The problems of converting the heat contained in fuel into useful work with greater economy have led engineers to consider some other fluid than water as a transmitting medium. Mercury is being used in the boilers and turbines in some power stations in the United States
MERCURY-STEAM-ELECTRIC POWER STATION erected by the General Electric Company of America at its works at Schenectady, New York State. Working on the principle devised by an American engineer, William Le Roy Emmet, this station is unusual in addition because it has been erected out of doors.
THE function of all generating plants driven by gas engines, petrol engines, heavy oil engines or steam engines is to turn into useful work the heat generated by the combustion of the different fuels. In all internal combustion engines, the heat of the burning gas, petrol or oil is used directly in the cylinders of the power unit.
In the steam engine, however, an intermediary has to be used between the fuel and the engine cylinder. The coal, oil, peat, wood or gas has first to impart its stored up heat to water, which, in the form of steam, then transfers the heat to the cylinder. The water is recovered, less the greater part of the heat it has carried, in the condenser, when one is fitted. Otherwise it escapes to the atmosphere as in the trailing clouds which accompany the locomotive on its hurrying way.
It may seem strange that the source of power should be called heat instead of the pressure of burning petrol or the pressure of steam, but the idea is basically sound and is no mere convention. The fundamental equation in power engineering is the simple one: Thermal efficiency = T₁ — T₂ / T₁
There is nothing mysterious about this plain algebraic expression. The term T₁ stands for the temperature of the medium entering the engine and T₂ for that when it is leaving. The result is a fraction, which is invariably reduced to a percentage when the engineer talks about efficiency.
This fraction never reaches unity, for that would mean that T₂ would have to be zero: in other words, all the heat would have to be abstracted from the medium before it left the engine — a physical impossibility. But the greater the engineer can make T₁ and the smaller he can make T₂, the nearer is the approach to 100 per cent thermal efficiency. The raising of the one and the lowering of the other have been the aim of successive generations of engineers. In the steam engine the final temperature depends upon the use of an efficient condenser. It would seem that no further progress can be made to lower the final temperature, so great have been the improvements during recent years in the design of the condenser and of its accessories.
The question then arises as to what can be done to raise T₁. Though much has been effected in this direction of late there are difficulties in the way. Every increase of temperature of either gas or steam is accompanied by a rise of pressure. When steam is being used the difficulties become formidable and steam boilers which work at a pressure of about 1,500 lb. per square inch, a fairly common pressure to-day — not infrequently cause their designers and users considerable concern and anxiety.
The major difficulty is that at the high temperatures attendant on high pressures the steel boiler tubes become red hot; when they are in this condition their strength falls off rapidly. The metallurgist has done a great deal of late years in providing steels which will not become unduly weak when hot, but there arise questions, which cannot be ignored, of first cost and maintenance.
FROM THE DRUMS of the mercury boiler radiate a number of tubes with closed ends. The vapour is collected in a header above the drums and passes thence to the turbines.
The problem has, however, been tackled from another angle, and to an American engineer, William Le Roy Emmet, is due the credit of working out in a brilliant and painstaking way a novel solution. He boldly discarded water as a means of heat transmission and, instead, filled his boiler tubes with that strange metal — mercury. He used the vapour from the mercury to drive the rotor of a turbine. Water turns into steam, at atmospheric pressure, at a temperature of 212° Fahr. Mercury, on the other hand, reaches a temperature of 675° Fahr. before it begins to vaporize in atmospheric conditions. In other words, it absorbs a greater amount of heat while still in a fluid condition. Therefore, at a temperature when steel begins to glow redly the pressure of the mercury vapour is only about 150 lb. per square inch, as compared with nine or ten times that pressure when steam is used. Here, then, is a means of getting a high T₁ without having to face greater difficulties of pressure than were successfully met when the ordinary triple-expansion steam engine was introduced.
This solution, however, was not in itself easy. Emmet and his devoted band of workers at the great Schenectady Works of the General Electric Company of America have, as have all pioneers, met with many troubles in perfecting the plant. The mercury-steam-electric plant, as it is called, may be considered as being yet in the early stages of development, though it is being watched with great interest by engineers.
