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near future, for comparison purposes, because the equipment required for their determination is generally available in chemical laboratories. It may well be that other properties, such as viscosity, surface tension, thermal conductivity, heat capacity, latent heats, dielectric characteristics, absorption spectra, and x-ray diffract,ionpatterns (S), are more distinctive and sensitive than some of these listed, and the time may come when the chemist will be in a position to employ them more generally than at present.
Vol. 22, No. 9
Literature Cited (1) Cottrell, J . A m . Chem. Soc., 41,721 (1919). (2) Shepherd, Bur. Standards J . Research, 2 , 1169 (1929). (3) Stewart, Chem. Rev., 6, 500 (1929). (4) Swietoslawski, Bull. acad. pol. sci. l e K , [AI,434 (1929). ( 5 ) Washburn, Z . physik. Chem., 130, 692 (1927). (6) Washburn, Bur. Slandards J . Research, 2 , 476 (1929); gives method of distilling without decomposition.
(7) Washburn and Read, J . A m . Chem. Soc., 41,799 (1919). (8) White, J . Phys. Chem., 24, 393 (1920).
Trends in Heat Transfer’ D. F. Othmer EASTMAN KODAKCOMPANY, ROCHESTER, N. Y.
I
N T H E innumerable places where heat is generated, applied, or dissipated in various ways in almost every
type of industry it must be passed from some relatively hot body to one a t a lower temperature. This process is called “heat transfer” and its aspects are among the most familiar of physical phenomena. It is not the purpose of this article to consider the physical laws governing the molecular, electronic, or wave motions involved in the theory, nor yet to consider the mechanics, often empirical, involved in the practical utilization of such abstruse discussions. Leaving such scientific aspects, however, there have been numerous examples of interesting applications of the principles of heat-transfer technology in recent years in that wide field involving the effects of heat, from the circulation of a warming fluid in the body or building to the cooling coils intricately blown of glass in some high-powered thermionic valves for radio transmitters. Boilers and Power Generation
The commercial utilization of heat received from the sun and stored in coal and oil has required the development of the modern boiler. Inefficient as the production of power from such combustion is, it converts a tremendously larger amount of the heat received than does that other solar-heat engine: the evaporation of surface water, condensation as rain and utilization of water power developed as the streams run down hill to the sea. There are recent experimental successes with a more direct type of solar-heat energy utilization from the thermal standpoint-i. e., the reflection by huge parabolic mirrors of the heat of the sun’s rays (7 B. t. u. per minute per square foot) on water tubes at the mirror’s focus. These boilers transform solar heat into power without waiting for the passage of geological eons as with coal, nor earthly seasons or lunar months as with hydroelectric or tidal energy transformation. Another of the heat engines in which the actual thermal units are as free as the air, but not quite so immediately valuable, is the proposed Claude power plant utilizing the difference in the temperature of surface and deep sea water in tropical seas. A continuous stream of warm surface water is drawn into a closed chamber maintained under high vacuum. Part of the water flashes into steam at this pressure, and the very low pressure steam is drawn through the blading of special turbines and thence through condensers cooled by water drawn up from the bottom of the sea.. The effective head on these water pumps supplying condensing water is very low, even though the water is lifted for miles, because the hydrostatic pressure nearly balances the water inside the suction pipe. The power required for water 1
Received May 21, 1930.
