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[kiwimodeller]

Greetings all, does anybody have any idea what sort of temperature (preferably taken at the top of the funnel) indicates a good, clean burning mixture of gas and air when operating a Propane/Butane mix burner? I was thinking of getting a high temp digital thermometer to see if I could tell whether the gas jet size is correct in relation to the volume of air passing through the flue. I have tried an infra red laser thermometer but that was all over the place as it was probably measuring the temperature of the metal bits it was pointing at rather than the gas. I have been offered a thermometer which takes a "K" type stainless thermocouple probe. It measures to 350 degrees C but I can get an alternative probe that goes to 1300 degrees. Just thought this might be a good way to see if my burners are running rich or lean. I have also heard that different proportions of Propane in the mix give different amounts of heat so the thermometer might show if this was true. I would also out of curiosity try the temperature of solid fuel, Sterno and meths in some of the smaller plants. Any input most welcome. Cheers, Ian.

[derekwarner]

Hullo Ian....an interesting question ....however with limited technology 

A number of years ago on PD's I posted my boiler temperatures from a number of locations when the relief valve was exhausting @ 45 PSI
From the steam tables......this confirmed approx. 152 degrees C steam temperature...& yes the boiler steam discharge valve body registered ~~~150 degrees C
The instrument I used was a simple digital infrared point & shoot pyrometer [maybe a $300.00 unit]
The downside here is ....these instruments measure temperatures of fixed or static components
They are not designed to report on the actual temperature  ....in this case ...of a fast moving body of elevated temperature gas .........
A number of years ago...I was involved in the temperature analysis of hydraulic cylinders ....within water jacket's...& then within heat shields...over BOS furnace skirts......
We used K type thermocouples in different locations within the cylinder, heat shield & water jacket
These results required a 24/7 datalogger to extrapolate the actual temps attained.........the attachment shows one page of results......Derek

[AlexC]

Hi Guy's,
 
Ian asked...

 Greetings all, does anybody have any idea what sort of temperature (preferably taken at the top of the funnel) indicates a good, clean burning mixture of gas and air when operating a Propane/Butane mix burner

The temperature of the gas leaving the funnel would not prove the clean burning of the gas... at best it may give you an indication of the efficiency of the boiler to use the available HEAT... but even this would be dependant on other factors.

It is easily possible that any given burner fitted in 2 different boilers could produce a higher funnel temperature in one boiler even with the burner not optimised... than it did in the other boiler running at optimum... it all depends on boiler HEAT transfer capability.
 
Take a given boiler... fit a large burner, correctly adjusted for clean burn...(this could be done by measuring the flame temperature directly)... during boiler warm up measure the exhaust gas at the funnel... you will see that it gradually increases until it reaches a stable level when the boiler reaches stable pressure... this temperature gradient is caused by the boiler absorbing more heat from the burning gas at lower water temperatures than it will/can absorb when the boiler has reached working pressure.
 
Using the same boiler... but a smaller burner and using exactly the same gas mix... again adjust to give maximum flame temperature at the burner...repeat the boiler warm up process and you will observe the same temperature gradient... But it will be much greater... with the funnel temperature being much lower... this is due to the difference in the available HEAT being provided by the burner... However, the BURNER FLAME TEMPERATURE in both cases would be identical.
 
Exhaust flue gas temperature would only be meaningfull if: -
 
a. the exact temperature of the correctly burning mixture in the burner was known.
 
b. the precise amount of HEAT in BTU's being produced by the burner was known.
 
c. the precise heat absorption capability of the particular boiler was known.
 
Note... this would be dependant upon available heated surface area, distance from burner, gas turbulence, distance between gas flow volume and heated surface, skin effect... amongst others...
It would also be a variable depending upon initial water temperature and final operating pressure.... Heat absorption/transfer is proportional to temperature difference, and the thermal coefficients of the media.
 
Hope that all makes sense.
 
Best regards.
 
AlexC

[kiwimodeller]

While I accept that the above are valid points I do feel that provided comparisons are made under similar conditions and for a fixed period of time then the exhaust temperature should give an indication of whether a different jet in a gas system will provide more or less heat to input in to the water. As long as I only change the jet I should be able to tell which gives the hottest temperature over the time needed to get up to pressure. I agree that this could probably be measured as flame temperature at the burner rather than temperature at the funnel exit except that I have found in the past that some burner designs perform differently when they are in open air to when they are in the flue so the exhaust temp might be more accurate. To begin the process I have been doing some experimenting with a little Midwest vertical boiler which burns "Sterno" gel as this is easy to work with. As Alex suggested the temperature varied depending on the water temperature. I was quite surprised at some of the temperatures reached. The temperature of the flame stayed almost constant at about 640 degrees C. The temperature at the funnel exit quickly stabilised at 350 to 360 degrees until the boiler started producing steam at which time it climbed to 460 to 480. As Alex suggested there was obviously less heat transfer to the water once it began to steam and therefor more heat out the funnel. Obviously if this had been a gas fired system I would have been able to turn the gas down once the boiler reached a stable temp and pressure. Next step will be to start playing with the gas fired boiler (using the golden rule of only changing one thing at a time) but that will have to wait for a bit more free time. Cheers, Ian.

