Mechinopedia For Mechanical Engineers

The process of increasing the pressure of  air, gas or vapor by reducing its volume is called Compression and the device used to carry out this process is called a Compressor. Compressed air is mostly used in pneumatic brakes, pneumatic drills, pneumatic jacks, pneumatic lifts, spray painting, shop cleaning, injecting fuel in diesel engines, supercharging internal combustion engines, refrigeration, and air conditioning systems.   

The energy supplied to steam turbine is not fully utilized to transform it into mechanical energy. This is due to various losses occurred in the turbine and energy dissipated away from the turbine. The losses  which occur in the steam turbines are given below.  1. Losses in regulating Valves : Steam before entering the turbine passes through the main valve and regulating valves where it gets throttled adiabatically with constant enthalpy. As a result of this, some pressure drop occurs. Thus, some available energy of steam is lost. The pressure drop varies from 3 to 5 % of the inlet steam pressure.  2. Losses due to steam friction : As stated earlier, friction occurs both in nozzles and blades. In nozzles, the effect of friction is considered by nozzle efficiency. Losses in moving blades are caused by various factors such as impingement losses, frictional losses and turning losses. These losses are taken into account by blade friction coefficient.,   (K = V b / V l ) 3. Losses due to

 This method is adopted in modern high - pressure impulse steam turbines which contain a large number of stages of small mean diameter in high pressure stages. Such turbines are usually designed for a definite load known as economic load at which the efficiency is maximum. This load is taken as about 80% of the maximum continuous rating. According to the principle of  by-pass governing , some extra quantity of steam is by - passed to the far down stages of the turbine when the load exceeds the economic load.            Nozzle control governing is not preferable because of small enthalpy drop in the first stage of a high- pressure turbine. Further , incase of  higher loads the extra steam required cannot be admitted through additional nozzles in the first stage due to many practical reasons. Those difficulties are overcome by using by-pass governing.   Steam through a throttle valve enters  the nozzle box or steam chest. The throttle valve is controlled by a speed regulator or governor.

 Nozzle Control Governing is used in large power steam turbines to which very high pressure steam is supplied. In this method, the total number of nozzles of a turbine is grouped in a number of groups varying from two to twelve groups and each group of nozzle is supplied steam controlled by valves. the  valves are poppet valves and are opened or closed by automatic devices. Number of  groups of nozzle in operation at a particular instant depends upon the land on the steam turbine. The nozzles are divided in four groups N1 , N2 , N3 , N4 and are controlled by valves V1, V2, V3, V4 respectively. At full load on the turbine, steam flows through nozzles of all groups . At part load, only the required number of groups of nozzles is operated. The arc of  admission is limited to 180* or less. The Nozzle control governing is restricted to the first stage of the turbine, the nozzle area in other stages remaining constant . It is suitable for simple impulse turbine.

Steam Pressure at inlet to a steam turbine is reduced by throttling process to maintain the speed of the  turbine constant at part load and hence this method of governing is called "Throttle Governing".  Construction : Throttle governing system consists of  a centrifugal governor, a lever , an oil pump , a pilot piston , control valve , a relay piston and a throttle valve. The throttle valve is moved by a relay piston. The relay piston is actuated by pilot piston control valve. There are two piston valves covering ports in the pilot piston control valve without any overlap. These piston valves are operated by lubricating oil supplied by a gear pump at 2 to 4 bar. The oil returns to the drain from this chamber.  Working : When the turbine works on full rated load, the throttle valve will remain open. When the load is decreased, the energy output of the turbine becomes in excess and the turbine shaft speed increases. Hence, governor sleeve will lift. The upward movement of the

The method of  maintaining the speed of  the turbine is constant irrespective of variation of the load on the turbine known as governing of  turbines. The governors regulate the supply of  steam to the turbine in   such a way that the speed of the turbine is maintained as far as possible a constant under varying load   conditions. The principal methods of steam turbine governing are as follows :  (i) Throttle Governing  (ii) Nozzle Control Governing  (iii) By- pass governing   (iv) Combination Of throttle and nozzle governing or throttle and by - passing governing. 

