T H E J O L l R N A L O F I ; V D C ‘ S T R I A L AiVD E N G I N E E R I - Y G C H E M I S T R Y
860
In a centrifuge the gaseous medium should possess an angular velocity of w . The two forces t h a t will operate on the particle when its distance r from the axis increases will be that of fricional viscosity and the centrifugal force. The centrifuge process is frequently used when the particles to be separated from a gas possess considerable mass. Under these conditions w need not be so great. It is very doubtful if a process of this kind would be practical in the case of small smoke particles. A combination of a washing (or an electrical discharge) and a centrifugal method might be found to be better than any single method used by itself. The efficiency of the “volume” forces depends upon the pressure required to force the gas through the sieves. The nature of the differential “radiations” have been worked out mathematically but these have no value as a practical method of removing suspended matter from gases. Neither is polarization a practical method. Probably one of the most important from the efficiency point of view is the differential ionic force THE DIFFERENTIAL “IONIC”
Trans. A . I . E . E.. June, 1913.
j,
NO.
IO
million particles. Assume that the particles are to be given a velocity of 2 0 centimeters per second and that the average radius is 0.0ooo1 centimeter: F = 67r(0.00001) ( p = 0.000178) (V = 2 0 ) Let the particles be moved a distance of 8 centimeters aiid suppose there are 30,000,000 particles per cubic centimeter. Since 1000 cubic feet contain 2.8317(10)? cubic centimeters, the required work in watts will be: W = 8F X 30,000,000 X 2.8(10)’ = about 40 watt5 This would indicate that possibly 15 per cent of the electrical energy may be expended in cleaning the gas, this energy being converted again into heat energy through the agency of the viscosity forces. I n the above calculation i t must be recognized that the data are extremely meager. I t is t o be hoped that theoretical investigators will take up a detailed study of these problems. DCPARTXENT OF
INDUSTRIAL
v S l V E R S I T Y OF
RESEARCH
PITTSBURGH
FORCE
The use of electromagnetic fields for removing suspended matter from gases and liquids is practically limited to the action of the electric field. This condition is due to the fact that very few suspended particles possess any appreciable magnetism. Even nickel dust would be removed with difficulty by the action of a magnetic field. This leaves only iron and a few magnetic compounds of iron that can be acted upon by a magnetic field. In general it may be said that the application of electric fields is restricted to a non-conducting medium, such as a gas or a dielectric liquid. There are no gases that conduct naturally. T h e ions or carriers in a gas must be produced artificially. Conducting liquids are electrolytes and these contain spontaneously generated ions. These liquids support only a small difference of potential and conduct by electrolysis. When electrolysis takes place gas usually accumulates a t the electrodes, thus causing an added resistance t o the flow of current. The separation of suspended solid and liquid particles from a non-conducting gas or liquid is therefore dependent upon the action of electric fields possessing a n intensity ranging between several hundred to several thousands of volts per centimeter. This action of an electric field is not as simple, however, as it might appear to be. The suspended particles may be caused t o combine with each other (aggregation) ; they may be in a state of polarization and they may become charged. In the case of suspended matter in gases the important item is that of charging the particles. The efficiency of the process .depends upon the thoroughness of this charging process. Like the other processes used for cleaning gases, the electrical process causes an expenditure of energy that is proportional to the volume of gas rather than proportional to the amount of matter suspended in the gas. No one has devised any cleaning process of the latter type although certain automatic devices promise to make the electrical process active only when there is a certain density of the suspended matter. The writer’ has determined the energy loss in the electrical precipitation method to be about 400 watts for treating gas at the rate of 1000 cubic feet per minute. This energy loss does not vary greatly between o o and zooo C . ; does not vary with the velocity of the gas, with the density of the suspended matter or with the composition of the gas. I n order to compare this efficiency with the “perfect” efficiency given by Stokes’ law i t would be necessary to determine the number of particles and the average amount of energy required to remove each particle from the gas by using Stokes’ formula. A very rough calculation of the efficiency of the electrical method may be made by assuming Broghk’s data for cigarette smoke. The radii of these smoke particles range between 0 . 1 to O . O O I ~and in each cubic centimeter there are several 1
Vol.
THE UTILIZATION OF SEWAGE’ B y GEORGE A. SOPER-)
It is often supposed t h a t the discharge of sewage into rivers and harbors represents a great economic waste and many persons, among them some eminent scientists, have, in times past, proposed that cities should conserve the manurial value of their sewage. Victor Hugo was a n ardent advocate of this form of conservation and Liebig, Hoffman and Crookes have raised their voices in solemn warning against the waste. Such warnings are well remembered and the question is frequently asked-Why should the sewage of our cities be discharged without any attempt being made t o utilize it and return the useful ingredients to the soil? To answer that it costs more to utilize the manurial ingredients than they are worth does not answer the question with sufficient fullness. The principal reason why it is i
