Aug., 1916
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NOTES AND CORRESPONDENCE ON EFFICIENCY OF AIR DRYERS Editor of the Journal of Industrial and Engineering Chemistry: I have read with considerable interest the letter of Prof. W. K. Lewis on the Efficiency of Air Dryers,l wherein he refers t o a discussion of this subject in the columns of the Journal of Metallurgical and Chemical Engineering in the years I go9 and 1910. Inasmuch as I was guilty of starting some of the discussion referred to, in the issue of April and May, 1910, I beg permission to say a few words further on the subject. The discussion arose from a published report2 of a test of a drying machine wherein an “efficiency” of 85 per cent was claimed by the late Mr. W. B. Ruggles I criticized the use of the word “efficiency” in the sense in which it was employed, and I take this opportunity to say that my criticism was directed solely to what I regarded as a misleading use of a scientific term, and that I very clearly recognize the valuable services of the late Mr. Ruggles to the practical art of drying. Mr. Lewis finally arrives a t the conclusion that it is desirable to express the results attained by drying machines in terms of “performance” rather than “efficiency,” “Performance” as defined by Mr. Lewis being the actual heat consumption in terms of B. t. u. per lb. of water evaporated. With this I heartily concur and it really differs in no way from what I contended was the correct definition of “efficiency” in 1910. Such a statement of performance is exact and definite, and precisely expresses what a drying machine accomplishes towards the purpose for which it is designed, ziz., the evaporation of water by expenditure of heat. Further, the form of statement as proposed by Mr. Lewis makes it possible directly to compare the operation of two or more machines of different types. The “performance” as defined by Mr. 1,ewis is indeed a direct statement of energy expended and useful work performed thereby. The difficulty pointed out by Mr. Lewis OF stating the “efficiency” of multiple-effect evaporating apparatus is a real one and we may as well boldly face the fact that such apparatus (multiple-effect) really has an efficiency as heat using machinery of greatly over 100per cent actually, and its theoretical “efficiency,” as Mr. Lewis shows, is infinity. This is really only another way of stating the doctrine of conservation of energy. Ordinarily our utilization of energy is confined to one transformation from kinetic to potential or vice versa. Multipleeffect apparatus is one instance (practically almost a singular instance) where we are able to repeat the change cyclically. Hence a definite amount of heat can be made to eyaporate an indefinite amount of water. Of course in practice the physical limits are somewhat sharply drawn a t five or six cycles, but theoretically there is no limit to the quantity of water that can be vaporized by a given amount of heat. Such, however, is not the case with “air dryers,” which is the titular heading of Mr. Lewis’ letter and which was the subject matter of the discussion of several years since, referred to by him. The consumption of heat in an “air dryer” for evaporative purposes is a perfectly definite thing and just as definite as in the case of a steam boiler. We have to deal only with one possible transformation of heat from sensible to latent form and the work that can thereby he performed (as evaporation) is perfectly definite. It is perhaps well to define an “air dryer,” and by the term I understand a machine utilizing artificial heat to evaporate water from some solid substance, wherein air a t substantially atmospheric pressure is usually the medium by which heat is THISJOURNAL, 8 (1916), 570. J Met and Chem. Eng , March, 1910.
