TURBINES Turbines are rotating fluid machines of the dynamic kind that produce shaft power (Torque x angular velocity) from primary sources of energy (prime mover). A Turbine is a device which converts the heat energy of steam into the kinetic energy & then to rotational energy. The Motive Power in a steam turbine is obtained by the rate of change in momentum of a high velocity jet of steam impinging on a curved blade which is free to rotate.
Transcript
TURBINES
Turbines are rotating fluid machines of the dynamic kind that produce shaft power (Torque x angular velocity) from primary sources of energy (prime mover).
A Turbine is a device which converts the heat energy of steam into the kinetic energy & then to rotational energy.
The Motive Power in a steam turbine is obtained by the rate of change in momentum of a high velocity jet of steam impinging on a curved blade which is free to rotate.
TURBINES
The common types: Steam, gas, hydraulic and wind turbines.
Steam and gas turbines: Thermal turbines due to high temperature of the working fluid. (The temperature of steam could around 540°C and that of air in gas turbine is1400°C).
Steam Turbines are mainly used for electricity generation.
Gas turbines are used to propel aircrafts because of their high power to weight ratio.
Steam and Gas turbines are also used to drive compressors and pumps and other high speed machinery.
STEAM TURBINESCommonly used prime mover - runs large generators to produce electric power in Thermal or Nuclear power plants.
High pressure, high temperature steam is generated in boilers.
This steam continuously interact with alternate rows of blades.
The blades are stationary (stator /nozzle blades) and rotating (rotor blades)
This interaction produces the required torque and speed (shaft power).
In stator blades energy transformation takes place while in rotor blades energy transfer takes place.
The two aspects of the analysis:- Energy consideration - Momentum consideration
ENERGY CONSIDERATION
Steam enters the turbine with high specific energy e1 (energy for unit mass) at section (1) and leaves the turbine at section (2) with a lower specific energy e2.
Conservation of energy principle:Power, P = m(e1-e2) = T x ω = mws (W)
where, ws – Specific work (work done by unit kg)
Turbine coupled with an alternator
PROBLEM
In a steam turbine, the mass flow rate of steam is 15 kg/s. The specific energy at inlet is 3000 kJ/kg while that at the outlet is 2500 kJ/kg. Determine the power output. Also determine the torque, if the speed is 3000 r/min. Neglect the losses.
Solution:
Power = 15 (3000 – 2500) = 15 x 500 = 7500 kW = 7.5 MW
Torque = power / ω = (7.5 x106/ 2 x3000) x 60 = 23.87 kN-m.
MOMENTUM CONSIDERATIONThe power developed in the turbine can also be calculated by considering the momentum change.
This analysis leads to an important equation - Euler turbine equation.
Specific work (work done/unit mass) Ws= u (Cu1-Cu2)
Power, P = mws
where u is the blade speed = dn/60
Cu1 and Cu2 - components of steam velocityin the direction of rotation at inlet and exit to the rotor.
IMPULSE and REACTIONIMPULSE:
The force obtained on an object when a jet of fluid strikes the object with a velocity.
REACTION:
The force obtained on an object when a fluid leaves the object with a high relative velocity.
IMPULSE AND REACTION TURBINESTwo types of Turbines : Impulse and reaction turbine. Energy transfer normally takes place in the rotor of the turbine.
Impulse turbine:Steam possesses only kinetic energy while entering the rotor (no pressure energy) and leaves the rotor with reduced kinetic energy. The total energy transfer is only due to the change in K.E.
Reaction turbine:The rotation is due to reactive forces rather than the direct push or impulse. Degree of reaction: 1 – fraction of energy transfer due to change in KE. i.e. if the total energy transfer is 100 units and of this, that due to change in kinetic energy is 40 units, then degree of reaction
DEGREE OF REACTION
Defined as :
1 – fraction of energy transfer due to change in KE.
Ex: Total energy transfer is 100 units and of this, energy transfer due to change in kinetic energy is 40 units.
Degree of reaction,R = [1- (40/100)] = 0.6 or 60%.
SIMPLE IMPULSE TURBINE (DE LAVAL TURBINE)
Consists of a rotor with a single row of blades. High pressure & high temperature steam enters through section ‘A’ and expands through a few nozzles (that produces a high velocity jet).
At the exit of the nozzle (2), the steam possesses only kinetic energy.
This high velocity steam jet impinges the rotor blades and transfers the kinetic energy to the rotor.
SIMPLE IMPULSE TURBINE (DE LAVAL TURBINE)
Energy transfer takes place only in the rotor (reduced specific energy).
In the nozzle energy transformation takes place (conversion of pressure into KE).
Pressure (p), velocity (v) and sp. energy(e) change for steam flow through the turbine.
IMPULSE / REACTION TURBINES
IMPULSE / REACTION TURBINES
COMPOUNDING OF TURBINES
The speed of a simple impulse turbine is very high (30,000 r/min.)Most applications require much lower speeds. Hence we go for compounding of impulse turbines.
Compounding reduces the speed of the turbine without any drop in its efficiency.
Types of compounding:
- Velocity compounding
- Pressure compounding
- Pressure velocity compounding.
VELOCITY COMPOUNDING (CURTIS STAGE)
A nozzle, two rows of rotor blades and a row of stator blades in between them.
The high pressure & high temp. steam passes through few nozzles.
The entire energy of steam is converted into kinetic energy (velocity).
Thus pressure energy is transformed to kinetic energy in the nozzle.
The specific energy in the nozzle remains the same since there is no energy transfer.
The steam leaving the nozzle passes through the first row of rotor blades.
Velocity decreases while pressure remains the same and specific energy decreases indicating energy transfer.
VELOCITY COMPOUNDING (CURTIS STAGE)
Steam then passes through the stator row of blades.
The direction of velocity is changed for smooth entry into the next row of rotor blades.
In this row there is no change in specific energy, pressure and velocity.
Finally, steam passes through the second row of rotor blades.
The velocity decreases, pressure remains the same and specific energy decreases indicating energy transfer.
VELOCITY COMPOUNDING (CURTIS STAGE)
2-stage velocity compounding
(KE transfer takes place in 2-stages)
PRESSURE COMPOUNDING
Actual steam turbines will have pressure-velocity compounding.