[AlexC]

Hi Ian,
 

While I accept that the above are valid points I do feel that provided comparisons are made under similar conditions and for a fixed period of time then the exhaust temperature should give an indication of whether a different jet in a gas system will provide more or less heat to input in to the water. As long as I only change the jet I should be able to tell which gives the hottest temperature over the time needed to get up to pressure

 
With all due respect, I feel you are perhaps missing the point a little.

Flame temperature has little to do with HEAT output and even less to do with funnel exhaust temperature.

Heat is measured in Btu’s (Joules in metric) not degrees F or C.

70/30 Butane/Propane max flame temperature in air is approx 3600deg F. (1982deg C).
70/30 Butane/Propane mix has a calorific value of approx 21,400 Btu’s per lb.

‘Sterno’ (methanol/ethanol mix), as you have discovered has a much lower flame temperature but has a calorific value of around 12,500 Btu’s per lb.

Changing the jet size will not change the flame temperature of the chosen gas (assuming the gas is perfectly burnt) it will only increase/decrease the HEAT output in BTU’s by burning more/or less gas in the same time period.

The real question is… what can the particular boiler do with the heat available, in a given time? (See below).

This is what will determine funnel exhaust temperature.

Boiler absorption rate.

A larger burner cannot change the MAX rate at which the boiler can absorb heat.

A typical centre flue model boiler can evaporate water into steam at a MAX rate of around 1.5 cu in water per minute per 100sq in of heated surface.

This translates to an absorption rate of 63.4 BTU’s per minute per 100sq in heated surface at a steam pressure of 45psi.
(Note… the number of Btu’s required per minute changes with steam pressure)

Changing the burner cannot change this absorption rate unless the burner is not producing sufficient Btu’s in the time period.

A boiler with the same evaporation rate, but having only 50 sq in heated surface area, can only absorb 31.7 Btu’s per minute, hence, it can only evaporate ¾ cu. in per minute at 45psi.

However, since model boilers are at best, around 50% efficient then in order to achieve the design evaporation rate the number of Btu’s produced by the burner would need to be twice the above figures respectively.

I.e. 126.8 Btu’s per minute @ 100sq in heated surface area.

Or 63.4 Btu’s @ 50 sq in heated surface area.

Assuming that the burner can burn the supplied gas fully… a gas jet capable of passing the exact amount of gas required to produce the above Btu’s from the burner would be the correct size for the boiler.
This would be determined by the pressure of the gas and the cross sectional area of the jet orifice.

Such a burner would be optimised for the boiler.

Burning more gas, by increasing the jet size, thus increasing the Btu’s produced even further, would have no effect on the max evaporation/absorption rate of the boiler… the additional HEAT would just pass out of the funnel and be wasted and would tell you nothing about the burner efficiency... and this would give higher exhaust temperatures.

If the burner were incapable of burning the supplied gas fully then the increase in jet size would be detrimental.

The only way to produce more steam in a given time period at a given pressure is to increase the HEATED SURFACE area or change to a different type of boiler with a higher conversion rate.
 
The experiments you are doing, whilst certainly interesting, will not result in a common or typical exhaust temperature value which can be generally applied... which was the original question asked... at best they will allow you to optimise your burner for that particular boiler, if not already at optimum,  and any result you do come up with would only be applicable to that particular burner and boiler combination.
 
Sadly, there is no magic number... I wish there was, since it would make boiler design a bit simpler in some areas.
 
I hope this helps your understanding a little more.
 
My very best regards.
 
Sandy. (AKA AlexC)

[Jerry C]

For a given boiler with common capacity, start temperature and ambient temperature wouldn't the time taken to lift the safety be a good indicator of which burner arrangement is better?
Jerry.

[AlexC]

Hi Jerry,
 

 For a given boiler with common capacity, start temperature and ambient temperature wouldn't the time taken to lift the safety be a good indicator of which burner arrangement is better?