conveyed to the material to be dried and aZways the medium by which the vapor produced is removed from the material or machine. I believe this definition covers all practical types of atmospheric drying apparatus. It will be noted that it does not cover vacuum drying apparatus; but in all practical vacuum drying apparatus the evaporative possibilities of a given amount of heat are also definite and limited. The theoretical possibility of indefinitely repeated utiliiation shown by Mr. Lewis applies only to multiple-effect machines. As far as I am aware multiple effect has not been applied to drying, but only to the evaporation of solutions. We see, therefore, that either in “air dryers” or in vacuum dryers (single-effect) the quantity of water that can be evaporated by a given amount of heat is quite definite-not only practically but as a theoretical matter, and hence it is quite possible to state the performance of all practical dryers as a percentage “efficiency.” I t requires practically 1 1 2 0 B. t. u. to evaporate I lb. of water from average normal temperature (I speak of course in round numbers) and any air-drying machine or vacuum dryer that evaporated I lb. of water for an input of I I Z O B. t. u. would have an efficiency of IOO per cent. In practice there are various losses, due to radiation, to heating the dried material to the discharge temperature and to heating the outgoing air to the discharge temperature-this last being usually the greatest. All the losses are unavoidable; all can be greater or less according t o the type and perfection of the machine and on their balance depends the efficiency of the machine. My contention in the discussion of several years since was that any drying machine should be charged with the total heat supplied, whether in the form of steam or from direct combustion of fuel, and credited solely with the heat usefully expended in evaporation of water. This view is, I believe, entirely in accordance with Mr. Lewis. The second example given by Mr. Lewis to demonstrate that there is no definite efficiency in air drying, appears to me quite irrelevant. The case is given of spreading material under a shed and allowing air to blow over it. Manifestly there is in this case no expenditure of artificial energy and it can have no relation to efficiency of fuel because fuel is in no way concerned. Nevertheless energy is expended and quite definitely; and the only reason we are not able to determine the efficiency of such a system is the impossibility of determining the factors. Could we determine with any accuracy the amount of air coming in contact with the material under such circumstances and the amount of (low-grade) heat brought into the system, also the drop in temperature and the increase in saturation (both of which would be very small), we should have in hand all the data necessary to determine the efficiency of the system. Mr. Lewis’ letter really treats of much more than the efficiency of air dryers and goes into the field of evaporation in general. TT’hile this is of equal interest, i t appears to me that the two things are quite distinct, although they do indeed involve the common factor of evaporation of water Nevertheless the other factors are so different that they had best be treated independently. Mr Lewis has himself clearly shown one reason for separate treatment, in pointing out that a theoretically perfect multipleeffect evaporator has an unlimited efficiency. I do not think this has been scientifically shown before, but we see in trade customs a dim realiiation that the standards applicable to drying machines, or in general single-effect machines, are not equally applicable to multiple machines. It is quite a common custom for manufacturers of drying machines to claim for their
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machines a certain “efficiency,” as a percentage. As I remember, this is never done with multiple-effect evaporators, but instead a certain evaporation per pound of steam supplied is specified. This it will be seen is practically identical with the “performance” proposed by Mr. Lewis. As regards the last portion of Mr. Lewis’ letter, treating of the evaporation of solutions, I frankly confess that I utterly fail to understand. He assumes a solution boiling under a definite pressure, p , which is less than the pressure, fro, of the pure solvent a t the same temperature. The vapor from the boiling solution is to be compressed isothermally and “can be introduced into the coils of the same effect from which the vapor itself came and there condensed a t a temperature differentially higher (italics are mine) than that of the solution on the other side of the heating surface. The heat of condensation will thus be available for the evaporation of more water from the original solution.” Now the temperature of the vapor is that of the pure solvent boiling a t the assumed pressure and is lower than that of the boiling solution, owing to the lowering of the vapor tension by the substance in solution. This vapor is compressed isothermally, that is, i t remains a t the same temperature, but it is required to give up its “heat of condensation” ( ? latent heat) to a solution a t a temperature higher than its own. Unless I have misread this passage, it looks like a fiagrant violation of the second law of thermodynamics, and indeed involves the direct transfer of heat from a !ower temperature to a higher. I hasten to say this is not criticism and that my failure to understand the mechanism of the described process is undoubtedly due to my personal weakness in the science of thermodynamics. Vir, E. WADMAN BAYONNB, KEW JERSEY June 24, 1916
