Monotube Flash Boiler Design

[Steamchick]

The "regular" condenser is directly connected back to the engine and boiler... Should it fail to condense adequately, the pressure may not be adequately dropped. As it is a safety control, not a normal running device, the safety relief valve should vent to its stand alone system dump or atmosphere.
Think "SAFETY SYSTEM = INDEPENDANT SYSTEM IF SOMETHING FAILS, THAT RETURNS THE SYSTEM TO A SAFE CONDITION".
A lesson I received during training in the 1970s. I assume it is still valid?
K2

 

[Toymaker]

First Boiler test: Great clouds of steam,...this thing makes a LOT of steam!!
For this first test of the boiler, the input is connected to a garden hose faucet in my yard. There are no restrictions on the 5/8" (16mm) output tube, so very little pressure build up; still, steam shot about 3 to 4 meters into the street. The video below is only 5 seconds long; I posted a longer video here: Boiler Test #1

 

[Steamchick]

Showy, but I don't think this test tells you anything.... unless you know how much water is being boiled?
K2

 

[Toymaker]

Showy, but I don't think this test tells you anything.... unless you know how much water is being boiled?
K2

Several of my design goals were verified during this first test.

Notice the 18 small holes pointed to by the red arrow in the photo below. Those are the exhaust holes for the burner gases, and collectively, the area of all the holes is approximately 3 times the area of the blower output. During operation, the air blown in expands greatly during combustion, then cools and contracts as it transfers its heat to the boiler tubes. Carefully placing my hand into the exhaust stream revealed the flow to be slow and gentle. This first test has verified the exhaust holes collective area is large enough such that the exhaust gas flow is not restricted.



Second goal achieved (so far). Clearly, the more BTUs transferred from the combustion gases through the copper tubing and into the water yields greater efficiency; a good indication of this efficiency is to measure the exhaust gas temperature as it come out of all those little holes. Although I neglected to have a thermometer within easy reach, I did place my hand into the flowing exhaust gas stream and noted it was barely warm,....honestly, I think my cat is hotter than the exhaust gases. Exhaust temperature will no doubt change when steam flow is partially blocked allowing pressure to build, and when the burner is operated at higher levels, but this first test is very encouraging.

Finally, during the one minute the YouTube video was recording, I was slowly closing the water valve until I saw no more liquid water dripping from the steam output, as can be seen in the video; at which time I walked around and turned off the boiler. Once I put down my camera/cell-phone, I pulled off the quick-disconnect from the water faucet to observe the flow rate. I was quite surprised that such a large amount of steam had been generated by what I perceived to be a rather low rate of water flowing from the faucet, and in a senior moment of forgetfulness, I instinctively reached down and turned off the faucet. What I had planned to do, was place the near-by bucket to catch the water coming out of the faucet and time how much water was flowing out of the faucet,...oh well, no worries, that measurement will now wait for test #2

The final lesson learned from this test: as long as I'm using water as the working fluid, and not R123 or any other organic fluid, the required feed pump needs a much smaller flow rate than is required for R123.

I will talk about required flow rate in a separate post.

 

[Toymaker]

Feed Pumps, Flow Rates, Pressure, & Boiler Horse Power.

Question: what size feed pump is needed for the boiler/burner in this thread? Although my final goal is to use a Freon as the working fluid, my plan has always been to get most everything working, with no leaks, over-pressures, over-temperatures, etc., using water as the working fluid. So, for this discussion, I'm using water as the working fluid.

