Full Steam Ahead!
I
AT the present moment, with the birth of James Watt two hundred years in the past, it may seem a project in mere reminiscence to invite attention to steam. Is not the subject mostly historical? Have we not graduated long since into the electrical age? And are there not more modern and more ingenious schemes for generating electricity than by bridling the puffing vapor of the familiar teakettle? Newspaper headliners and political orators acquaint us with the mighty potentialities of ‘free’ falling water as dramatized along the Tennessee Valley in the South, at Boulder Dam in the West, and at the projected St. Lawrence waterway in the East — to say nothing of the clever exploitation of nature’s ‘free’ resources implicit in the Passamaquoddy project for harnessing the rise and fall of the tides. Even on the railroads, where steam has ruled for a century, the flare of modernization is linked in the public consciousness with spectacular now trains powered by diesel oil-driven electric generators. In the news, and consequently in the public mind, steam is inconspicuous, mostly absent, hardly ever mentioned in accounts of the new engineering.
But among the engineers themselves, and in the technical press, the case is far different. In the field of mechanics there is scarcely any branch of invention that is more rapidly advancing to-day than that of the innovators in steam. Truly sensational introductions are on the horizon, some of them already at work in a few adventurous industrial plants here and abroad. There are improved mechanisms for utilizing the expansive force of steam in power production, new types of boilers for producing the steam at high temperatures and enormous pressures, and even new liquids to boil in the boilers as auxiliaries to water. Underlying all this is a new-won body of exact knowledge of the properties of steam, a tabulation of data which has just been formulated out of the pioneering of a few research scientists in America and Europe.
Years ago someone called steam ‘that great civilizer.’ Never has it been more alive, more usefully in the service of man, more puissant, than in this fourth decade of the twentieth century.
II
First of all, let us understand that steam is still by overwhelming odds the prime mover of industry. In the United States, more than 60 per cent of the central power stations are actuated by steam, and 90 per cent of the individual industrial power plants are steam-driven. The largest singleshaft units for the production of kilowatts are not any of the mighty water wheels designed for Boulder Dam, but yet more powerful steam turbines recently installed in Philadelphia and New York. And the power plant with the largest actual output is a steampropelled manufactory of electricity on the East River front in Brooklyn.
At Niagara Falls, and some other places where dense populations and large industries are conveniently near water-power developments, the hydroelectric generator has demonstrated its economic superiority. But New York cannot afford to buy Niagara Falls power. So great is the resistance of copper wire to the long-distance transmission of electricity that in general it is cheaper to buy the coal to produce a kilowatt than to transmit that kilowatt over three hundred miles of wire. Coal can be shipped from five to ten times the distance that electricity can be transported for the same cost. And so successful have been the stratagems of the steam engineers that in every year of our decade the fuel costs of a unit of electricity have been lowered another decimal point. To-day the steam men are extracting from each ton of coal more than double the output of electricity that was obtained in 1920.
A kilowatt-hour is that amount of electricity which will keep alight for an hour forty of the ordinary twenty-fivewatt incandescent lamps, or one such lamp for forty hours. It is equivalent, in energy available for work, to one and a third horsepower for an hour. And just as you can figure the cost of a horse’s power in terms of food consumed, so you can reckon a kilowatt in terms of coal burned.
Back in 1920, for every kilowatthour that pulsed over the wires of the utility plants, three pounds of coal burned in their furnaces. By 1925 the average fuel requirement had been brought down to two pounds. By 1930 it was a trifle under 1.7 pounds. In 1935 the average for all utilities was 1.46 pounds. For the top dozen of these plants, the twelve most efficient steam producers of the United States, the present toll of fuel is less than one pound of coal for each kilowatt-hour of electricity.
To have tripled the economy of fuel utilization within the brief space of fifteen years would seem to be a very great advance indeed. And when we are reminded that Watt’s fame and fortune were won with engines which required thirteen pounds of coal to produce the equivalent of power which our moderns are getting from less than one pound, the steam plant of to-day can hardly be denied its pa?an of praise.
