Energy assumed vs energy produced

Jim Davis

I thought this might create a good interaction and discussion between the Wallies.
Theoretically a cubic foot of gas can produce approximately 1000 BTU, or a gallon of #2 oil 140,000 BTU. This only occurs at stoichiometric combustion or Zero Oxygen and Zero CO. Proper combustion for the equipment we work on should occur at 6-9% Oxygen, which means the BTU actually produced by a cubic foot of gas is closer to 900-950 BTU before it is even transferred. The question I ponder is if a piece of equipment is rated 100,000 BTU input, is it really? Or is it just 100 cubic foot of gas input and 90,000-95,000 BTU actual capability?

Glen

I would say ...

that the rating is correct - and not just the cfh one could stuff through it. And I base this thinking on fuel types available, eg. nat gas, propane, butane, propane/air mixes. All have different calorific values and would translate into different volumes of gas needed to produce the same 100,000 btuh. This also brings to bear the variations of calorific values of any fuel type based on altitude of installation and the resulting derating necessary to have the appliance fire correctly. And - each burner type will have a different target oxygen values, eg. an atmospheric burner in a scorched air furnace will have relatively high oxygen in the flue gaes, some conversion burners offer slightly better combustion. Once you approach high end burners such as the new Riellos or Weishapts where 5% or less oxygen is expected throughout the complete modulation range - truer comparisons could be made as to rating versus throughput of gas. I have setup a number of dual fuel Weishapt/Viessmann VSB boilers - and I found numerous consistencies regarding fuel consumption, excess air, oxygen and burner efficiencies between the nat gas and propane fuels. If fact - once the equipment was setup on nat gas - the predictability of propane consumption and flue gas expectations was easily calculated and then confirmed by clocking the appliance and flue gas analysis.

Mike Kusiak

Jim, you bring up an interesting point here. If a boiler is rated at a 100000 BTU input, how is that heat input actually measured. I would think that you would clock the amount of gas used in an hour, and then multiply it by the the theoretical heating value of the gas used. This would, as you say, give you the heat of combustion under ideal stoichiometric conditions. If due to the excess air, the actual combustion efficiency is less than stoichiometric, then this loss of efficiency would show up in the overall gross output of the boiler, which would be reduced by the losses due to the excess air. So for example if with a theoretical 100000 BTU in, the boiler actually measured 80000 BTU out into the water, then the 20000 BTU loss would be a combination of the loss due to excess air, and the loss due to imperfect heat transfer in the exchanger. I dont really know if you can separate these two factors, since the net effect of the excess air is to reduce the flue gas temperature and increase its volume, which reduces the efficiency of transfer in the heat exchanger. I guess its all in how you define it. Perhaps some of the guys from the boiler manufacturers can shed some light on this.

Boilerpro_2

I like your question....

I was alluding to this in the previous discusion when I said that I size my boilers very tightly and have never had one that was undersized in my short time in this field. ARE the output ratings real or just theorectical?


Boilerpro

Tony Conner

There Are...

..."higher" and "lower" heating values assigned to fuels. I've worked in operations where the lower heating value for natural gas was taken as 904 BTU/cu ft. This value assumed that a portion of the hydrogen combined with the oxygen to form water vapour. The same type of factor was applied to #2 fuel oil.

It doesn't really matter, as long as one value or the other is consistently used by everyone.

Glen

Do the values change ???

Theoretically - 1050 should be the calorific value for nat gas - in this area the gas supplier tells us it's a max of 1000 - more likely 980+. It's enough to throw off your clocking calculations by a bit. Which changes the amount of excess air required - excess air being there for complete combustion and some level of safety to ensure complete combustion. But is it the gas or the appliance that assumes the responsibility of efficiency? I too like the question.

Tony Conner

I Think...

...you'd need to see the EXACT wording for the conditions and calculations manufacturers use for their efficiency ratings.

I know a number of low water content high pressure steam boiler manufacturers make claims of over 90% eff. This is no doubt the case under carefully controlled lab conditions, with a nice, flat load. In the real world? There is no way on earth they're hitting anything even close to that. But nobody can say for sure, because I've never seen an installation yet with these little boilers, where there's any proper steam output metering. The vast majority don't even have just crappy steam metering - they have none at all. And the fuel input to each boiler would have to be "guesstimated" off the gas bill for the whole plant, including a huge portion of direct fired process equipment. It'll be largely the same for residential boilers. Does everyone who clocks the gas meter make sure that the water heater/dryer/range hasn't come on or gone off, while their taking readings? Somebody turn on a ventilation fan in the bathroom, or over the stove? Holding the door open while they bring in the groceries? None of these things will actually affect the boiler eff. Other gas burners will show up on the meter reading. Things like vent fans and open outside doors won't actually affect the boiler eff, but the boiler WILL use more fuel during that time. And unless you've got an output meter, all you'll see is an increase in fuel consumption. But you won't necessarily know why.