Looking at the burner first, the fuel nozzle is spec'd to burn 14 L/hr when using 6psi air pressure, but that rate can easily be doubled to 28 L/hr by increasing the air pressure. If my calculations are correct, burning 28 liters of Diesel in one hour yields 400 HP = 300 KW = 1,018,679 BTU. IF the boiler was 100% efficient, and remembering that one BHP (Boiler Horse Power) = 13.155 standard HP, then my boiler would be rated for 30.4 BHP.
Using the flow rate formula: BHP x 0.069 = gpm (gallons per minute), 30.4 BHP x 0.096 = 2.1 gpm = 8 Lpm.
Therefore, assuming 100% boiler & burner efficiency, my feed pump needs to deliver 8 LPM (liters per minute).
I've previously decided my boiler will operate at 530 psi max, but to keep from operating the pump at max pressure continuously, lets work with 1000 psi which is = 70 bar. So the feed pump needs to deliver 8 LPM at 70 bar. Many home-use pressure washers exceed those requirements; the Delton S9 is rated for 400 bar at 10.3 LPM and is priced at $35 in Thailand. I need to determine if the pressure is adjustable.

Useful fact: 1 BHP = 34.5 lbs/hr of steam or water at 0 psi & 100 C.
Therefore, if I measure the water flow rate into the boiler, and keep the steam output temperature at 100 C, I will be able to experimentally determine the boiler's BHP.

 

[HMEL]

Several of my design goals were verified during this first test.

Notice the 18 small holes pointed to by the red arrow in the photo below. Those are the exhaust holes for the burner gases, and collectively, the area of all the holes is approximately 3 times the area of the blower output. During operation, the air blown in expands greatly during combustion, then cools and contracts as it transfers its heat to the boiler tubes. Carefully placing my hand into the exhaust stream revealed the flow to be slow and gentle. This first test has verified the exhaust holes collective area is large enough such that the exhaust gas flow is not restricted.

View attachment 150885

Second goal achieved (so far). Clearly, the more BTUs transferred from the combustion gases through the copper tubing and into the water yields greater efficiency; a good indication of this efficiency is to measure the exhaust gas temperature as it come out of all those little holes. Although I neglected to have a thermometer within easy reach, I did place my hand into the flowing exhaust gas stream and noted it was barely warm,....honestly, I think my cat is hotter than the exhaust gases. Exhaust temperature will no doubt change when steam flow is partially blocked allowing pressure to build, and when the burner is operated at higher levels, but this first test is very encouraging.

Finally, during the one minute the YouTube video was recording, I was slowly closing the water valve until I saw no more liquid water dripping from the steam output, as can be seen in the video; at which time I walked around and turned off the boiler. Once I put down my camera/cell-phone, I pulled off the quick-disconnect from the water faucet to observe the flow rate. I was quite surprised that such a large amount of steam had been generated by what I perceived to be a rather low rate of water flowing from the faucet, and in a senior moment of forgetfulness, I instinctively reached down and turned off the faucet. What I had planned to do, was place the near-by bucket to catch the water coming out of the faucet and time how much water was flowing out of the faucet,...oh well, no worries, that measurement will now wait for test #2

The final lesson learned from this test: as long as I'm using water as the working fluid, and not R123 or any other organic fluid, the required feed pump needs a much smaller flow rate than is required for R123.

I will talk about required flow rate in a separate post.

The accepted way to calculate boiler efficiency is to measure the exhaust gas temperature, and the o2 and Co2/ You then calculate the theoretical heat release and subtract from it the stack losses. The theoretical heat release use fuel calculated in weight, volume of mcf. You will need an ultimate fuel analysis listing carbon, nitrogen, oxygen, hydrogen and ash in the fuel as the bare minimum. There is a short cut version and a long version for the calcs. There used to be a short from ASME which you could use but its not considered accurate enough anymore. The other calculations are used in evaluating the thermodynamic cycle.

 

[HMEL]

Feed Pumps, Flow Rates, Pressure, & Boiler Horse Power.

Question: what size feed pump is needed for the boiler/burner in this thread? Although my final goal is to use a Freon as the working fluid, my plan has always been to get most everything working, with no leaks, over-pressures, over-temperatures, etc., using water as the working fluid. So, for this discussion, I'm using water as the working fluid.

