In former times, before electricity had reached its present state of development, a supply of compressed air was one of the greatest conveniences that could be installed at a furnace plant. It was very frequently used for operating bell cylinders, for drilling the tapping hole, for operating pneumatic tools on repair jobs, and many similar operations. For this purpose an air compressor of good design was installed in the power plant, and the air piped to the points of utilization. In good practice branches with valves were inserted in the main at different points so that air could be taken wherever it might be needed, with a short temporary line or hose. Electric power is more economical than compressed air and its general introduction has tended to supplant compressed air installations but the conveniences of the latter is so great that they are well worthy of consideration in spite of somewhat lower economy.

The modern designs of air compressors are greatly improved over those of a few years ago, being more economical and more reliable, and air supplies a very desirable form of power, being absolutely safe and extremely convenient. Especially for repair and construction work no substitute for the compressed air riveter and other tools of that kind has as yet been discovered. Where compressed air plants are installed as a permanency an after-cooler should always be provided just outside the power house to cool the air to or below the temperature of the atmosphere so that the moisture condensed out of the air by compression may be removed at that point and not go through the line as vapor to condense in some distant pocket where it can make trouble in winter by freezing up.

Many types of compressors have been brought out, and one known as the straight-line type, has had considerable popularity, this is of the same general design as the long cross-head blowing engine, but made horizontal, but compressors are very uneconomical unless they are compounded as to both steam and air. This means two cylinders for each, and therefore the straight-line type must have four cylinders all in a line to embody these features. This makes a rather complicated and extremely inaccessible type of apparatus, and as this type of engine has two cranks and two connecting rods virtually no more machinery is involved to make a cross-compound, two-stage tandem apparatus, with the advantage of vastly greater accessibility and generally a more economical design. The space required, and the foundation cost are greater with the cross-compound type, but its operating advantages are well worth the difference.