May, 1926
I S D U S T R I A L AND B N G I N E E R I S G CHEJIISTRY
505
Power for Chemical Plants' By J. I,. McK. Yardley WESTINGHOUSE
ELECTRIC & MANUFACTURING CO.,PHILADELPHIA,
Where considerable steam is required for heating and process work, the chemical plant should generate its own steam at high boiler pressure and then obtain electrical power for its mechanical drives and electrolytic circuits from that steam, before turning i t over as exhaust or bled steam to the processes. Very highly efficient turbo-generator units of medium size are now available. If the boiler and prime-mover rooms are in close proximity to the electrolytic cell or tank room, the electrolytic power should be generated as d. c. power. Very highly efficient geared d. c. turbo-generating units can now be obtained. Where appreciable distance intervenes between the boiler and prime-mover rooms, and the tank house, or cell room, or where appreciable steam is
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BEMICAL plants are not all dependent, upon cheap power, such as might be obtainable from a hydroelectric development. Many of those that are not thus dependent actually produce, or could produce, cheap power for their own motor drives as a by-product of process steam which they require in large quantities. Chemical plants might be differentiated as those requiring large quantities of cheap power and those requiring large cpantities of steam. The former, generally considered as electrochemical or electrometallurgical plants, carry on some electroIytic or heat-reduction process, only possible or commercially feasible a t the high temperature obtainable by electricity and a t the same time having few operations requiring low-temperature heat. Plants of the second class conduct some process in which the solution of chemicals, the reaction between chemicals, or the evaporation of excess moisture requires low-temperature heat such as is obtainable from steam. This class comprises what are generally spoken of as chemical plants. The first class of plant usually produce. some intermediate product from some raw material. The second class usually produces combined, refined, or finished products from an intermediate material. The dependence of some materials upon cheap power is indicated by the fact that electric ferromanganese of domestic manufacture disappears from the market when the price level drops much below $150 per ton. Canadian and Norwegian furnaces can market here a t $115 per ton and pay a duty of over $33 per ton. I n the first ten months of 1925, 5000 tons of manganese came from Canada and 8000 tons from Norway, or a total in terms of 80 per cent alloy of 16,500 tons. The electrochemical or electrometallurgical plant is located first with relation to power and second with relation to raw materials. The chemical plant is located w i t k relation to markets, labor, supplies of fuel, and the intermediate products. Plants of the first class we accordingly find restricted to localities such as Niagara Falls, Shawinigan, and the Saguenay on this continent; while plants of the second class are distributed throughout the United States, especially in the East. Cheap hydroelectric power is being increasingly absorbed by electrochemical and electrometallurgical plants in electrolytic processes and in high-temperature electric furnace heat-reduction processes, which do not have large low-
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Recei\ed January 2 6 , 1926.
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not required for heating and process work, so that the electrical power is purchased, then the best economy will usually be obtained by converting the electrolytic power from alternating current by means of synchronous converters. Comparative results of d. c. generating, converting by synchronous converters, motor generators, and mercuryarc rectifiers are discussed. The article is written primarily to attract attention to possible economies and stimulate interest in the study of specific problems. Owing to the limited extent to which the practice here recommended as best has been put into effect, the information is given more in the form of illustrations than generalizations.
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temperature operations in parallel. Chemical plants having principally low-temperature operations are not likely to develop in such localities, as they usually can operate more economically with steam heat elsewhere. I n such plants the efficient use of steam should be encouraged, and this use may be extended by generating steam in a t least moderately high-pressure superheating boilers, which will permit the extraction of electrical power for the mechanical drives and high-temperature heat applications throughout the plant, before the steam is turned over at process pressure to the chemical processes. Industrial plants have only recently awakened to the possibilities of getting their electrical power for practically nothing. They have only recently begun the installation of high-pressure boilers and pressure-reducing prime movers, from which the process steam may be obtained, either as bled steam or as high-pressure exhaust steam, while a t the same time considerable electrical power is being generated. The manufacturers of turbo-generators are beginning to recognize the possibilities and requirements of this industrial power field, and have designed automatic bleeder type units of moderate capacity suitable for such plants and having efficiencies more nearly comparable with the larger units furnished to central stations. Value of Steam in Process Work
I n most chemical plants the steam produced in the boiler plant has a very great value for something besides the generation of electric power. For example, a copper refinery, A , is installing a 2800-kilowatt, 136.5-volt d. c., geared turbogenerator unit to take steam a t 260 pounds gage, 100' F. superheat. This unit is supplied with a condenser and will have a straight steam consumption on a 28.5-inch vacuum of 13.8 pounds per kilowatt hour. However, it is a bleeder type unit, and actually 20,200 pounds of steam per hour will be bled at 5 pounds gage for process work. As the total steam furnished will be 51,200 pounds per hour, that chargeable to power is 31,000 pounds or 11.1 pounds per kilowatt hour. At 2500-kilowatt load with 51,200 pounds total steam supplied and 26,300 pounds bled, the steam consumption will be only approximately 10 pounds per kilowatt hour. I n a rubber mill, B , using large quantities of steam a t about 100 pounds in manufacturing processes, it was found possible. in spite of low initial boiler pressure (250 pounds, 125' F. superheat), to install a 5000-kilpwatt turbo-generator
506
INDUSTRIAL A N D ENGINEERING CHEMISTRY
between the boiler head and the pipe supplying the 100pound pressure process steam. The unit is regulated as to load by a back-pressure regulator and a standard centrifugal type of governor. Power from Straight Back-Pressure Units
