Modulating burners??

captainco

I have talked to Tom Butcher from BNI back in the 90's. I sent my oil carbon monoxide diagnostic sheets to him. He said their testing involves new stuff not existing.
Anyway, when I see someone listing a calculated combustion efficiency number as something meaningful, I have to question how they came up with their results. The btus listed in the BNI report are suspicious. I see thermal efficiencies listed but no method of how they were calculated or measured.
I agree that certain process boilers may need to be kept online which is not the point. It just cost more fuel but sometimes fuel is less than loss of product.
However, when I hear the reason, they don't want them to shut off is because they are afraid they won't come back on, then someone needs to find a better mechanic.

captainco

I am not a scientist or a mathematician but I can add, subtract, multiply and divide. I have a combustion analyzer that I can simulate combustion readings and get calculated efficiency numbers. The percentages on this article may not reflect 100% the actual btus, but they certainly have made me question these for the past 40 years.

SlamDunk

I would like to drive my company's operation toward a method described in the article I mentioned. That is to get rid of the 600HP boilers and replace with more 250's or 300 HP boilers and let them stage up and run at higher firing rates as needed by demand and weather. It seems we would achieve both efficiency and redundancy.

As to the find a better mechanic comment, first they are hard to find and when you do find them, you have to wait in line. And second, sometimes the burners are junk. I have seen pilots blown out in cold night time weather. Once they fail to light, they shut down until reset. It isn't always a maintenance issue.

From what I have read, Limpsfield burners are a dream but if I was a betting man, I would bet they fail to light too. Does anyone have experience with them?

Jamie Hall

All fascinating. I like @SlamDunk 's thoughts on process boilers -- if nothing else, it would mean that they could be optimized to run very close to their best performance (I am NOT using the term efficiency here -- what I mean is pounds of steam per BTU of fuel, nothing else). But the other consideration is this: if a heating boiler fails for whatever reason, somebody is going to get cold. That's no fun, but it's manageable. If a process boiler fails, however, depending on the process, you could be in a world of hurt financially.

captainco

I have helped many industrial engineers on equipment that efficiency was the least of their worries. Drying ovens, kilns, pig iron furnace, bake ovens etc.. The product was more important.

Steamhead

The statement in several posts here regarding a constant steam flow are on to something.

Go to our Find a Contractor ad, the link is in our signature below. On that job, we upgraded the burners to lo-hi-lo firing, not full modulation. These are PowerFlame C-series burners that modulate the air as well as the gas.

The active boiler (only need one of the two) runs full fire up to 5 ounces. At that point the pick-up factor is satisfied- the mains are full of steam and the radiators are heating, and a Vaporstat drops it to low fire. The burner continues to run at low fire until the pressure reaches 12 ounces, at which point another Vaporstat stops it. This low-fire period may last up to an hour on a very cold day.

Avoiding the start-stop of an on-off burner is key here. The primaries on these burners have 90-second pre- and post-purge times. This is a Code requirement, but can blow a lot of heat up the chimney if the boiler shuts off on pressure.

We have these burners tuned so the fuel-air mix on low fire is close to that of high fire. The stack temp is less on low fire and the calculated efficiency is a bit more, so we're saving a bit there too.

We saved that building about a third on its fuel consumption. They've been through a couple different property managers since then, but the Board doesn't want them using any other heating contractor. Quite a vote of confidence, eh?

captainco

Steamhead - I will agree a lo-hi-lo-off burner is better than modulation because it is easier to set the fuel/air ratio in two firing rates instead of 10 or 20. A modulating burner needs to be set going up the ramp and then double checked going back down. The crank arm and cam linkages had a lot of slop in either direction. The new direct linkage is tight but I have not found one set correctly yet!
Usually if the btus of the burner were less than 4 million but more than 2 million the low-hi-low was the best. Above that I recommended a modulating burner but set the controls for low-hi-lo-off.

Something I like to ask - Do more btus go up the flue during pre-purge and post-purge or when the burner is operating?

The hospital boiler I talk about goes back to 1983. It was 600 HP. The engineer at the hospital just attended a combustion seminar at the hospital and his boilers were tuned up by 4 engineers from the boiler institute of Louisiana. Here are the numbers in high fire: O2 - 12% Flue T - 320 degrees Eff - 79% This was a modulating burner and after seeing high fire there was no reason to check low fire. Boilers made 100# steam. What do you think?

SlamDunk

Not my original thoughts @Jamie Hall. I want to say they are @RayWohlfarth's. I hope I am giving credit to the right person.

@captainco, we got rid of our linkage/modutrol system and replaced with servos on each the air damper, gas valve, oil valve and fgr. We can place as many points on the firing rate curve and tune the burner to optimal as we go from low, to high, back to low. The results are repeatable because there is no backlash. The servos are pinned to shafts.

As far as your hospital boiler, We don't even light off with 12% o2. 10% at low fire because it makes the burner more reliable. Then 3-5% throughout. The better techs can tune to 3% at all points above low fire but it takes skill and patience.