 This method is a combination of  pressure and velocity compounding. The total pressure drop is carried   out in two stages and velocity obtained in each stage is also compounded. Steam pressure from boiler   pressure to condenser pressure is dropped in stages through convergent - divergent nozzles . Velocity  compounding is done by using a guide blade rings in between every two moving blade rings.    High - pressure steam expands through first ring nozzles, does work on the first row of moving blades   and enters guide blades. Through the guide blades the steam comes out with a changed direction of flow. Then the steam flows through the second row of moving blades where it does work. The remaining   reduction of pressure up to condenser pressure takes place in the second set of nozzles and the process of doing work on two set of moving blades and guide blade is continued. Thus, total pressure drop is  obtained in stages through nozzles sets and velocity changes takes place through mov

In this method, a number of  simple impulse turbine stages is arranged in series. Each of these simple Impulse turbines consists of one set of Nozzles ( F.N ) and one row of moving blades (M.B). The   exhaust from each row of  moving blades enters the succeeding set of nozzles. The steam from the boiler   is passed through the nozzles and moving blades. The steam velocity increases when it passes through nozzles and pressure drops. The steam velocity decreases without much alteration in pressure as it flows over the moving blades. Finally, the pressure falls down to condenser pressure. Both the variation in pressure and velocity will vary while the steam flows through fixed nozzle and moving blades. The pressure is reduced in each stage of nozzle rings and hence this is called Pressure compounding. Examples of this type of turbines are Rateau turbine and Zoelly turbine. 

In this method, there are number of moving blades (M.B), separated by rings of fixed blades (G.B) , Keyed in series on a common shaft. The stem from the boiler is passed through the row of  Nozzles from  the boiler pressure to condenser pressure and attains high velocity.  The high velocity steam jet passes over the rings of moving blades and fixed blades alternatively. During   which, a part of kinetic energy is absorbed in each ring of moving blades. The direction of  steam is   changed without altering much its velocity in the rings of fixed blades. Thus, all the Kinetic energy is utilized in moving blades. Since there is no pressure drop as the steam passes over the moving blades, the   turbine is thus of impulse type.  The pressure drops fully at the nozzle itself and the pressure is kept remains constant in moving blades   and fixed blades. The velocity of  the steam coming out of nozzle is very high and it is reduced in stage  -   by - stage of moving blades. Hence, it is known

If the expansion of steam from the boiler pressure to the condenser pressure takes place in single stage Turbine, the velocity of steam at exit of turbine is very high. Hence, There is a considerable loss of    kinetic energy  ( i . e about 10 to 12% ). Also, the speed of  the rotor is very high ( i .e up to 30000rpm ). There are several methods of reducing this speed to lower value. Compounding is a method of absorbing   the jet velocity in more than one stage when the steam flows over moving blades.  The different methods of compounding are:  1. Velocity Compounding.  2. Pressure Compounding.   3. Pressure - Velocity Compounding.

 Impulse Turbine :                         * It consists of nozzles and moving blades. * Pressure drop occurs only in nozzles not in moving blades. * Steam strikes the blades with kinetic energy.  * It has constant blade channel area. * Due to more pressure drop per blade, number of stages required is     less. * Power developed is less. * It occupies less space for same power output. * Velocity of Turbine is more. * Lower efficiency. * Blade manufacturing is not difficult and Thus it is not costly.  Reaction Turbine : * It consists of Fixed Blades and moving blades. * Pressure drop occurs in fixed as well as moving blades. * Steam passes over the moving blades with pressure and Kinetic         energy. * It has varying blade channels area. * Number of stages required is more due to more pressure drop.  * Power developed is considerable.  * It occupies more space for same power . * Velocity of Turbine is less. * Blade manufacturing process is Difficult.   

 In Reaction Turbines, There is no sudden pressure drop. There is a gradual pressure drop and takes place continuously over the fixed and moving blades. A number of wheels are fixed to the rotating shaft. Fixed guide ways are provided in between such pair of rotating wheels.  The function of fixed blades (F) is that they guide the steam as well as allow it to expand in a larger velocity. It is similar to nozzles as in case of  Impulse Turbine.  The moving blades serve the following functions : * It converts the Kinetic energy of  the steam into useful mechanical energy. * The steam expands while flowing over the moving blades and thus     gives reaction to the moving blades. Hence, The turbine is called       as Reaction Turbine. * The Velocity of the steam decreases as the kinetic energy of  the         steam absorbed. ' * The velocity of  the steam decreases as the kinetic energy of  the       steam absorbed.     Since the pressure of steam reduces continuously as it follows o

It consists of one set of nozzle followed by one set of  moving blades. A rotor is mounted on a shaft. The moving blades are Attached to the rotor.  The steam from the boiler at high pressure and low velocity enters the nozzle which is fitted in the casing. The steam expands in the nozzle where the pressure drops to p1 and velocity increases to V1. This high velocity steam jet impinges over the blades mounted on the rotor attached to the shaft. This causes the rotation of the turbine shaft and thus useful work is obtained. It is noted that the pressure of the steam when it moves over the blade remains constant but the velocity decreases. The upper portion shows the longitudinal section of the upper half of the turbine. The middle portion shows the development of the nozzles and blading. The bottom portion shows the variation of velocity and pressure of the steam during which it passes through nozzles and blades. DISADVANTAGES OF SIMPLE IMPULSE TURBINE : * Since all the kinetic energy o

In Reaction Turbines the steam expands both in fixed and moving blades continuously as the steam   passes over them. As it expands, there is some increase in steam velocity thereby resulting reaction force.   The pressure drop occurs gradually and continuously over both moving and fixed blades. The examples   of such turbine are parson's Turbine. 