Vol. 18, No. 5
Power Supply a t Copper Refinery
A copper refinery, C, in the market for power generating and converting units, gave its conditions as in Table I. Table I-Load Conditions a t Copper RefinerylC Copper production, tons per month 15,000 22,500 A. c. generation, kilowatts 5,500 9,800 Lbs./hour Lbs./hour Saturated steam at 165 pounds from waste heat boilers 28.600 35.500 Steam to Drocess at:
If the power required is not too great in proportion to the low-pressure steam requirements, it may be obtained economically by a straight noncondensing back-pressure unit operating upon a boiler pressure selected to meet the conditions. Assuming a perfect machine for every drop to half the initial absolute pressure, 1000 pounds of steam per hour passing through a turbine or engine will produce 21 The boiler feed make-up (40’ F. winter, 60’ F. summer) plus the turbohorsepower, or assuming a little less than 50 per cent effi- condensate at 80’ F. would go to the condenser hot-well, and from there be ciency, 10 horsepower. This rule may be used throughout pumped through the bearing oil cooler, generator air cooler, primary (extraction) heater (6 pounds absolute) open heater (2 pounds gage), and closed the range 15 to 1000 pounds absolute, though the error may be heater (30 pounhs gage) between feed pumps and economizers. appreciable if the initial pressure is more than twenty times the exhaust pressure. Suppose a plant has a constant deThis company is in the business of refining blister copper, mand for 800 horsepower and 20,000 pounds process steam which is supplied from a number of smelter sources. This per hour at 45 pounds absolute. Then 800/20 or 40 horse- blister copper is melted down and refined somewhat in the power must be obtained from each 1000 pounds of steam, and “anode furnaces” and then poured into anode molds. The the initial steam pressure must be about 45 x 2 x 2 x 2 x 2 anodes go to the electrolytic tank house, where the silver and = 720 pounds. The extra fuel required would be only gold are separated from the copper, and the copper is deabout 200 pounds per hour, or insignificant. Assume posited upon cathodes in an acid sulfate electrolyte, the tem600 horsepower required and 100 pounds absolute back presperature of which, to get best efficiency, is maintained by sure, then 100 x 2 x 2 x 2 = 800 pounds initial pressure steam heating a t from 140” to 150” F.2 The cathodes, after required. Applying this rule backward to the rubber mill being washed free from electrolyte, are melted down and given B , the total steam would be about 500,000, or about 15,000 further refinement in the “cathode furnaces,” from which the boiler horsepower. copper is poured into molds of commercial shapes. I n any plant the desirable pressure will, of course, depend Steam from Waste Heat Boilers upon the relation between the steam and the power requirements and upon the degree to which they fluctuate up and These anode and cathode furnaces are essential to the down together. Where they do not keep in step an automatic bleeder type unit should preferably be supplied with a con- process, and for some time it has been the practice to equip them with waste heat boilers. The economv of these furdenser. or a steam accumulator might be employed. naces has been greatly inAt the Boston Edison Comcreased since the introduction pany’s Weymouth s t a t i o n , of pulverized coal, but still we find one of the latest exfrom 0.7 to 0.9 pound of steam amples of c e n t r a l s t a t i o n is obtained for every pound efforts to obtain more power of plant output of copper, per unit of steam. Here the though the steaming rate Rankine cycle efficiency has varies greatly with the furbeen increased 14.7 per cent nace conditions throughout by installing a 1200-pound the 24 hours. To make good pressure, 700” F. boiler to use of this steam, therefore, supply a 3150-kilowatt turbothe waste heat boilers must generator. T h e 133,000 be run in parallelwith another pounds (42.2 pounds per kilocontrolled source of steam. watt hour) exhaust steam a t I n this way large quantities 350 pounds gage pressure is of heat are conserved for use heated up again to 700” F. either in power generation or and supplied to turbo-generain plant processes such as tors running full condensing, electrolyte heating (0.8 to 29 inches vacuum, and taking 0.85 p o u n d of s t e a m p e r 9.8 pounds per kilowatt hour. Figure l-One of T w o 3-Unit, Synchronous Motor-Generator pound of copper) and evapI n this way 13,600 kilowatts S e t s to Operate f r o m a 50-Cycle Circuit at 5000 Volts, 80 Per c e n t orating as required in thevariPower Factor i n Making Starting S h e e t s for Electrolytic Copper more are generated, or a total T a n k House ous operations included in the of 16,750 kilowatts, of which D. c . rating of each set is 780 kilowatts 6 5 volts d. c . 12 000 amperes, other departments of the 18.8 per cent is generated a t 500 r. p. m. Voltage range, 0 to 65 v d t s d. c . Effihency, 77.5% at half load; 80.8% at three-fourths load; 82.2% at full load. plant, It should be possible the high pressure and 81.2 per to build these waste heat cent a t the low pressure. The reason for such a large percentage of the total power being boilers for any commercial boiler plant pressure, and the generated by the high pressure unit is understood to lie in copper refinery A is installing a waste heat boiler on its the high efficiency obtained in the high-pressure moisture-free, reverberatory melting furnace for 260 pounds gage to straight impulse type stages. This high-pressure power is operate in parallel for control with the boilers supplying Obtained by the addition Of Only 8*7per cent 2 brought out in the paper on “Electrometallurgical Applicaplants may profit tions” before the Amerjcan Institute of Electrical Engineers, October, more heat’ Some Of the larger by this example. 1924, an appreciable power saving is effected by steam heating a t this point. I_
I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
May, 1926
the 2800-kilowatt d. c., geared electrolytic unit. Obviously, such plants as refineries A and C cannot afford to purchase power. At refinery C the boiler plant pressure established by the power plant before waste heat boilers were installed was 165 pounds gage. The waste heat boilers were therefore installed for this pressure and the steaming rate, which varies from practically nothing to 1.5 pounds per pound of copper, averages 0.7 pound per pound of copper. A new boiler plant and new generating units are to be installed with steam conditions 365 pounds gage, 200" F. superheat, and arranged to equalize the steam output of the existing 165-pounds waste heat boilers by bleeding or receiving steam automatically at that pressure. The needs of the boiling or evaporating tanks in the silver refinery and bluestone department and elsewhere will thus be taken care of, as wanted, while at the same time steam bled a t 30 pounds gage, 2 pounds gage, and 6 pounds absolute will be used in heating the electrolyte, make-up water for the tanks, and also the buildings in winter. Such a flexible supply of heat can scarcely be obtained in any way other than by the automatic bleeder turbine, and it is difficult to see how a central station could sell power to such a plant. Power at Chlorine and Caustic Soda Plants
