The hose that firefighters drag to a burning building is filled with water almost to the bursting point. The nozzle, however, is turned off. One person can hold it easily. Then as soon as the nozzle is opened and the big stream starts to spurt, the hose straightens and jumps like a giant snake. Two or three firefighters struggle to hold it and to keep it turned on the blaze. That kind of power, coming from the flow of a fluid, is used in turbine engines water turbines, steam turbines, and gas turbines. The power depends upon the momentum of the water, steam, or gas. (See Windpower.)
Water Turbines
If someone wanted to use the tremendous power within the firefighter's hose, it could be done in two different ways. A wheel could be set up with vanes on its rim, and the full force of the water could be directed against these vanes, making the wheel spin at great speed. This would be an example of the simplest form of turbine. The simplest form is called the impulse turbine.
On the other hand, imagine a wheel shaped like a windmill wheel inside the hose. Such a wheel would cause the water to slow while the water made it turn. It would not turn as fast as the one described above, which receives the full force of the stream all at once, but it would deliver a steadier and equally powerful motion. This would be an example of the reaction turbine.
Water turbines are of immense value where a swift and plentiful supply of water is available. They are used principally for generating electricity. Some of the largest hydroelectric plants in the world are situated at Niagara Falls (on the United States-Canadian border), where the vast flow is harnessed by mammoth reaction turbines of the Francis type. In the high Sierra Nevada in California, where streams can be dammed to get high pressures, impact turbines called Pelton wheels are favored. Where the volume of water is great but pressure is low, as at Louisville, Ky., on the Ohio River, reaction turbines of the propeller type are used (see Waterpower).
Steam Turbines
Steam has one great advantage over water it expands in volume with tremendous velocity, often as much as 4,000 feet (1,200 meters) per second. No wheel made can revolve at any speed approaching this velocity. Various devices, however, are used to subdue the steam. This is usually done by sending it through successive turbine wheels of increasing size. The Curtis type of steam turbine uses the impulse system, while the Parsons type is built on the reaction plan.
To be efficient, steam turbines must run at high speed. This causes difficulty in using them on ships, as the propellers cannot run at the turbine's rate of speed. Most ships meet the difficulty with speed-reducing gears between the turbines and the propellers. Separate turbines are used to reverse the propellers, for turbines cannot be reversed. In ships with turboelectric drive the turbines run electric generators, which can operate at turbine speed. The current from the generators is used in motors to drive the propellers.
Gas Turbines
The gas turbine has many of the advantages of internal-combustion engines without the complications associated with reciprocating motion. It basically directs a continuous stream of hot gases against the blades of a turbine shaft. In most cases the air is compressed before it is directed into combustion chambers. In these chambers fuel is mixed with some of the air and burned. A portion of the air is directed around the burner and is then mixed again with the combustion products. The emerging gases are forced through nozzles and against the blades of the turbine shaft, thereby providing enough rotary power to drive the compressor. There is enough power left for turning the propeller of a ship or driving an electric generator. Some gas turbines, most notably turbojet and turbofan engines for aircraft, use the energy from the exhaust gases for power forward thrust is produced by the reaction to the high-speed, rearward ejection of the hot gases.