IMPULSE TURBINE : In Impulse Turbine, the steam at high pressure and temperature with low velocity expands through nozzles where the pressure reduces and velocity increases. This High Velocity jet steam which is obtained from nozzle impinges on the blades fixed on a rotor. The blades change the direction of  the steam flow without changing its pressure. This causes change in momentum and the force developed drives the turbine rotor. Here, the nozzles are stationary and fitted in a casing. The examples of Impulse Turbine are De-Laval, Curtis, and Rateau Turbines.

  Steam Turbines are Classified as follows :  1. On the basis of method of steam expansion ,     a.) Impulse Turbine.     b.) Reaction Turbine.     c.) Combination of Impulse and Reaction Turbine. 2. On the basis of Number of Stages ,     a.) Single Stage Turbines     b.) Multi - Stage Turbines 3. On the basis of Steam Flow Directions ,     a.) Axial Turbine.     b.) Radial Turbine.     c.) Tangential Turbine.     d.) Mixed Flow Turbine.  4. On the basis of pressure of steam,     a.) High Pressure Turbine.     b.) Low Pressure Turbine.     c.) Medium Pressure Turbine.

  Steam Turbine is a device which is used to convert kinetic energy of steam into mechanical energy. In   this, enthalpy of  steam is first converted into kinetic energy in nozzle or blade passages. The high  velocity steam impinges on the curved blades and its direction of flow is changed. This causes a Change of momentum and thus force developed drives the turbine shaft.  The steam turbine has been used as a prime mover in all steam power plants. Now-a-days, a single steam   turbine of 1000MW capacity in built in many countries. In larger Sizes , it is used for driving electric   generators. In small sizes, it is used to drive   pumps , fans , compressors etc. 

When the superheated steam expands in the nozzle, the condensation will occur in the nozzle. Since the steam has more velocity, the condensation will not take place at the expected rate. So, the equilibrium between  the liquid and vapor phase is delayed and the steam continues to expand in a dry state.  The Steam in such set of condition is said to be supersaturated or metastable flow.  EFFECTS OF SUPER SATURATION : The following effects in a nozzle on steam in which supersaturation occur may be summarized as follows : 1. The dryness fraction of  the steam is increased. 2. Entropy and specific volume of the steam are increased. 3. Exit Velocity of the steam is reduced. 4. Mass of steam discharged is increased.  

There is only one value of the ratio (p2 / p1), which produces maximum discharge from the nozzle. This ratio is called critical pressure ratio.  Where  p1 = Inlet pressure.             p2 = Throat Pressure. i) For saturated steam n = 1.135    We know that the critical pressure ratio     (p2 / p1) = (2 / (n+1))^ (n / (n-1))      (p2 / p1) = ( 2 / (1.135 +1))^ (1.135 / (1.135 -1)) Critical pressure ratio,  (p2 / p1) = 0.577 ii) For Super heated steam n = 1.3     (p2 / p1) = (2 / (n+1))^ (n / (n-1))                     = (2 / (1.3 -1))^ (1.3 / (1.3 -1 )) Critical Pressure Ratio , (p2 / p1) = 0.546  iii) For gases n = 1.4     Critical Pressure ratio , p2 / p1 = ( 2 / (n+1)) ^ ( n / (n-1))                                          p2 / p1 = ( 2 / (1.4 + 1))^ (1.4 / (1.4 -1))                                            p2 / p1 = 0.5282 

When the steam flows through a nozzle the final velocity of steam for a given pressure drop is reduced  due to the following reasons.  1. Due to the friction between the nozzle surface and steam.  2. Due to Internal fluid friction in the steam.  3. Due to shock losses. Most of these frictional losses occur between the throat and exit in convergent-divergent nozzle. The effects of these frictional losses are listed below : 1. The expansion is no more isentropic and enthalpy drop is reduced resulting in lower exit velocity 2. The final dryness fraction of the steam is increased as the part of the kinetic energy gets converted  into the heat due to the friction and is absorbed by steam with the increase in enthalpy. 3. The specific volume of steam is increased as the steam becomes drier due to this frictional reheating. This can be best understood with the help of h-s diagram or mollier chart.  The point A represents the initial condition of steam. It is a point, where the saturation line