Another plant requiring large quantities of steam as well as power in process work is the plant for the production of caustic soda, chlorine, and derivative products. Although the principal use of power is in the electrolysis of salt solution to obtain chlorine, caustic soda, and hydrogen, and in the graphitizing of carbons to form anodes for the electrolytic cells, cheap hydroelectric power is not essential and purchased steam-generated power not permissible, since the large amount of lowtemperature heat used in the heating and evaporating of solutions is cheaply obtainable from exhaust or bled steam. The caustic evaporators alone take from 9 to 10 pounds of steam per pound of 50" B6. liquid caustic produced. At plant D about 62,650 pounds per hour a t 3 pounds gage pressure are required in treating the product of about two thousand electrolytic cells. Of the total average plant load, 5700 kilowatts, the cell room takes an average of 1600 kilowatts a t 250 volts d. e. The maximum load on the plant in the course of a year is about as follows: Electrolytic Graphitizing M o t o r drives
Kilowatts 6000 2000 1100
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TOTAL
9100
I n the past this process steam was obtained from the exhaust of two out of three 900-kilowatt Corliss engine-driven generators taking about 35 pounds of steam per kilowatt hour upon 150 pounds initial steam gage pressure. The remaining load was carried by two 3125 KVA., 2300-volt, Westinghouse, 46-CW turbo-units, supplied with 4250 square foot condensers and taking, according t o plant tests a t full load, about 15 pounds of steam per kilowatt hour, with initial steam conditions of 200 pounds and 100" F. superheat. For this application a 3000-kilowatt, 80 per cent powerfactor unit capable of passing 105,000 pounds of steam was proposed. Under the conditions, this unit would take 'O"
O"
0.746 X 0.945
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13.8 pounds of steam per kilowatt hour, or a total of 41,500 pounds
With 62,650 pounds bled at 3 pounds gage, the total steam would be about 76,500 pounds. Instead of obtaining, as by the old Corliss units, 1800 kilowatts for 63,000 pounds heating and 63,000 pounds total steam, the new unit would give 3000 kilowatts for 6:3,000 pounds heating, and 76,500 pounds
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total, or a difference of 1200 kilowatts would be obtained for 13,500 pounds additional steam, or a t a rate of 11.25 pounds per kilowatt hour. The power plant a t D contains sufficient boiler capacity to supply 200 pounds pressure, 100" F. superheated steam, and some of the older 150-pound pressure boilers could be placed in reserve or eliminated. Table 11-Present Class No. 1 4 2 1 3 4
Horsepower 300 822 508
Boiler Capacity a t Plant D Gage pressure Superheat Efficiency Pounds OF. Percent 150 62 200 (operated at 150 pounds) 100 81 200 100 75
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The Class 3 boilers supply the two Westinghouse 46-CW units. The Class 2 boiler is run at an overload of 100 per cent when necessary to carry the maximum load required by the Corliss engines. The installation of the proposed new unit would permit the scrapping of considerable inefficient power equipment. I n addition to the three Corliss engine sets, the plant contains three 1250-kilowatt, geared d. c. generator units, two of which are driven by low-pressure turbines and the third by a high-pressure turbine taking steam a t 150 pounds pressure. The low-pressure units would, of course, be done away with as a result of scrapping the Corliss engine units; and, as the third 1250-kilowatt unit is of comparatively old design and has a steam rate of 19 pounds per kilowatt hour, it should also be scrapped. The Problem of Old Equipment This case is typical of conditions existing in many industrial steam-power plants and represents the biggest problem in installing replacement equipment today. The problem is what to do with the old equipment, which was written off the books years ago but which still runs satisfactorily, even though inefficiently from a heat standpoint, and with large maintenance. Chemical and metallurgical industries figure large returns upon investment, owing to the rapid changes and advancements in the art and technic of making their products and because chemical products themselves are so rapidly superseded by other products. Witness the extensive substitution of liquid chlorine for bleaching powder. Many chemical plants must have had to make appreciable changes in methods and in plant equipment on this account. Owing to these high amortization charges, therefore, equipment that has been in use only a few years has been written off (on the books a t least), and new equipment must show a greater saving in order to get consideration. The matter of buying new power equipment really comes down, then, to the actual needs of increased capacity, or insurance that existing capacity may be maintained. Fortunately, the need of increased manufacturing capacity has recently been felt in many lines of chemicals, the increases during war time having been largely absorbed. There are also a number of plants, installed from fifteen to twenty-five or even thirty years ago, where the matter of insurance of continuous output is getting to be a serious consideration. The writer has recently visited a t least six chemical plant power plants that are full of inefficient, antiquated, or obsolete power equipment; two or three of these are already taking steps to modernize. The prime-mover rooms themselves are often objects for admiration, having been constructed with ample overhead space and crane accommodations back in the days of massive reciprocating units. The room a t plant D , for example, is a splendid room, with a t least four times the floor space occupied by the two comparatively modern 3125-KVA. units, or three times that necessary were modern equipment installed to carry the entire load and old stuff torn out. Here, insurance of security of operation is shortly going to demand that something be done;
INDUSTRIAL A N D E;VGINEERING CHEMISTRY and in addition to an appreciable saving in operation, the change could be credited with floor space abandoned for power generation and turned over to some process department. The same thing to a large degree might be said of the power plants Of the Other plants' The are in general, and the floor space now occupied by inefficient or antiquated equipment is much more than sufficient for modern equipment to meet any reasonable future requirements.
