TYPES OF TURBINES
Steam turbines may be broadly grouped into three types, the classification being made in accordance with the conditions of operation of the steam on the rotor blades.
The groups are as follows:
1. Impulse. This may be divided into
a) Simple impulse. Pressure compounded
b) Compound impulse. Velocity compounded
c) Combined impulse. Pressure velocity compounded
2. Reaction subdivided into
a) Axial flow
b) Radial and axial flow
3. Combination of 1 and 2.
1. Impulse Turbines. In an impulse turbine the potential energy in the steam due to pressure and superheat is converted into kinetic energy in the form of weight and velocity by expanding it in suitably shaped nozzles.
The whole of the expansion takes place in the fixed nozzle passages. As there is no expansion in the passage between the rotor blades, the steam pressure is the same at the inlet and outlet edges of these blades. The steam impinges on the wheel blades causing the wheels to rotate. The expansion is carried out in stages referred to as "pressure stages", each stage being separated from the next by a diaphragm with nozzle openings through which the steam passes on its way through the turbine.
a) Simple impulse. This type has a considerable number of pressure stages, a wheel in each stage having one row of blades. To obtain high economy it is necessary that the steam should flow through the turbine with high velocity. This is attained by provision of a large number of pressure stages, the greater the available heat drop, the greater the number of stages. In the simple impulse turbine a wheel of comparatively large diameter is used in the first stage which can deal efficiently with a large energy drop. This large wheel, under nozzle control of the steam can maintain a higher efficiency over a wider range of load than a small one could and is less liable to be affected by changes of steam conditions. An added advantage of a large wheel is that the maximum rating of the machine can be obtained without by-passing which results in a flat consumption curve being maintained over the whole output range.
b) Compound impulse. This turbine has comparatively few pressure stages, a wheel in each of them provided with two or more rows of blades. Low velocity steam is obtained by the provision of what are usually termed "velocity stages" in each of the pressure stages. In these velocity stages the steam after passing through the first row of blades on a wheel is re-directed on to the second row of blades on the same wheel, and successively on to the other row of blades on this wheel, if provided. The steam is re-directed by arranging stationary blading between each two adjacent rows of wheel blading so that the steam leaving the first row of blades on a wheel in a backwards direction, enters the first row of stationary blades where its direction is reversed ready for entering the second row of blades on the wheel and so on. This action is repeated in each pressure stage on the turbine.
c) Combined impulse. This turbine is a combination of the types a) and b). It consists of one or more pressure stages with a wheel in each of these stages provided with two or more rows of blades. In the velocity compounded impulse turbine the "carry-over" velocity and the speed of, the shaft are much less than with the simple impulse machine. Each disk carrying the moving blades is perforated, thus maintaining the same pressure on both sides of the wheel. The pressure velocity compounded design is generally known as the "Curtis" type. The pressure compounded turbine has a higher efficiency since the pressure drop per stage may be arranged to give the most suitable, jet velocity for a given speed of the machine.
2. Reaction Turbines. In the reaction turbines expansion takes place in both the stationary and rotating passages and the pressure at entrance to the rotor blades is therefore greater than at exit.
1 a) Axial flow. In a pure reaction turbine expansion should take place only as the steam passes through the moving blades, the turning-effect being due to the reaction consequent on the increase in velocity which accompanies expansion. The reaction turbine has a ring-of stationary blades instead of a diaphragm with nozzle passages between the blades of each pair of adjacent wheels. The steam expands in the fixed blades, increasing its velocity, which is imparted to the moving blades on the impulse principle.
Steam is supplied direct to the blading system without expansion in nozzles and the rotation produced is chiefly due to the reaction set up by the steam between the stationary and rotating blades while expanding in them.
b) Radial flow. The Ljungstrom turbine is really a combined radial and axial flow machine. The flow of steam is radial, being admitted at the center of the blade discs and flowing outwards, the steam then being inverted to axial flow in the last stages. The turbine may be constructed for single or double motion. With the double motion design the discs rotate in opposite directions at equal speeds and the relative speed of the blades is therefore equal to twice the running speed. This design consists of one group of radial flow double rotation blading and two groups in parallel of low pressure axial flow single rotation blading, the divided flow in the final stages assisting-in the reduction of the "leaving losses". Each steam rotor is coupled to an alternator which carries half the total output.
3. Combination Turbines. This type consists of a machine embodying the "impulse" and "reaction" principles, the high-pressure turbine being the impulse section and the intermediate and low-pressure turbines being the reaction section. Where the term reaction is used it is to be understood that this refers to the "impulse-reaction" type of turbine. The practice in large output high speed sets is to include reaction blading at the low pressure end. The blade areas are large and therefore the leakage areas proportionately small, and as a double-flow exhaust is used the end thrust is balanced. These arrangements enable the length of the turbine to be reduced.
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