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Steam Turbine

Steam spinning the world's generators

Date
1884
Era
Electric age
Rebuild stage
V Electric age
Difficulty

Plate

Legend
  1. 1Steam inlet and stop valve
  2. 2Fixed blades on the casing
  3. 3Moving blades on the rotor
  4. 4Rotor shaft
  5. 5Exhaust to the condenser
  6. 6Dynamo

Anatomy

Moving blades, on the drumSteam; each streak shows its speed and directionFixed blades and casing, cut throughThe last four stages of a condensing turbine, with the half of the casing nearest you cut away along the shaft. Each stage is a ring of fixed blades on the casing followed by a ring of moving blades on a drum 1.6 m across. These are Parsons reaction stages: the two rings have similar profiles, curved opposite ways, and each takes about half of the stage's drop in enthalpy, the energy the steam gives up as it expands. The fixed blades speed the steam up and turn it toward the rotation; the moving blades turn it back and are pushed round. In an impulse stage, as in de Laval's and Curtis's turbines, the fixed nozzles take nearly the whole drop and the moving blades only catch the jet. Steam arrives from the earlier stages at 1.2 bar and 115 °C and goes to the condenser at 0.05 bar and 33 °C, by then with about 11 % of it condensed into droplets. Since it fills more room at every step, the blades grow from 11 cm in the first ring to 54 cm in the last, whose tips move at about 420 m/s. The gauges read the pressure in front of the first stage, after each of the first three and at the exhaust; they read 0 to 1.5 bar absolute. The circle on the left shows one stage and the next fixed ring opened out flat at mid-height of the blades, with the rotation running down the page: the fixed blades turn the steam downward, the way the moving blades run, and the moving blades turn it back. On a 50 Hz grid the two-pole generator holds the shaft at 3,000 rpm, so a change of load changes the steam flow, not the speed. At 36 kg/s these four stages give about 13 MW. With less steam the pressure in front of each stage falls nearly in step with the flow (Stodola's ellipse law): at 30 % of the flow the inlet is down to 0.36 bar and the last stage does little work. The model uses IAPWS-IF97 steam properties and a row efficiency of 90 % in dry steam, less 1 % for each 1 % of water droplets, and counts the speed the steam still has on leaving as lost. Blades and steam are drawn 600 times slower than they move, both by the same factor, and with fewer blades per ring than a real turbine.

A steam turbine lets steam expand through rows of fixed and moving blades, turning its pressure into speed and that speed into the rotation of a shaft. Charles Parsons coupled one to a dynamo in 1884; turbines of this kind now drive the generators that produce most of the world's electricity.

What it is

A steam turbine is a wheel driven by steam, with no piston and no crank. Steam flows through rows of blades set around a shaft and leaves the machine at a much lower pressure; the energy it gives up on the way turns the shaft. The motion is smooth and fast, which suits an electric generator.

Coal and nuclear plants boil water to drive these machines. Gas-fired combined-cycle plants raise steam from the hot exhaust of a gas turbine, and solar thermal and most geothermal plants use heat from the sun or the ground. Together they make steam turbines the source of most of the world’s electricity.

How it works

Steam gives up its energy by expanding. In a nozzle, a passage that narrows, its pressure falls and it speeds up. At the narrowest point, the throat, it can reach the speed of sound; to go faster, it needs a passage that widens beyond the throat, the flaring shape of de Laval’s nozzles. In each stage, a ring of fixed blades on the casing works as a set of nozzles and throws the steam onto a ring of curved moving blades on the rotor, which turn the jet aside and are pushed round.

A steel disc with a ring of small curved blades on its rim and brass nozzles aimed at them De Laval’s impulse wheel: flaring nozzles send jets through a single ring of blades.

In an impulse stage almost all the pressure drop happens in the fixed nozzles, and the moving blades only catch the jet. In a reaction stage, the kind Parsons chose, the passages between the moving blades narrow too, so the steam keeps expanding as it crosses them and pushes them back as it leaves. In a Parsons stage the expansion is shared about equally between the fixed and the moving row.

Released through one nozzle, a large pressure drop gives a jet of over 1,000 metres per second, and blades work best at about half the jet’s speed, so a single wheel must spin dangerously fast and needs gearing. Parsons split the drop into many small steps, so the shaft turns at a rate a dynamo can use.

As the pressure falls, the steam fills more room, so the blades grow longer towards the exhaust: 1.9 metres in the last row of the turbines built for Hinkley Point C, in England.