Vol. 18, No. 5
Table 111-Cost of Power b y Two S y s t e m s D . C. Generation 3 units (including one spare) each consisting of two 1400-kilowatt, d. c. generators geared to turbine 3 condensers
$285,000 46,000
TOTAL 81330,000 Efficiency (beyond turbine), generator 93.5 % ' X gear 98.5% = 92.1% A . C. Generation
crzge
u n ~ a ~ ~ ~ ~ ! 3000-ki10u ~ $13!,000 i ~ 3 condensers 40,000 5 converting units (including one spare), consisting of synchronous converters, transformers, regulators, and switchboards; or else consisting of motor-generator sets 150,000
Development in Use of Electrolytic Power Equipment TOTAL $330,000 (beyond turbine. turbine efficiency same in The equipment manufacturers have been getting ready to Efficiency both cases), generator 95% X motor-generator set compete for these replacements and additions to generating and exciter 90.7% 8 6 . 1 per cent Or generator 9 0 7 ~X synchronous converter, transcapacity by developing lines of bleeder type of turbo-genformer and regulator, 92.5% 5 7 . 9 per cent of plant using d. c . generation over that using erating units of moderate capacity. The type of plant Superiority a. c. generation (depending upon whether synchrothat is installed should depend largely upon the distance benous converters or motor-generator sets are used for conversion) 4 . 2 or 5 9 per cent tween the prime-mover or generating room and the tank With tank room and power plant practically adjacent so house. As many of these plants were built when direct current generation by means of engine-driven units was that d. c. bus costs and losses would be about the same in common, and the cost of copper for extensive busses was either case, there is obviously no advantage in installing quite an item as now, the distance between tank house and the a. c. generating plant, as its operating labor and main Dower plant was usually made quite short and sometimes tenance cost would also be greater owing to the greater number of pieces of apparawas eliminated entirely by tus. I n case the power and making the two buildings electrolytic plants could not contiguous. It is therefore be contiguous, so that d. c. necessary in planning bus costs and losses would increases or replacements of offset the efficiencydifference, equipment to consider d. c. t h e a. c. g e n e r a t i n g plant generation of the electrolytic would become the more depower and especially to coms i r a b l e. 0 t h e r m i s e t h i s pare d. c. bus losses with would not be so, although conversion losses. s o m e e n g i n e e r s have conThe whole development in' sidered that under light load the use of electrolytic equipconditions it would permit ment has been a logical one shutting d o w n o n e p r i m e from the user's point of view. mover and shifting the cirIt began with direct current cuits to the others to keep generation by means of directthem loaded and therefore connected e n g i n e d r i v e n o p e r a t i n g a t highest effiunits because they were the ciency. However, for reasons most efficient units obtainshown later, favorable condiable. When the limits of size t i o n s permitting high effiFigure 2-Nine 2500-Kilowatt, 500-Volt, D. C., 5000-Ampere, and efficiency of such units 60-Cycle, 400 R. P. M., Synchronous Converters for Electrolytic ciency with a. c. generation were reached, and the equipService. cannot be counted on in ment manufacturers began to These are part of an installation of 33 total. The d. c. switching is shown on the left and the low-tension a. c . switching on the right. T h e practice. develop high speed, direct transformers are not shown. Three of these synchronous converters, in parallel, are connected to one cell circuit. Larger a. c. turbo-unitsc o m e c t ed. alternating curi. e., one large enough to carry rent turbo-generators, was logical that electrolytic plants such as refmeries or chlorine the whole load-might be used in the hope of increasing effiand caustic soda plants should consider for their power plants ciency, but it will be shown later that this efficiency could the installation of a few fairly large, highly efficient, alternat- not be utilized or would be offset by other inefficiencies. ing current turbo-units, and that the direct current for the The investment would also be greater, as in Table IV. tank-house circuits should be obtained by conversion from Table IV-A. C. Generation for Larger Unit6 alternating current, utilizing either synchronous converters 2 units (including one spare), each consisting of one 6000-kilo~1so,ooo watt, a. c. generator, direct connectcd to turbine or motor-generator sets. At present, however, the situation 2 condensers 60,OOo 5 converting units (including one spare) consisting of synis somewhat changed, in view of modern developments in chronous Converters, transformers, regulators, and switchhigh powered reduction gearing, and because equipment boards, or else consisting of motor-generator sets 150.000 manufacturers can now build moderate-sized steam turbines TOTAL 8390,000 of very high efficiency. These changes make logical, necessary, Either of these a. c. plants couM utilize the spare unit in and desirable the consideration of direct current generation by means of moderate speed generators geared to high-speed supplying power to motors driving the mechanical loads turbines, taking advantage of low d. c. bus losses and thus throughout the plant, or the total load-the electrolytic load plus the mechanical load-could be so divided between eliminating conversion losses. the units as to get the best efficiency obtainable under the Comparison of A. C. a n d D. C. Systems conditions from all. I n practice, however, it is found diffiAn electrochemical plant having four electrolytic circuits cult or not advantageous to combine the loads. With the of 1400 kilowatts maximum, each totaling 5600-kilowatt d. c. generating plant for the main electrolytic circuits, a load with bleeding required for process purposes, could gen- separate unit of proper size, either a. c. or 250 d. c., would be installed to handle the motor load. erate power in either of the ways shown in Table 111:
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INDUSTRIAL A N D E,VGI,VEERING CHEMISTRY
May, 1926
Table V-Data Kilowatts Pounds per b. h. p. a t 365 Ibs. gage, 28.5 inches vacuum Same conditions, 200' F. superheat A. c. generator, e5ciency a t 80 per cent power factor Pounds steam per kilowatt hour Efficiency of motor-generator set conversion, % Pounds per kilowatt hour at d. c. bus Efficiency of synchronous converter, transformer, and regulator conversion, 7, Pounds steam per kilowatt hour at d. c. bus D. c. generator e5ciency, 7, Reduction gear efficiency, yo Combined e5ciency! % Pounds steam per kilowatt hour at d. c. bus Best obtainable motor-generator set conversion Superiority of d. c. generation % Best obtainable synchronous c6nverter conversion, Superiority of d. c. generation, 7,