III
An engineer told me of a remote Marconi station in Africa where two natives work daily at a strange task. Their job is to sit tandem fashion at a pedal-driven dynamo and tread out the electric power necessary to actuate the wireless apparatus. In an eight-hour day, with a half hour off for lunch, they may pump into the lines as much as one kilowatt-hour of electricity.
For a contrast to this African treadmill consider a massive steam turbine in a Connecticut power plant. It whirls with the speed of a planet under the hot breath of a blast of mercury vapor and produces a kilowatt-hour of power in a third of a second, and at a fuel expense of three fourths of a pound of coal. If anyone is looking for a contrast of muscle labor versus mechanical power as a side light on economic ills, point him to these two contemporary examples of performance in the field of electrical generation: three quarters of a pound of bituminous coal (cost about two tenths of a cent) balanced against fifteen hours of hard human labor !
And yet, with all our advances, with brilliant attainments in fuel economy and in the mass production of power, the wastes are colossal. For the glaring fact is that most of the energy of the burning coal never gets converted into the kilowatts of the copper wire or the horsepower of machinery.
The life of steam is heat, and the potential heat stored in fuel is reckoned in a standard quantity known as ‘the British thermal unit.’ Every pound of good coal contains approximately 14,500 thermal units in potential form. Every horsepower-hour represents the conversion of approximately 2500 thermal units into mechanical power. Thus, by simple division, it appears that every pound of coal has the heat equivalent of more than five horsepowerhours. But in actual performance the best steam plants are able to get only about one and a half horsepower-hours, and even those which have the added help of mercury vapor get only about two horsepower-hours. In other words, the heat efficiency of the most capable plants to-day is only 25 to 35 per cent.
It is the remaining margin of 75 to 65 per cent of possible improvement that lures the engineers to ever new experiments with steam. The water wheels already are 90 per cent efficient — some of them better than that. If the hydroelectric plant at Niagara were improved to the extent that it converted every unit of energy in its falling water into harnessed horsepower, it would add only 10 per cent or less to its present output of electricity. But if the great steam-electric plant on the Brooklyn river front were to be improved to the extent that it converted every calorie of its burning coal into harnessed horsepower, it would add more than 200 per cent to its output of electricity. Since steam, with a present efficiency of 25 to 35 per cent, can compete successfully with water power, of an efficiency of 90 per cent and better, is it strange that steam should continue to dazzle the imagination of inventors and industrialists? Steam is so much more promising, it contains so many more wild horses of potential power, so much more energy waiting to be tamed.
We hasten to add that it is too much to expect 100 per cent utilization of thermal energy by any heat engine. The second law of thermodynamics tells us that, and it speaks imperiously. Heat will move only from a body or system at higher temperature to one at lower temperature, and in the transition some energy will be lost. Just as the copper wire demands a toll for its transmission of electricity, and will deliver at the end of its line less power than was pumped into it at the dynamo, so the steam cycle demands its toll for the transfer of heat, and will deliver to the engine or turbine less than was fired into it at the furnace. But the engineers believe that this toll is greater than it need be. Therein lies the story of our progressive advances and our hopes of future conquests.
In 1763, when Glasgow University asked the young instrument maker James Watt to repair one of its pieces of laboratory apparatus, a Newcomen steam engine, very little was known of the behavior of steam in the scientific sense. The laws of thermodynamics were yet to be formulated. But the careful Scot was immediately impressed with the heat losses which occurred in Newcomen’s engine. Its operation was a process of causing a piston to move upward in a cylinder by the pressure of steam, and then condensing the steam to cause it to move downward. In order to condense the steam, Newcomen cooled the cylinder with a jet of cold water — and thus for each cycle of its operation the steam had to reheat the cylinder before it could do its full work of pressing up the piston. What a waste, thought Watt.
He would build a better heat machine. It would be better because of a certain principle which was to be fundamental in its design: to keep the cylinder as hot as the steam which entered it. So, instead of cooling the steam inside the cylinder, Watt led it off in a pipe to a separate cooling chamber some distance away, and did the condensing there. And in order to guard against heat losses to the atmosphere, he surrounded his cylinder with a hollow jacket through which steam circulated and kept the interior mechanism as nearly as possible at a constant temperature. The reciprocating steam engine, the little giant that overthrew the old handicrafts and brought to pass the industrial revolution, was born of that simple idea.
