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SUMMARY A brief resum6 of previous vapor pressure determinations has been made. A detailed description has been given of the method employed in obtaining vapor pressure values for naphthalene, anthracene, phenanthrene, and anthraquinone, and tables
Vol. 14, No. 1
and curves of observed vapor pressures of these compounds have been recorded. Boiling point determinations have been made on anthracene, phenanthrene, and anthraquinone with the following results: Anthracene, 342" C.; phenanthrene, 340.2" C.; anthraquinone, 379.8" C.
Experiments with Heat Interchangers' By F. Russell Bichowsky DEPARTMENT OF CHEMISTRY, UNIVERSITY OF CALIFORNIA, BERKELEY, CALIFORNIA
In liquid air machines and other refrigerators which depend on the Joule-Thomson effect for cooling, the efficiency depends, for a given intake pressure, temperature, and rate of flow of the gas, on the character of the heat insulation, and especially on the efficiency of the heat interchange between the comparatively hot incoming and comparatively cold outgoing gas. In a perfect heat interchanger, the gas entering the interchan'ger and the gas leaving the interchanger will have the same temperature. Assuming the intake temperature of the gas (in this case 02) to be 2138" A,, and the intake pressure to be 200 atm., and assuming perfect heat interchange and perfect heat insulation, and taking for the value of the Joule-Thomson effect under these conditions -50°, for the molal heat capacity of the gas at 1 atm. G.7, and for the heat of vaporization per mole 1800 cal., it ran be shown that 11 per cent of the gas will be liquefieda2 If the outgoing ga8 leaves the interchanger at temperatures less than that of the incoming gas, a similar calculation will show that the yield of liquid 0 2 (and the same values hold approximately for liquid air and liquid Nz) will be decreased 20 per cent for each 10" difference of temperature between outgoing and incoming gas. Ordinarily, even for the comparatively poorly insulated, small-size liquefiers found in many laboratories, the loss of efficiency due to poor thermal insulation is small, compared with that due to poor interchange. The heat interchanger commonly used on liquid air machines consists of a coil of one or more copper tubes either wound one tube inside the other, as in the original Linde design, or, as is more common in laboratory interchangers, of the Hampton type, in the form of flat coils, the firzt coil being mound from the center out and the next from the outside in, either one tube being used or several tubes wound in parallel. In the Linde design the compressed gas, after expansion at the lower end of the coil, passes back the entire length of the coil through the annular space between the inncr and outer tubes. I n the Hampton type, the gas, after expansion, passes back over the tightly wound coil through the interstices between layers left during winding. Oneeighth inch 0. d. drawn soft copper tubing is about as small as is practical to use, and one-fourth inch 0. d. copper tubing is a convenient size. To study the efficiency of heat interchange in such small tubes between the walls and the rapidly moving high pressure gm, the apparatus shown in Fig. 1 was constructed. The wire AC was of tested No. 40 constantan which was carefully stretched so as to be exactly central in the tube. The small leads were No. 40 copper, the whole forming a differentid thermoelement giving the difference of temperature of the gas between A, B, and C. The initial temperature was given by a thermoelement not shown. The mean inside diameter of the tube was 0.124 in. (0.315 cm.), the outside 0.251 in. (0.638 1 Received
March 7, 1921. based on the equation 0 8 (g. 298-50' A,) =zOa(l, 90' A.) (1-x)Oz (9. 298" A.); 4H-0. Here z is per cent of gas liquefied and H is heat content. Details will be given by Lewis and Randall, "Treatise on Thermodynamics."
+
* This is
cm.). The length between junctions A-B and B-C was 1.0 ft. (30.5 cm.). Table I gives typical results of many tests. I n this table, temperatures are given in degrees Centigrade absolute, pressures in atmospheres, rate of flow in cu. m. per min. free gas. The figures in parenthesis in Columns 4 and 7 are corresponding values in cu. ft. per min. and O F. per ft. per deg. TABLEI Intake Temp. TEmp. of Bath Pressure Rate of Flow Tem A. OA. Atm. Cu.M./Min. A-C?