Showing Efficiency of Turbo-Units 1400 1750 2100 9.89 9.59 10.34 8.48 8.22 8.88 91.0 92.2 89.0 11.95 13.35 12.45 82.3 84.3 85.6 14.00 16.25 14.75 89.3 14.95 90.6 96.3 87.2 13.62 13.7 0.5 12.60 -5.0
The point of best efficiency of turbo-units for such plants is usually at about two-thirds of the maximum continuous load capacity. Taking into account the generator efficiency, i t will be only 2 per cent less efficient a t maximum load, but 18 per cent less efficient a t one-third maximum load. The data in Table V are given on a turbine capable of carrying a maximum continuous load of 4375 kilowatts. It might be a d. c. geared unit consisting of turbine, reduction gear, and two 2187.5-kilowatt, 275-volt, d. c. generators, each supplying an electrolytic circuit; or it might be an a. c. directconnected turbine unit supplying the same two electrolytic circuits through converting units such as motor-generator sets or synchronous converters, transformers, and regulators. I n selecting a. c. generating equipment for a plant where electrolytic and generating rooms are in close proximity the electrolytic layout and process being conducted must be considered because if several a. c. units are carrying the total load, a relatively high efficiency may be maintained in the generating equipment under light load conditions on the plant by shutting down part of the generating equipment. I n using a large unit, it is apparent from Table V that it is important to keep it loaded. I n many plants, light load on the plant means that all circuits are operating but a t greatly reduced loads. This is particularly true in copper refineries or electrolytic copper or zinc plants, or in fact any electrolytic process where reaction between the electrolyte and the products of electrolysis begins to cause trouble as soon as the current is shut off, or the voltage is lowered below the critical voltage of the cell circuit. It is customary to keep voltage and some current on all cells so long as business conditions or maintenance conditions permit. I n chlorine and caustic soda plants, however,
90.2 13.8 91.5 97.0 88.7 12.78 13.40 5.0 12.55 -2.0
90.9 13.15 92.2 97.5 89.8 12.28 13.37 8.5 12.60 $2.5
509
2625 9.27 7.95 93.5 11.4 87.4 13.05
2800 9.19 7.88 93.8 11.25 87.9 19.8
3500 9.33 8.00 94.8 11.3 89.4 12.65
91.75 12.45 93.05 98.0 91.2 11.7 13.05 11.6 12.45 +6.5
91.90 12.25 93.2 98.1 91.45 11.53 12.80 11.0 12.25
92.2 12.25 93.5 98.5 92.2 11.6 12.65 9.0 12.25 +5.5
+6.0
4375 9.54 8.18 95.8(100%P.F.) 11.45 90.7 12.61 92.5 12.39 93.8
98.8
92.6 11.8 12.61 7.0 12.39 +5.0
it is usual for a number of cell circuits to run in parallel on one converting or d. c. generator unit, or on one bus to which all such units operating supply power in parallel. Independent control is therefore obtained only by cutting out or cutting in sections of cells in individual circuits and reductions in output are frequently made by cutting out whole cell circuits. The electrolyte is then shut off and drained from these cells and they are turned over for repair or rebuilding, as is required about every twelve months. The voltage is kept up fairly well on the remaining circuits, so that the efficiency of the d. c. generators or converting equipment in actual service is kept high; and if it is possible to reduce the number of a. c. turbo-generators in proportion to the reduction in total load, the turbo-generator or generators in service may be kept operating a t the most economical point or within the economical range. Probably the most economical a. c. generating plant for such an electrolytic plant would be one having three duplicate high-pressure, superheated steam bleeding, a. c. turbogenerators, for conditions in which the sustained average load would be equal to or not less than about two-thirds full load of two units, and the seasonal maximum load would not exceed two-thirds full load of three units or the full load capacity of two units. I n this way, should six fully loaded converting units represent full load (business boom) plant conditions, and four fully loaded converting units represent the sustained average (normal business) conditions, then a fairly high generating and converting efficiency would be obtainable at any plant load as represented by six, five, four, three, or two fully loaded converting units. I n electrolytic copper refineries or reduction plants, however, the situation is different. At such plants it is the
Table VI-Effect of Load on Efficiencies 8000 amp.--Av., 7000 amp.M. g. set M. g. set efficiency efficiency Volts Kw. 75 Volts Kw. % Circuils I and 2 , T a n k House I Max. 2i.5 2200 90.7 255 1780 89.5 210 1050 86.3 Av. 250 2000 90.34 230 1610 89.0 190 950 85.5 A4in. 225 1800 89.6 205 1430 88.12 170 8 s 84.52 Ratio of min. to max. load = 3 8 . 6 % Ratio of av. min. to av. max. load = 4 7 . 5 % Circuil I , Tank House 2 Mar. 205 1640 SS.i8 190 1330 87.5 160 83.85 4v. IS? 1440 87.6 165 1150 86.3 140 82.39 hlin. 15, 1240 86.2 140 980 84.7 80.95 120 600 Ratio of min. to max. load = 36.6% Ratio of av. min. to av. max. load = 4 8 . 6 7 , Circuit 2 . Tank House 2 Max. 225 1800 85.6 2 io 1470 88.3 1i5 875 84.8 Av. 200 1600 88.58 185 1300 87.3 15.3 775 83.47 Min. 17.5 1400 8 7 . 07 160 1120 86.02 133 675 82.02 Ratio of min. to max. load = 37.57, Ratio of av. min. to av. max. load = 4 8 . 4 7 , Kw. Max. 7840 Total load for 4 circuits 2976 5670 Ratio of total min. to total max. load for 4 circuits = 387,. Ratio of total av. min. to total av. max. load for 4 circuits = 46%. The corresponding efficiencies of synchronous converter, transformer, regulating units to meet the same conditions would range from 887, at the minimum condition of 5000 amperes a t 120 volts, to 92.5% at the maximum condition of 8000 amperes at 2 7 5 volts-that is, a difference of from 7 % at the lightest load to 1.8% a t the heaviest load. 3-unit motor-generator set speed 720 r. p. m.; synchronous converter speed 450 r. p. m. -Max.,
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