Tony Conner

I Would Bet...

...that manufacturers, trying to make their appliance look as eff as possible, would use the lower heating value of any given fuel, unless the standards imposed by whatever the governing body is, specifically identifies those heating values.

And you still need to know the exact wording of the eff statement provided by manufacturers (or anyone else, for that matter). Is it burner eff, as defined by combustion tests or boiler eff, as defined by heat out over heat in?

I saw a high pressure industrial steam boiler last week. I don't know what the burner eff was, and there was no steam meter, or individual gas meter for that boiler (there are 3 boilers, plus a bunch of other gas fired stuff on site), but with a 700*F stack temp, I'm guessing boiler eff is not great. This is a new installation, too. I can't imagine the factory start-up guy not setting the burner up, so it's probably right on. It could be the greatest burner in the world, but it's installed on a pretty crappy boiler design, and a ton of heat is going right up the stack, instead of into the water.

Glen

It depends ...

I set up a industrial burner a couple of years back, (nat gas/wood cogeneration) 18,000 HP, 750 PSI @ 700 Deg F. It was running a max of 4.5% oxygen flue gas measurements and it's flue gas temp was very high - if the flue gas temp was lower - it would cool the steam. Operating costs per hour were $1000 + on gas - so they would switch over to wood stokers ASAP.

Tony Conner

This Boiler...

...runs at 125 PSIG, and it's saturated - no superheat. Stack temp shouldn't be more than about 600*F on the one here. Both the one you worked on, and the one I saw last week would benefit from an economizer. But it depends on the payback. On wood, it'll likely be tough. On gas, it's likely a sure thing. And the economizer will improve system eff, but boiler eff will remain unchanged.

And even on the much bigger boiler you worked on, I'll bet they still used crappy old orifice plate for metering the steam. Unless something like vortex shedding units with pressure compensation are installed, the output from the boiler will be pretty much fictional. I can't remember is the vortex shedders will take the 700*F steam temp though. If they tune their orifice plate meters in, and install a pressure compensator, they'll probably get a steam output number that's at least close.

Mike Kusiak

I believe the higher heating value assumes condensation of the water vapor produced by combustion, while the lesser heating value does not include the latent heat of condensing the vapor. This may be part of the reason the quoted efficiencies of condensing boilers seem so much higher than conventional ones, especially if the manufacturer is required to use the higher heating value for rating the conventional boiler. The conventional boiler is at a disadvantage in rated efficiency since the input includes the higher value, but never receives the benefit of the latent heat, since it does not condense the vapor. Again, it is all in how you define, measure and calculate efficiency.

Tony Conner

I Never...

...heard that definition of higher vs lower heating value applied to the steam boilers I've worked on. The water was deemed to be formed from hydrogen and oxygen getting together, and since we very much did NOT want condensing in those boilers, it never came up. It was just water vapour heading out of the stack with the rest of the combustion products. I always assumed that the fact that the flue gases were cooled to the point of condensation, in a condensing boiler because so much heat had been transferred to the boiler water, as opposed to less eff boiler designs. I would think that all of the boilers in a given size range (if not all classes as a group) should use the same heating value of the input fuel. Otherwise, there's no effective comparison.

To me, boiler (or furnace) eff is very simple - output BTU over input BTU. This formula, of course, doesn't identify WHERE the losses are. It could be poor combustion eff. It could be fouled heat transfer surfaces. It could be skin losses, etc.

I can't remember all of the information provided by industrial steam boiler makers with their units, but I do recall it was pretty complete regarding the rated output. It mentioned that the output of a given boiler was based on a specific feedwater temperature, air intake temp, even temps for whatever fuel was to be fired. I don't specifically remember, but if I was the manufacturer providing that level of detail, I also spec a minimum BTU rating per unit of fuel. If the installation failed to provide the required conditions, then the boiler could have reduced capacity, and that wouldn't be, in all fairness, the fault of the boiler maker.

[Deleted User]

I've seen situations...

under controlled labratory conditions, and in my own monitored home where the actual output of a water heating appliance was less (substantially less) than was physically being seen through the eyes of a combustion analyzer.

When you're monitoring water mass flow and delta T conditions, and comparing that against combustion efficiency numbers and see a big (20%) difference, it's enough to make you stand back and go "HMMMmmmm....."

