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 Post subject: Boiler Output & Efficiency
PostPosted: Wed Apr 10, 2013 6:27 am 
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Output & Efficiency of the Boiler

As applied to steam launch practice, several types of steam boilers are in use, and they typically have output steam flows ranging from about fifty Pounds Per Hour (PPH) to several hundred PPH.

Boilers are generally classed as two types, although there are many variations available.

Firetube Boiler type, where the hot flue gas from the fire passes through the inside of tubes, and boiling water is on the outside of the tubes. These boilers usually have a large inventory of boiling water, are rather heavy, and as a result are usually easier to control than other boiler types. About half of the steam launches use this type of boiler. Firetube boilers nominally produce about 3 PPH steam generation per square foot of heat transfer surface exposed to the hot flue gasses.

Watertube Boiler type, here the hot flue gasses pass over the outside diameter of steam generating tubes, with boiling water inside the tubes. These boilers generally have a smaller inventory of water, and hence can raise steam more quickly. They also need closer control of the fire and water level, as they do not gave the high thermal “mass” associated with firetube types. Watertube boilers nominally produce about 6 PPH steam generation per square foot of heat transfer surface exposed to the hot flue gasses.

A properly designed boiler, with the nominal outputs delineated above, will have an efficiency (conversion of fuel heat into steam output energy) in the vicinity of 60% - 70%. Various features, such as economizer tubes and superheat tubes can increase the efficiency, as well as extra steam generating heat transfer surface area, but overall efficiency of 80% is about the practical limit.

Very roughly, each pound of steam generated in the boiler requires about 1,000 British Thermal Units (BTU) of boiler output. For example, a boiler producing 100 PPH of steam needs an output of 100,000 BTU per hour, and would need a fuel input of 140,000 to 165,000 BTU per hour.

All boilers can be “forced” to high outputs with a sacrifice in efficiency. Having an intense fire in the boiler might allow 200% to 250% of the nominal output listed above, but efficiency will generally be much lower.

As to the heating value of fuels, these also vary widely, typical examples in British Thermal Units (BTU) per pound: Random Wood, 3,000 – 4,000, Dry Hardwood, 6,000 – 7,000, Coal, 11,000 – 12,000, Coke, 14000 Fuel Oil, 19,000, Propane, 22,500


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 Post subject: Re: Boiler Output & Efficiency
PostPosted: Mon Nov 11, 2013 4:54 am 
High efficiency condensing boilers are commonly installed with new central heating systems nowadays because they minimise energy waste. Condensing boilers do not release the steam that is made when water is heated up, whereas old boilers would expel it through the flue. Instead, condensing boilers capture the water vapour and turn it back into liquid water, so no energy is lost and less energy needs to be used.


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 Post subject: Re: Boiler Output & Efficiency
PostPosted: Mon Nov 11, 2013 5:14 am 
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There are central heating boilers that dump steam up the stack?? Weird science! Who knew?

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 Post subject: Re: Boiler Output & Efficiency
PostPosted: Tue Nov 26, 2013 10:54 pm 
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what scottmatthew is saying is that the products of combustion (for a natural gas heating boiler) contain water vapor (what scott refers to as the "steam being made"), and these high efficiency heating boilers condense this water vapor in the exhaust gas, to capture the heat of condensation. Actually this condensation occus on the heat exchange surfaces of the heating boiler, not actually in the stack. The inside of the stack does however run wet, as the exhaust gas has been cooled to the "dew point" temperature.

This type of boiler is generally not applicable to steam launch boilers, as the stuff that is being heated in a launch boiler (the high pressure boiling water) is well above 212F (100C), and in a high efficiency heating boiler the stuff being heated has to be well below 212F (100C) temperature.


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 Post subject: Re: Boiler Output & Efficiency
PostPosted: Wed Nov 27, 2013 1:37 am 
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Interesting. So they then collect the water and drain it off?

Mike

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 Post subject: Re: Boiler Output & Efficiency
PostPosted: Wed Nov 27, 2013 2:56 am 
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Yes, the collected water is drained off, and often dumped into a small plastic condensate pump for installations in a basement without floor drains. I have had small flooding in my basement several times when this little pump stops working. The same pump collects condensation from the air conditioning "A coil" mounted above the heater in summer.


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 Post subject: Re: Boiler Output & Efficiency
PostPosted: Tue Jan 28, 2014 6:42 pm 
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I have the same condensate pump arrangement that Fred described on my high efficiency forced air, gas furnace. The pump exits into the sewer pipe directly.


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 Post subject: Re: Boiler Output & Efficiency
PostPosted: Sat Oct 04, 2014 3:57 am 
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I think I'm missing something here on the calculations.

My VFT has 33 sq. ft. of heating area. At 3 PPH/sq, ft. that comes to around 100 PPH.

Times 1000 to get BTU's gives 100,000 BTU.

But dividing by about 2500 for H.P. gives 40 h.p.!!! Yeow! Beckman rates this boiler at 4.5 h.p. and that agrees reasonably well with my experience.

What am I doing wrong here? This a disagreement of around a factor of nine.

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 Post subject: Re: Boiler Output & Efficiency
PostPosted: Sun Oct 19, 2014 2:22 am 
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All heat engines convert only a portion of the boiler heat output into usable power, the remaining heat shows up as low temperature heat in the engine exhaust. As always and in everything, what goes in must come out, but the energy split in our small steam launches is to convert only a few percent of the input fuel heat to shaft power. 2500 BTU per hour is equal to one horsepower, the mechanical equivalent of heat.

In a typical small steam launch cycle, one would burn perhaps 100,000 BTU per hour of fuel, and with boiler efficiency 60,000 BTU per hour goes into the generation of steam, and the rest of that fuel heat is wasted as hot stack gas. Then the steam cycle converts only perhaps 4 percent of the boiler heat into mechanical power, 2500 BTU per hour in this case, equaling one horsepower shaft output. The remaining 57,500 BTU per hour shows up as exhaust steam heat, plus heat losses from the engine, the piping, etc. The actual efficiency of our small steam launch engines is typically around a few percent, while larger compound engines with vacuum exhaust can have efficiencies of a few percent more, but I would think 8% efficiency is a rare number in our hobby steamboats.

The most advanced real steam cycles are able to convert about 40% to 45% of the fuel input heat to shaft power, and that is how about 80% of all the world's electric power is produced. These cycles however use steam pressures ranging upwards of 2500 PSI, with steam temperatures well above 1000F. And these engines are not reciprocating machines, they are steam turbines.


Last edited by fredrosse on Sat Nov 07, 2015 2:44 am, edited 1 time in total.

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 Post subject: Re: Boiler Output & Efficiency
PostPosted: Sun Oct 19, 2014 4:13 am 
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I understand about the comical efficiency of our little engines. And that the input of the boiler (fuel) mostly goes up the stack. But that 100 pounds per hour is boiler steam power output. So what you are saying is that when Beckman rates the boiler at 4.5 h.p. they are not just rating the boiler but rating the boiler and engine together? Telling the customer that this boiler would be a decent match to an engine rated at 4.5 h.p.?

That seems like that would be subject to significant error due to the difference between some small single poorly insulated and with the valve gear running at a long cut off and, at the other extreme, a multiple expansion engine with good lagging and well balanced gear and running at moderate cut off. Hmm.

So anyway back to where I am evaporating 40 h.p. worth of steam and getting perhaps 5 h.p. at the shaft. That sounds like 12.5% efficiency. Way too good. Still a factor of around four too good.

Somewhere along the line there are assumptions that cannot be right. Or I am dumber than a sack of hammers tonight. Probably the latter.

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