290 240 290 240 290 290 240 290
90 90 240 90 240 90 90 240
200 200 200 100 100 200 200 200
0.93(33) 0.93(33) 0.93(33) 0 93(33 0:93 331 0.47{16.5) 0.47(16.5) 0.47(16.5)
33 25 8 24 8 68
42 15
Diff Temp Modulus A-E Deg.jM.lDeg. 17 0r26 (0.085)
11 4 12
4 35 20 8
0 42 0 13) 0 : 5210: 16)
The first six columns are self-explanatory; the last column gives the temperature modulus, i. e., the fall of temperature per nieter of tube per Centigrade degree difference of temperature between the temperature of the bath and the temperature of the incoming gas, found by dividing the values in Column G by the temperature head, i. e., by the temperature of the bath minus the intake temperature. These results, which are only approximate, show that the temperature drop per meter of tube is independent of the pressure, inversely proportional to the rate of flow, and proportional to the temperature head from bath to incoming gas. Knowing the modulus as defined above for a given sized tubing and given rate of flow, it is possible to calculate the length of that tubing which will be necessary to construct an ideal interchanger of given thermal efficiency. =temperature of incoming gas =temperature of gas just before expansion = tempeEature after expansion (for liquid air machines this will be T8 temperature of liquid air) T4 =temperature of gas leaving exchanger Cgi =molal heat caparity of incorning gas, under pressure Pi at any point along interchanger c g 0 =molal heat capacity of outgoing gas, under pressure Po at same point along interchanger ti and to = temperatures of incoming and outgoing gas at same point Mi =mass of gas, in moles, entering interchanger in any interval of time Mo =mass of gas leaving interchanger in same interval Then for a liquid air machine Mi-Mo = A , the mass of air liquefied in that @interval;or, choosing the interval so that Mi = 1, and assuming that the liquefier has been running long enough so that conditions do not change with the time, I - M o equals the efficiency of the liquid air machine. Now Cgi and Coo will in general be functions of the temperature, i. e., cgi = fgi (T) and Cg, fg, (T). The function will depend on kind of gas and pressure. This may be determined by experiment. Let 1 be the length of the interchanger, and let p be the temperature modulus as defined above, i. e., the drop of temperature per unit length per degree temperature head. Now the heat transferred from the incoming to outgoing gas for any small segment of the interchanger is Let Ti
Tz
-
(ti
-10)Pdl
where dl is length of segment. This equals MCPo dt and 4- CPi dt, where d t is change of temperature of the gas in length dl. Integrating gives:
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THE JOURNAL OF INDUSTRIAL AND ENGINEERING CHEMISTRY
If for any gas Cp is known as B function of T and P, these equations can be solved by usual methods. For D first approximation the value of Cg at the geometrical mean temperature may be assumed. Treating C@*I constant, and integrating. the equation bemmea
For the p-ent
erne e-0.26, CP., for air &tone atmo3phere and 160- K. (the meso temperature of the outgoing gas) equals 7.1; for air at 200 atm. (the most convenient pressure) and 240- K. (the mean temperature of the incoming gw) Cp, = 8.5.' Assuming 60 per cent theoretical yield of liquid i O0.40air, i . ~ . , l i q u i d a i r e 5 ~ ~ n c g o f 7 p e r c u l t . M = 9 3 a n d h - - T ~ = f X 20' c. using these values. 1 = 180 m. (550 it.). If SO per cent theoretical yield is assumed, 1-250 m. (770 ft.).