and vacuum pumps is subtracted to give the net amount available. Much interest is evidenced in this system, not alone because of the previous achievements of the proposer, but also because of the limitless power made available to a system which can use it between such narrow limits of high and low temperatures. Turning from equatorial islands to Arctic steppes, another Frenchman, Barjot, desires to take the cold of Canadian nights and make it turn the wheels of industry or fire the arc of an electric furnace. Insulated from the rigor of Arctic winter by the surface ice pack, vast quantities of fresh water in northern rivers never fall below 0” to 4” C. The average atmospheric temperature during months of the cold winters may be approximately -20” C., and various types of heat engines may be devised to operate between these two temperatures. A very ingenious one proposed by Doctor Barjot uses a material which is gaseous a t ordinary temperatures and boils a t approximately 0” C. Water pumped from the river bottom is intimately mixed with a substance such as liquid butane, the water gives up its latent heat of fusion to freeze and the butane absorbs it as heat of evaporation to boil at a pressure just below atmospheric. Again a special turbine wheel is turned and the vapors pass to a vacuum condenser space filled with blocks of cryohydrate, the ice-salt solid formed by freezing saturated brine. Through the use of a working substance mutually insoluble with water or brine, and therefore readily separated by gravity decantation, heat is transferred from ice water to the low-boiling liquid in the boiler by direct contact, and from vapors to blocks of cryohydrate in the condenser. This minimizes the loss of available temperature range caused by the use of heat interchanging surface. If the difference of temperature between subsurface water and cryohydrate in this paradoxical use of ice water to supply rather than remove heat were equal to the difference of temperature available to a Claude system, thermodynamics shows that the Arctic zone process would supply more power in a perfect engine than that of the torrid zone. Even the terrestrial heat stored under the thick crust of the earth has been utilized in this age of power. I n various cold countries in which hot springs are located it has been the practice to circulate such hot water to heat adjacent homes. I n Italy the mixture of vapor and spray formed by the flashing of superheated geyser water is separated and the steam used for power and heating. So-called “volcanic boilers,” in which water is circulated and heated in pipes suitably placed where the earth’s internal heat may be tapped and used near the surface, have been successfully experimented with. None of these newer methods are worrying producers of power from coal or waterfalls because of the small amount of
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INDUSTRIAL AND ENGINEERING CHE.MISTRY
power demanded in those parts of the world where i t has been
so obtained, and mainly because of the high investment cost required for equipment to obtain a "free B. t. n." The more orthodox methods are, nevertheless, producing more and more power from heat of combustion. Higher and higher steam pressures are continually being used with the power technologist transferring the heat of flue gases to tubes of increasingly higber temperatures. The strength and life of the tube and other materials of boiler design have been the limiting factors, but with increased strides by the steel maker and fabricator the pressures advance each year. There seems to be no reason for steam power to pass the laurel of progress to internal-combustion engines in the light of recent advances. We have successful tests finished on power generation a t the critical temperature of water and equipment available for its commercial realisation, huge plants in qmation just under 2000 pounds pressure, other boilers producing almost a million pounds of steam an hour per unit, single furnaces liberating the heat of 40 tons of coal in the same length of time, one steam turbine doing the work of a quarter million horses, and iron horses of the rails whistling with steam having over a hundred times the absolute pressure inside the tubes as is in the flue gases. In various industrial plants unusual methods of obtaining and transferring heat for steam generation have developed, two of which will be mentioned here. Some chemical plants using large bloeks of hydroelectric power have found it economical to prodnce steam electrically and have used either resistance heaters or the electrical resistance of the water itself. I n this country of abundant fuel such a system would be used only because of some special convenience to be d e rived, and not for power generation on the grand scale, aC though the efficiency of such conversion of electrical to thermal energy may approach 100 per cent. With operators of coke ovens the practice of cooliiig and prevenbing combustion of coke coming from the ovens by means of a water bath is being eliminated in favor of dry quenching to conserve the available heat and produce a better product. Uninflammable gas circulated through a bed of such discharged coke and t h m through a special boiler cools the coke and produces 1000 pounds of l40-pound steam per ton of coke at the plant of the Rochester Gas and Electric Company. Not only is this economy desirable, but a coke haviug better physical properties is obtained. Tbermodynamic considerations have shown that in power generation the difference between tho boiler and condenser temperatures-the numerator of the familiar expression for efficiency (T,- T2)/Ti-had best not be undertaken in a jump with a single fluid, however high the upper temperature be made. Binary liquid systems, one always being water, with mercury, diphenyl, and various low-boiling organic solvents as the complement, have been suggested, tried, and put in operation. The outstanding success in this field in this country has been in operation for several years by the Hartford Electric Light Compsny and in it heat is transferred from flue gases a t around 1200" F. to mercury boiling a t 70 pounds pressure and about 900" F. The boiling temperature of the mercury is almost 200' F. above the critical temperature of water a t which, with water, a heat-transfer surface would have to be designed to withstand a pressure of over a ton and s half per square inch. The peculiar physieat properties of mercury, its cost, and its poisonous nature have all contributed to the design of an unusual type of boiler, not the most ordinary feature of which is the large use of welded joints in its fabrication. After passage through special turbines, the mercury vapor is condensed to boil water and produce steam, which in its turn is also expanded through turbine van-. The power-generation field has not yet been invaded by
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diphenyl (an organic solid a t ordinary temperatures, formed by joining two molecules of benzene together), although a periscope would probably show such usage to be just around the corner. The only reported commercial uses have been in such heat-transfer equipment as requires the advantages of steam beat a t much higher temperatures than can be obtained by steam without excessive pressures.