Only up to the point where the burner output matched the boilers evaporation capacity, plus any external losses, was reached... beyond this point any additional output from the burner would be wasted and would not reduce the time needed to get to safety valve lift.
It would certainly differentiate between a weak burner and a strong burner, since the weaker one would take longer to reach safety lift... but it would not tell you if the stronger burner was too strong... or indeed how weak the weaker one was.
 
Best regards.
 
AlexC. 
 
 

[kiwimodeller]

Hi Ian,
 
 
With all due respect, I feel you are perhaps missing the point a little.

Flame temperature has little to do with HEAT output and even less to do with funnel exhaust temperature.

Heat is measured in Btu’s (Joules in metric) not degrees F or C.

70/30 Butane/Propane max flame temperature in air is approx 3600deg F. (1982deg C).
70/30 Butane/Propane mix has a calorific value of approx 21,400 Btu’s per lb.

‘Sterno’ (methanol/ethanol mix), as you have discovered has a much lower flame temperature but has a calorific value of around 12,500 Btu’s per lb.

Changing the jet size will not change the flame temperature of the chosen gas (assuming the gas is perfectly burnt) it will only increase/decrease the HEAT output in BTU’s by burning more/or less gas in the same time period.

The real question is… what can the particular boiler do with the heat available, in a given time? (See below).

This is what will determine funnel exhaust temperature.

Boiler absorption rate.

A larger burner cannot change the MAX rate at which the boiler can absorb heat.

A typical centre flue model boiler can evaporate water into steam at a MAX rate of around 1.5 cu in water per minute per 100sq in of heated surface.

This translates to an absorption rate of 63.4 BTU’s per minute per 100sq in heated surface at a steam pressure of 45psi.
(Note… the number of Btu’s required per minute changes with steam pressure)

Changing the burner cannot change this absorption rate unless the burner is not producing sufficient Btu’s in the time period.

A boiler with the same evaporation rate, but having only 50 sq in heated surface area, can only absorb 31.7 Btu’s per minute, hence, it can only evaporate ¾ cu. in per minute at 45psi.

However, since model boilers are at best, around 50% efficient then in order to achieve the design evaporation rate the number of Btu’s produced by the burner would need to be twice the above figures respectively.

I.e. 126.8 Btu’s per minute @ 100sq in heated surface area.

Or 63.4 Btu’s @ 50 sq in heated surface area.

Assuming that the burner can burn the supplied gas fully… a gas jet capable of passing the exact amount of gas required to produce the above Btu’s from the burner would be the correct size for the boiler.
This would be determined by the pressure of the gas and the cross sectional area of the jet orifice.

Such a burner would be optimised for the boiler.

Burning more gas, by increasing the jet size, thus increasing the Btu’s produced even further, would have no effect on the max evaporation/absorption rate of the boiler… the additional HEAT would just pass out of the funnel and be wasted and would tell you nothing about the burner efficiency... and this would give higher exhaust temperatures.

If the burner were incapable of burning the supplied gas fully then the increase in jet size would be detrimental.

The only way to produce more steam in a given time period at a given pressure is to increase the HEATED SURFACE area or change to a different type of boiler with a higher conversion rate.
 
The experiments you are doing, whilst certainly interesting, will not result in a common or typical exhaust temperature value which can be generally applied... which was the original question asked... at best they will allow you to optimise your burner for that particular boiler, if not already at optimum,  and any result you do come up with would only be applicable to that particular burner and boiler combination.
 
Sadly, there is no magic number... I wish there was, since it would make boiler design a bit simpler in some areas.
 
I hope this helps your understanding a little more.
 
My very best regards.
 
Sandy. (AKA AlexC)

Here endeth the first lesson thanks very much Sandy - perhaps we should have a masterclass section on choosing and using boiler/burner combinations. I guess that I did not really ask the right question which probably makes it hard to get the right answer. The more you give me to think about the more I realise that what I was trying to do was to check the burner was optimised for the boiler it was being used in but I was looking at it as if it was a petrol engine where the exhaust temperature is closely linked to whether or not there is complete combustion. I feel that at least one of my burners is fitted with too large a jet and therefore is wasting gas so will try "tuning" it by recording time to reach pressure as well as taking its temperature and maybe its pulse too! If you have any suggestions on how to optimise the burner for any given boiler it is operating in they would be most welcome. Cheers, Ian.

[xrad]

if you have access to a CO meter, you measure that parameter for burn quality.  I find that with all things being equall(position of burner,full tank of gas, room temp water, room temp boiler, same water level at start) the best method for 'getting things right'  is time to pop off.
 
you can check the burner flame and look for the best flame front when adjusting the air fuel mixture, as a starting point.