These figures accord with our experience with interchangers of the best design. Ordinarily the interehange from the low pressure return air to the tubes is by no means as efficient that assumed here, and it is therefore not safe, unless special precautions are taken, to reduce the length of an interchanger of 7 per cent capacity to less than, say, 250 meters. It was not convenient to repeat theexperiments above for different sizes of tubes, since the results for larger tubing would have no practical value to ns (larger size tubing being unduly difficult to coil), and since it is doubtful that our type of apparatus would give results ofmuchvalue for smaller sizes of tubing. It is thought, however, that for a given flow in free feet per minute the rate of interchange woudd be indo pendent of the diameter, a t least for sizes of tubing not too greatly different from one-eighth in. i. d. This approximation is based on the assumption that for constant mass velocity the rate of interchange is proportional to circumference. inverselv Droportio& to dimiter, and directly proportional to the linear velocity of gas flowing down the tube. The problem of heat interchange between the ION pressure cold gas and the copper tubmg carrying the high presqure gas is not so well adapted to simple experimental inyestigation as the problem of heat interchange between the copper tubing and the compressed gas. I n the last case the flow of gas is approximately of stream type. I n the flrst case we have conditions which vary from nesrly stream flow itl the Linde
-
&s
I
Holborn and Jakob, 2. Va. d&. IIC. 68 (1914). 1429.
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type interchanger to extremely turbulent flow in the Hampton type. It is obvious that if the flow is turbulent enough so that the temperature at every point in any cross section normal -to the axis of flow is the same, no matter what is the shape or length of ttie coil of the interchanger, provided its beat conductivity longitudinally is negligible, the same length of tubing d l give the same efficiency. This makes the problem of effective interchange of heat. from tube to outflowing gas entirely a problem of pto. 2 care in construction, particularly care in preventing the format.ion of stream channeis causing local differences of temperature in a given cross section, and in so winding the coils that the outflowing air always has a resultant of velocity opposite to the velocity of the compressed gas. The necessity for effective heat insulation of the whole interchanger is obvious, but deans of providing it are familiar and need not be discussed. THENELSONINTERCHANQER The chief objections to the type of interchanger described above are the length of copper tubing required, making the entire apparatus bulky, and increasing the dficulty of heat insulation, and, more important, the skill and time required to wind the %t coils. Such interchangere are therefore necessarily expensive. To obviate these difioulties Mr. GeorgeNelson suggested on-June 11,1919, the use of short, twisted, flattened tubes in place of long ordmary tubes of theregular interchanger. The advantage of the flat tube over the round is that while the surface area of the tube is constant, tho mean distance of heat:flow is very much decreased, so that the efficiency of an interchanger, which for conditions of constant flow in grams per minute is proportional to area and roughly inversely proportional, to the distance between walls, is greatly in excess for a flat tube than for a round tube of same area. The reason for twisting the tubes, as shown in A or B, Fig. 2, is to allow them to stand the high presMe. a sure without b;giig.
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I n the first experirnents (Xovember 8, 1919) there were used 15 in. of twisted tube, made by flattening down completely a section of three-sixteenth in. 0. d. copper tube and twisting it uniformly on a lathe. In place of the elaborate expansion valve customary in liquefiers, various sorts of valves and orifices, the details of which are not important here, were used. Thermocouples were soldered on a t the top, middle, and bottom of the twisted tuhe, in order to gain some idea of the thermal distribution. This tube was placed in a loosely fitting test tube insulated with cotton wool. The flow of gas used was about 0.2 cu. m. (6 cu. ft.) per niin. The pressure was 220 atm. After thermal equilibrium had been reached, the temperatures of the tube at the top, middle, and bottom were, respectively, +20", -5", and '-30" C., about what would be expected if there were no interchange. It was thought that this unfavorable result might be due to the fact that the flow outside the tube was not turbulent. Therefore the experiment was repeated, packing the space between the twisted tube and the test tube with coarse copper and aluminium powder. The temperature dropped very rapidly, and within 15 min. liquid air was observed in the bottom of the test tube, the temperature then being at the bottom - B O " , a t the middle -80", and at the top $20" C. However, as the temperature of the outgoing gas was - 10") the eIEciency of this interchanger was only 40 per cent of theory. I n other
Vol. 14, No. 1
experiments the aluminium powder was replaced by winding a spring of copper wire, as shown in Fig. 2-C. The efficiency for a 46-em. (18411.)interchanger at 0.2 cu. m. (6 cu. ft.) per hr. was 60 per cent. Several twisted tubes in parallel were also tried, and finally Mr. Kelson constructed a complete liquid air machine (Fig. 3), using 2 m. (6 ft.) of twisted tube in the interchanger. This machine had a capacity of 3.51. per hr. liquid air, using an intake pressure of 250, an intake temperature of OD, and a free flow of air of 0.72 cu. m. (24 cu. ft.) per min., or an efficiency of about 70 per cent. Such liquefiers would seem to have a great use for laboratory purposes. Their cost is but a fraction of the usual liquefier, their size is small (the height of the can in Fig. 3 is 12 in.), and as they use only 2 m. (6 ft.) of copper tubing for the interchanger, the time necessary to run the machine before it begins to form liquid air is much reduced, i. e., 4 min. as compared with 30 min. or so for apparatus of the old type. Interchangers using the twisted tube were also employed in a successful small-scale liquid hydrogen machine. The first designs of this machine (by Rodebush and Latimer and by the author) have, however, been superseded by a very ingenious design of Dr. Latimer's, which uses also the same twisted tube interchanger, and which will shortly be described by him.