It is the key idea of all our modern gains in steam-power efficiency — the principle of heat conservation. The guiding rule of our engineers still is to keep the power mechanism as hot as the steam which enters it, and, we may add, to generate that entering steam at a temperature as high as is possible to control. The higher the temperature, the greater is the pressure of the expanding steam, and in general the more rapid is its velocity.
Also, a high temperature at the beginning of the system permits a longer range for the temperature to fall through in its progress to the cool end of the system. As the hot vapor bombards a modern turbine, starting at 925 degrees Fahrenheit under a pressure of 1200 pounds to the square inch and with the speed of a pistol bullet, the spinning vanes of the stainless-steel rotor literally extract heat from the cleverly directed hurricane of steam. So great is the superheat that the metal glows a dull red; you can see it shining in the dark. Each stage of the whirling mechanism is larger, more extended, expands the steam some more, until at the end of its journey the spent vapor rushes into the condenser’s vacuum at 80 degrees, so cool that it chills the hand!
IV
Three things must be engineered in getting the maximum heat into steam: the water, the fuel, and the air. For every ton of coal that burns in the furnace, ten tons of water must be supplied the boiler, and twelve tons of air must be sucked into the combustion chamber. Incidentally, thirteen tons of combustion gases go up the stack. In a big steam plant, these items assume huge proportions. Thus the Hudson Avenue Station in Brooklyn consumes an average of 3750 tons of coal daily. To cool its steam at the end of the cycle, this station pumps 3,600,000 tons of East River water through its condensers, a daily pumpage that is well in excess of the total daily water supply of the city of New York. There are now ten steam power plants on the shores of this short stream, all using its water to cool their condensers, and in consequence the average temperature of the East River has been raised sixteen degrees in the last thirty-five years. In even the coldest winter it does not freeze over.
The water for the boilers must be as pure as can be got; minerals in suspension would soon accumulate from the continual evaporation. So distilled water is used, and it is used over and over again.
The fuel is mostly coal, though sometimes oil, and in certain regions gas; the coal is pulverized to an impalpable powder and then blown into the furnace through special burners designed to promote quick combustion. Some of the modern burners spray their powdered coal downward, and the black mist catches fire in mid-air. Temperatures as high as 3000 degrees are generated.
The temperature that can be generated depends not only on the state of the coal, but also on the state of the air which supplies the necessary oxygen. If the entering air is cold, some of the heat will be absorbed to bring it up to the kindling temperature, and to avoid this loss the engineers preheat the air.
Here is where those thirteen tons of hot gases become useful. As the gases pass out of the furnace into the flue, they are surrounded by pipes and hollow walls through which pass the twelve tons of air necessary to the combustion of the one ton of coal. By this means heat, which ordinarily would escape up the stack, is used to bring the air supply to temperatures of 400 to 500 degrees — and it is this preheated air that passes on to the furnace, to leap to its union with the falling particles of coal in the thousandth of a second.
Also surrounding the flues, and providing obstructions and heat absorbers to the waste gases of combustion, are other hollow walls and pipes for the preheating of the water before its admission to the boiler. The hotter the water, the hotter the air, and the finer the particles of coal, the more rapid will be the generation of steam in the water walls surrounding the furnace and in the coils of tubing between which the flaming gases pass in their swift flight to the stack.
There are other economizing devices, but perhaps the current trends in this direction can be suggested most concretely by citing certain new types of steam-producing mechanisms which have emerged from the experiments.
One of these is the Benson boiler. It dispenses with the familiar boiler drum and stakes all its expectations on a coiled steel tube hundreds of yards in length which surrounds the fire of the furnace. As first designed by Dr. Mark Benson, the boiler was planned to operate at a pressure of 3200 pounds to the square inch. At that terrific squeezing, steam has practically the same density as water, and the inventor anticipated certain advantages in this once-through tubular spiral, with water entering at one end and the equally dense vapor emerging at the other and ‘raring to go,’ packed with its maximum of heat and pressure. I believe the Benson has been operated at that pressure; and there are other boilers in Europe to-day that are generating steam at 3200 pounds; but in a factory in a Berlin suburb where a Benson boiler is now in commercial use the pressures that have been found most practicable are 1200 to 2500 pounds to the square inch. Operation there proved so satisfactory that a larger unit was designed for installation last autumn. Rumor has it that Russia is building five units of the Benson type — only they call it the Ramsin boiler in Moscow. A once-through boiler of somewhat similar type has been under development at the research laboratory of Purdue University, with important experimental results.
