INDUSTRIAL AND ENGINEERING CHEMISTRY
786
-
VOL. 31, NO. 6
Accuracy
Error in H ,
The ohief source of errors in this method lies in the determination of the Duhring line slopes. The order of magnitude of A H for steam is 1000 B. t. u. From Equation 3 a n error of one thousandth in D will introduce a n error of about 1B. t. u. in AH’. This error will be numerically transferred to B1.The resultant error in H , the relative enthalpy, is a function of the concentration at which the error occurs. From Equation 5, for an average error of E B. t. u. in AH’ betaween concentrations W4and W,, the error in H is:
w4
Wh
0.1 0.2 0.3
0.2 0.3
0.001 error
0.4 0.5
0.4
in D
1.0 0.50
0.33 0.25
B. t. u.
0.01 error in D 10 5.0
3.3 2.5
These errors become cumulative as Equation 5 is integrated from low to high concentration. This, combined with t h e fact that vapor pressure data are likely t o be less reliable at high than at low concentrations, will tend t o give larger errors at higher concentrations.
Literature Cited
or E (I
- Wb/wa) Be t. U.
The following table shows the order of magnitude of errors in H due to errors of one thousandth and one hundredth in D over a 10 per cent concentration range:
(1) Bertetti, J. W., and McCabe, W. L., IND. ENG.CHEM.,28, 242 (1936). (2) Ibid., 28, 375 (1936). (3) Brown, G.G.,J. Franklin Inst., 219, 406 (1935). (4) Fricke, R., in Landolt-Bornstein physikalisch-chemische Tabellen, Berlin, Julius Springer, 1931, Eg. I1 b, p. 1332; Hsyward and Perman, Ibid., p. 1333. (5) Keenan, J. H.,“Steam Tables”, New York, John Wiley & Sons, 1937. (9) McCabe, mi. L., Truns. Am. Inst. Chem Engrs., 31,129 (1934).
CORRESPONDENCE Study of Liquid Flow SIR: I n The Du Pont Magazine [33,No. 3, 3 et seq. (1939)] we find an article by W. T. Collins “Research Employs Plastics.” This publication is primarily devoted to a discussion of the outstanding advantages to be gained by the use of transparent plastics-for example, “Pyra1in”-in the construction of models for hydraulic research work. The possibility of being able to follow visually the flow of a liquid in systems of involved construction should be of great advantage to the engineer engaged in studies of liquid flow. Models such as those described by Collins will permit the detection of extreme cases of turbulence, usually responsible for erosion, cavitation, etc. Frequently the addition of dyes, insoluble in the liquid, will assist in detecting turbulent zones of flow in the system. However, these methods are not sufficiently accurate to permit a study of the type of flow encountered or the detection of turbulence in cases of low rates of flow. These factors, moreover, are not only of importance to the civil engineer, but equally so to the chemical engineer in the construction of stills, rectifiers, piping in general, columns, etc., since maximum efficiency will largely depend on flow characteristics of the liquid. We have found that the phenomenon of birefringence, or stream double refraction of colloidal dispersions consisting of anisometric particles, can be advantageously applied in such studies. It is known that anisometric particles will always tend t o orient with one axis parallel t o their direction of flow, and that
such orientation becomes readily detectable if we place the container through which the liquid flows between crossed Polaroid films or plates and illuminate it with a strong source of diffuse light. Highly diluted water dispersions of natural bentonite have proved especially suited for such work. Other colloidal sols exhibiting stream double refraction-i. e., vanadium pentoxide, ferric oxide, soap solutions, etc.-have also been tested. However, they have certain disadvantages which make their use less attractive. The most important are color, change of the surface tension of the liquid, difficulty of production, and cost. A well prepared one per cent dispersion of bentonite in water obtained by fractionating bentonite in a supercentrifuge and selecting fractions with particles below 50 mp will be practically clear to the eye, have a viscosity and a surface tension close to that of water, and exhibit pronounced birefringence even at extremely low rates of flow for temperatures up to the boiling point of water. A detailed study of liquid flow under different conditions is in progress and will be published later. E. A. HAWSER AND D. R. DEWEY,~ W D MASBACHUSETTS INSTIlUTE
CAMBRIDQE, MABEL Maroh 29, 1939
OF TECENOLOQY
