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A BTU is the energy required to raise one pound of water by one degree Fahrenheit. A US gallon of water weighs 8.3 pounds (3.8 kg). To raise 230 L (60 US gal) of water from 10 °C (50 °F) to 50 °C (122 °F) at 90% efficiency requires 60 × 8.3 × (122 − 50) × 1.11 = 39,840 BTU. A 46 kW (157,000 BTU/h) heater, as might exist in a tankless ...
During the Han dynasty (202 BC – 220 AD), most, if not all, iron smelted in the blast furnace was remelted in a cupola furnace; it was designed so that a cold blast injected at the bottom traveled through tuyere pipes across the top where the charge (i.e. of charcoal and scrap or pig iron) was dumped, the air becoming a hot blast before ...