By Pauline Gill

Modern steam boilers create easily utilized energy for many heating, processing, and mechanical and electrical power-generation applications. They can derive their incoming energy from many types of fuels: electricity, co-generation, sunlight, nuclear and waste heat from other processes. Being able to calculate boiler steam flows from any of the parameters surrounding a boiler, including boiler horsepower, kilowatts, fuel-flow rates, feedwater flow, combustion air flow, or steam use data provides a complete picture of boiler performance.

This industrial boiler serves many users with pressurized, saturated steam.
Modern process plants use steam for heating and cooking.

Step 1

Use boiler horsepower to calculate steam flow. Boiler horsepower is not related to mechanical horsepower. It is a boiler industry rating that predicates the amount of saturated steam a boiler will generate starting with water at 212 degrees Fahrenheit and 0 pounds per square inch gauge (psig--meaning atmospheric pressure) and ending with steam at 212 F and 0 psig. For example, if a boiler is rated at 50 bhp, then it will produce 50 bhp x 34.5 pounds per hour (lb/hr) = 1,725 lb/hr of steam under these conditions.

Modern power plants have huge boilers to feed turbine generators.

Step 2

Use btu/hr (British thermal units/hour) heat input rate to calculate boiler steam flow. It takes 970.28 btu/hr to produce 1 pound of steam at the above conditions. If 50 million btu/hr are being applied to the boiler, divide by the 970.28-btu/lb to yield 51,531-lb/hr steam flow. This is an average industrial boiler.

Step 3

Calculate boiler steam flow with a known kilowatt hour (kWh) usage in an electrically heated boiler. If 1-kWh can produce 3.517 pounds of steam at the above zero-psig and 212-deg-F conditions, then 1,000 kWh will produce 3,517 lb/hr of steam.

Step 4

Compute the flow rate of steam from a boiler based on burning 20 gallons/hour of No. 6 fuel oil, assuming normal 85 percent boiler combustion efficiency. The accepted btu/gallon conversion rate for No. 6 fuel oil is 150,000 btu/gal. A boiler burning 20 gallons per hour would produce [(150,000 btu/gal x 20 gal/hr) / 970.28 btu/lb] x 0.85 (efficiency) for a total of 2,628.1 lb/hr of steam.

Step 5

Calculate the combustion-air mass required to burn the 20 gallons of No. 6 fuel oil above. Since the air-fuel stoichiometric ratio is 14.4 pounds of air per pound of fuel, the boiler's burner would need 20 gallons x 7.95 lb/gal for No. 6 fuel oil, or 2,289 pounds of air to burn the oil. This would in turn yield a total flow of 2,628.1 lb/hr of steam. Therefore, knowing the amount of combustion air flow during an hour for a given boiler, you can back-calculate to compute the steam flow. This means a boiler will produce about 2,628.1 lb/hr / 2,289 lb/hr of combustion air consumed, or about 1.15 pounds of steam for every pound of air.