Courlcsy slvcnson I3uaporiilar compnny
Figure I-Rorced
Circvlatfon Evaperator for Cauatfc Soda Steam pressure 80 pounds, caustic conceotration 70 per cent
h notable installation in the oil industry uses the lieat supplied by dipbenyl vapors to distil lubricating oils. In the design of boilers for this light straw-colored solid-liquid-vapor, the natural convectioii currents supplying liquid movement through the tubes of ordinary water tube-type boilers are insufficient and recourse is taken in forced circulation. The heated liquid is pnmped through vertical tubes surrounded by flue gas. Although the liquid absorbs heat in ascending the tube, boiling is prevented by the pressure due to the high velocity and the hydrostatic head. The pressure decreases rapidly as the liquid rises in the tube, and a t 8 point just below the top the superheated liquid flashes into a vaporous spray dashing against a parabolic deflector. The liquid is returned to the suction of the pump, the vapor passes to the high side of the binarv fluid Dawer svstem or. as in this case. to t h c ~ heating tub&. This Droccss at the Indian Refinina ComDanv uses the vapors outside another system of tuges th;ough which a lubricating oil stock is circulated and dist,illed under vacuum in a manner similar to the action with diphenyl in the first boiler. Thus, two forced circulation heat-transferring machines are used in series: the heat of the flue gases is dissipated to the diphenyl which, in boiling, produces vapors in their turn condensing to pass heat to boil the oil, and the vapors from this distillation are condensed to produce an
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Figure 2-Foster
Vol. 22, No. 9
Wheeler Flash Heat Interchanger
especially fine lubricating oil. Diphenyl boils a t approximately 500’ F., and at 640’ F., where water has a vapor pressure of over 2000 pounds per square inch, diphenyl has barely 50 and may be handled in safety in comparatively light walled tubing.
