A Hand-Crank Blower Sized for a Charcoal Forge: Air, Pressure, Gearing and the Forge Size at Which the Hand Runs Out

A hand-crank blower for a charcoal forge is usually sized by copying an old machine, and the copies fail in two directions: fans that cannot make the pressure the tuyere demands, and gearing that asks the operator for a cadence no arm will hold. This paper sizes the machine from the forge outward. The stated bed, 0.30 m in diameter and 0.18 m deep, needs a working flow of 12 L/s against a total pressure of 310 Pa, of which the fuel bed itself accounts for only 30 Pa even after a threefold fouling allowance; the tuyere nozzle accounts for 240 Pa, just over three quarters of the whole. An impeller of 0.45 m diameter with a pressure coefficient of 0.55 makes that pressure at 920 rev/min, which from a comfortable crank speed of 70 rev/min requires gearing of 13.1 to 1, a two-stage drive. The surprise of the arithmetic is how little strength the work takes: the crank input at the working point is 9.72 W, the torque is 1.33 N·m, and the tangential hand force on a 0.20 m crank is only 6.6 N. What the operator must supply is speed, not force. A boost flow of 18 L/s for heat-up raises the pressure to 691 Pa and the input to 32.5 W, sustainable for about twenty minutes but not indefinitely, and with fixed gearing it demands a crank cadence of 105 rev/min, which is the genuinely awkward requirement in the design. Scaling the forge at constant superficial air velocity, one operator cranking indefinitely at 30 W can supply a bed up to 0.53 m in diameter; beyond 0.61 m even twenty-minute working spells fail, and a 0.70 m bed needs 52.9 W, in the five-minute band. Leg drive at 60 W extends the indefinite limit to 0.75 m. Past those diameters the hand-crank blower does not work, and a treadle, a second operator or a motor is required.