CHtANDLER M E b A L AWARD' Introductory Remarks
Chemistry is a basal science underlying the practice of so many human activities that a large proportion of those who By George B. Pegram start with a chemical training must ultimately add to their COLUMBIA UNIVERSITY,NEW YORK,N. Y. equipment other kinds of expert knowledge before qualifying It is our pleasure t o be assembled this evening t o hear as for their life's work. It is a pity that so few up to now have the Chandler Lecturer for the present year a gentleman who is chosen biological qualifications. Hitherto, the primary trainwidely known among chemists as a pioneer in the very rema-kable ing of most of those who have investigated biochemical problems developments of our knowledge that have been brought about has been biological or medical. Such workers have done very through the study of food accessories such as vitamines. well, but as knowledge progresses it becomes more and more I have the honor of introducing Professor Frederick Gowland netessary that at least some of the work should be done by those Hopkins, of Cambridge University, England. whose chemical knowledge is primary and not secondary. But I have referred to certain disadvantages suffered by biochemistry and you will think of one of them. It has hitherto been difficult to point clearly to a professional (as distinct from Medal Address an academic) career for the young man who thinks of devoting Newer Aspects of the Nutrition Problem himself to the subject. Only in connection with medicine has it hitherto offered professional opportunities, and medical By F. Gowland Hopkins PROFESSOR OR BIOLOGICAL CHEMISTRY, UNIVERSITY OA CAMBRIDOE, CAM- qualification is often first demanded of its votaries. This BRIDGE,ENGLAND state of affairs is rapidly altering. Medical practice can and will NUTRITIONAL STUDIES AS A BRANCHOF APPLIEDCHEMISTRY in the future be helped by workers whose training has comprised something less than a complete medical course. Biochemical The study of nutrition is most productive when it is followed knowledge, moreover, is being sought in many unexpected as a branch of applied organic chemistry. As such it doubtless quarters. The scientific representatives of a firm that manusuffers certain disadvantages. It calls for workers fully ac- factures explosives on a great scale asked me some time ago quainted with the technic of the chemical laboratory and pos- to supply them with a biochemist. At first.it seemed difficult sessed of all that is special in the chemist's mental equipment to know why; but the explanation was simple enough. There and mode of thought. Yet it calls for the application of these is, or was, some anxiety about the supply of glycerol. Fats possessions in a region which is perhaps more remote from the which used t o be hydrolyzed are now being used intact in all chemist's experiences during his training than are any other of the sorts of fresh ways, and there is less glycerol as a by-product. many regions in which his science is applied. The successful Hence a desire to develop the methods by which it is produced by pursuit of biochemistry, of which science nutritional studies microorganisms, and the biochemist gets a n opportunity. This form a part, calls for a second discipline. The young chemist is but one illustration. I can say with certainty that, in Great having received his primary training must be content to become Britain a t any rate, there is a demand for professional biochemnext something of a biologist; he must know enough about ists which is greatly in excess of the present supply. This I animals and plants to appraise the problems which their or- find satisfactory, for if a profession opens up we shall find it ganization presents; he must acquire a biological outlook. easier to obtain workers who during one period at least of their 1 Presented at Columbia University, New York City, April 18, 1921. career will help advance the science itself.