practice for all cell or tank-room circuits to be kept alive and taking current a t reduced voltage as long as possible. Individual tank-room circuits are controlled by means of individual converting or d. c. generating units. Reduced plant output means lowered voltage upon some, or all, of the tank-room circuits, usually all in proportion. If a. c. generation for electrolytic power is selected, it is still practicable to maintain fairly high efficiency in the generating plant at all reasonable plant loads by proper selection of the size of prime mover; but it is difficult, or practically impossible, to maintain it upon the converting equipment, particularly if motorgenerator sets rather than synchronous converters are employed. This is due to the relatively low efficiencies of motorgenerator sets under light load conditions. It is generally impracticable to put tank circuits in parallel or series in light load conditions for the purpose of raising the load upon, and therefore, the efficiency of, the converting equipment in operation. In other words, while it may be possible to keep the prime-mover unit operating above 60 per cent load, it is not possible to keep the converting unit thus operating, and particularly, if the converting unit is a motor-generator set, the over-all efficiency as represented by pounds of steam per kilowatt hour a t the d. c. bus to the tank room drops off appreciably. Steam consumption a t such a condition (see Table V) would be a t 700 kilowatts to each cell circuit and four cell circuits upon one prime mover. For motor-generator set conversion, this consumption would be 11.25/82.3 = 13.7 pounds per kilowatt hour. For synchronous converter, transformer, regulator conversion, this consumption would be 11.25/89.3 = 12.6 pounds per kilowatt hour. The corresponding figure for d. c. generation is 13.62 pounds per kilowatt hour. It appears, therefore, that in such plants, d. c. generation is more efficient than a. c. generation with motor-generator set conversion under any reasonable load conditions. It is less efficient than a. c. generation with synchronous converter, transformer regulator conversion under the very light load conditions, but appreciably more efficient than a. c. generation with synchronous converter, transformer, regulator conversion a t one-half load and above on the d. c. generator unit, which in this case is one-half load or more on the tank circuits. Inasmuch as it is frequently impracticable to maintain most efficient loading of a. c. generating units, the actual operating efficiency of d. c. generation is likely to show up even better in comparison with a. c., including conversion, than indicated by the table. To show that consideration must be given to light load operation for long periods, the figures in Table V I are given as conditions cited for refinery C , which must be met by the electrolytic power equipment, together with the efficiencies of motor-generator sets offered to meet them. of Cost8 of Equipment for A. C. or D. C . Generation of Electrolytic Power D. C. Generation 3 units (including one spare) each consisting of two 2187-kilowatt, d. c. generators geared to turbine (including gear and $393,000 switching) 65,500 3 surface condensers
Table VII-Comparison
TOTAL 2200-kilowatt, a. c. turbo and condenser for the motor load
$458,500 57,500
TOTAL A . C . Generation 3 units (including one spare) each consisting of 4375-kilowatt, 100~o power. facto:, a. c. generator, direct connected to turbine and includinq switching 3 surface condensers 5 converting units (including one spare) consisting of synchronous converters, transformers, regulators and switchboards; or else consisting of motor-generator sets and switching
'$516,000
$192,000 65,500
TOTAL 2200-kilowatt, a. c. turbo and condenser for motor load
$475,500 57,500
TOTAL
$533,000
218,000
Vol. 18, No. 5
One equipment manufacturer offered to meet the steam and electrolytic conditions by means of two mixed-pressure, multi-extraction turbo-generators of 6250-KVA., 95 per cent power factor capacity each. Only one unit would be operated under light load conditions, but both would be operated under heavy load conditions (when, of course, there would be no spare unit) and the motor load of the plant would be carried by the main units. A . C. Generation 2 a. c. turbo-generator units (no spare) 2 condensers 5 converting units (including one spare), consisting of synchronous converters, transformers, regulators, and switchboards; or else consisting of motor-generator sets and switching
$180,000 60,000
218,000
TOTAL
6458,000 44,000
TOTAL
$414,000
Omitting spare converting unit
Another equipment manufacturer offered to meet the steam and electrolytic conditions and to carry the motor load by means of two 5000-KVA. and one 3125-KVA. a. c. turbogenerating units. This arrangement also lacked spare capacity with plant operating under maximum business conditions. Maintenance of Electrolytic Equipment
Where the distance between power-generating plant and electrolytic tank room is so great that bus losses and costs offset the efficiency difference in favor of d. c. generation, the figures are much in favor of an a. c. generating system in which the electrolytic power is obtained by conversion by means of synchronous converters rather than motor-generator sets. With synchronous converters, furnishing the electrolytic circuits and operating at unity power factor, the power factor of the whole plant is so high, owing to the electrolytic load being such a large share of the total, that power factor correction by means of synchronous motor generator sets, or otherwise, is seldom, if ever, required. The greater maintenance has, however, been brought up in several cases as a deciding argument in favor of motor-generator sets. Several years ago, an electrical engineer operating over 250,000-kilowatt synchronous converters in one of the largest electrometallurgical companies in America, and the writer made a thorough investigation of the cost of maintenance. The records over a period of years of three plants in different localities were examined. These records showed the maintenance costs of three or four different sizes of large synchronous converters, the product of two different electrical manufacturers. The average of all was found to be 15 cents per kilowatt year of synchronous converter capacity. The operation x-as a t 500 volts or higher, and it is reasonable to assume that for 250-volt electrolytic service this figure might be doubled as commutators and collector rings would be double the size required for 500 volts. A figure for maintenance thus obtained might be considered as having the highest reliability possible. At another electrolytic plant, the property of another large mining and metallurgical company, where six 5800kilowatt synchronous converters have been in operation for a period of years, also delivering 500 volts or higher to the tank circuits, the maintenance was reported to be $5.00 per day per converter. This would be 31.5 cents per kilowatt year, or if doubled, 63 cents per kilowatt year for 250-volt service. Despite this high reported maintenance, additional duplicate units were rewntly purchased without competition for this plant, so that apparently the maintenance of synchronous converters did not militate against them. The report from this individual plant, however, should not be accepted as a criterion of what should be expected as the
May, 1926
INDUSTRIAL A N D ENGINEERING CHEMISTRY
proper maintenance cost for synchronous converter operation in electrolytic service. Large Electrochemical Plant Developments in 1925 A number of large electrolytic plants have been developed within the past year, and it is of interest to note the type of equipment purchased in each case.
