Here's ABMAs answer to the 5 sq. ft. question

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Posted by Joye Chizek on December 20, 2001 at 18:59:50:

In Reply to: five square foot of heating surface boiler standard posted by Joye Chizek on December 18, 2001 at 10:45:01:

Re: Industrial Boiler Heat Transfer Surface Area

Your question on the above subject was forwarded to me for a response.
Basically the 10 square foot per Boiler Horsepower standard was used when the predominant fuels were solid fuels, such as coal, and combustion characteristics were relatively poor. This resulted in significant generation of smoke and soot by today’s standards. A coating of soot on the heat transfer surfaces approximately ¼ inch thick can increase fuel use by some 20 to 25%. This also had the effect of tending to block the flow of flue gases through the boiler. In order to combat these effects, relatively large flue diameters and heat transfer surfaces had to be used.
In the 1940 - 1950’s era, the liquid and gaseous fuels started to become the fuels of choice, and as heat transfer and combustion technology improved there was less need to use large heat transfer surface areas to achieve desired efficiencies, etc. Thus it was found that 5 square feet of heating surface per boiler horsepower was perfectly adequate, in order to achieve efficiencies of the order of 80%.
In today’s world with continued development in the areas of heat transfer and combustion, higher efficiencies are being generated with heat transfer surface areas of the order of 2.5 to 4 square feet per boiler horsepower. The role of the computer in modeling the combustion and heat transfer processes has also obviously contributed to these improvements. The 5 square foot standard is still often quoted today, and it is a good, safe standard, however there is a general trend toward smaller, more efficient boilers, and it seems likely that this standard will go the way of the 10 square foot standard.

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