Making pig iron requires intense heat, which in turn demands a continuous, high-volume blast of compressed air forced into the bottom of a blast furnace. At the historic Sloss Furnaces in Birmingham, Alabama, this monumental task was handled by massive blowing engines powered by double-acting steam cylinders. Unlike simpler single-acting engines that only apply power on one direction of the stroke and rely on gravity or flywheels for the return, double-acting engines generate mechanical force on both the forward and backward movements of the piston.
In a double-acting cylinder, high-pressure steam is alternately admitted to opposite sides of the internal piston head. When steam enters the rear chamber, it forces the piston forward; at the end of that stroke, precision Corliss valves shift, admitting fresh steam into the front chamber while exhausting the spent steam from the rear. This pushes the piston back to its starting position with equal force.
At Sloss, the steam cylinder's piston rod extended directly into an adjacent giant blowing tub containing a air-compressing piston. Because the double-acting steam setup provided continuous double-sided power, the air piston was kept in near-constant high-pressure motion. It delivered tens of thousands of cubic feet of air per minute directly into the furnace's bustle pipe, keeping internal temperatures at a roaring 3,000 degrees Fahrenheit without deadly power drops.