I have been asked over the years “What is a boiler horsepower” and what is the design criterion of square feet of heating surface per boiler horsepower.
To understand boiler horsepower it is necessary to review the genesis of the term “boiler horsepower” (Bhp). The term originally related to the quantity of steam necessary to operate a one horsepower steam engine. Due to variations in engine efficiencies, this quantity of steam was itself a variable. Tests conducted in 1876 on a modern (for the time) steam engine determined that it took approximately 30 pounds of steam per hour to produce 1 horsepower (mechanical) of work. In 1889 the American Society of Mechanical Engineers (ASME) standardized the term “Boiler Horsepower” as being based on a conventional steam engine steam rate of 30 pounds of steam per hour (PPH) at 70-psig pressure and feedwater of 100 degrees F. This definition was later modified to: “Boiler Horsepower = the unit of capacity expressed as equivalent evaporation of 34.5 pounds of water per hour from and at 212 degrees F (33,475 Btu per hour.)”
In the early days of brick firing surfaces the rule of thumb was that it took 10 square feet of heating surface to create 1 boiler horsepower. In todays modern boiler designs the heating surface necessary to create 1 boiler horsepower has been reduced to 5 or in some cases less that 5 square feet of heating surface.
As we all know before machines we did all the heavy work… then “Joe” one day hitched a horse to a rock to move the rock. Without knowing it at the time “Joe” created the term motive power. Motive power is a term used to describe something (air, water, steam, or the horse) that has the energy (power) to create movement in something else. So, as steam engines replaced the horse as a means of motive power steam engines were were rated in horsepower instead of “Joe Power”. Sorry Joe !
Hope you enjoy this bit of horsepower history.