Looking at the burner first, the fuel nozzle is spec'd to burn 14 L/hr when using 6psi air pressure, but that rate can easily be doubled to 28 L/hr by increasing the air pressure. If my calculations are correct, burning 28 liters of Diesel in one hour yields 400 HP = 300 KW = 1,018,679 BTU. IF the boiler was 100% efficient, and remembering that one BHP (Boiler Horse Power) = 13.155 standard HP, then my boiler would be rated for 30.4 BHP.
Using the flow rate formula: BHP x 0.069 = gpm (gallons per minute), 30.4 BHP x 0.096 = 2.1 gpm = 8 Lpm.
Therefore, assuming 100% boiler & burner efficiency, my feed pump needs to deliver 8 LPM (liters per minute).
I've previously decided my boiler will operate at 530 psi max, but to keep from operating the pump at max pressure continuously, lets work with 1000 psi which is = 70 bar. So the feed pump needs to deliver 8 LPM at 70 bar. Many home-use pressure washers exceed those requirements; the Delton S9 is rated for 400 bar at 10.3 LPM and is priced at $35 in Thailand. I need to determine if the pressure is adjustable.

Useful fact: 1 BHP = 34.5 lbs/hr of steam or water at 0 psi & 100 C.
Therefore, if I measure the water flow rate into the boiler, and keep the steam output temperature at 100 C, I will be able to experimentally determine the boiler's BHP.

Boiler Feed Pump Size:

Requires establishing boiler firing rate in terms of mass.
Requires establishing boiler maximum pressure in units of your choisce
Requires sizing pump for 120% or more for mass flow at a pressure greater then allowable boiler pressure.
Freon will probably require a special pump as it must be sealed completely probably a positive displacement pump finding one will be a challange. If you use water the extra flow is needed for blowdown. Given the tube boiler design you will need very good water quality a subject that has been discussed yet.

And finally forget about the archaic boiler hp calculation as it was based on older boiler technology. Just size it for your thermodynamic cycle. A good boiler efficiency will be around 85%. You will be lower unless you burn coal-- has to do with water of combustion -- however European standards ignore comustion water loss so there is some difference in technique.

 

[Toymaker]

The accepted way to calculate boiler efficiency is to measure the exhaust gas temperature, and the o2 and Co2/ You then calculate the theoretical heat release and subtract from it the stack losses. The theoretical heat release use fuel calculated in weight, volume of mcf. You will need an ultimate fuel analysis listing carbon, nitrogen, oxygen, hydrogen and ash in the fuel as the bare minimum. There is a short cut version and a long version for the calcs. There used to be a short from ASME which you could use but its not considered accurate enough anymore. The other calculations are used in evaluating the thermodynamic cycle.

Thanks for providing the specific method to calculate boiler efficiency; at some future time when I've had a chance to measure exhaust gas temperature with a thermometer, (instead of my hand) it will be useful to know the more precise boiler efficiency.

But for this first test run, I'm more than a little pleased knowing the exhaust gases were so cool as to be barely warm; which means the boiler was able to drop the combustion gas temperature from about 2500 C down to around 45 to 60 C, at a fuel burn rate of over 14 liters per hour, which is over 148 kWh.

 

[Steamchick]

Thanks for adding the messages from 244 onwards. That was the "Missing stuff" I wanted to hear about. I realise now what the video was actually showing, as you were varying the water flow rate to balance the steam generation to fuel being burned. There was an earlier phase when the boiler was priming, but you have explained that as well. There's more to the story than just pictures.
Thanks!
K2

 

[ajoeiam]

The accepted way to calculate boiler efficiency is to measure the exhaust gas temperature, and the o2 and Co2/ You then calculate the theoretical heat release and subtract from it the stack losses. The theoretical heat release use fuel calculated in weight, volume of mcf. You will need an ultimate fuel analysis listing carbon, nitrogen, oxygen, hydrogen and ash in the fuel as the bare minimum. There is a short cut version and a long version for the calcs. There used to be a short from ASME which you could use but its not considered accurate enough anymore. The other calculations are used in evaluating the thermodynamic cycle.

I would be willing to bet that for non big dollar purposes the short from ASME would work quite well.

Do you have such info?

 

[HMEL]

Thanks for providing the specific method to calculate boiler efficiency; at some future time when I've had a chance to measure exhaust gas temperature with a thermometer, (instead of my hand) it will be useful to know the more precise boiler efficiency.

