The difference between watertube boilers and firetube boilers is closer to 2:1, certainly not 10:1!
Some boring history for those who are looking outside at snow, with no place to go on their steamboat, as I am:
A "Boiler Horsepower" is clearly defined as 33,475 British Thermal Units per hour energy delivered to the steam/water. This is equivalent to generating about 30 pounds per hour of steam at 70 PSIG.
A common early 1900's rule of thumb standard (USA) was 10 square feet heating surface per boiler horsepower, generally applied to the "Rating" of all boilers. When watertube boilers became popular about that time, they typically could be capable of being fired to higher outputs than industrial firetube boilers, but they were still "Rated" at 10 square feet surface area per boiler horsepower. Literature of that period would often state that watertube boilers were capable of exceeding their "Rating", and would typically state that a watertube boiler was capable of "170% of Rating, 200% of Rating, etc." Shortly after WWII, the small boiler manufacturers changed the standard to 5 square feet of heating surface per boiler horsepower, generally because more modern firing equipment allowed greater output per square foot surface area. This is the current "pseudo standard" in the USA, but clearly there are wide variations among boiler manufacturers. This was in normal industrial practice, and almost any boiler can be forced with a heavy fire to much higher output.
Perhaps the best perspective as to steam generating capacity of various heat absorbing surfaces in power boilers is the current ASME Boiler and Pressure Vessel Code. When determining the required safety valve capacity, the code requires consideration of the following rules:
Steam Generating Capacity in Pounds Per Hour (PPH) based on 1 square foot (non furnace) heating surface.
Hand Fired Solid Fuel Boilers – Firetube 5 PPH, Watertube, 6 PPH
Stoker Fired Boilers Firetube 7 PPH, Watertube, 8 PPH
Oil, Gas, PC Fired Boilers Firetube 8 PPH, Watertube, 10 PPH
Waterwall / Furnace Surface, receiving radiant heat from fire.
Hand Fired Solid Fuel Boilers – Firetube 8 PPH, Watertube, 8 PPH
Stoker Fired Boilers Firetube 10 PPH, Watertube, 12 PPH
Oil, Gas, PC Fired Boilers Firetube 14 PPH, Watertube, 16 PPH
From these numbers you can see that radiant heat absorbing surface carries much more evaporation potential than ordinary tube surface that does not “see” radiant heat from the fire. Note that one could easily build a firetube boiler with a big furnace that would out-perform a typical watertube boiler that has allot of ordinary (non-radiant) tube surface.
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