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MERCURY IN THE NINETEENTH CENTURY

This is part of a British Association Paper called "Some of the Developments of Mechanical Engineering during the Last Half Century" By Sir Frederick Bramwell. It appeared in The Scientific American Supplement No. 312, on 24th December, 1881. While the mercury is not being used here as the working fluid (steam is) it's an interesting account of a bizarre idea.

"There was another kind of marine engine that I think should not be passed over without notice; I allude to Howard's quicksilver engine. The experiments with this engine were persevered in for some considerable time, and it was actually used for practical purposes in propelling a passenger steam-vessel called the Vesta, and running between London and Ramsgate. In that engine the boiler had a double bottom, containing an amalgam of quicksilver and lead. This amalgam served as a reservoir of heat, which it took up from the fire below the double-bottom, and gave forth at intervals to the water above it. There was no water in the boiler, in the ordinary sense of the term, but when steam was wanted to start the engine, a small quantity of water was injected by means of a hand-pump, and after the engine was started, there was pumped by it into the boiler, at each half revolution, as much water as would make the steam needed. This water was flashed on the top surface of the reservoir in which the amalgam was confined, and was entirely turned into steam, the object of the engineers in charge being to send in so much water as would just generate the steam, but so as not to leave any water in the boiler. The engines of the Vesta were made by Mr Penn, for Mr Howard, of the King and Queen Ironworks, Rotherhithe. Mr Howard was, I fear, a considerable loser by his meritorious efforts to improve the steam-engine."

"There was used, with this engine, an almost unknown mode of obtaining fresh water for the boiler. Fresh water, it will be seen was a necessity in this mode of evaporation. The presence of salt, or of any other impurity, when the whole of the water was flashed into steam, must have caused a deposit on the top of the amalgam chamber at each operation. Fresh water, therefore, was needed; the problem arose how to get it; and that problem was solved, not by the use of surface condensation, but by the employment of reinjection, that is to say, the water delivered from the hot well was passed into pipes external to the vessel; after traversing them, it came back into the injection tank sufficiently cooled to be used again. The boilers were worked by coke fires, urged by a fan blast in their ashpits, but I am not aware that this mode of firing was a needful part of the system."

The idea of using an amalgam of mercury and lead as a reservoir of heat was not a good one, and one regrets that Mr Howard was a considerable loser. The liquid with the highest specific heat, and so greatest storage capacity, is not some exotic chemical. Remarkably, it is plain water. Much cheaper and much safer!

MERCURY IN THE TWENTIETH CENTURY

The man behind the use of mercury vapour turbines in electric power stations was William Le Roy Emmet of the General Electric Company. Emmet devoted a great deal of time and energy to their development and promotion, "as a more efficient power generation system than steam turbines." The difficulties and risks in using the new technology led to termination of the development after his retirement. He died in Erie, Pennsylvania, on September 26, 1941, at the age of 82, so he had presumably managed to avoid mercury poisoning.

Mercury has a much higher boiling point than water, so the Carnot efficiency of a thermodynamic cycle that uses it is higher, in theory at least. The plants were dual-cycle, (also known as binary cycle) the hot metallic exhaust from the mercury turbines being used to produce steam for a second stage of steam turbines.

1914 Plans are made to build an experimental mercury boiler and power plant. (see below)

1923 The Hartford Electric Light Company (Helco) installs an experimental General Electric mercury-steam generating unit at Hartford, Conn.

THE BRITISH THOMSON HOUSTON PATENT: 1914

In September 1914 the British Thomson Houston was granted British patent GB21689 for 'Improvements in and relating to power systems' which is an obfuscatory patent title if I ever saw one. Clearly BTH thought they has something worth hiding, insofar as you can hide a patent. The patent probably roused little interest in Europe, as there other matters going on at the time.

The British Thomson-Houston Company was the British arm of the General Electric Company of the USA.

Left: The BTH patent of 1914 The text below is taken from the patent: A plant comprises a mercury-vapour turbine and a steam turbine, the exhaust from the mercury turbine being utilized to generate steam for the steam turbine. A mercury-vapour generator 8, provided with tubes 6, is heated by a furnace 5. The mercury vapour passes by way of a pipe 20 to a turbine 15, whence it is exhausted by way of a pipe 22 to a condenser 21, which also functions as a steam-generator. The condensed mercury returns to the generator 8 by way of a pipe 25 and economizer 27, situated in the furnace flue. The water for the steam turbine is heated in a coil 40 situated in the furnace flue, and afterwards passes through the generator 21 where it is formed into steam by the heat given up by the mercury. The steam is led by a pipe 45 to a steam turbine 18, which exhausts by way of a conduit 46 to a condenser 47. A by-pass valve 41 is provided to divert mercury vapour from the turbine when the pressure rises above a predetermined limit. A cooler 48, divided into two compartments by a perforated partition 52, is connected as shown to the mercury and steam condenser systems, and to an air-pump 49 so that one pump serves to extract air from the mercury and water. The action of the cooler prevents any mercury vapour from being withdrawn by the pump. The valves of the two turbines are under the control of a single governor, but are so set that the mercury valve is first opened and consequently the regulation due to variations of load is mainly effected through the steam turbine. The turbines may be arranged upon a common shaft or upon separate shafts.

