How many BTU per hour does one boiler horsepower represent, and how many pounds of steam per hour does that roughly equate to at saturation?

• A. 33,475 BTU/hr; about 34.5 pounds per hour
• B. 15,000 BTU/hr; about 10 pounds per hour
• C. 25,000 BTU/hr; about 20 pounds per hour
• D. 50,000 BTU/hr; about 60 pounds per hour

Answer: (A)

Boiler horsepower is a defined heat transfer rate used in steam practice. One boiler horsepower represents the energy needed to evaporate 34.5 pounds of water per hour into saturated steam at 212 °F (0 psig). That amount of energy is 33,475 Btu per hour. Since the latent heat of vaporization of water at the boiling point is about 970 Btu per pound, 34.5 lb/h × 970 Btu/lb ≈ 33,465 Btu/h, which aligns with the exact 33,475 Btu/h when rounding is taken into account. Therefore, one boiler horsepower equals 33,475 Btu/hr and corresponds to roughly 34.5 pounds of steam per hour at saturation. The other options don’t match this standard definition.