Do more btu's go up the stack during purges or while firing?...that is a good question. I'll say it's about the same because most btu's are used to boil water while firing. The more passes your boiler has, the fewer btu's go up the flue.

captainco

Slamdunk - yes the new servos are amazing but there are few people like yourself that knows how to adjust. 2 years ago at Milton Hersey school they had that type of set-up. Whoever set it up was clueless!!

Forget the 12% O2 which is bad. What is another real problem with the numbers I gave?

RayWohlfarth

Thanks for bringing this up. When I was doing research for a book, I read in a Honeywell manual that a Low High Low burner uses 15% less energy than a modulating burner. According to the EPA, Boilers are most efficient when operated between 50-80% and efficiency drops significantly when operated below 50%

SlamDunk

I was better tuning the linkages. we get pros to tune the servos unless we're in a pinch. the 79%eff. The excess air had to be high and that cools the flame; stack temp is low. A cold flame doesn't heat as much.

Looking at an old saved analysis from one of our 600 hp, 80 psi boiler, at high fire: O2-5.5% (higher than i would like); Exair-31.7%; Eff-83.8; COaf-1ppm; CO2-8.6%; stack temp 375.

Personally, I think the efficiency number high. My gut tells me 80 is about as good as you can get but, my gut gets it wrong sometimes.

captainco

I guess I will answer my own question. The temperature of 100# steam is 338 degrees. The flue temperature on the hospital boiler was 320 degrees and that was in high fire.

My combustion rules for flue temperature on a boiler is the low fire flue temperature has to be higher than what you are making and I usually like to see at least 50 degrees higher. On a 4 pass boiler in high fire the flue temperature should be at least 100 degrees higher than the water or steam. On a 2 or 3 pass boiler I have found 170 degrees higher is the best range. These numbers found themselves after testing hundreds of boilers in the field operating at their best. Not the stupid calculated efficiency that analyzers provide.
80# steam is 334 degrees so I would be looking for a flue temperature of at least 434 degrees.

captainco

Ray - I don't remember whose book I read years ago, whether it was a boiler manufacturer or someone else but their chart showed that efficiency of boilers drops off if you fire below 15% of its rated capacity. Whether it is 15% or 50%, low fire costs energy.

captainco

Thanks to all who are commenting here. I was hoping this would be educational for me and others.

I like having fun! Can anyone tell me the purpose of a vent valve on a commercial gas train?

SlamDunk

You mean the block and bleed? I believe it was required by IRI or Factory Mutual. To vent any fuel trapped by the blocking valves. And direct any leaks passed the first blocking valve out doors. I bubble check the vented side of blocking valve periodically. Do you know how to check that the vent valve actually opens?

captainco

The vent valve was necessary because of piloted diaphragm valves that needed a differential to close. If there was pressure between them only one would close. Since the use of motorized valves the vent valve has no functional use but I still see it listed. \
Actually, had a job in Cincinnati where the gas train supplied by the boiler manufacturer had the vent valve first. It vented gas 100% of the time when the boiler was off. I can't believe the customer didn't complain about their gas bill.

Steamhead

captainco said:

I guess I will answer my own question. The temperature of 100# steam is 338 degrees. The flue temperature on the hospital boiler was 320 degrees and that was in high fire.

My combustion rules for flue temperature on a boiler is the low fire flue temperature has to be higher than what you are making and I usually like to see at least 50 degrees higher. On a 4 pass boiler in high fire the flue temperature should be at least 100 degrees higher than the water or steam. On a 2 or 3 pass boiler I have found 170 degrees higher is the best range. These numbers found themselves after testing hundreds of boilers in the field operating at their best. Not the stupid calculated efficiency that analyzers provide.
80# steam is 334 degrees so I would be looking for a flue temperature of at least 434 degrees.

@captainco , at what point in the firing cycle were these readings taken? I can see getting these stack temp readings (NOT the excessive O2 reading) if the boiler is starting up cold. Obviously when making that 100# steam, the stack temp ought to be higher.

Remember that our company almost never works on process boilers, and the few we encounter are still low-pressure.

Like you, we've encountered some horribly tuned burners on larger boilers. And one factor we've encountered is that our local gas utility has been known to spike its gas with whatever it can find during very cold weather. This can raise the BTU per cubic foot value, which can make a burner run too rich and create CO and soot. So we use the oilman's trick of finding the point at which we start to make CO, then adding enough air to drop the CO2 by half a percentage point, which gives some headroom- thanks @Firedragon and @Alan R. Mercurio , who taught me this.

EBEBRATT-Ed

Engineers generally get paid by a % of how much the job costs. Their always afraid of putting in something too small.

Both of the above lead to oversizing

STEVEusaPA

captainco said:

…Forget the 12% O2 which is bad. What is another real problem with the numbers I gave?

I’d guess the low stack temp giving them the false sense that they are running 79% efficiency.

captainco

The boiler was in high fire and the pressure was almost at 100#. Many of you know this but the people that everyone believe don't have a clue. What percent of contractors do a combustion test on every furnace, boiler and water heater? How many those know how to do it correctly?

