Applications of Dowtherm Vapor Heating D. K. DEAN Foster Wheeler Corporation, New York, N. Y.
A
of Dowtherm vapor is much less than that of steam; on the EUTECTIC mixture of diphenyl (C6H& and diphenyl other hand, the density of Dowtherm vapor is correspondoxide (CgH&O, called "Dowtherm A", has shown ingly greater, The net result is that, volume for volume, the excellent properties for the transmission of heat in the two gases have approximately the same latent heat content a t range between 400" and 700" F. Its boiling point a t atmoscorresponding pressures in the range of normal commercial pheric pressure is 500°, and it does not solidify until the temusage for Dowtherm vapor. At first glance this would indiperature is lowered to 54" F. Neither does i t break down cate that pipe sizes for Dowtherm vapor and steam a t a chemically during long periods of operation while generating given pressure should be the same at any given heat load. vapor within this range and in well-designed equipment. However, friction losses in piping vary directly with the The use of a condensing vapor for the transmission of heat density as well as the square of the velocity. It is therefore in process work offers the advantage of close regulation of essential that due consideration be given this factor in detemperature in the heated vessel. The high rate of heat signing Dowtherm piping. This requirement is of prime imtransfer accompanying condensation of the vapor ensures portance inasmuch as most commercial Dowtherm installaeconomical use of the heating surface and permits operation tions operate at a relatively low pressure, even though in with close temperature differences between the heating memost cases to date the lengths of pipe lines have been reladium and the product heated. I n this way all danger of tively short. All lines must be figured with care in order to overheating due to change in the rate of throughput is miniavoid pressure losses in the piping which would render the mized. Additional advantages are that the heat transmitting system inoperative.. surface exposed to a condensing medium will be subjected to the same temperature throughout, and "hot spots", which might injure the product, are avoided. Regenerative Air Heating Cycle Steam is an excellent medium for such heating service up The first large commercial use of Dowtherm was in a reto temperatures of 350" or 400" F., but for higher temperagenerative air heating cycle at the Bremo station of the Virtures the pressure becomes excessive with the result that ginia Public Service Company, Extended surface econoserious difficulties and expenses are encountered. Hence, mizers (Figure 2) of the type normally used for water heating the present trend to the use of Dowtherm vapor a t high temare installed between boilers and stacks of two 1980-horseperatures and low pressures. Figure 1 gives a comparison of the pressure-temperature power steam generators, for heating 98,500 pounds per hour of Dowtherm from 120" to 500" F., utilizing stack gases a t an characteristics of steam and Dowtherm A vapor. Saturated i n i t i a l t e m p e r a t u r e of Dowtherm vapor a t a 630" F. The Dowtherm temperature of 500" F. is then circulated through has an absolute pressure TABLE I. PROPERTIES OF DOWTHERM A VAPOR 64nch pipes to fin-type of 15 pounds per square -Pressure7 -Heat ContentSpeoifio Density, air heaters about 100 Lb./Sq. In. B. t . u./Lb. Heat Lb.!Cu. Ft. inch, while s a t u r a t e d Temperature Abs. Gage Liquid Latent Total Liquid Liquld Vapor feet below. The advansteam a t that temperaF. C. tages of the system lie in ture has a n absolute pres0.28 0.63 54.1 345 222.0 123 15.0 0.0 500.0 258.0 the reduction of space 53.7 0.32 349 0.63 sure of about 679 pounds 228.0 121 510.0 266.0 17.0 2.0 0.36 0.64 53.2 234.0 120 354 19.0 4.0 520.0 271.0 and cost for ducts that per square inch. At 0.40 0.64 53.0 240.0 119 359 21.0 6.0 530.0 277.0 52.7 0.44 0.65 118 365 540.0 282.0 24.0 9.0 247.0 would have been required 700" F. saturated steam 0.48 0.65 52.3 27.0 12.0 253.0 117 370 550.0 288.0 for the transmission of has a pressure of 3100 51.9 0.54 375 0.65 30.0 15.0 260.0 115 560.0 293.0 heated air, and the virtual pounds per square inch 0.60 114 381 0.66 51.6 33.0 18.0 267.0 570.0 299.0 386 0.66 51.2 0.67 274.0 112 36.0 21.0 580.0 304.0 elimination of radiation absolute whereas saturated 392 0.66 50.8 0.75 281.0 111 39.0 24.0 590.0 310.0 losses. Dowtherm vapor has a 50.4 0.88 398 0.66 43.0 28.0 288.0 110 600.0 315.0 1.00 0.67 50.1 47.0 32.0 295.0 109 404 610.0 321.0 These air heating sysvapor pressure of only 103 49.8 1.10 