STEAM USE IN PUMPING ENGINE PRACTICE.
IN pumping engine practice high steam pressures and high piston speed are important elements in the attainment of economy of steam plant. Both contribute to the saving of steam and fuel. High initial steam pressure causes the waste by back-pressure to form a smaller proportion of the total horse-power effect developed in a given sized cylinder, while high piston speed, or rather great rapidity of piston reciprocation, causes the waste by condensation of steam in cylinder to be reduced to small proportions.
The main advantage of these two adjuncts to economy is the fact that they enable the securing of the required pumping capacity for a smaller first cost of plant. The result of such reduction in first cost is not only that the smaller original outlay conduces to the introduction of the plant, but the smaller annual interest and depreciation which the investment represents materially decreases the current expense account of the pumping plant of given capacity. Of course, the piston speed must not be so excessive as to seriously encroach on the durability and life-time of machinery, or to prevent proper flow, suction and discharge of water.
Compounding also has the effect of reducing the waste by cylinder condensation, and furthermore conduces to smoothness of running, but it has the disadvantage that it increases the first cost. It is a matter of determination in any special case whether the fuel saving effected by compounding is equivalent to the additional interest and depreciation chargeable to extra first cost.
The use of moderately superheated steam, if the degree of superheating does not exceed ioo° F., is an undoubted advantage in pumping engine practice, materially decreasing the amount of, and waste by, cylinder condensation. While the fuel saving is very appreciable, the cost of superheating appliances is comparatively small. The pumping engine is, of course, essentially of same construction, whether steam is superheated or not.
The use of high ratios of expansion, when not carried too far (with ordinary high steam pressure not exceeding, say, ten expansions), secures a fuel saving, but the difference in amount of fuel consumed when ten expansions are used, and when six is the ratio adopted, is so small that when expressed in money value it does not equal the extra interest and depreciation expense, due to larger plant required for a given pumping effect
From an extended experience and many careful comparisons relating to this special matter, we feel warranted in asserting with some degree of confidence that, taking everything in consideration, an expansion of six is about the most economical ratio for the steam cylinders of pumping engines, with ordinary high initial steam pressure in use, and even in those cases where the boiler pressure does exceed ioo pounds per square inch ; and this statement is put forth with full knowledge of the fact that Mr. Corliss and other engineers have not hesitated to expand steam of 125 pounds initial pressure twenty times in steam cylinders of pumping engines. But when such high ratios are used, the plant must be so large, and consequently of such great first cost, that it is paying too much to secure such saving of fuel as may be attained.
In the above we have only touched briefly on a few of the points involved, and no attempt has been made to be exhaustive in any particular. We shall have occasion, ere long, to recur more in detail to the several points mentioned, and to others of perhaps equal import in the consideration of the problem of “ Steam Use in Pumping Engine Practice.”