But for this first test run, I'm more than a little pleased knowing the exhaust gases were so cool as to be barely warm; which means the boiler was able to drop the combustion gas temperature from about 2500 C down to around 45 to 60 C, at a fuel burn rate of over 14 liters per hour, which is over 148 kWh.

So you know while building this thing, high excess air will drastically lower flue gas temperatures. This will increase stack gas losses and will distort your analysis. You will need a mass air flow meter on the fan and a o2 sensor in the exhaust gas to measure this.-Boilers will work with high excess air. When you get around to trimming the air fuel ratio you might find that the temperatures will climb significantly. Based on what you have described I believe, just guessing here that you are approaching 50% excess air. The leaf blower fan is designed to deliver lots of air. To control this you need either a variable speed motor or a damper. The fan is probably ok from a static pressure point but things will change when you bottle up the combustion gas flow. But from where I sit I do not have the numbers to advise one way or another but my experience is telling me just looking at your system your residence flue gas time has to be quite low meaning your gas velocities are very high.

 

[Toymaker]

So you know while building this thing, high excess air will drastically lower flue gas temperatures. This will increase stack gas losses and will distort your analysis. You will need a mass air flow meter on the fan and a o2 sensor in the exhaust gas to measure this.-Boilers will work with high excess air. When you get around to trimming the air fuel ratio you might find that the temperatures will climb significantly. Based on what you have described I believe, just guessing here that you are approaching 50% excess air. The leaf blower fan is designed to deliver lots of air. To control this you need either a variable speed motor or a damper. The fan is probably ok from a static pressure point but things will change when you bottle up the combustion gas flow. But from where I sit I do not have the numbers to advise one way or another but my experience is telling me just looking at your system your residence flue gas time has to be quite low meaning your gas velocities are very high.

Your assumption of excess burner air is certainly possible, but let me provide you with a little more data concerning the test. Perhaps you'll form a new opinion,....perhaps not

Two benefits of using a computer to control the burner; are repeatability and easy changes to the parameters which control air flow and fuel flow. The leaf blower's DC motor's speed is controlled by the computer via a PWM motor speed controller; a separate, but identical, PWM motor speed control board controls fuel flow. Both of these motors are variable from 0 to their max rpms and are under computer control.

Several days before I placed the burner into the boiler, I adjusted the software to provide the best blue-white flame I could get from the burner, while operating at 70% rpm for the leaf blower...the below photo is the result. Reducing airflow quickly turned the flame Yellow, while increasing air flow beyond the first appearance of the blue-white flames both increased flame length and reduced the amount of white flame. By slowly increasing airflow to the point where all the yellow flames are replaced by blue-white flames, I believe there is very little, if any, excess air in the exhaust flames. This video shows part of the adjustment process and clearly shows the color changes of the exhaust gases: Burner Test & Adjustment

These settings of fuel and air flow are what the computer used to fire the burner during the initial test video, meaning the photo below shows burner operation inside the boiler tubes. My only input was to slowly reduce water flow until I observed no more water dripping from the steam flow.

Perhaps the exhaust gases were still cool because the boiler had been running for less than a minuet when I placed my hand into the exhaust to determine how hot they were.

I'm still in the early stages of testing,...much more testing will follow, and I'll post more data as I get it.

 

[NapierDeltic]

The last posts sparked me some questions/ideea, though I am not ideal person to raise them:
"one BHP (Boiler Horse Power) = 13.155 standard HP" seems to reflect an overall cycle efficiency of about 7.6% which comes from experience with older steam engines. Your real efficiency could be very different.
I would chose a feed pump with at least twice the flow and adopt partial recirculation. This bypass flow, once set primarily, could be controlled by pc through a proportional valve controlled by whatever sensor according whatever circumstances.
Could a lambda sensor from a car exhaust at least monitor if not control the air-fuel mixture? I agree with visual adjustment of flame, but this is a primary setting; additional gas flow losses and variable work load could shift it from ideal.
The video (which I liked very much) shown fine water droplets in the steam. Maybe some are so fine that you cannot see even when steam looks to be "dry" and my concern is: wouldn't these droplets at sonic speed be harmful to turbine's blades long term? Something similar to cavitation effect. Would a liquid water separator aid in this matter? At least I think you can clarify this (is/is not an issue) with people from that steamers forum.

