Turbo means spinning or whirling around. Turbo machinery, in mechanical engineering, describes machines that transfer fluid energy to a rotating wheel, including both turbines and compressors. While a turbine transfers energy from a fluid to a rotor and compressor transfers energy from a rotor to a fluid. The two types of machines are governed by the same basic relationships including Newton's second Law of Motion and Euler's energy equation for compressible fluids.
Centrifugal pumps are also turbo machines that transfer energy from a rotor to a fluid, usually a liquid, while turbines and compressors usually work with a gas.
ENERGY TRANSFER IN TURBO MACHINES
FIRST LAW OF THERMODYNAMICS
The law states that if a system executes a cycle transferring work and heat through its boundary, then the net work(W) transfer is equivalent to the net heat(Q) transfer. Therefore,
Application of First law to turbo machines:- When a machine system executes a process, the change in stored energy of the system is numerically equal to the net heat interactions minus the net work interaction during the process.
E2 – E1 = Q – W
ΔE = Q – W
Where, E represents the total internal energy. In the absence of electric, magnetic and chemical energy, neglecting the change in Potential Energy (PE) and Kinetic Energy (KE) for a closed system, the equation can be written as
ΔU = Q – W=U1 –U2
Entropy ( Second law of thermodynamics) – The second law of thermodynamics states that for a fluid undergoing a reversible adiabatic process, the entropy change is zero. Due to the increase in entropy, the power developed by a turbine is less than the ideal isentropic power developed. Similarly, the work input to a pump is greater than the isentropic or ideal work input.
MOMENT OF MOMENTUM EQUATION AND EULER TURBINE EQUATION:-
According to Newton’s law of motion, the sum of all the forces acting on a control volume in a particular direction is equal to the rate of change of linear momentum of the fluids across the control volume.
m = mass of the body (Kg)
V1 = initial velocity of fluid (m/s)
V2 = Final velocity of fluid (m/s)
This equation is a modified form of Newton’s second law of motion. The left hand side of this equation represents the impulse acting on the body. The Right hand side equation represents the change in momentum of the body in the time period dt (short time interval). Hence, this equation is known as impulse momentum equation. It is used to study the impact of fluid jet striking a stationary or moving plate and also to study the flow characteristics, namely the head loss in a pipe due to change in area, hydraulic jump, etc.
EULER TURBINE EQUATION:- Consider the adiabatic flow of a fluid as shown in Fig.1 The fluid in a stream tube enters a control volume at radius r i with tangential velocity v i and exits at r e with tangential velocity v e. For a compressor or pump with steady flow, the applied torque is equal to the change in angular momentum of the fluid, or
The input power of a general turbine is
This equation is often referred to as the Euler pump equation. Application of the first law of thermodynamics to the flow through the control volume gives
Combining this expression with Eq. gives
Likewise, for a steady-flow turbine, the output torque is equal to the change in angular momentum of the fluid, or
This equation is often referred to as the Euler turbine equation. Application of the first law of thermodynamics to the flow through the control volume gives
PRINCIPLES OF IMPULSE AND REACTION MACHINES:- An impulse stage is one in which the static pressure at the rotor inlet is the same as that at the rotor outlet. Then, it is defined as one where the relative velocity of fluid flow is constant on the rotor. These two definitions are very nearly equivalent since there is very little pressure drop during flow through a rotor when the relative velocity is constant. A reaction stage is one where a change of static pressure occurs during flow over each rotor stage. The machine containing such stages is termed as reaction machine.
Figure 2 Working of Impulse and Reaction Turbine
DEGREE OF REACTION:- The degree of reaction is parameter which describes the relation between the energy transfers due to static pressure change and energy transfer due to dynamic pressure change.
ONE DIMENSIONAL ANALYSIS:- Dimensional Analysis uses our knowledge of the systems of measuring units and the dimensions of physical quantities in the solution of engineering problems. If we have a theory, dimensional analysis complements it, but we can also get close to the answer without one. Dimensional Analysis allows us to:
Convert from one system of units to another.
Check the units of an equation.
Simplify problems by reducing the number of parameters.
Plan experiments so as to reduce the effort required to investigate the situation under consideration.
Design and use models for experimental tests.