107 409 0.67 51.0 36.0 302.0 620.0 327.0 tems were installed in pounds per square inch. 309.0 415 0.67 49.3 1.17 56.0 41.0 106 630.0 332.0 49.1 1.24 0.67 105 421 640.0 338.0 62.0 47.0 316.0 1931 and are still the The properties of Dow1.29 0.67 48.6 650.0 343.0 68.0 53.0 323.0 104 427 largest Dowtherm applicatherm A in the saturated 48.4 1.34 74.0 59.0 330.0 102 432 0.68 660.0 349.0 47.9 1.40 81.0 66.0 337.0 101 438 0.68 670.0 354.0 tions in point of heat vapor form are given in 47.5 1.5 88.0 73.0 344.0 99 443 0.68 680.0 360.0 transfer; the hourly rate Table I. 1.6 0.68 47.2 351.0 98 449 95.0 80.0 690.0 366.0 of e a c h is 1 6 , 0 0 0 , 7 0 0 A comparison of the 103.0 88.0 358.0 0.68 46.9 1.7 700.0 371.0 97 455 46.3 111.0 96.0 365.0 0.68 710.0 95 460 1.8 377.0 B. t. u. The Dowtherm properties of Dowtherm 1.9 120.0 105.0 372.0 0.68 720.0 382.0 93 465 45.9 129.0 114.0 379.0 0.68 730.0 388.0 92 471 2.1 45.5 is tested annually for devapor and steam shows 139.0 124.0 2.3 44.9 386.0 0.68 740.0 393.0 90 476 terioration but in more that under ordinary con135.0 0.68 2.5 44.4 393.0 750.0 399.0 89 482 150.0 than seven years there ditions the latent heat 797
INDUSTRIAL AKD ENGIKEEKING CBEMISTI1Y
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F ~ G U R1.E PRESSURE AND TEMPE~ATURE Ca~ancTEEISTIUS OF STEAM A N D DOWTHERM VAPOR has been no trace of decomposition. During the same time there has been no sign of corrosion or scaling of the Dowtherm heating surface, which is conclusive proof that this material has no detrimental effect on steel tubing or pipe.
Vapor Generator at Midland Since Dowthem A is a n organic product, it is essential that vapor generators should be designed to provide positive and rapid circulation in order to avoid tube wall temperatures greatly in excess of the temperature of the vapor generated. It is al& imperative that the boiler shall contain sufficient storage capacity for liquid Dowtherm to maintain proper height of liquid when the dense Dowtherm vapors have filled the entire process system. This supply of Dowtherm liquid should be out of contact with the heating surfaces but should always be available for circulation within the tubes. To fulfill both
FIGURE2. INSTALLATION OF DOWTHERM HEATERA S PART OF A REQENERATATIVE AIR HEATINQ SYSTEM AT T R ~BREMO PWUUT OF THE VIRGINIA PUBLIC SERVICE COMPANY This heater ia made up ai extended surface elements of the type usually used in power taut economizers. Gases from the bo& p a up through the heater. The Dowtherm enters the heater at the top and flows through successive rows of elements, lesvin at the bottom. The heater itiequippe8 with eight mechanical soot blowers, the heads of which esn be seen outside the setting.
VOL. 31, NO. 7
(not the economizer~regenerativeair heater described above which operates on the liquid pliase only) was designai for forced circulation with the vapor and liquid storage drum placed helow the heating surface as shown in Fi'iyre 3. Duwtherm liquid from the drum is forced illrough thc heating elements, passing first through radiant tubes and then through ennvection tubes and back to the drum where vapor becomes disengaged from the liquid. In this system, installed in the Dow Chemical Company's plant at Midland, Mich., vapors pass through a vapor header serving several stills and process units; the condensate is collected and flows by gravity from the units hack to the drum. When this unit was designed, it was felt advisable to have the drum below the boiler so that the boiler could he instantly drained into the drum in the event of tube failure. I n seven years of service i t has never been necessary to make use of this feature, and Dowtherm vapor generators are now built somewhat along the lines of water tube steam boilers. All installations, however, are provided with rundown tanks large enough to hold the entire supply of Dowtherm in the system. This first system was installed in the Dow plant in 1932 and was used intermittently for t.hree years, operating 30 hours continuously every 2 weeks. Since 1935 i t has been in service 24 hours daily, 5 days a week. Normal operating pressure is 15 pounds per square inch in the generator giving a temperature of 560" F. a t the three vacuum stills i t serves, On occasion the pressure is carried to 45 pounds per square inch to give a temperature of 630" a t the stills. Normal heat output of the vapor generator has been 1,000,000 B. t . u. per hour for the last 4 years, hnt i t has run as high as 2,000,000, The generator is designed for a maximum of 4,500,000 B. t. n rx?r hour to take care of future plant requirements.
