The problem of compounding blowing engines has been solved in the case of several of the types of engines above described, by building two separate units, one driven by the high-pressure steam cylinder, the other driven by the low, these units being connected only by the steam receiver. These engines are thus enabled to run as compounds and enjoy the benefit of that system, and at the same time retain the flexibility due to small units, because each engine can be run separately. The high-pressure engine has two dispositions for its exhaust, one to the receiver and the other direct to the condenser, or atmosphere. Then, if the low-pressure engine is not needed, or needs repairs, it may be shut down and the high-pressure then exhausts direct. Similarly, the low-pressure unit has a double source of steam supply, one the receiver steam from the high pressure, the other, live steam throttled through a reducing-valve, so that this engine can be run entirely independent of whether the high-pressure unit is in operation or not.

This type of compounding has been applied to the horizontal tandem, the quarter-crank vertical, to the horizontal tandem, and particularly to the long cross-head types of engines. In fact, for several years the long cross-head type of engine regained at least in part its old preeminence through the introduction of this system.

When this system is used one engine may be used on one furnace and the other on another, if desired. Where three cylinders are required to blow a furnace, as is not infrequently the case, six of these units, constituting three complete compounds, blow two furnaces, one complete unit on each furnace, the high-pressure of the remaining unit on one furnace and the low on the other. The arrangement of the valves is such that if one furnace is to be shut down, or if the blast be taken off for even a short time, the exhaust valves on the high pressure can be operated simultaneously, the receiver exhaust being closed while the direct exhaust is opened; the steam valves on the low pressure can be similarly operated so that either half of the unit may be kept going on its furnace irrespective of what the other unit is required to do. The governor operates on the high-pressure cylinder only and the cut-off of the low-pressure unit is regulated by hand to bring its speed the same as that of the high pressure. This adjustment will hold for a considerable variation of pressure without very much variation from the desired speed on the part of the low-pressure unit, ♦when both are on one furnace, and, consequently, blowing against the same pressure, but when the two units are used on different furnaces whose pressures vary in different ways, the conditions are not so simple and more frequent adjustment of the cut-off on the low-pressure unit is required.

The writer designed several years ago a means whereby these engines could be made to run at the same speed irrespective of the conditions under which they operated, but the manufacturers took no interest in introducing the device, and nothing was ever done with it.

Review Of Different Types Of Engines

We may now review these different types of engines in the light of the general conditions set forth above, and pass on their desirable and undesirable characteristic.