Correlate experimental data.
Graphically present the results of experimentation or analysis more concisely
STEAM TURBINE; - Steam turbine is a power-generating machine in which the pressure energy of the fluid is converted into mechanical energy. This conversion of energy is due to the dynamic action of steam flowing over the blade (Figure 3).
Figure 3: Schematic Diagram of an Impuse Turbine
Figure 3:Velocity Diagram of Impulse Turbine
Compounding of impulse turbine: This is done to reduce the rotational speed of the impulse turbine to practical limits. (A rotor speed of 30,000 rpm is possible, which is pretty high for practical uses.) - Compounding is achieved by using more than one set of nozzles, blades, rotors, in a series, keyed to a common shaft; so that either the steam pressure or the jet velocity is absorbed by the turbine in stages.
Three main types of compounded impulse turbines are:
a) Pressure compounded, b) velocity compounded and c) pressure and velocity compounded impulse turbines.
Figure 4: Variation of Diagram/ Blade Efficiency with Speed Ratio
Blade efficiency or Diagram efficiency or Utilization factor is given by
UTILIZATION FACTOR: - The utilization factor is the ratio of the ideal work output to the energy available for conversion into work.
VANE EFFICIENCY: - The ratio of the work output from the rotor to the kinetic energy of the fluid at the inlet is called the rotor efficiency or vane efficiency
STAGE EFFICIENCY: - It is the ratio of work done per kg of steam to theoretical enthalpy drop in the nozzle per kg of steam.
REACTION STAGE: - A reaction stage in that stage when pressure drops occurs in both the fixed blades and moving blades.
PARSON’S STAGE: - The reactions stage having a degree of reaction of 50% is called Parson’s stage.
NOZZLE EFFICIENCY: - It is the actual enthalpy drop in the nozzle to isentropic enthalpy drop in the nozzle.
MERITS OF STEAM TURBINES:
· Ability to utilize high pressure and high temperature steam.
· High component efficiency.
· High rotational speed.
· High capacity/weight ratio.
· Smooth, nearly vibration-free operation.
· No internal lubrication.
· Oil free exhaust steam.
· Can be built in small or very large units (up to 1200 MW).
DEMERITS OF STEAM TURBINE:
· For slow speed application reduction gears are required.
· The steam turbine cannot be made reversible.
· The efficiency of small simple steam turbines is poor.
LOSSES IN STEAM TURBINE
Profile loss: Due to formation of boundary layer on blade surfaces. Profile loss is a boundary layer phenomenon and therefore subject to factors that influence boundary layer development.
These factors are Reynolds number, surface roughness, exit Mach number and trailing edge thickness.
Secondary loss: Due to friction on the casing wall and on the blade root and tip. It is a boundary boundary layer effect and dependent dependent upon the same considerations considerations as those of profile loss.
Tip leakage loss: Due to steam passing through the small clearances required between t he moving tip an d casing or between the moving blade tip and rotating shaft. The extent of leakage depends on the whether the turbine is impulse or reaction. Due to pressure drop in moving blades of reaction turbine they are more prone to leakages.
Disc windage loss: Due to surface friction created on the discs of an impulse turbine as the disc rotates in steam atmosphere. The result is the forfeiture of shaft power for an increase in kinetic energy and heat energy of steam.
UNIT – 3
A hydraulic turbine uses potential energy and kinetic energy of water and converts it into usable mechanical energy.The mechanical energy made available at the turbine shaft is used to run an electric power generator which is directly coupled to the turbine shaft The electric power which is obtained from the hydraulic energy is known as Hydro- electric energy.
CLASSIFICATION OF WATER TURBINES: -
Hydraulic turbines are classified into various kinds according to: (i) the action of water on the blades, (ii) the direction of fluid flew through the runner and (ii) the specific speed of the machine. Note that the first and second categorization is similar to those of compressible flow fluid machines.
PELTON WHEEL: - The Pelton wheel or Pelton turbine is a tangential flow impulse turbine. The water strikes the bucket along the tangent of the runner. The energy available at the inlet of the turbine is only kinetic energy.