The Loeffler boiler, the invention of the late Professor S. Loeffler of Charlottenburg, Germany, is now under development at Vitkovice, Czechoslovakia. It resembles the Benson boiler in that it applies heat to a coiled tube, but it differs from the former in that only steam — and no water — is heated in this tube. The vapor is raised to 925 degrees, and then this superheated steam at high pressure is discharged into a steel reservoir of water. The hot steam literally boils the water, generating new steam of a lower temperature but with a full saturation of water, whereupon this saturated steam is pumped to the heat coil to be superheated. A pressure above 1900 pounds to the square inch is maintained throughout the system, and in the cycle of changes from saturation to superheat the steam is made to twirl a turbine and generate power. Loeffler boilers are in use in Germany and Russia, and one was installed in England in 1936.
Yet more radical is the Velox boiler, a Swiss innovation. All the accepted schemes of economizing are used, and more. Not only is the air preheated, but it is pumped into the combustion chamber under pressure and thus is in concentrated form when it meets its fuel. Chemical reactions are speeded by pressure, and the result of this and other devices is to hasten combustion. The hot gases generated by this combustion are forced at high velocities through multiple-walled boiler tubes, and this quickens the rate of heat transfer. Not only that, but before they escape up the flue the hot gases are harnessed to do other work: they are expanded into a gas turbine and made to turn its rotor, and this drives the pump which compresses the preheated air into the furnace. Another pump forces preheated water into the boiler tubes — ten to twenty fold as much as can be evaporated within the time. The total effect of all this forcing is a compact unit which delivers power in a hurry. So rapid is the heat transfer that steam can be raised from cold water to a temperature of 850 degrees in eight minutes or less. More than twenty Velox units are now in industrial use in Europe, and preparations have been on the way for the first American installation.
Boilers produce steam. Turbines turn steam into power at high velocities and in enormous quantities and with the minimum demand upon supervision, and for these reasons the rotating turbine has almost entirely supplanted the familiar reciprocating engine (with its back-and-forth motion) in plants where mass production of power is essential. Even in small plants the turbine is edging a way in. Philip Swain, editor of Power, tells of a 1000horsepower steam turbine which he saw some time ago in course of construction in Berlin — a unit that weighs only five ounces to the horsepower! ‘It is built to operate at 20,000 revolutions per minute and with steam at a pressure of 1400 pounds to the square inch,’ explained Mr. Swain. ‘Only the combination of high speed and very high pressure makes it possible for five ounces of turbine to generate a horsepower.’ But, he added, ‘if you suggest that this steam machine would be suitable for airplane drives, no one would deny it.’
The layman’s thought of steam is associated with the bubbling of water, but engineers are experimenting with other liquids which absorb heat and boil at other temperatures. Some of these are being used in industrial power production to-day.
There is, for example, mercury — the familiar liquid metal whose principal commercial use in the past has been in the manufacture of thermometers. You must heat mercury to 677 degrees in order to get it to boil at atmospheric pressure, whereas water boils at 212 degrees. Several years ago William L. Emmet began to experiment with a combination mercurywater system. His idea was to heat mercury to its boiling point, and use its vapor to turn a turbine. Then he would conduct the still-hot mercury vapor to a water boiler and use its heat to evaporate the water to steam, and employ the steam to drive another turbine. In this way the original heating of the mercury would cause two turbines to turn instead of one. Mr. Emmet’s idea is now in practical operation in an electric power plant in Hartford, Connecticut, in another near Newark, New Jersey, and in a new power plant recently erected at the General Electric Works in Schenectady, New York (where the inventor conceived and developed his device). As it has worked out in use, the mercury is held under moderate pressure to 958 degrees; from the left-over heat of the mercury vapor the steam is generated at 450 degrees and then separately superheated to 750 degrees. A hundred and fifty-five tons of quicksilver (cost about $200,000) bubble in the boilers of the Schenectady plant. This fluid metal is used over and over again in the hermetically sealed system, completing its cycle from boiler to condenser and back again hundreds of times each minute.
