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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. Combinatio n 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 other rows 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.

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 are 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.

Further Classification. As the output capacities and working conditions have affected the construction of each particular make it has been suggested that


the following particulars be given for each turbine: 1) number of shafts, 2) number of cylinders, 3) number of exhausts, 4) the speed.

Many types of industrial turbines are in use today, depending upon the conditions under which they must operate. They are classified as high-or-low- pressure turbines, according to the inlet pressure of the steam, and as superposed, condensing, and noncondensing turbines, according to the exhaust steam pressure. A superposed or high backpressure turbine is one that exhausts to pressures well above atmospheric pressure, 100 to 600 psi. A superposed turbine operates in series with a medium-pressure turbine. The exhaust steam of the superposed turbine drives the medium-pressure unit. The noncondensing turbine has lower exhaust pressures, but the steam still leaves at atmospheric pressure or above 15 to 50 psi. The exhaust steam may be used for drying or heating processes.

The condensing turbine operates at exhaust pressures below atmospheric pressure and requires two auxiliaries: a condenser and a pump. The condenser reduces the exhaust steam to water. As the steam is condensed and the water is removed by a pump, a partial vacuum is formed in the exhaust chamber of the turbine. This type of turbine is used chiefly for the low-cost electric power it produces.

If steam is required for processing, a turbine may be modified by extracting or bleeding the steam.

Extraction takes place at one more point between inlet and exhaust, depending upon the pressures needed for the processes. The extraction may be automatic or nonautomatic. Generally, factory processes require steam at a specific pressure, in the case, and automatic-extraction turbine is necessary. When steam is needed within the power plant itself for heating boiler feed-water, nonautomatic extraction is generally used.

Turbines may be classified according to their speed and size. Small turbines, varying in size from a few horsepower to several thousand horsepower, are used to drive fans, pumps, and other auxiliary equipment directly. The speed of these units is adjusted to the speed of the driven machinery or is converted by a suitable gear arrangement. These turbines are used wherever steam is readily available at low cost or where exhaust steam is needed.

Turbines for the production of electric power range in size from small units to those of over 500, 000 kw, and the trend is toward even larger units.

Sometimes turbogenerator units are constructed to operate at 3, 600 or 1, 800 rpm. The selection of the speed depends almost entirely on the size of the turbogenerator desired. The speed of 3, 600 rpm is preferred whenever the size of the turbine permits. The turbine operating at the higher speed has the following advantages: lighter weight, more compactness, and great suitability for high- pressure, high-temperature operation.

With a few exceptions turbines larger than 100, 000 kw will operate at 1, 800 rpm. All turbines of smaller capacity will run at 3, 600 rpm. However, because of the advantages of the 3, 600 rpm unit and because of the greater efficiency of large units turbine manufacturers will continue to raise the upper limit of speed and capacity.


Generally, turbogenerators on a single shaft and within a given speed range are constructed, with either a single or a double-rotor.

The double-rotor arrangement is used for only the largest turbines falling within a given up speed range. A double-rotor unit is c1led tandem-compound turbine, and the flow is double-exhaust to accommodate the large volumes of steam occurring at the low-pressure end.

 






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