The largest of the three plants erected by 1938 is at the Schenectady Works of the General Electric Company. There are two boiler units, one of which generates steam, the other being the mercury boiler. Large quantities of steam, as well as electric power, are required for manufacturing and heating purposes in the works. The major part of the steam demand, however, is met by the mercury unit. Both boilers are fired by pulverized coal, that is, by coal ground to a fine powder and blown into the furnaces by jets of air — a method of firing which causes an intensely hot torch-like flame.
The lower part of the furnace of the mercury boiler is lined with water tubes, in which steam is generated; the upper part is lined with tubes containing mercury. These linings are for the purpose of using as much of the heat of the fuel as is practicable, the mercury being evaporated mainly in the upper part of the boiler. Here there are seven drums, from the underside of each of which radiate 440 tubes closed at their outer, ends and projecting into the hot gases of the furnace. The mercury vapour generated in the drums and tubes is led to the mercury turbine, whose rotor it drives just as steam drives that of the ordinary turbine, the mercury vapour having a pressure of 125 lb. and a temperature of 958° Fahrenheit. On its passage through the turbine it spins the rotor round at the rate of 900 revolutions a minute and reaches the mercury condensers, of which there are two, at a pressure of 1·4 lb. per square inch absolute and at a temperature of 480° Fahr.
INSIDE THE FURNACE of the mercury boiler. This view is looking upwards towards the drums seen in the sectional drawing above. The hot gases from the burning pulverized coal pass over the tubes and between the drums on their way to the chimney.
The condensers are not unlike those used for a steam turbine as they contain a number of tubes through which water is circulated, the mercury vapour flowing over the outside of the tubes. But there is one important difference. The cooling water in a steam condenser is discharged as soon as it has done its task, carrying away with it a large amount of heat, which goes to waste in the sea in marine installations or in a river with land plants similar to Battersea Power Station. With the mercury vapour at an exhaust temperature of 480° Fahr., however, the cooling water is turned into steam and the mercury condensers are, therefore, at the same time steam boilers.
The mercury from the condensers, now in the fluid metallic state, is pumped back to the mercury boilers, just as condensed steam is pumped back into boilers as feedwater. The steam generated in the mercury condensers joins the supply from other sources. The output of the mercury unit is, then, 20,000 kilowatts of electric current generated in the alternator driven by the mercury turbine and 325,000 lb. of steam an hour, of which 240,000 lb. are generated in the mercury condensers and 85,000 lb. in the water walls of the mercury boiler. The steam pressure is 400 lb. per square inch and the steam from the water walls and from the condensers is then passed through a superheater situated in the uptake above the drums of the mercury boiler, in which it is joined by the steam from the steam boiler. The steam then passes, at a temperature of 750° Fahr., to a non-condensing steam turbine coupled to an alternator generating 6,000 kilowatts. This turbine exhausts at a pressure of 225 lb., the pressure necessary for the works supply.
Built on a Concrete Raft
At this point it may be objected that, as the steam is exhausted from the steam turbine at so high a pressure and at a correspondingly high temperature, the object of obtaining a widely-separated T₁ and T₂ has not been realized. But, steam at this pressure and temperature being needed, the gain in thermal efficiency by the use of mercury is not apparent in this particular installation. Suppose, however, that no process steam is required but that the installation is an electric power station pure and simple. A condensing steam turbine, driving an alternator, would in this instance have been fitted and the advantages of the lowest possible final temperature would have been secured. The fact that a high initial temperature, unaccompanied by dangerous pressure, has been obtained is not affected by the degree of final temperature in the plant.
Apart from the novelty of the operating principle, this power station is notable as being an outdoor one. The outdoor transforming station is a familiar enough sight nowadays, but this is probably the only example of a generating station with its boilers and turbines situated in the open air and unprotected. The object of this arrangement was twofold — first, to avoid the cost of a large enclosing building and, secondly, to have a structure to which extensions could be readily made.
The plant is carried on a concrete raft supported on piles. The foundations of the turbine set go down to this base, as do those of the boilers, and the circular coal bunkers, surmounted by the chimneys, are also supported directly by it. A glazed building surrounding the lower part of the plant was erected largely to prevent the coal dust from the pulverizing plant from blowing about and to afford cover for the coal wagons.
