A Treatise On The Principles And Practice Of Dock Engineering

DOCK ENGINEERING. 510 electricity is generated, and air and water are pumped under pressure. This distinction, however, is not of sufficient moment to call for more than a passing remark, and need not invalidate the tabular arrangement adopted above, of which it will be convenient to take each item in detail, seriatim. Compressed Air.—Air, like steam, is an elastic fluid, and, cousequently, in its capacity as a transmissive medium, has the advantage of accommo- dating its volume to the resistance of the load—in other words, the work done is commensurate with the power employed. But this alteration of volume entails corresponding disabilities. Compressed air never effectively reproduces all the work which is done upon it ; partly, because it is not capable of expansion to the same extent as its previous compression and, also, because some of the energy imparted to it is dissipated in the form of heat. Then, again, leakages are rapid and difficult to detect, so that in long lines of communication there is inevitably much loss. Apart, however, from these drawbacks to its use on a large scale, compressed air has many advantages to offer for the working of small portable appliances, such as those employed in connection with ship repairs in graving docks, and the fact tliat sufficient power for the purpose can generally be obtained from a small air-pump renders it desirable, in the absence of more important installations, to equip such docks, especially if in isolated situations, with a pump, pipe lines, and branch couplings, so that the pressure may be transmitted readily to any desired point. This, however, apparently marks the limit of utility of compressed air in connection with dockwork. Steani.—The most useful characteristic of steam power is the con- venience with which it can be adapted to detached locomotive machinery. It necessitates no central generating station, although such can be employed in cases where the circumstances render it permissible. The general practice is for each machine to be entirely independent and self- supplied. In this way the loss of energy arising from long lines of communication and multiple connections is entirely obviated. Steam has the further advantage of supplying each machine with its own means of mobility, whereas in the case of other systems conforming to the exigencies of dockwork,* transportive power has generally to be obtained from extraneous sources. On the other hand, for intermittent operations, unless carried out in connection with a central station, steam power is not always readily available, nor indeed without due préparation. A boiler has to be heated, and some delay is inevitable before the requisite pressure is obtained ; furthermore, there is considerable waste of heat in the cooling down of the boiler after the allotted duty has been performed. A central generating station certainly does away with these defects, but the loss of heat from the steam supply due to its transmission through pipes to outlying positions is excessive, so much so that in no case will any * Trolley wires and underground cables are considered inapplicable to these special conditions.