The turbine is a rotary prime mover which converts thermal energy into mechanical energy by using steam with a certain temperature and pressure as working medium. When it operates, the thermal energy of steam is converted into kinetic energy, and then the kinetic energy of steam is changed into the mechanical energy.
Ⅰ.The principle of impulse action and reaction
It can be known from mechanics that, when a motive object collides with another object, it is obstructed and its speed and direction are changed, and it exerts a force on the object hindering the movement at the same time, this force is called impulsive force. The amount of this force depends on the variation of the mass and speed of the object, as shown in figure 1-3, the steam with high speed flows from nozzle, impulses the wood block on the table, the steam speed changes, an impulsive force then exerts on the wood block and causes it to move forward, this principle is called principle of impulsive action.
The generation of reaction force is different from that of impulsive force, it results from a sudden speed increment obtained by an object in stationary state or with small speed, which is leaving or passing through another object. In figure 1-4, the high pressure gas generated by the fuel combustion spouts from the rear of the rocket, the high speed gas stream applies a force on the rocket; the direction of it is opposite to the gas flow, rocket moves upward by the push of this force, which is called reaction force. In turbine, when steam speeds up and expands inside the path formed by blades, the steam flow also exerts a reaction force to push the blade to move and does mechanical work; this principle of doing work is called theory of reaction.
In turbine stage, the conversion from kinetic energy to mechanical energy is usually fulfilled by the above two different principles.
Ⅱ.Basic working principle for turbine
The simplest turbine is shown in figure 1-5, it consists of nozzle, blade, impeller, shaft, etc. It can be seen from the figure that the steam with a certain pressure and temperature expands and speeds up when it is flowing through nozzle, the steam pressure and temperature reduce at the same time, the thermal energy of the steam is converted into kinetic energy, then steam with high speed flows out of nozzle, changes the direction and enters into the bending blade path, in which the steam direction changes and exerts impulsive force on the blade, as shown in figure 1-4, the generated torque drives the main shaft to rotate, and export mechanical work, i.e., steam pushes blade to rotate and does work, the conversion from kinetic energy into mechanical energy is fulfilled.
It can be seen from above description that when turbine operates, first the thermal energy of steam is converted into the kinetic energy in the nozzle cascade, and then the steam kinetic energy is changed into the mechanical energy in the blade cascade. The nozzle cascade and the blade mated with it accomplish the whole process of the energy transformation, thus the basic element doing work for turbine is formed, and this element for doing work is called the stage of turbine
Ⅲ. Single stage turbine
The turbine consisted by one stage is called single stage turbine, the types are as follows according to the working principles:
(1) Steam only expands in the nozzle, and the movable blade only endures the steam impulsive force, this turbine is called pure impusive turbine.
(2) Half or the thermal energy of steam converts into kinetic energy in nozzle, and another half converts into the kinetic energy in movable blade, the movable blade endures both the impulsive force and reaction force, this kind of turbine is called turbine with reaction type
(3) Besides most thermal energy converts into kinetic energy in nozzle for steam, a small part of steam expands in the movable blade, besides enduring the impulsive force for movable blade, it endures a small part of reaction force, this turbine is called impulsive turbine with degree of reaction, and it is called impulsive turbine for short.
For single stage impulsive turbine, the steam kinetic energy cannot be completely converted, and large residual speed occurs when steam leaves off the movable blade, which causes heavy residual speed loss, to utilize the kinetic energy of residual speed of the discharged steam, the combined velocity stage turbine can be used. Figure 1-6 is the illustration for single stage turbine with double-row velocity stage, which means that a row of guiding blade fixed on the cylinder and a second row of movable blade is mounted on the same impeller after the single stage of impeller type, and its process is that：When steam expands and speeds up in nozzle, it enters into the first row blade to do work, thus the steam speed reduces, then steam enters into the guide blade fixed on the cylinder, enters into the second row of movable blade after changing the direction, utilizes the residual speed and continues doing work, thus the turbine power is increased and the residual speed loss reduces, this kind of turbine is used for the single stage turbine whose enthalpy drop is large.
The power of single impulsive turbine is small, even the velocity stage is adopted, the increased power is limited, and loss for single stage turbine is large. To ensure the turbine to give out larger power, many stages should be parallel connected to form multi-stage Turbine, figure 1-7 is the illustration for turbine structure. It mainly consists of cylinder, rotor, diaphragm, etc. and each stage is aligned according to the sequence, when it works, steam enters into the multi-stage turbine, flows through all stages in sequence, expands and does work, the pressure reduces by stage, when steam flows off the final stage movable blade, it changes into the discharge steam with low speed and discharges off the cylinder.
Ⅴ. The classification of turbine
Turbine is not only used in the power station, it is also widely used in other fields, so there are various types of turbine, and the classifications of turbine are as follows:
1. Work principle
(1) Impulsive turbine: turbine works in impulsive principle is called impulsive turbine. When it works, the steam expansion is chiefly carried out in nozzle, and small portion of steam expands in the movable blade.
(2) Reaction turbine: turbine works in reaction principle is called reaction turbine. Each half of expansion is carried of in nozzle and movable blade.
(3) Combined turbine: turbine combined by impulsive stage and reaction stage is called combined turbine
2. Thermal process:
(2) Back pressure turbine: when steam does work in turbine, it is discharged for industrial use or heating, this kind of turbine is called back pressure turbine. If the discharge steam is provided for turbine with middle pressure or low pressure, it is called boost type turbine.
(3) Extraction steam adjusted turbine: steam having done work is extracted with one or two kinds of pressure (which can be adjusted in a certain range), and other steam is discharged into condenser, this kind of turbine is called extraction steam adjusted turbine. The combination term of steam adjusted turbine and back pressure turbine is heat providing turbine.
(4) Intermediate reheat turbine: steam having done work in HP cylinder goes to reheater to be heated to a certain temperature and returns to the MP & LP cylinder to continue doing work, this kind of turbine is called intermediate reheat turbine. Times of reheat may be one, two or more, but once intermediate reheat turbine is generated adopted.
3. Steam initial parameter
(2) MP turbine : live steam pressure is 1.96～3.92MPa.
(3) HP turbine : live steam pressure is为5.88～9.8MPa.
(4) Super high pressure turbine: live steam pressure is 11.76～13.72MPa.
(5) Sub-critical pressure turbine: live steam pressure is 15.68～17.64MPa.
(6) Super-critical pressure turbine: live steam pressure is 22.06MPa or above.
4. Direction of steam flow
(2) Radial flow turbine: the general direction of steam flow is approximately vertical to the shaft.
(3) Circumferential flow turbine: steam flows circumferential along the impeller approximately.
On the other hand, there are other kinds of classifications, e.g. according to the number of cylinder, it can be classified as single cylinder, double-cylinder, multi-cylinder type, according to the number of shaft; it can be classified as single shaft, double-shaft turbine, etc.