Vapor Generator at Bound Brook The second Dowtherm vapor installation was made for the Bakelite Corporation at Bound Brook, N. J. The vapor generator consisted of two drums connected by symmetrical banks of tubes with the furnace placed between the banks, the flow of flue gases being horizontal through the furnace and back through t.he tube banks in parallel. This pioneering
JULY, 1939
INDUSTRIAL AND ENGINEERING CHEMISTRY
unit, and more than thirty-five built since, has depended upon natural circulation; this provides unobstructed natural flow of liquid in the boiler. The vapor generator a t the Bakelite plant is used in connection with a batch system of resin heating in jacketed kettles. The system was equipped with a stesm-jet vacuum pump to permit operation at temperatures below 500" F., which is desirable in the first and last stages of the heating; Dowtherm is also used in a heater to superheat steam for processing within the kettles. The success of the first two vapor generators and the processes with which they were installed was such that there has been a consistent increase of new applications accompanied by refinements in operation. Oil or gas is customarily used as fuel and is governed by semiautomatic control. A high-low burner regulated by a thermostatic, or pressure, element in the vapor line maintains a constant temperature of the vapor by operating on high or low flame. An overpressure control, set to function at some pressure in excess of the usual operating pressure, protects the system by shutting off the fuel supply if the thermostatic or pressure control for the burner fails to act. A safety valve safeguards the apparatus in case all of the other control devices fail.
799
Gravity Return System
Coincident with the wide growth in the use of indirect heating employing Dowtherm as a medium, many different types of systems have been developed to meet various processing requirements. Figure 4 illustrates the simplest form of Dowtherm vapor heating system. I n this case the customary still or high-temperature heater, known as the user, is located sufficiently high above the vapor generator to return the resulting condensate to the generator by gravity. Such a system may be operated by controlling the burner serving the generator in order to give the desired outlet vapor temperature. Within limits, also, the supply of heat to the user and the temperature of the vapor entering the user may be controlled by throttling the supply of vapor.
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FIGURE4. SIMPLEARRANGEMENT OF DOWTHERM VAPORGENERATOR, SHOWINGMETHODOF INSTALLING HARTFORD LOOP Distance M re resents the head of the friction loss in the vapor fine, heater, and condensate line.
I n the design of such a system it is essential that the sizes of the vapor and condensate lines and heating coil or jacket space shall be such that the friction loss will be less than the available static head between the bottom of the user and the liquid level in the boiler drum. This involves the flow of vapor in the vapor line and within the user, plus the friction loss of the condensate in the return line to the boiler. I n normal operation the condensate in Figure 4 will establish a level in the return line a t B. The distance, M , between this point and liquid level A in the boiler drum is really a P measure of the total flow losses in the system. By throttling the supply of vapor to the user, the resistance to flow in the vapor line is increased, with a reduction in pressure within the user. Full generator pressure, however, would still be imposed upon the lower end of the condensate column. Consequently the height of the column might be increased even until it should reach the bottom of FIGURE3. DIAGRAMOF FORCED-CIRCULATION DOWTHERMthe user. The height, H , in this case would be the measure VAPOR GENERATORAND ONE OF THE THREE VACUUM STILLS WHICHIT SERVES AT THE MIDLAND PLANTOF THE Dow CHEMI- of the total flow losses in the system. With the vapor line throttled so that the height of the return column is just below CAL COMPANY
INDUSTRIAL AND ENGINEERING CHEMISTRY
800
VOL. 31, NO. 7
preciably above atmospheric, a simple vent valve can be used to relieve the system of these noncondensables. If, on the other hand, the system operates under a vacuum, or if the normal operation is so close to atmospheric that a vacuum is likely to exist in the system during certain periods of operation, it is desirable to have some evacuating means to draw out these noncondensables from this vent line. Figure 5 illustrates a gravity return sys\ tem with a number of users. User A is assumed to be operating with full vapor pressure and the height H , is the static head S necessary to return condensation to the generator. User B is throttled somewhat and requires a greater static head, Hb. User C is throttled to such a n extent that the level, H,,of the condensate floods a certain amount of the heating surface. Figure 5 indicates how several users may be operated in a TEMPERAWRE gravity return system from a single Dowtherm supply main. It is essential that with a LO DOWTHERM multiple system particular attention be paid to venting. It is usually desirable that an evacuating means be provided. Figure 5 shows a steam-jet ejector connected to a vent line which has branches leading to hot CHECK wells a t the ends of the several condensing /VALVES\ zones. A condenser is provided ahead of TURN LINE CONDENSA E the ejector to condense any Dowtherm vapors which may be carried along with the noncondensables and to return the Dowtherm to the FIQURE 5 . ARRANGEMENT OF DOWTHERM VAPORGENERATOR WHEN APPLIED system. Figure 5 also shows a Dowtherm TO THREE DIFFERENT USERS AND DESIGNED FOR THROTTLING so AS TO storage tank. With small systems a n ordiOPERATEAT DIFFERENT PRESSURES AND TEMPERATURES nary handcharging pump may be used to deliver fresh Dowtherm to the generator if the capacity of the user since it would result in a considerrequired or to empty the generator for inspection purposes. able reduction of the temperature of the heating surface which Certain products that are very sensitive to heat may necesis flooded because of stagnation of the liquid. This might be sitate keeping all of the heating surface a t the same temperaobjectionable in the handling of certain sensitive products. ture all of the time. Figure 6 illustrates a system in which Whenever control of the vapor supply by means of throtseveral users may be operated a t different temperatures and tling is contemplated, it is essential that there shall be a sufficient volume of liquid Dowtherm available in the boiler USERS