511
converters when the power supply is a t 25 cycles. From Table VI11 it is apparent that synchronous converters would be the best application a t plant E. The comparative efficiencies over the range of operating voltage at the normal load current of 32,000 amperes are as follows: Voltaee 280 265 250 235 220 190 160 130 IiotoF-generator 89.3 RS.0 8 6 . 4 84.4 9 0 . 3 90.1 8 9 . 7 sets, per cent 90.6 Synchronous converters 9 2 . 5 5 9 2 . 5 9 2 . 4 92.05 9 1 . 8 5 91.2 9 0 . 4 8 8 . 9 ~~
(1) Refinery A , already referred to, requiring process steam, and havmg tank house and power plant adjacent purchased a Efficiency of Electrolytic Power Conversion Equipment 2800-kilowatt, 136.5-volt, direct current, geared turbo-generating unit. Men operating or managing electrochemical plants and (2) A large electrolytic reduction plant F , located several miles from the 60-cycle hydroelectric plant, purchased 65,000 those laying out extensions or new plants for the future want kilowatts of synchronous converters. to know the efficiency of available equipment, which repre(3) Refinery C, already referred to, requiring process steam sents as nearly as possible what will be obtained in service and having tank house and power plant in reasonable propinquity, after installation-that is, the efficiency which will actually purchased 60-cycle, a. c. generating turbo-units, and 8000 kilowatts of 100 per cent power factor motor-generator sets for con- figure in their cost of operation. They do not seem to care how this efficiency is measured or tested. version. (4) A large electrolytic reduction plant, located T a b l e VIII-Data on E l e c t r o l y t i c R e d u c t i o n Plant E nearly 100 miles from the 50-cycle steam power (4) (3) (1) (2) plant, purchased 22,000 kilowatts of 90 per cent 3 m. g. sets 5-7200 amp. 6-7200 amp. 4-25 cyc. power factor motor-generator sets for conversion. svn. conv. 300 r. u. m. (no soare) 6600 v.. transformers synchronous Same m. g. sets ( 5 ) The Union Miniere Haut de Katanga in and regulators 12,000 amp. converter Central Africa purchased 12,000 kilowatts of 80 transformers Same as 280 v eff. 91% and regulators 500 r. p. m. (2) (no spare) per cent power factor, 50-cycle, motor-generator $258,550 $310,100 $214,500 $283,800 sets for conversion for electrolytic copper reduction. Price f . 0. b. factory 920,000 677,000 766,500 887,800 weight, pounds (6) A large electrolytic reduction plant, E , Shipping $333,750 $231,900 $278,250 $306,600 Price f . 0. b. destination located a considerable distance from the 25-cycle Efficiencyunder normal op92.5 90.3 92.5 90.3 eration % power plant, purchased 11,500 kilowatts of 100 per Allowable' addition to first cent power factor motor-generator sets for conver$; 92,000 ... $ 92,000 cost for better efficiency sion. Synchronous converter
(Y
I
maintenance assumed in
.
.
excess of motor-generaApparently in cases (4) and (5) motor-genertor set maintenance cap$ 18,700 ... $ 18,700 ... ator sets for conversion were selected primarily italized figures f. 0. b. on account of the necessities for power factor Comparati,ve $260,450 $204,950 $306,600 $231,900 destination correction. It is doubtful that the suDerior effiActual input-output tests to check efficiency guarantees are ciency of synchronous converter conversion was overlooked, or that maintenance was a decisive factor. Obviously, when costly, and very difficult to make accurately. On the other power factor correction is required the synchronous motor- hand, separate loss tests, to determine the efficiency by totalgenerator set is excellent apparatus for obtaining it. Prob- ing the losses separately determined, are reasonably inexpenably the question of' maintenance was influential in cases sive to make, and easy to make accurately. Assume that the (3) and (6), although it is difficult to see how it could efficiency of conversion is 93.5 per cent. The output is then have been decisive. Any figure on increased maintenance about 100 per cent, and the input 107 per cent. Meters with an error less than 1 per cent plus or minus are not commercial, for synchronous converters as compared with motor-generator sets, as experienced by any operating company, can be and are not obtainable except when special care is given to more than written off by the savings obtained from greater calibration. To get correct readings, the supply voltage and frequency, and the direct current load amperes and voltage efficiency. The following analysis was made for electrolytic reduction must be held absolutely constant. If either of the meters is off, or the voltage fluctuates so that the input actually plant E . The conditions are as follows: read is 1 per cent plus or minus from 107, or the output 1 One tank-room circuit taking a maximum of 36,000 amperes per cent plus or minus from 100, then the efficiency actually a t 280 volts obtained by test may vary anywhere from 91.7 per cent to Range of voltage, 130 to 280 volts 95.4 per cent. On the other hand, if by means of a small Normal operating condition, 32,000 amperes a t 265 volts belted motor driving the rotor of the electrolytic set, the Cost of power, $70 per kilowatt year Freight charges, $2.57 per 100 pounds difference between the input and the output is being directly Synchronous-converter maintenance assumed to be 30 cents per kilowatt year and motor-generator set maintenance nothing measured-that is, if, instead of measuring the 100 and the 107, we are measuring 7-then any error from the same causes Capitalization, 16 per cent is the same per cent of 7 it would be of 100 and 107, and the Three-unit construction is taken advantage of in applying losses measured would only vary between approximately motor-generator sets in electrolytic service; that is, an elec- 6.85 and 7.15 instead of between 5 and 9. Obviously, it trolytic unit or motor-generator set consists of one motor with is desirable to keep away from input-output methods of test, a generator on either side and the necessary exciter or ex- and to adhere to separate loss methods. citers, direct connected beyond the generators. This conThe American Institute of Electrical Engineers has destruction permits the highest speed and lowest cost prac- veloped a method of stating and determining the efficiency of ticable in units of larg& kilowatt rating; and in the case of equipment of this type which takes all losses into considera25-cycle power supply it permits furnishing generators of tion and accurately indicates what may be expected of the appreciably higher inherent frequency (around 60 cycles)apparatus in service. that is, with an appreciably greater number of brush arms and E~ciency--Classification of Losses reduced magnetic circuits as compared to the corresponding Those accurately measurable: 25-cycle synchronous converters. This tends to reduce the (1) No-load core loss, including eddy current loss in conduccosts of motor-generator sets in comparison with synchronous tors a t no load
512
INDUSTRIAL A N D E,VGISEERING CHE;MISTRY Load 12R losses in windings No load 12R losses in windings Miscellaneous losses, such as field rheostat loss, ventilating blowers, if any, and other auxiliary apparatus.
