Steam Kettle Heat-Up Time and Steam Demand

The first half is the ordinary sensible-heat relation: mass is gallons x 8.34 x specific gravity, the heat required is mass x specific heat x temperature rise, and the time is that heat divided by the rated input times the jacket efficiency. What makes it worth computing is the second reading. Dividing the same heat by the latent heat of the steam gives pounds of steam per batch, and dividing that by the heat-up time gives the pounds per hour the boiler must supply DURING the come-up. Kettles are sized on batch volume and boilers on peak steam rate, and the peak is the come-up, not the simmer, so a kitchen with four kettles that all start at 6 am has a boiler problem that never appears on the equipment schedule. At 15 psig the latent heat of saturated steam is about 945.6 BTU/lb; higher pressure carries slightly less latent heat per pound but a larger temperature difference across the jacket, so it heats faster. A 40 gal kettle of water-like stock from 60 F to 200 F rated at 100,000 BTU/hr with an 85% jacket holds 333.6 lb, needs 46,704 BTU, comes up in 0.55 hr or 33 minutes, and draws 49.4 lb of steam per batch, which is about 90 lb/hr or roughly 3 boiler horsepower. Four of them starting together want about 10 boiler horsepower for half an hour; stagger the starts and the peak halves. A sizing estimate; the kettle's rated input and the boiler manufacturer's data govern.

mass_lb = gallons x 8.34 x specific_gravity; heat_btu = mass_lb x specific_heat x (final_temp_f - start_temp_f); heatup_hr = heat_btu / (rated_input_btuh x jacket_efficiency); steam_per_batch_lb = heat_btu / latent_heat_btu_lb; steam_rate_lb_hr = steam_per_batch_lb / heatup_hr; boiler_hp = steam_rate_lb_hr / 34.5.

The sensible-heat relation Q = m c dT with 8.34 lb per gallon of water, and steam demand from Q divided by the latent heat of saturated steam at the operating pressure (about 945.6 BTU/lb at 15 psig per the ASME steam tables), cited by name and not reproduced; boiler horsepower at 34.5 lb/hr of steam. The kettle's rated input and the boiler manufacturer's data govern.

The sensible-heat relation and the boiler-horsepower definition are public. The kettle volume, product properties, temperatures, rated input, jacket efficiency, and steam latent heat are the user's values.

Estimate. AHJ and licensed professional govern.

Field names used by the API: gallons, specific_gravity, specific_heat, start_temp_f, final_temp_f, rated_input_btuh, jacket_efficiency, latent_heat_btu_lb, mass_lb, heat_btu, heatup_min, steam_per_batch_lb, steam_rate_lb_hr

• Water density 8.34 lb per gallon, scaled by the product's specific gravityphysical properties of water
• Latent heat of steam about 945.6 BTU/lb saturated at 15 psig; cited to the ASME steam tables, not reproducedASME steam tables
• Boiler horsepower 34.5 lb/hr of steam per boiler horsepowerstandard boiler rating definition
• Peak is the come-up boilers are sized on the come-up steam rate, not the simmerfood-service mechanical design practice