Oversizing a boiler is a common mistake. An oversized boiler short-cycles: it fires, reaches pressure, shuts off, and repeats. Each cycle includes a purge that carries heat out of the boiler, and frequent cycling wears the burner and controls faster.
One boiler horsepower is 33,475 Btu/hr, equal to 34.5 lb/hr of steam from and at 212°F. At higher operating pressures and with cooler feedwater, one boiler horsepower produces somewhat less steam, so use your actual conditions when converting.
To size the boiler, list each piece of steam-using equipment and its demand in lb/hr at its operating pressure. Apply diversity: not all equipment runs at once, so base the load on how the plant actually operates rather than the sum of nameplate ratings. Add piping losses and blowdown. Add pickup, the extra steam needed to warm cold piping and equipment at startup. Include future load only if it is planned. Then convert the total to boiler horsepower at your operating conditions.
Consider whether one boiler or two fits better. With two smaller boilers, one can carry light loads, the second covers peaks, and the plant can keep running while one is down for service. Sizing so the plant can carry its load with any one boiler out of service is called N+1 redundancy.
Burner turndown also matters. Turndown is the ratio of a burner's maximum to minimum firing rate. A burner with 10:1 turndown can follow a light load with less on-off cycling. It's a burner specification, separate from boiler size.
For a new plant or a major expansion, have a mechanical engineer confirm the sizing. Once the boiler is installed, we handle burner startup and tune it across its full firing range.
Related reading: Boiler and Burner Formula Cheat Sheet and Combustion Tuning: What It Is and How Often. See all of our services on the What We Do page.
Request service, or email service@combustionspecialists.com.
