I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
368
Acknowledgment
The author is indebted to the courtesy of the Eastern Cuban Sugar Corporation and the American Sugar Refining Company for the opportunity given him, while guest on their
VOl. 21, Yo. 4
estat'es, to carry out these investigations. His work was, mat>eriallyexpedited by the kind cooperation of the official6 of the centrals visited, and the freedom of the laboratories of the establishments.
Lord Kelvin's Law in Chemical Manufacture' Herbert H. Dow Dow CIIEMICAL Co.,MIDLAND, MICH.
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N THE course of my remarks a t the dinner in my honor given at the Chemists' Club in New York on February 27, I chose to elaborate somewhat upon the application of Lord Kelvin's law with a few instances that had come to my attention during my experience as a chemical manufacturer. The interest manifested in these statements has prompted me to write them down for the benefit of those who were not present a t the dinner. How large should a copper wire be for transmitting electrical energy? This was answered many years ago by Lord Kelvin, and the considerations which determine the best cross section of an electrical conductor also apply to like determinations of size and cost of many pieces of apparatus used in a chemical plant. For example, how much heating surface should a vacuum pan have for a given capacity? The governing factors are the capital charge that goes along with a bigger and more expensive pan or with multiple effects and the accompanying saving in operating cost which the larger pan or multiple effect makes possible. When an additional investment is required to improve the pan and the saving through Herbert operation of this pan is just equal to the increase in the capital charge that goes with such additional investment, then we shall arrive a t the lowest total cost for conducting the evaporation. Another example is the fixing of the elevation of a bridge crossing a river, such as the one near Albany. If we assume that all the freight comes downhill to the bridge and then rises again on its way to a higher elevation, the number of foot-pounds that will be exerted upon every passing train is a definite figure probably known with considerable accuracy by railroad engineers. Raising the elevation of this bridge would reduce the power required to transport freight over the river. It would also add to the capital charge. When the amount of such additional capital charge is just equal to the value of the power and other savings realized, we have the right elevation for the bridge above the valley below. For many years this rule has been used to fix the number of electrolytic cells that should be placed in series on a given voltage. If we increase the number of cells and thereby lower the voltage per cell, we have increased the energy efficiency of the electricity used, but a t the same time we have decreased the capacity of the cells. The right voltage a t which to operate is the one where a change either to increase or to decrease this voltage would increase or decrease the energy expense by the same amount as the capital charge would be varied by such change. 1
Received March 8, 1929.
Many of the problems are less simple-the size of a steam pipe, for example. Shall we carry a higher pressure on our boilers, use smaller steam pipes and a greater drop on our line and thereby reduce radiation? If we get the pressure above the point where equipment is reliable, then an uncertain factor comes into the problem, and in any event the higher s t e a m p r e ssu res and higher temperatures are in general more troublesome and this is a factor sometimes hard to handle. When all these variables are valued at the best figure that experience indicates, it would seem that our steam pipes are, as a rule, too large and our drop in steam pressures too small, and that in the future we will probably see a great change toward much higher steam velocities in steam pipes. It is evident from the above that the law can very readily be applied to determine the size of equipment in which chemical reactions take place and to set the point a t which we should switch from batch processes to continuous processes. Steam engineers generally recognize that it is possible to spend too much money on equipment to get the last attainable degree of vacuum in a power plant. The H. DOW cost of the condensers and pumps - - and their maintenance and operation are sometimes greater than is conducive to a minimum total cost of power. I n an experimental plant the possibility that the equipment will become out of date within a very short time is such a big and uncertain factor that it makes a quantitative determination under the Kelvin law of relatively little use. Practically every chemical plant during the war was working under the assumption that the amortization might be 100 per cent more or less. The uncertainty in connection with this figure outweighed all other considerations to such an extent that it was hardly worth while using the Kelvin law, but now that our plants are working under normal conditions and processes are being stabilized, amortization no longer dominates our calculations to such an extent that other factors are unimportant, and we can now use this law effectively. I think we should give credit tb the engineers who are building our modern steam power plants for taking a fuller advantage of the Kelvin h w than do the engineers in other lines. Let us wisely follow. The Bureau of Standards of the Department of Commerce has announced the development of a precise method for measuring the unit of x-ray dosage which is important since the success of x-ray treatment depends upon the accurate control of the dosage given the patient.