EB - you are correct that architects and engineers get paid based on the cost of the job so everything is big. When it comes to A/C, which building are the units way oversized? Some or most of which is cause by Code!

captainco

Steve - Hope to see you in King of Prussia in Dec.

Steamhead

captainco said:

The boiler was in high fire and the pressure was almost at 100#. Many of you know this but the people that everyone believe don't have a clue. What percent of contractors do a combustion test on every furnace, boiler and water heater? How many those know how to do it correctly?

100% of my company.

EBEBRATT-Ed

Another common issue I have seen with techs is they start up a boiler and it's cold. They test and the C0 is high. They want to add more air to bring the C0 down and the boiler/burner is really making C0 because it's not warmed up enough......has not reached steady state.

Just like a cold pan of water on a gas stove will make C0.


Burners will also make C0 if the flame has too much air. The excessive air is "chilling" the fire.

captainco

Definitely can make high levels of CO with too much air especially when underfired. But there is a pretty blue flame. Not had the cold boiler CO problem unless there is some kind of impingement. If it is part of the design it will go away. But if it is because of a mechanical issue it may go down but not enough.

Starting a boiler in low fire when cold is a major mistake. Only two thing cause boiler shock. Dry firing and then adding water or bringing up a boiler to full temperature before turning on the circulating pumps.

STEVEusaPA

captainco said:

Steve - Hope to see you in King of Prussia in Dec.

It's on my wish list. My only problem being owner/sole operator is it's almost impossible for me to not work 3 days in the winter. But I'm negotiating coverage, so fingers crossed! Or I might just travel to a fall class in Ohio.

captainco

I would just like to meet you whether you sign up for class or not!!

SlamDunk

I have worked and installed hundreds of block and bleed gas trains since 1994, but I don't think I ever saw or heard of a "piloted diaphragm valve". Never heard that explanation for the bleed valve.

captainco

Never seen a Honeywell V88 diaphragm gas valve?

EBEBRATT-Ed

The only bleed or vent valves I ever saw were solenoid valves, usually made by ASCO or General. Those were required by IRI (Industrial Risk Insurers) now called FIA. Usually, the vent valve pipe size is 1/2 the size of the gas train. So a 3" gas train would use a 1 1/2" vent valve

I never saw a Factory Mutual or UL gas train with a vent valve.

captainco

Gas valves, not vent valves were piloted diaphragm.
I guess it was only IRI that required vent valves and that standard is gone.

Does anyone know that if you install the pilot solenoid valve before the pressure regulator you can minimize light-off failures? In longer off cycles the pressure at the outlet of the regulator equalizes to line pressure, so initial light-off can blow out the pilot if it lights at all.

GroundUp

My $.02 here lies in low load zones. Let's just use a basement as an example, and that basement has an 8,000 BTU load. The rest of the house has a 60,000 BTU load, all on the same zone.

Option A: 80k mod/con locked into high fire- every time the basement calls for heat at design outdoor temp, the boiler cycles 10 times before meeting thermostat setpoint due to the lack of emitter.

Option B: same 80k mod/con allowed to modulate down to 8k and run continuously with no startup or shutdown during the same scenario as above.

I'm no genius here and have no data to support my theory, but wouldn't the short cycling of option A drop the overall efficiency to a lower point than the modulated efficiency, despite a higher combustion efficiency?

Call it a hunch, but that's why I feel modulation exists for the most part. Do you burn more gas starting your truck every mile and holding it wide open in between, or setting the cruise at 55 and just getting to the destination? The combustion efficiency is higher at WOT than at idle, but there is no start/stop to contend with when the cruise is set and inertia is being used to advantage meaning less fuel is used overall for the same proverbial trip.

captainco

Modulation was invented to keep boiler running because of light-off failures and for process equipment with high loads and requirement for faster recovery. There was never any measured efficiency with most of the older equipment. Mod-cons are the first devices that modulate with and decent efficiency. Slightly less in low fire but much better because of the low mas heat exchangers. But I have measured at least one Mod-con that was less than 30% efficient in low fire. Most modulating furnaces operate at less than 50% efficiency in low fire.

There was one case some NCI people were looking at the gas usage of two houses almost identical in size, next door to each other. One had a modulating furnace the other a single stage. The person with the single stage furnace gas bills averaged over 35% lower for three years. After 3 years the modulating furnace was replaced with a single stage.

STEVEusaPA

captainco said:

I would just like to meet you whether you sign up for class or not!!

It would be my pleasure. If I can’t do the class I can come up and take you out to dinner one night. I promise not to ask too many of the hundreds of questions bouncing around my brain 😀

SlamDunk

captainco said:

Gas valves, not vent valves were piloted diaphragm.
I guess it was only IRI that required vent valves and that standard is gone.

Does anyone know that if you install the pilot solenoid valve before the pressure regulator you can minimize light-off failures? In longer off cycles the pressure at the outlet of the regulator equalizes to line pressure, so initial light-off can blow out the pilot if it lights at all.

I had to check. All my regulators are before the solenoid valves.

captainco

Yes they all are. I just have situations that switching them eliminated a nuisance.