 

[Toymaker]

The last posts sparked me some questions/ideea, though I am not ideal person to raise them:
"one BHP (Boiler Horse Power) = 13.155 standard HP" seems to reflect an overall cycle efficiency of about 7.6% which comes from experience with older steam engines. Your real efficiency could be very different.
I would chose a feed pump with at least twice the flow and adopt partial recirculation. This bypass flow, once set primarily, could be controlled by pc through a proportional valve controlled by whatever sensor according whatever circumstances.
Could a lambda sensor from a car exhaust at least monitor if not control the air-fuel mixture? I agree with visual adjustment of flame, but this is a primary setting; additional gas flow losses and variable work load could shift it from ideal.
The video (which I liked very much) shown fine water droplets in the steam. Maybe some are so fine that you cannot see even when steam looks to be "dry" and my concern is: wouldn't these droplets at sonic speed be harmful to turbine's blades long term? Something similar to cavitation effect. Would a liquid water separator aid in this matter? At least I think you can clarify this (is/is not an issue) with people from that steamers forum.

Yes, allowing an ECU (Engine Control Unit) to monitor a lambda sensor (O2 sensor) placed in the burner's exhaust stream, would allow the ECU to make real-time changes to the fuel-air mixture, but there's really no need to do this on a steam engine's burner, as the burner's function is completely independent of engine load. The burner will operate at whatever level it's been set to whether the engine is turned off, running at full capacity, or over-loaded and slowing down. So, once I determined the best fuel-air mix over the burner-blower's entire operating range, I plugged those values into the computer's software.

My burner doesn't like running at blower settings below about 50% rpm; as I'm unable to get the blue-white flame at those lower conditions; I believe this is due to the fixed exhaust size of the burner. The obvious fix would be to install a device which would allow for a variable size exhaust,...but that's a project I don't wont to get into today.

You're absolutely right about wet steam destroying turbine blades; this major problem was discovered shortly after engineers began using steam turbines. I'm pretty sure the steam will be much drier once I install an valve on the steam output and allow pressure and temperature to build-up to their target values of 500 psi and 200 C.

 

[TimTaylor]

So you know while building this thing, high excess air will drastically lower flue gas temperatures. This will increase stack gas losses and will distort your analysis. You will need a mass air flow meter on the fan and a o2 sensor in the exhaust gas to measure this.-Boilers will work with high excess air. When you get around to trimming the air fuel ratio you might find that the temperatures will climb significantly. Based on what you have described I believe, just guessing here that you are approaching 50% excess air. The leaf blower fan is designed to deliver lots of air. To control this you need either a variable speed motor or a damper. The fan is probably ok from a static pressure point but things will change when you bottle up the combustion gas flow. But from where I sit I do not have the numbers to advise one way or another but my experience is telling me just looking at your system your residence flue gas time has to be quite low meaning your gas velocities are very high.

Actually, under normal circumstances, with the increase of excess air the temperature of boiler exhaust gas increases. Reducing the excess air to the minimum achievable to ensure complete combustion will reduce the exhaust gas temperature. This of course assumes that the boiler has been properly designed for the expected load conditions - over-firing a boiler will always result in increased stack gas temperatures. There always needs to be some excess air to insure complete combustion - how much typically depends on the type of fuel.

Another factor people often fail to consider is that combustion is a mass based reaction - it takes x lbs of O2 with y lbs of fuel to achieve complete combustion. Air density (and the mass per unit volume of it's gas components) changes significantly with temperature, so using cfm only gives you an approximation. Commercial/industrial boilers will use an O2 trim system to compensate for these variations. In situ O2 measurement is the most cost effective approach for small to medium sized boilers. Larger boilers will often add CO measurement to monitor unburned products of combustion to allow optimum control of the process.