LAYOUT OF THE SCHENECTADY PLANT
To prevent loss of heat, the turbines and accessories are completely covered with insulating material; this treatment gives them a white appearance. Above them is a 50-tons travelling gantry crane for handling the parts during overhaul. An angled structure above the steam boiler and two others at the side of the coal bunkers house the flues through which the gases are drawn by induced-draught fans situated in chambers at the bases of the chimneys. Above the mercury boiler is the superheater and in the flue leading from this is the air preheater, the air for combustion being heated in the preheater before being discharged to the boiler furnaces by a forced-draught fan.
The difficulties experienced in the initial experiments were, in the main, due to defective circulation of the mercury, which caused some of the tubes to be burned out. The question of leakage from the mercury boiler and turbine was troublesome also. These difficulties have now been surmounted. The tubes are welded into the drums; each of them is made with an inner tube, not extending to the bottom of the outer one, which causes a brisk circulation of the mercury inside them. Yet the engineers have to guard carefully against the loss of mercury. It is expensive material and, though the boiler contains rather more than 12½ tons of it, none of it should escape. This precaution is necessary, not primarily because of expense, but because mercury vapour is poisonous to those exposed to it for any length of time, even though it be present in minute quantities.
As the presence of the vapour cannot be detected in such quantities by scent, irritation or colour, chemical means of detection have had to be devised. The first device used incorporated paper indicators soaked in a solution of selenium sulphide. Mercury vapour in the air caused the strips to turn black even when it was present in such a minute quantity as one part in 500,000 parts of air. This was not, however, considered satisfactory, especially as examination of the men working on the plant showed that they had absorbed mercury in what seemed to be alarming amounts. Then it occurred to some one that dentists are accustomed to handling mercury in connexion with tooth-filling operations, and so several perfectly healthy dentists were examined. As they were found to contain a larger amount of mercury than the plant attendants, that bogey was laid.
Early Troubles Overcome
The work, however, of perfecting a more sensitive detector went on and the existing device, which uses resonance radiation, can detect mercury vapour when present in such microscopic amounts as one part in 100 million parts of air. Pipes from various parts of the plant run to a central instrument and an interesting daily test takes place.
To find out whether the instrument is in good working order it is necessary deliberately to produce mercury vapour in the flues and watch it report its presence to the instrument. The test is carried out by the use of calomel pills. First one, then two and finally three of these pills are thrown into the boiler furnace when it is under full load, and an operator at the instrument records
the result. As calomel pills contain a strictly measured quantity of mercury, the amount liberated by the different doses is known and the instrument can thus be checked.
The mercury is free from a disability which causes trouble with the high-pressure steam boiler — deposition of scale in the tubes from hard feed water. At first, however, some analogous troubles developed. It was found that the circulation of the mercury was being interfered with by accumulations near the tops of some of the tubes. This trouble was traced to iron oxide (rust) and iron scrubbed from the tubes by the stream of moving mercury being held in the mercury and finally separating out at the points where the vapour formed. The cure lay in the addition of a small amount of another metal — sodium — to the mercury. This was found to dissolve the oxides and the resultant iron particles floated harmlessly in the drums. The tubes now appear to be silvered inside and the rate of heat transfer is improved.
What may be the future of the mercury plant cannot well be predicted at this stage of its development, but it is safe to say that with the enormous resources for scientific investigation enjoyed by the General Electric Company, its possibilities are certain to be explored to the utmost limits of practicability. The question of fitting mercury plant to large transatlantic liners, for instance, has recently been discussed.
MERCURY-BOILER CONTROL BOARD. The quadrant gauges at the top centre indicate light pressures such as chimney draught. The circular gauges flanking them show heavier pressures—mercury vapour, steam, feedwa^er and the like. The large dials in a row are recorders in which a pen traces on a chart curves of pressure and flow. At the bottom, push buttons with indicating lamps enable the electrically driven auxiliaries, pumps, coal pulverizers and so forth to be started and stopped as dictated by the indications on the gauges.
