Boiler Output & Efficiency
Posted: Wed Apr 10, 2013 6:27 am
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
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