 

[Steamchick]

As I only with testing of cars for exhaust emissions, I have never really considered the similar Exhaust feedback loop for boilers. - Most interesting! Especially considering the complexity of primary boiler, superheater, economiser and condenser and the thermal inertia of the whole system, varying the burner output to suit the variable demand must be a bit of a control challenge!
Toymaker: Your comment "I'm pretty sure the steam will be much drier once I install an valve on the steam output and allow pressure and temperature to build-up to their target values of 500 psi and 200 C." I think I should remind you that "wet steam" - from whatever evaporated medium - will occur in any engine that expands the initial "steam" so the temperature and pressure fall below the saturation point at some condition in the engine cycle. By this I mean that modern steam turbines run Superheated steam at inlet temperature "x" that passes through the turbine and exits as Superheated steam at temperature "y". The temperature drop x to y is the work done in the engine. It is in the gas laws, and how the expansion of gases works...
I.E. inputting steam at 200c and 500psi that is at the saturation point (not superheated) will expand as it passes through the valve into a chamber/pipework that is below 500psi, so the amount of expansion below the saturation temperature of the gas at the downstream side of the valve will cause some condensation to occur. Furthermore, as this wet "steam" passes through the engine and further expands, the heat withdrawn from the medium will cause further pressure and temperature drop and more condensation will be created in the wet steam. Study "Steam" tables (Enthalpy tables) for your medium, with some estimated temperatures and pressures, and see what is happening...It may not be so bad, but I "am sure" (95%) it will be a "wet steam" turbine from what I have read.
Curiously, modern turbines that run purely on superheated steam "in and out" use purely aerodynamic blades at very high velocities (as do gas turbines!), and the pressure drop across the turbine blades (leading face versus trailing face) powers the shaft. With a "wet-steam" turbine, there is a weird momentum exchange from the droplets of condensate hitting blades and using momentum exchange as part of the mechanism from "high velocity steam to low velocity steam". The slower droplets after losing speed to a blade, then are accelerated by the stream of "wet steam" before hitting another blade... One can only guess at what is happening with high velocity tiny droplets of condensate hitting the blades, in terms of wear, surface erosion, or whatever?...
- I do not have a clue how to calculate what will happen!
K2

 

[TimTaylor]

Yes, allowing an ECU (Engine Control Unit) to monitor a lambda sensor (O2 sensor) placed in the burner's exhaust stream, would allow the ECU to make real-time changes to the fuel-air mixture, but there's really no need to do this on a steam engine's burner, as the burner's function is completely independent of engine load. The burner will operate at whatever level it's been set to whether the engine is turned off, running at full capacity, or over-loaded and slowing down. So, once I determined the best fuel-air mix over the burner-blower's entire operating range, I plugged those values into the computer's software.

My burner doesn't like running at blower settings below about 50% rpm; as I'm unable to get the blue-white flame at those lower conditions; I believe this is due to the fixed exhaust size of the burner. The obvious fix would be to install a device which would allow for a variable size exhaust,...but that's a project I don't wont to get into today.

You're absolutely right about wet steam destroying turbine blades; this major problem was discovered shortly after engineers began using steam turbines. I'm pretty sure the steam will be much drier once I install an valve on the steam output and allow pressure and temperature to build-up to their target values of 500 psi and 200 C.

If you find yourself needing to restrict combustion air, the most energy efficient method (depending on blower design) is usually to damper the combustion air blower inlet, not the outlet.

Water entrainment (wet stream) can be minimized a couple of ways. Most boilers are designed with a steam drum of sufficient volume to reduce the steam velocity, letting the heavier droplets drop out. This is often combined with other design elements such as deflector plates (de-mister section). This can be done internally in the drum, or externally in combination with an appropriately sized steam trap. Another method is to add some superheat to the steam by running the steam from the drum through a coil in the boiler exhaust. Another option is to run the boiler at higher than process pressure and use a pressure reducing control valve, which also will leave a bit of superheat(if there is any there originally) to the lower pressure process pressure steam. In any case using superheat, you need to make sure the pipes are properly insulated to maintain it.