ACTIVE AND LATENT SOIL ACIDITY vs. IMMEDIATE AND CONTINUED LIME REQUIREMENT Editor of the Journal of Industrial and Engineering Chemistry: Recently there appeared an article in THISJ o v ~ s a L 8, (1916), 572, entitled “Immediate and Continued Lime Requirement vs. Active and Latent Soil Acidity,” by W. H. MacIntire, of the University of Tennessee. I n this article the inference is made 8 (1916), that the present writer in a report in THIS JOITRNAL, 341, has not given sufficient credit or prominence to MacIntire’s recognition of his so-called “Immediate and Continued 7 (1915), 864. Lime Requirement” as given in THISJOURXAL, I n regard to the statements that are made and the questions that are raised the writer deems it appropriate to state as follows: Although possibly unknown to MacIntire, the writer published an article in Science, 42 ( I g I j ) , jog, in which the distinction is made between active and inactive soil acidity. This article appeared in the issue of October 8, 191j , and MacIntire’s article, in which he gave the expressions immediate and continued lime requirement, appeared also in the October, 1915, number of THISJOURNAL. I n the Science article, the writer used the term inactioe, for which was later substituted the term latent, since that seemed more desirable. It is well to note further that Veitch, in an excellent discussion of the nature of soil acidity in the A m . Chem. Jour., 26 (1go4), 659, used the terms active or actuel acidity and inactive or negative acidity to designate different kinds of soil acidity. Veitch states that the total active and inactive acidity is estimated by his lime-water method. This distinction of Veitch is thus plainly a different one than that made by the writer in 8 (1916), 341, since the lime-water method does THIS JOURNAL, not estimate even all the acidity that is designated by the
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writer as acthe acidity, to say nothing of the acidity designated as latent acidity. Possibly other references could be given in which a recognition has been made of differences in the activity of t h e acids causing soil acidity. One of the most significant facts in regard to the various methods proposed for determining soil acidity is that the results by any one method are seldom the same as by any of the others. These differences have been recognized and attributed by some to differences in the nature and activity of the acids causing soil acidity. The writer used the term active acidity and recognized differences in actizlity of soil acids in Wis. Sta. Bull. 249, p. 15. This bulletin appeared eight months beCore MacIntire’s publication in which was used the expression inzmediate lime requirement. Perhaps the expressions chosen by MacIntire are superior to those chosen by the writer. If such is the case the criticism may be made that the writer should have changed his expressions in the subsequent publications after IllacIntire’s article appeared. The writer is as yet not convinced of this superiority, and even if this superiority is granted other difficulties arise as follows: Determinations made lately indicate that MacIntire’s method for his so-called imnaediate lime requirement does not indicate the same amount of acidity as the method given by the writer for active acidity. The methods thus clearly do not indicate the same point in the complete neutralization of the soil acids. The writer 4-ould thus have been open to serious criticism had he stated, subsequent to the publication in Science, that the method proposed measures the so-called inzmediute lime requirement as defined by MacIntire. In fact, the writer wishes to state definitely that the method proposed does not measure this immediate lime requirement as apparently defined by MacIntire. Whatever practical or scientific meaning MacIntire may wish to convey by the expression, immediate lime requiremenf, it may be well to state clearly that the method (in which is used a solution of Ba(OH)2)proposed by the writer for active acidity does not, and never was intended to indicate, in any way, the amount of lime that should be used under practical field conditions. I t simply indicates the amount of acid substances present which are sufficiently active to react quickly with bases like Ba(OH)2or Ca(OH)*. The expression, amount of a c h e acidity, has no connection or relation to the expression, lime requirement, in the sense in which the latter expression has been used in the past. The writer confesses that he does not clearly understand what MacIntire means by continued lime requirement. I t hasnot been possible to find a description of a laboratory method for the determination of this so-called continued lime requirement and hence had the writer appropriated the expression continued lime requirement in place of latent acidity, or stated that the two expressions mean the same thing, he would again have been open to serious criticism. Further than mentioning that MacIntire had described a method in which a solution of CaC03 in carbonated water is used, the writer purposely avoided either favorable or unfa7rorable comments. Since MacIntire did not present data, the relation of immediate lime requirement to active soil acidity could not be intelligently discussed at that time. The field was left clear, and as far as the present writer is concerned, MacIntire should have little trouble in getting full recognition for the merits of his work. MacIntire states that he is unable to find in the writer’s 8 (1916), 341, any data which indicate article in THISJOURNAL, that soil acidity is due to true acids and not selective ion adsorption by colloids. The writer believes that this is entirely possible. For further explanations the reader is referred to an article by the writer entitled “The Cause and Nature of Soil