expansion. The dependence of the rate of heat transfer on the surface films rather than on the material of which tubes are made has been well shown by the fact that, though the actual resistance of glass to the passage of heat is three hundred times that of copper, the amount of heat passed under comparable conditions from steam on the outside to Evaporation liquid on the inside is only two and a half times as great with It was, in fact, the utilization of vaporous heat rather than copper as with glass. I n both cases the largest resistances its generation which led to the development of the Swenson are offered by the fluid films; and that of the solid itseIf, forced-circulation evaporator for the transfer of heat from a glass or copper, is comparatively small. tube wall to the body of a liquid t4herein. The stagnant Multiple-effect evaporation was developed in the first half condition of liquid immediately adjacent to a solid s u r f a c e of the last century. Such a system depends on passing vapors enormously emphasized by the adhering film left when a glass from an evaporating chemical solution to heat and boil the of oil or molasses is inverted to empty-may be greatly same or a different chemical solution a t a lower temperature changed by forcing the body of the liquid past the surface or and pressure, and to continue this procedure on to the limit through a tube at a high velocity. Heat passes with great set by the practical vacuum which may be maintained in the reluctance through a quiet or slowly moving liquid, but the last evaporator or “effect” of the series. Several pounds of convection currents in a swiftly flowing turbulent liquid ab- water may thus be evaporated by one pound of steam. More sorb heat comparatively readily. Seemingly, it is impossible recently commercial installations have proved practicable the to eliminate the stagnant i l m immediately adjacent to a solid “vapor-recompression” system, which reuses the vapors from surface such as the wall of a tube, but its effect in decreasing evaporation in a single evaporator body. The vapor$, inheat transfer may be minimized to a remarkable extent by stead of degrading in pressure successively while passing positive and rapid circulation. It is the chemical engineer from effect to effect, are compressed to the pressure of the who meets most frequently the necessity of such methods of initial steam used for heating. This compression is accomaccelerating heat transfer. since the solutions he must heat plished by a high-velocity turbo-blower or a steam-jet comand concentrate often have viscosities hundreds or thousands pressor, in either case the auxiliary steam used being supplied of times that of water. The effects of surface films in such by a pressure line and exhausted to the heating space. Evapoheavy liquids are comparatively more difficult-and more ration by the recompression method may be accomplished necessary-to minimize. The forced-circulation evaporator largely by mechanical work, and where electrical power is for this reason has recently proved its worth, usually for d e cheap and steam expensive the turbo-blower is motor-operated watering chemical solutions. The most spectacular reported and only a small amount of make-up steam is used beyond success of this apparatus, which resulted from a studied that required to heat the solution to the boiling point. application of the theories of heat transfer, has been in the Another evaporation system, until recently applied only in concentration of caustic soda solutions, in which it has been the manufacture of a fine brand of table salt but now used in possible to remove all but 1 or 2 per cent water, and fin- other chemical process industries by the Peebles Process ish lye dry enough for market. Company and others, heats the liquid to be evaporated or Certain other chemical solutions are so corrosive that no concentrated to a high temperature under pressure in a series metal may be used in contact with them. Glass or ceramic of heaters, only the last of which is supplied with steam from materials are then the only available materials for use, and the boilers. The liquid is then allowed to evaporate by evaporators with only Pyrex glass in contact with the heated flashing into successive enclosed chambers a t progressively material have been operated with steam at 80 pounds gage. lower pressures and supplies steam a t these lower pressures to In these, the desired high velocity is maintained by early heat the incoming liquid. The k a l one of these flash chamallowing a part of the solution to evaporate and drive the bers supplies steam a t approximately atmospheric pressure remainder through a long, tortuous, tubular path by its to the first heater for the cold liquid and earlier flash chambers
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INDUSTRIAL A N D ENGl .NEERING CHEMISTRY
supply steam for later heaters. The original development, for the production of Diamond Crystal salt, was to take adv a n t a g of the fact that certain impurities in raw salt brine
. .Courier~Bcihlchrm Povidry and Machine Corntony Frederklng Jacketed Kettle
Figure 3-Orlglnal
are comparatively insoluble at high temperatures and may be settled out and removed after preheating but before evaporation. The multiple-effect flashing gives steam economies comparable to the more usual multipleeffect pan evaporation. Perhaps the most spectacular type of recent evaporator, comparable to the Humphrey internalcombustion pump since both involve cornbustion gases in immediate contact with the liquid handled, is in the application of the Brunler flame. Air and liquid or gaseous fuel are supplied under pressure and burned under the surface, in immediate contact with the liquid to he evaporated. Usually the process is conducted under a pressure of about 170 pounds and the mixture of 45 per cent steam and 55 per cent combustion gases is utilized to run a special non-condensing steam engine. With the flame totally surrounded by the liquid none of the heat is lost, and the directness of its application and the small size of contingent equipment makes a mhst economical operation. There are apparent disadvantages from the standpoint of power generation as a primary object, but the power derived is an important by-product in several interesting chemical processes using this flame. The high temmrature obtainable in a liquid solution has proved not only a means of e