This is most interesting because it shows that even at this early date the technology for later plants was in place, notably the use of Field tubes in the mercury boiler, and a turbine bypass valve rather than a conventional safety valve venting to atmosphere. (The latter not being a good idea in a mercury plant)

THE DUTCH POINT EXPERIMENTAL POWER STATION AT HARTFORD, CONN: 1923

Emett first proposed the use of mercury vapour in 1914, and got the backing of T H Soren, the vice-president of the Hartford Electric Light Company. (HELCO) Following a deal with the General Electric Company, a 5000 HP experimental system was built at the company's Dutch Point generating station in 1923. The system contained a hefty 20 tons of mercury, and used mercury vapor at 35 psi, (gauge) and 812 degF to drive a single-wheel mercury turbine. The mercury condensed at 1.5 psi, (absolute) and 485 degF, and in doing so generated steam in excess of 200 psig to drive a steam turbine. To improve efficiency, the steam was reheated in the mercury boiler to about 700F.

(Info from Connecticut, the Industrial Incubator 1982, published by the History and Heritage Committee of the Hartford section of the Americian Society of Mechanical Engineers.

There were some problems; it was reported in Popular Science (March 1931) that at one point a boiler head blew out with the escape of thousands of dollars worth of mercury, only part of which was recovered. Also a turbine disc disintegrated and put the unit out of action for several months. It was stated that "workers were overcome by the fumes of the poisonous mercury. There were, however, no fatalities, nor was anyone permanently injured, because of the unusual precautions taken by the company to protect the men who had charge of the work." That does not sound like mercury poisoning as we know it, Jim. One wonders about the long-term health of the workers who were "overcome".

After this plant had been fully studied a second experimental plant was built at South Meadow, giving improved results but also suffering some problems.

Info from Popular Science Sept 1929 and March 1931

SOUTH MEADOW POWER STATION AT PORTSMOUTH, NH: 1928

In November 1928 the first commercial mercury cycle generating unit in the world began operation at Helco's South Meadow Station in Hartford.

Left: The first commercial mercury plant installed at South Meadow in 1929 The mercury turbine ran on mercury vapour at 884 degF. The power output was 13,000 HP. More than a thousand gallons of mercury, weighing nearly 70 tons, were in use in the cycle. The current price of mercury was about $200 per gallon; the working fluid represented an investment of more than $200,000. Leaks were discouraged. According to Popular Science for September 1929, the plant had recently been dismantled for engineering improvements to be made. From Popular Science September 1929

Left: The two mercury-condenser/steam-boilers at the plant at South Meadow in 1931 The snail-shaped thing between them is the mercury turbine. From Popular Science, March 1931

Left: The mercury boilers at the plant at South Meadow in 1931 Given the general absence of pipework, this photograph was probably taken during construction. From Popular Science, March 1931

Left: The mercury part of the system at South Meadow in 1931 It appears from the picture that the mercury vapour safety valve does not exhaust to atmosphere, (which would be a really bad idea) but just bypasses the turbine. This might prevent turbine overspeed, though presumably there were conventional safety devices that measured the rotor speed directly, but it would not stop the boiler exploding. (which would be an even worse idea) I suspect that in reality the safety valve was connected to a big empty tank. This magnificent illustration is taken from Popular Science, March 1931

Left: The mercury part of the plant at South Meadow as at 1949 The mercury vapour leaves the mercury boiler drum, and drives the turbine at the top of the drawing; it then exhausts to the two adjacent mercury-condenser/steam-boilers; these are as close as possible because of the difficulties in handling large volumes of mercury vapour. Feed water passes through the economiser, leaving at 410 degF, and is converted to steam at 450 degF and 410 psi by the condensing mercury. This steam passes through a superheater in the top of the mercury boiler and then goes to the station steam header at 700 degF and 385 psi, which powers conventional steam turbines. The condensed mercury goes back to the mercury boiler drum down a spiral path to prevent the heavy mercury building up excessive momentum and wrecking the pipe-work. Note the use of air pre-heating; air was supplied to the burners at 523 degF. The South Meadow plant was reported in Popular Science March 1931 as producing 143 kiloWatt-hours of electricity per 100 pounds of coal burned, while the best conventional steam plant produced only 112 kiloWatt-hours. The average efficiency acrosss the whole country was only 59 kW-h per 100 lb of coal. The legacy of these experiments is still with us. Mercury contamination is in sediment of the the Connecticut River. Wethersfield Cove just south of Dutch Point is a former bend in the Connecticut River (it is an oxbow lake) that has up to 3 ppm Hg in its sediment and is stated to contain more than half a ton of mercury From Power Generation March 1950