Those approximately measurable or determinable : (1) Brush friction loss (2) Brush contact loss (2 volts considered as standard) (3) Windage and bearing friction losses Those that are indeterminable-that is, “stray load losses .” (1) Iron loss due t o flux distortion (2) Eddy current loss in conductors due to transverse fluxes occasioned by the load current (3) Eddy current loss in conductors due to tooth saturation resulting from distortion of main flux (4) Tooth frequency losses due to flux distortion under load ( 5 ) Short-circuit loss of commutation
The American Institute of Electrical Engineers Rules, formulated as a result of test data obtained from all the electrical equipment manufacturers, provide that in determining efficiency 1 per cent should be allowed a t all loads for the “stray load losses.” This concludes a method for determining efficiency by the separate loss method, which appears to be as true an efficiency as it is practicable to indicate. For electrolytic motor-generator sets having direct-connected exciters, an additional allowance should be made for the exciter losses. I n general, the subtraction of a n additional 0.1 or 0.2 per cent will provide for this factor. Mercury-Arc Rectifier for Electrolytic Service
The mercury-arc rectifier has reached an appreciable commercial development in the past few years, owing to the efforts of European companies who were forced, as a result of the war, to find a substitute for synchronous converters which would not require much copper in the manufacture. American equipment manufacturers had practically stopped the development of the mercury-arc rectifier when it was found that the vacuum could not be maintained without continuous pumping. As a result of European work, however, a very reliable piece of apparatus has been developed, which in the sizes that have been principally built-from 500 to 1000 kilowatts, at 600 volts-has only about one-third the weight of the corresponding synchronous converter. Chief among the advantages are the limited amounts of active materials and the comparative ease with which the active parts may be replaced; in other words, the amount of actual labor and the time taken in changing important parts is very small. However, this advantage is offset somewhat in that considerable time is required to establish the vacuum in the first place, or after extensive changes or repairs, and that 20 minutes or so are required to start the rectifier after it has been out
Vol. 18, No. 5
of operation for a few hours or days. After the rectifier is once in operation, the pumps are operated only a t intervals. Their operation can, in fact, be made automatic as necessary to maintain the vacuum. The mercury-arc rectifier is very efficient for high (600 volts and above) d. c. voltages. Its efficiency is largely proportional to the d. c. voltage, and its size to the amperes output rather than the kilowatts. This is due to a practically constant voltage drop (approximately 23 volts d. c.) through the rectifier. The efficiency of a 300-kilowatt, 600-volt, mercury-arc rectifier outfit has been determined as follows: Loss
Per cent Rectifier (23 volts) Transformer Reactors Excitation Vacuum pump Voltaee .. reeulator -
3 2 0 0 0 0
84
80 1.5
on 28
24 __
TOTAL
7.4 Efficiency a t 600 volts 9 2 . 6 per cent For 300-volt, d. c. electrolytic service the corresponding losses (the 2Jvolt drop being constant) would he 11.24 per cent, and the efficiency 88.76 per cent. For 150-volt, d. c. electrolytic service the corresponding losses would be 15.08 per cent, and the efficiency 84.92 per cent.
Therefore, although the mercury-arc rectifier outfit will probably be used extensively in railway service and deserves consideration for electrolytic service a t 500 or 600 volts d. c., it would not be the most efficient application for most electrolytic plants that use voltages less than 300 volts d. c. Furthermore, as the size and costs are proportional to amperes output rather than kilowatts, costs for low-voltage mercury-arc rectifier installations need not be at all comparable to those of high-voltage installations. The constant voltage-drop loss in the mercury-arc rectifier is somewhat analogous to the much smaller but also practically constant voltage-drop loss which occurs in the brushes of direct current generators. I n the d. c. generator, the per cent loss due to brush drop is directly proportional to the voltage of the machine. The brush friction loss is also directly proportional to the voltage. It is of interest to compare these losses with the constant voltage-drop loss of the mercury-arc rectifier (Table IX). c e n t Losses w i t h D. C . G e n e r a t o r and M e r c u r y - A r c Rectifier 125-Vo~rSERVICE 250-vOLT SERVICE Full load Half load F u l l load Half load D. c. generator. 1 . 6 a t all loads 0 8 a t all loads Brush I2.R Brush friction 1.4 2.8 0.7 1.4 Total losses a t commutsltnr .?n A 4 1 5 2 2 Constant voltage-drop loss of mercury-arc rectifier 18.4 18.4 9.2 9.2
T a b l e IX-Per
-
.I__.
_ _
Census of Dyes Production of coal-tar dyes in the United States in 1925 aggregated 86,000,000 pounds valued at $40,000,000 compared with 79,000,000 pounds, valued a t $36,900,000 in 1924, according t o a preliminary statement of the Tariff Commission the increase is attributed t o a greater activity in the domestic textile industry and t o improvement in export demand. Prices continued t o decline during 1925, the report states, while the output of fast dyes increased steadily. Imports were extremely heavy during the year, and there was a substantial increase in exports. The outstanding features for the year were: Continued recessions in dye prices due largely t o severe competition between domestic manufacturers. Progress in the manufacture of fast dyes, many valuable dyes of high fastness being produced for the first time in the United States in 1925. The output of v a t dyes (other than indigo) exceeded 2,500,000 pounds, compared with 1,820,000 pounds in 1925. This class of dyes is used largely on cotton a n d yields shades of exceptional fastness. The imports of dyes recorded a 75 per cent increase b y quantity, and a 6 5 per cent increase b y value. This increase in imports since the tariff
reduction of 15 per cent on September 21, 1924, shows increased competition from foreign dyes, mostly of the higher cost types. Dye exports recorded a n increase amounting t o 64 per cent b y quantity and 19 per cent b y value over t h a t of 1924. The weighted average price of all domestic dyes sold in 1926 was nearly 14 per cent less than the 1924 average. The price recessions were of a general character and include the low as well a s t h e high price dyes. The average sales price of indigo in 1925 was 15.5 cents per pound, compared with 22 cents in 1924. T h e current price is about 12 cents per pound, a decrease from the pre-war price, when our entire supply was imported from Germany a n d Switzerland. The total dye imports during 1925 were 5,318,158pounds, with a n invoice value of $4,791,900. This represents a 75 per cent increase b y quantity and a 65 per cent increase b y value over t h a t of 1924. Since the tariff reduction, imports have conspicuously increased. An increase in the activities of the textile trade during 1925 a n d t h e latter p a r t of 1924 was a factor in the increased imports after the tariff reduction. There has been, however, an increased competition from foreign dyes, principally of the higher cost types used for special purposes. These dyes have been almost entirely of German a n d Swiss manufacture.
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