Comparing steam with compressed air it may be said, if steam is piped any distance from the boiler, the loss from condensation is great, further, the hot steam pipes are in the way and costly to maintain for machinery whose location must be often changed. If small detached units are used the consumption of fuel is under most wasteful conditions for it must constantly be burned at a rate which will meet the maximum power requirements instead of being adjustable for the frequent intervening low-load periods. It is expensive to deliver fuel and water and remove ashes from a number of detached boilers occupying all sorts of positions with relation to the work. For each such unit an engineer or fireman must be upon the work for an hour or so each morning to get up steam before the laborers start their shift. Lastly is the consideration, important in many localities, that licensed engineers must be employed for such use of steam, whereas such requirement is not made in case of using compressed air.
With a centrally-located compressed-air plant much more economical arrangements can be made for handling fuel and ashes. An appreciable saving of fuel results from using compound condensing engines. A further great saving follows from the fact that the power generated may not be over half the sum of the capacity of the engines supplied, for not all the engines are in use at the same time. This point will be elaborated later. The air may be easily piped for long distances and connections maintained or readily changed. Licensed engineers are not required and it is a comparatively simple matter to train plenty of men as engine runners.
Within the last few years electricity has been successfully applied to the rather severe requirements of a service, where the weight of load to be handled varies within such wide limits. Direct current was first used as it is readily controlled to afford a wide range of speed, i.e., a slow speed for a heavy load or for certain particular operations, and a much higher speed when light load or other circumstances permit. Recently, however, at the Kensico dam alternating current induction motors have been applied to hoisting engines on this class of work. The engines are constructed to obtain two speeds by a shifting of gears as in an automobile. Thus the derrick engines (in this case 75 h.p.) operating with one motor speed can obtain a rope speed on the drum of 250 ft. or 500 ft. per minute. This type of engine is said to give entirely satisfactory service.
As in any such change the amount of investment in old plant delays the adoption of the new, so the amount of steam and air plant in the hands of contractors has delayed the adoption of electricity. One argument, that a considerable part of the power must be in the form of compressed air (for drilling purposes) and that the remainder might as well be, has lost most of its force with the advent of the Temple drill. To be sure, this tool is air-driven but with a small electrically-driven compressor for each drill. The outfit complete costs about $1200, as compared to $300 for an ordinary air drill. On the other hand, the power cost is about one-fifth as much as for the ordinary drill.1
Doubtless there will always be some situations favoring the use of steam or compressed air, but electricity will come to be much more generally used as it often possesses very pronounced advantages.
The power may be transmitted for very much longer distances, thus vastly increasing the range of possible location of the central generating station with respect to the work. This wide range will often render available cheaper fuel, i.e., the location of the cheapest fuel may determine the location of the central station. It may permit the development and use of a water power too remote to be considered otherwise or it may bring within the range of possibilities the purchase of power from some existing company thus saving the expense of installing a central generating station.
For any distance of transmission the line losses in electrical transmission will be much less than the leakage of compressed air in mains and the smaller connections.
In the case of compressed air the distribution mains are much more costly to construct and maintain than the distribution wiring for electric power, especially where so many of the machines (the derricks) are moved so often.
1 At the Kensico dam twenty-one of these drills are employed. They are the Temple-Ingersoll Type 5F, with cylinder 5 5/8 X 8 in.; with pulsators driven by 5-h.p. 220-volt motors, giving at full speed 400 strokes per minute. The starting bits are 3 1/2 in. to 4 in. in diameter. For the longest (28 ft. 6 in.) steel the diameter is 1 3/4 in. With drills actually cutting for 51.1 per cent, of the time (the remainder being taken up in shifting drill, changing bit, and bailing hole) the power consumption was found to be 30 kw-hours to 40 kw-hours per drill shift of eight hours. The Engineering News of March 5, 1914, gives the drill records in that particular rock.
Air must be cooled during the process of compression and when used during cold weather it must be reheated at the point of use, at the cost of some fuel and some bother even if the engine runner is the only attendant.