There is a chemical compound (C12H10O), known as diphenyl oxide, which boils at about 496 degrees, and an interesting series of experiments by the Dow Chemical Company has used this liquid in a binary steam-turbine system, somewhat analogous to the use of mercury in the Emmet system.
V
The story of steam power’s recent advance is primarily a story of increasing temperatures and mounting pressures, as we have seen. Few of the present attainments were feasible fifteen years ago. Iron glows red in the dark at 750 degrees, and in 1921 a boiler that delivered steam at this temperature was rated a superpower plant. To-day 900 degrees and above is established practice, and an experimental plant in Detroit has been operated at 1000 degrees. An indispensable factor in these steam developments has been the new alloys. The tough durable metals lately born of the meeting of steel with chromium, molybdenum, nickel, and tungsten have provided the necessary materials for the construction of boilers, tubing, turbine blades, and other parts able to endure the high temperatures and resist the high pressures.
But much of the development has been a groping, so far as exact knowledge is concerned. Designers in the post-war decade had to risk new ground, venturing conditions for which there were no precise standards. In 1921 a group of engineers, activated by George A. Orrok of New York, met at Harvard University to consider a programme of fundamental research into the properties of steam. At that time electrical standards were known with an accuracy of one part in ten thousand, but the same could be said of heat units only to one part in three hundred. Precise data were lacking for conditions at pressures above 140 pounds to the square inch. In consequence, liberal tolerance had to be allowed for all uses of steam at higher pressures; and since the trend of design was in that direction, the situation was notoriously unscientific.
Out of that Cambridge conference came a committee which (1) listed the steam problems a waiting determination, (2) allotted them to scientists at the Harvard Engineering School, the Massachusetts Institute of Technology, and the United States Bureau of Standards, and then (3) raised $100,000 to finance the research. Moreover, the American committee coöperated with groups in Europe to form an international conference on steam, and correlated investigations have been carried on at London, Prague, and Berlin. These searchings into the minutiæ of steam are now yielding a practical result — a new series of tables for the densities and other properties of steam. It can be said that these data place steam engineering on a new basis of exact science for all temperatures between 152 degrees (pressure four pounds to the square inch) and 860 degrees (5500 pounds to the square inch). And by calculation from these exact data it has been possible to extend the tables downward to the freezing point, 32 degrees, and upward to 1600 degrees.
Calculation, however, does not satisfy these realists. To-day the investigators are planning to push their experimental inquiries beyond the present limits of exact data, and to substitute determinations for extrapolations. The thermometer problem becomes serious at high temperatures. The task is not merely to measure approximately, but to get readings accurate within a thousandth of a degree. A platinum electric thermometer was used in the recent investigations, and is serviceable for temperatures up to 860 degrees.
Thus, scientifically as well as industrially, steam is alert, on the march, continually occupying new frontiers. At several research centres experimenters are seeking to adapt a steam unit to airplane propulsion, and at Oakland, California, a steam-driven airplane has been flown. Radical changes in steam locomotive design are emerging from the laboratories — witness the streamlined locomotive of the articulated ‘Hiawatha’ train which daily travels the 410 miles between Chicago and Minneapolis. It is reported that a European innovator is trying the Velox boiler in railway locomotive use. Meanwhile, the Union Pacific Railroad Company has placed an order for two locomotives of the Steamotive type. In this a steam turbine takes the place of the familiar reciprocating engine of cylinders and pistons, and drives an electric generator which in turn communicates the power to the wheels.
In 1881, Sir Frederick Bramwell, eminent among engineers of that day, entertained the British Association for the Advancement of Science with a prophecy. He predicted that ‘the heat engine of the future will be independent of the vapor of water,’ and doubted if the steam engine would be spoken of ‘fifty years hence’ except as a museum piece.
Well, the fifty years have passed, and our museums would need to be spacious indeed to hold all the harnessed horses of modern steam power. At no time has so much of the world’s work been entrusted to steam as now. Engineers should never say never.