I don't believe it was instrumentation arror to that degree. 2 or 3 points maybe, but 20???

HMMmmmm.....

ME

Tony Conner

THANK YOU...

... FOR RUNNING THOSE TWO TOTALLY SEPARATE TESTS. I have absolutely no doubt whatsoever that your numbers are correct. You did a combustion eff test, and a boiler eff test, and man, are they ever NOT the same thing. What exactly were the numbers you got?

There's just nothing like an actual, physical field test to make your eyebrows go up.

[Deleted User]

The tests...

One was at the lab up at Red Rocks. It was a copper tank style of boiler. We were testing the DHW coil and the boiler for steady state conditions. The water was being flowed at a given rate of GPM as checked and verified by a calibrated flow meter. The gas was being metered by a temperature compensated gas meter. The combustion efficiency was being checked by a brand new electronic combusion analyzer. We knew what we were sending in from the gas meter. We knew what the "tested theoretical" combustion efficiency of the appliance was, and we were reading the differential in temperature of the water under a steady state condition. There was a good 20 percent difference between what the combustion analyzer said we should be getting versus what the water flow rate and delta T said we in fact WERE getting. That was case number 1.

Case number 2 was at my own scientific labratory (the wife calls it home...) I have a flow meter on my main space heating loop, so I know my water mass flow rate. I've clocked the appliance numerous times and have verified the reading for gas input per hour. When I got my HOBO time/temperature recording device, and a "State" recording device, I was watching my house and the system to see what my BTU/degree day of demand was. I'd checked the appliance efficiency and confirmed it to be up the the rating plate (81%) efficiency. After about a weeks worth of recording, I down loaded the numbers and started analyzing them. Again, based on known flow rates and delta tees, I came up around 20% short of the tested theoretical combustion efficiency.

I'm still scratching my head over both of these. When I discuss it with other knowledgable people, they tell me something is amiss. (NO KIDDING SHERLOCK!!) And I go back over the numbers, and verify the correctness, and calibration of the recording devices, and come up with the same numbers. As much as 20% off...

I've not had a chance to continue my studies yet, but I want to test a copper fin tube boiler, a cast iron boiler and a stainless steel condensing boiler using all the same equipment and parameters and see what comes out of them.

Sometimes, I feel like I'm messing with something that I shouldn't be "messing" with, and that the boiler mafia is keeping a close eye on me...Kinda like being in the Twilight Zone...

That's this mans experience.

ME

Jackchips

Do do do do do

ME, if your in the twightlight zone, the rest of us must have reached the Outer Limits.

Tony Conner

Nothing Is Amiss...

...because you were looking at two different things. Combustion efficiency, while important, is only PART of overall boiler eff. And boiler eff, while important, is only PART of overall system efficiency. You could have that burner firing into empty space. It could be the greatest burner in the world...but unless something else (like the boiler) is there to catch it - who cares? Combustion eff will NOT look at how effective the boiler (as in the actual heat exchanger) is. If you're firing that great burner into a coffee can of a boiler, a HUGE amount of the heat is just blowing up the stack. Combustion/burner efficiency only looking at how effective the actual combustion process is. How the heat gets transferred to the boiler water is another matter altogether. Of course, it goes further - how effective is the delivery system to get the heat to the emitters in the rooms? How about the actual emitters? How effective is the structure at keeping the heat from drifting out through the sides/roof/floor?

As far as the boiler efficiency is concerned, heat out over heat in isn't just the main thing, as far as I'm concerned, it's the ONLY thing. You can have a rockin' burner eff of 81%, but if the heat out over heat in is 61%, you've got a boiler that's only 61% eff. And that's a fact. You can have 100% burner eff, but if only 61% of that heat gets put into the water or steam heading out of the boiler, you've still only got a unit that's 61% eff. Period.

Your test results are correct.

LKE

Heating value

this is from Perry's Engineering Manual: "High or gross heating values of fuels are distinguished from low or net heating values by the condition of the water formed in the products of combustion; if it is condensed to a liguid instead of remaining in the vapor form, the latent heat becomes available.The difference between the high and the low heating values is computed by Difference=9h(1,090.7-0.545t) Btu/lb fuel where h=weight of hydrogen in fuel,lb/lb t=temperature of atmosphere,degrees F. In American practice it is customary to buy fuels, and to guarantee the performce of fuel-burning equipment, on the basis of the high heating value." This sounds to me like the high heating value is used for the input value.In the section under Boiler Rating and Steam Output, it say's "In American practice the heat supplied in the fuel is the high or gross heating value on the "as fired basis" ." The difference between these two values for natural gas and for methane which is the main componet of natural gas is almost exactly ten percent. This would make it virtually impossible for a noncondensing boiler to acheive 90% effiency!

