I pay much attention to, I am more concerned about the three I mentioned. Truth you can put a candle in a steam boiler conversion and get a good eff but you will not heat the building very well.

Unknown

MPF that 300 degree

is good for gas as a minimum figure, actually around 275 is were condensing may begin in the flue gases. As Jim staed oil should be around 400 degrees.

When readings are being developed by the analyzer you want to watch O2, stack temp and CO, making adjustments to bring them into line. O2 and CO2 will track one another as O2 (excess air) goes up CO2 comes down and conversly efficiency will be reduced. We say excessive excess air reduces efficiency. From the gas side we have primary air which is air mixed with the gas at the primary air opening (atmospheric burner)which has little to do with actual eff its purpose is to devlop the correct burning speed of the gas. Secondary air is air mixed at the burner which helps to have safe burning (low CO) excess air is the additional air required to balance the overall operation of the combustion process. Depending on the type of system we are talking about excess air will vary. Oil and gas conversion burners because they are power burners need less excess air than an atmospheric gas therefore they tend to have higher efficiencys. You also must be aware that gas and oil have different ultimate CO2 gas being 11.7% and oil being 15%. Mnay use this to say oil is more efficient when actually using CO2 as a criteria they are probably equal. The oil becuase it burns hotter has a small advantage in the heat direction.

Unknown

Thanks Tim and Jim,,,

When you say "ultimate" do you mean optimum as in best? Should I be shooting for 11.7% and 15%? In the past I've always gone for (on oil) 12% and considered below 10% too low and above 13% too high. Although I can't tell you why anymore. I must admit not considering analysis on gas till only recently as I was taught to adjust either by clocking or by the old reduce air till you see yellow tips and then increase till they just go away method. Yes,,,I'm very old.

Can't recall the Crown stack temps anymore. if they were below 400* I know they're the only ones I've ever seen that low. Mostly I see 500* and up on the 80%-ers.

Does the calculated efficiency bug relate to the old style bacharach kit as well?

Unknown

Ultimate CO2 is

related to what is called Stoichoimetric combustion (perfect combustion) that would be zero% excess air. Your figures for oil are correct and with gas typically 7.5 to 9.5% is realistic in fact 9.5 is about equal to 13% on oil.

With the old Fyrite bottles we took our readings and charted them out on a Fire Finder which was really just mathematical calculation and not real accurate. To get real accurate eff readings we need to get into a laboratory and test. What we do is good enough for our business however. The key is safety and if we are safe then we can also rely on pretty good eff. If you do not test you have no idea.

Unknown

Much clearer,,,

thanks again!

Jim Davis

The old method of CO2 was not really accurate because you would have to know the exact btu of the fuel to determine the maximum CO2. All fuels burn at their maximum CO2, whatever it is. Excess air dilutes this reading in the flue. By keeping the O2 or excess air to a minimum, we maximize efficiency. 3%-4% O2 equal 13%-14% CO2(approx). These are quite attainable in the field at zero smoke. Going totally by manufacturers recommendations won't always allow us to get maximum operation. NO pump pressure should be below 125#. Hollow nozzles perform the worst on new burners. Two line(supply and return) on oil systems are self-destructive. Just a few problems that make oil seem worse than it is.

Unknown

I'm gettin' it!

Believe me, having used the old kit for years and years noone loves his new analyzer more than me. The on site testing time has been dramatically reduced as I don't have to keep screwing around with that fyrite bottle over and over till things get right. I can watch the effects of my adjustments in real time. Not to mention the part about the unit cycling off on setpoint right in the middle of testing and having to wait for things to cool down to try try again. The info given here is greatly appreciated, now I can do my job just a little bit better.