The following was sent to me by Rich Colopoulos in August 2017:

"I would like to pass on some anecdotes about South Meadow Station. I was hired by Northeast Nuclear Energy Company in December of 1980 as a Mechanics Helper on Unit 2 of the Millstone Nuclear Power Station Complex in Waterford Connecticut. This is about 50 miles southeast of the South Meadow Power Station. South Meadow is located on the Connecticut River which empties into Long Island Sound which is were Millstone is located on. Some of the mechanics I worked with came from South Meadow Station after it was closed (I don't know the closing date but Unit 2 came online in the mid seventies)."

"I was assigned to help an older gentleman named Jimmy Preste who was in general just a great guy and loved to tell stories to pass the time while we were supposed to be working! He told me that they used to put wooden boards down on the mercury in the main condenser and that they would provide enough buoyancy so that could walk on them to service the condenser! I don't know if this is true but it sure is a great story and I don't know why he would have made it up, I never asked about the mercury he just volunteered the information. He and other mechanics also volunteered the ominous information that "there was a lot of mercury down by the river". I sure hope that this is not so true or that if so it has been cleaned up."

Left: A man sitting on top of a pool of mercury: 1971 Being a sceptical person, I questioned if an ordinary wooden board floating in mercury would have borne a man's weight. I got two answers: "A typical man of say 90kg standing on a board of say 600 x600mm (two foot square) on mercury, would depress the board less than 20mm (3/4") into the liquid." (Bill Todd) "Here’s a quick calculation. Mercury weighs in at 13.56 g/cm^3. If we need to hold up a 100kg person, thats 100,000 grams so we need to displace 100,000 grams of mercury. Thats about 7374 cm^2. Let’s call it 7500. If they were using a rough 2x10 (so actually 2” by 10), that has a cross section of 129 cm^2. Dividing through gives us 58 cm, or about 22”. Mercury is very heavy!" (Andrew Hughes) That seems to settle it; many thanks to both contributors. As further proof, here's a man sitting on mercury without any assistance from a board. The picture was published in National Geographic magazine in 1972/

SCHILLER MERCURY POWER STATION AT PORTSMOUTH, NH: 1950

MERCURY POWER IN SPACE: 1969

I have unearthed a NASA report from 1969 that describes a nuclear-heated mercury power plant intended to generate large amounts of electricity in space. It is called :"DESIGN AND FABRICATION OF A COUNTERFLOW DOUBLE-CONTAINMENT TANTALUM-STAINLESS STEEL MERCURY BOILER" from the Lewis Research Center, Cleveland, Ohio. It was part of the SNAP-8 program to develop a Rankine cycle power system for space applications.

The reactor is cooled with a mixture of sodium and potassium, (NaK) which heats a mercury boiler. The mercury vapour drives a turbo-alternator and is then condensed and subcooled by a secondary NaK heat rejection loop which transfers the waste heat to a radiator for rejection to space. Why is a second NaK loop used? I have no idea, but this is NASA we're talking about so I bet there was a good reason.
The report concentrates on the design of the potassium/mercury boiler, which is an exotic device indeed. It is a spiral stucture designed to fit in a small toroidal space in a cylindrical spacecraft; the heat exchanger tubes are of tantalum, which is readily wetted by mercury, making for good heat transfer. The net power output was 37 kW.

Left: The NASA nuclear mercury turbine system. Note the extra cooling and lubricating loop using polyphenyl ether.

Left: The performance data. The extraordinary thing is that the overall efficiency seems horribly low at 6.9%. A conventional terrestrial steam power station would give some where around 40%. So why is it so bad? The Carnot efficiency E = (T1 - T2 ) / T1 where T1 is the heat input (top) temperature and T2 the heat rejection temperature. Looking at the diagram above, the mercury leaves the boiler at 670 degC. The effective rejection temperature in the condenser is a bit less clear but the NaK coolant leaves it at 335 degC. This gives a Carnot efficiency of 35%. That's obviously a theoretical maximum, but a long step from 6.9%; there must be some serious losses somewhere. The rejection temperature has to be high because in space there are no rivers flowing by to cool your condensers- the only way to lose heat is to radiate it away from a flat plate, which needs to be pretty hot to radiate effectively. Hence the radiators in the flow diagram; these are nothing like central-heating "radiators" which work mostly by convection. This is one reason- possibly the most important- for the choice of mercury as a working fluid; it permits a high heat rejection temperature so the radiators are efficient, and therefore relatively small and light.

This project was only one of several nuclear-powered sytems that were intended to provide large quantities of electricity for long-duration space missions. One of the reasons that they have not so far been used is that no-one is very comfortable with the notion of launching a nuclear reactor on top of a rocket.