Tony Conner

Here's What...

..."Steam - It's Generation and Use" by Babcock & Wilcox says:

"Efficiency, the ratio of energy output to input, is usually expressed as a percentage. The output term for a steam generator (or hot water boiler) is the heat absorbed by the working fluid (the water) to produce useful energy external to the steam generator envelope. The energy input term is the maximum energy available when the fuel is completely burned, i.e., the mass flow rate (MRF) of fuel is multiplied by the higher heating value (HHF) of the fuel. This is conventionally expressed as:

% efficiency = (output/MRF X HHF) X 100"

Babcock & Wilcox have been around for probably 150 years, making industrial and utility steam boilers. The biggest of these will be 200 feet high, and operate at over 3,000 PSIG.

Note that there is no reference at all to combustion efficiency, except to assume that ALL of the fuel will be perfectly burned. Of course, it won't be, but it's available to be burned, so any that's not, or has any excess air supplied at all, will be considered a loss. As it should be.

Mike Kusiak

This is exactly the point I was trying to make. Even if the combustion was 100% efficient, with stoichiometric air fuel ratio (zero excess air) and the heat exchanger was 100% efficient (total heat transfer), the maximum efficiency could never be more than 90% if condensing did not occur.

Even with the condensing boiler, if the water temp is maintained at higher than 135 F, no condensing will occur, and the efficiency will never be greater than 90%.

Tony Conner

Where Are The Boiler Manufacturers?

Why are none of them contributing to this string? Why isn't there any information forthcoming regarding the basis, and actual wording of the efficiency claims of the various makes of boilers?

Has anyone has actually measured the water flow, and delta-T across a condensing boiler, while measuring the fuel input to determine actual boiler efficiency?

Glen

100% is only -

attainable in direct fired appliances - all others will have some sort of losses due to heat transfer capibilities or lack of them. As I have posted before - most Viessmann VSB's will indicate 99.9% combustion EF on cold startup - but there are still stack losses so the overall EF will be a bit lower. Viessmann specs a flue temp of 20 deg higher than boiler temp on some installs - this is on a positive pressure venting system. Still the best boiler out there - IMHO.

Tony Conner

But...

...have you run BOILER efficiency numbers to compare with other makes? The combustion numbers sound impressive (which would, of course, be the marketing objective), but what really counts - where the rubber meets the road - is boiler efficiency.

I have no doubt that the various makers have run proper boiler efficiency tests at their facilities - on their units, AND the competition's. Yet nobody seems to be rushing forward to disclose just what THOSE efficiency numbers are. That makes me go "Hmmmmmmmm". I'll bet that like ME's (not me, but ME...) tests on his boiler, there's something like a 20 percentage point spread between the combustion eff, and the actual boiler eff. And I'll bet more again that for forced air units, the spread is even wider.

Tony Conner

Back To The Top

I'd still love to hear from some boiler manufacturers regarding just what their published efficiency numbers are based on. Is it COMBUSTION efficiency, or is it BOILER efficiency?

Glen

Steady state vs -

startup. I make the distinction quite clearly that these are startup numbers. Steady state - when system is up to temp and returning water to the boiler at temps lower than the stack temp - as most condensing boilers should - average EF - is 88 - 94%. This is combustion EF - while I forget what our friends from Allendorf state the heat transfer rate is - it too is very low. The key is lower boiler water return temps to kick serious butt and gain the maximum latent heat. So many variables .......

Tony Conner

I'm Still...

...looking for that elusive boiler efficiency value, as opposed to combustion efficiency. You've got to have an accurate water or steam flow (or air flow for forced air furnaces) to be able to calculate a proper boiler eff number. Until people put actual, physical, water flow measuring devices on their hot water boilers, any efficency numbers being discussed, are for combustion efficiency. And that is very much NOT the same as boiler efficiency, as ME saw in his tests.

I'd really be interested in a comparision of the BOILER eff of a condensing boiler, vs a conventional boiler. Tune the burners in, and then literally forget them for the boiler eff test. All that matters is the fuel input off the gas or oil meter, the mass flow of water through the boiler, and the delta-T for that water. Convert both fuel and water values to BTU for whatever the time period was for the test, and divide. Multiply that number by 100, and there it is - boiler efficiency. And until people run THAT test, comparisions between any make or model of boiler, whether condensing or non-condensing, is pretty much meaningless.