Steam released into the air stops expanding at atmospheric pressure. A condenser, a chamber of tubes cooled by river, sea or tower water, turns the exhaust steam back into liquid and keeps a near vacuum at the end of the turbine, so the steam goes on expanding and working inside the turbine before it is condensed. Parsons’s first condensing turbines, at Cambridge in 1892, matched the best piston engines of their size.

This loop is the Rankine cycle: a pump forces water into the boiler, which turns it into steam; the steam expands through the turbine, and the condenser turns it back into water for the pump. Pumping a liquid costs little next to what the steam gives, so the cycle has a net output. The pump descends from the piston pump, boiler and condenser from the steam engine; a refrigerator runs a similar loop in reverse.

On a 50-hertz grid, a two-pole generator on the shaft turns at 3,000 revolutions per minute, at 3,600 on 60 hertz. Transformers then raise the voltage for the lines.

How to build

A power-station turbine is fed by a boiler, which is a pressure vessel: do not build one. This model runs on air. You need an aluminium pie dish, two corks, a knitting needle, straws, a cardboard box, glue, and a hair dryer on its cold setting or a bicycle pump.

Cut a bladed wheel. Cut a 10 cm disc. Make 12 to 16 cuts from the rim, stopping 2 cm short of the centre, and twist every vane the same way by about 30 degrees.

Mount it. Glue the wheel to a cork on the needle, which turns in two pieces of straw fixed across the box.

Compare a pipe and a nozzle. With the same pump, distance and angle, blow air at the vanes through a wide tube, then through a cut-down straw held at a slant, and note which turns the wheel faster.

Add a stage. Glue a second wheel, vanes twisted the same way, to a second cork on the same needle, 3 cm behind the first. Between them, fix to the box a card ring with vanes twisted the other way and a central hole wide enough to touch neither the needle nor the wheels. Blow along the needle through the front wheel, then remove the ring and compare: the fixed vanes should turn the air leaving the first wheel back onto the second.

Measure the voltage. Couple a small DC hobby motor to the needle with a piece of tube and connect a multimeter set to DC volts. If the wheel can turn the motor, the reading rises with its speed.

Do not try the model with steam. Boiling water turns into more than a thousand times its volume of steam, and a heated vessel that is sealed or blocked builds pressure until it bursts. Real boilers follow certified designs, carry safety valves and are examined regularly by a competent person; a home-made boiler has none of these safeguards.

Safety. Steam scalds in a fraction of a second, even from a kettle spout, where it is invisible: keep hands and faces away. Never seal, plug or block a heated vessel. A spinning foil wheel has sharp edges and can throw a vane, so wear glasses and keep hair clear. Use only a small hobby motor and a meter, never the mains, and keep the hair dryer away from water.

History and evolution

A bronze sphere turning between two pipes above a cauldron on a fire, steam jetting from two bent nozzles Hero’s aeolipile: two jets of steam spin the sphere on its pivots.

In the first century CE, Hero of Alexandria described the aeolipile, a hollow ball spun by steam from bent nozzles, the first known device to turn steam into rotation. In 1629 Giovanni Branca published the drawing of a machine in which a jet of steam blows on a paddle wheel; it was very likely never built. In Sweden, Gustaf de Laval patented a small steam reaction wheel in 1883, then in 1889 a geared single-stage impulse turbine.

In 1884 Charles Parsons, a partner at Clarke, Chapman in Gateshead, coupled a turbine of many reaction stages directly to a dynamo. It gave 7.5 kilowatts at 18,000 revolutions per minute, and about 200 such sets were lighting ships by 1888. In 1890 Forth Banks, in Newcastle, became the first power station to use turbines.

A long, narrow dark launch with one tall funnel, throwing up a white bow wave Turbinia at speed: three turbines on three shafts drove nine propellers.

To prove the turbine at sea, Parsons built Turbinia, a 31-metre launch that reached 34.5 knots, and on 26 June 1897 ran her unannounced past the warships at the Spithead review. In 1905 an Admiralty committee recommended turbines for every class of warship.

In the United States, Charles Curtis sold his rights to General Electric in 1896. His 5,000-kilowatt set of 1903, then the world’s most powerful, stood 7.6 metres high against 18 for a piston engine of similar power. Sets reached 660 megawatts at Drax in the 1970s, and each of the two turbines built for Hinkley Point C is rated at 1,770 megawatts.

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