Inside the Blast House

​While giving tours at Curtin, more than once I have been asked to explain the mechanics at work inside the blast house. I’m not a mechanic or an engineer, so this explanation is geared to those of us lacking such expertise.

In a charcoal-fired iron-producing blast furnace there was a constant stream of air pumped across the charcoal at the base of the furnace. We’ve all seen how a bellows works to heat up a fire. Smelting iron requires the furnace to reach about 2800ºF, and the bellows effect is necessary to heat the charcoal sufficiently. In fact, that “blast”of air is what gives a blast furnace its name and why there is a structure known as the blast house.

Below is a diagram of the mechanics inside the blast house. A flume carried water flowing from nearby Antes Creek, now called Nittany Creek. The water powered a water wheel, which in turn moved two reciprocating piston rods, one on each side of the waterwheel, up and down. (Reciprocating just means it moves up and down, as opposed to making a rotary motion.) Attached to the top of each piston rod was a metal crown with a one-way valve in its middle. As the piston rod rose, it powered the metal crown upward inside the piston cylinder, and the valve closed. Air was thereby compressed inside the piston cylinder and forced into the adjacent mixing box. As the waterwheel continued turning, the piston rod and the attached crown moved back down. When the piston crown started its descent, a second valve at the top of the piston cylinder closed, preventing the compressed air in the mixing box from re-entering the piston cylinder. Simultaneously, as the piston crown was pulled down, the valve at its center (that had been closed on the way up) opened, allowing the next gulp of air to enter the cylinder from outside.

Sketched by me. Borrowed from Eggert, reference below, p34.

​In synchrony, the piston rod and metal crown on the opposite side of the waterwheel was moving upward as the piston rod and metal crown on the first side were moving downward. Valves on the second side similarly controlled the direction of airflow into the mixing box and prevented air from moving back into the piston cylinder. As air was pushed into the mixing box from side two, the valve at the top of the piston cylinder on side one closed.

Since the two pistons worked in synchrony, and two valves on each side regulated air movement, the compressed air in the mixing box could only move in one direction, outward through the air duct leading to the tuyere at the base of the furnace. (Tuyere is a word borrowed from the French word meaning nozzle and is pronounced in English something like twee-air.) The air pressure inside the mixing box was only about one pound per square inch, but it was enough to move the air forward into the furnace and get the job done.

Note — For purposes of clarity, I’ve drawn the analogy to pistons in a car engine. In reality, what I’ve called piston cylinders in the diagram and description were fairly air-tight, leather-lined barrels called blowing tubs. The mechanics are the same.

Reference:
Eggert, GC, Making Iron on the Bald Eagle, Penn State University Press, 2000