Simply adding a discharge valve to the boiler outlet will not change the moisture content of the steam - the best you can get is D&S. That said, if you are simply running the outlet open to atmosphere now, it will make a difference.

Finally, to get the most efficiency from a turbine, as much of the pressure drop as possible must take place across the blades. You want to maintain the pressure in the steam chest feeding the nozzle(s) as high as possible. Taking a significant pressure drop across the governor valve will significantly reduce the turbine efficiency, as will having too many and/or too large of nozzles.

 

[TimTaylor]

As I only with testing of cars for exhaust emissions, I have never really considered the similar Exhaust feedback loop for boilers. - Most interesting! Especially considering the complexity of primary boiler, superheater, economiser and condenser and the thermal inertia of the whole system, varying the burner output to suit the variable demand must be a bit of a control challenge!

It's actually not that tough - it's been done for many years in industrial boiler systems. How complex the controls have to be depends on the characteristics of the system. For base loaded systems it is relatively straightforward for both the basic control strategy, and to add feedback trim based on residual O2 in the stack gas. For swing loaded systems, it can be quite a bit more challenging, and you wind up with a stacked control loop configuration with multiple feed-forward/feedback elements. You have to take into account all kinds of variables, including dynamic system response to load changes. It can be really challenging with analog control loops - the advent of DCS systems has made it significantly easier.

 

[Steamchick]

Tim, I like your modest "It can be really challenging"...
I went there designing a "Better" (>50% more pressure) Air cylinder for high voltage circuit breakers. It had to accelerate >100g - then decellerate to rest in <5msecs. - moving significantly more mass than it was originally design to drive, and push more gas through the arc to extinguish the arc at the business end. Computer modelling was by a Doctor of Maths and used the company mainframe computer. The challenge was like stopping a high speed freight train so it all came to rest without "shuffling" when it stopped. - A bit like landing a rocket booster. Achieving zero kinetic energy (aka velocity) at JUST the right point (zero potential energy)! Manual maths would have taken forever, but the computer did it in 15 minutes! That was a little challenging...
K2

 

[HMEL]

Actually, under normal circumstances, with the increase of excess air the temperature of boiler exhaust gas increases. Reducing the excess air to the minimum achievable to ensure complete combustion will reduce the exhaust gas temperature. This of course assumes that the boiler has been properly designed for the expected load conditions - over-firing a boiler will always result in increased stack gas temperatures. There always needs to be some excess air to insure complete combustion - how much typically depends on the type of fuel.

Another factor people often fail to consider is that combustion is a mass based reaction - it takes x lbs of O2 with y lbs of fuel to achieve complete combustion. Air density (and the mass per unit volume of it's gas components) changes significantly with temperature, so using cfm only gives you an approximation. Commercial/industrial boilers will use an O2 trim system to compensate for these variations. In situ O2 measurement is the most cost effective approach for small to medium sized boilers. Larger boilers will often add CO measurement to monitor unburned products of combustion to allow optimum control of the process.

The best way to calculate the required combustion air is on a molar basis (100lb/mole) especially if you need to consider Nox formation. The energy from bond release heats the gases and the heat from the gas and flame is released by radiation and convection across the heat transfer surfaces. If you add more air the bulk temperature will go down because of the mass increase of the incoming gases. If you overfire a furnace with an oversized burner temperatures will go up but if the burner is fixed at a firing rate and you increase the excess air the temperature will go down. This is pure thermodynamics. Did a lot of this type of testing over the years with all types of fuels and all types of furnaces. But only a few of the boilers fired at a rate below 20,000 lbs/hr. So increase in back pass temperature usually means to me there is an issue either in residence time, heat transfer surface or operation. But so often the smaller boilers are stuck in a corner somewhere, virtually ignored and run until they have a major issue. I found them to be the most dangerous to be around simply because the people around them only turned them on and off and paid no attention to their actual condition.