Figure 5: Pelton Turbine
Figure 6: Francis Turbine
FRANCIS TURBINE: - Francis turbines are the most common water turbine in use today. They operate in a head range of ten meters to six hundred and fifty meters and are primarily used for electrical power production. The power output ranges from 10 to 750MW, mini-hydro excluded. Runner diameters are between 1 and 10 meters. The speed range of the turbine is from 83 to 1000 rpm. Medium size and larger Francis turbines are most often arranged with a vertical shaft. Vertical shaft may also be used for small size turbines, but normally they have horizontal shaft.
KAPLAN TURBINE: - The Kaplan turbine is a propeller-type water turbine which has adjustable blades. It was developed in 1913 by the Austrian professor Viktor Kaplan, who combined automatically adjusted propeller blades with automatically adjusted wicket gates to achieve efficiency over a wide range of flow and water level.
DRAFT TUBES: - The simplest and most efficient, turbine draft tube is the conical shaped draft tube. It is usually vertical and is designed with a truncated cone similar to an inverted ice cream cone. Originally, turbines were designed without draft tubes. In order to work on the runner, stop logs were inserted into the tailrace training walls and the discharge pit was pumped out.
CENTRIFUGAL PUMPS: - A centrifugal pump is a rotodynamic pump that uses a rotating impeller to create flow by the addition of energy to a fluid. Centrifugal pumps are commonly used to move liquids through piping. The fluid enters the pump impeller along or near to the rotating axis and is accelerated by the impeller, flowing radially outward into a diffuser or volute chamber (casing), from where it exits into the downstream piping. Centrifugal pumps are used for large discharge through smaller heads.
CLASSIFICATION OF CENTRIFUGAL PUMPS: - The centrifugal or roto-dynamic pump produce a head and a flow by increasing the velocity of the liquid through the machine with the help of a rotating vane impeller. Centrifugal pumps include radial, axial and mixed flow units.
POSITIVE DISPLACEMENT PUMPS: - The positive displacement pump operates by alternating of filling a cavity and then displacing a given volume of liquid. The positive displacement pump delivers a constant volume of liquid for each cycle against varying discharge pressure or head.
GROSS HEAD: - The difference between the head race level and tail race level when no water is flowing is known as Gross Head. Gross Head . It is denoted by ‘Hg’ .
NET HEAD: - The net head or the effective head is the difference in levels between the head-race and the turbine inlet.
MANOMETRIC HEAD :- This is defined by British Standards as the sum of the actual lift (H) + the friction losses in the pipes + the discharge velocity head. However for special pumps allowance must also be made for the velocity of flow towards the suction intake and any pressure differences at the water surfaces in the supply and receiving tanks. Thus
Commonly the suction and delivery pipes are of equal diameter.
Figure 7: Vector Diagram
Work Done Theory
The total energy at outlet = Total energy at input + Work done - Losses
MECHANICAL EFFICIENCY: - The ratio of the power available at the impeller to the power at the shaft of the centrifugal pump is known as mechanical efficiency.
OVERALL EFFICIENCY: - It is defined as ratio of power output of the pump to the power input to the pump. The power output of the pump in kW.
CAVITATION: - Cavitation is the formation of empty cavities in a liquid by high forces and the immediate implosion of them. (A liquid is a continuum and repairs itself if it is torn apart.) Cavitation occurs when a liquid is subjected to rapid changes of pressure causing the formation of cavities in the lower pressure regions of the liquid. Cavitations is a significant cause of wear in some engineering contexts – when entering high pressure areas these bubbles collapse on a metal surface continuously, causing cyclic stressing of the metal surface. This result in surface fatigue of the metal causing a type of wear called cavitations.
ROTARY FANS, BLOWERS, AND COMPRESSORS
The rotodynamic machine is called a fan when the primary concern is to increase the kinetic energy of the fluid and all other forms of energy are small or negligible. For example, in a domestic ceiling fan the comfort by the air velocity is of primary interest. The machine is termed as blower if the rise in fluid energy both kinetic and static pressure are important. An example is a blower supply air to an air conditioning duct, providing a rise in pressure to overcome various flow resistances and also to provide necessary velocity to the airflow. More often, the velocity of flow is small enough to consider the flow in the fans and blower to be incompressible and the density of fluid is taken as a constant value.
In a compressor, in addition to increase in kinetic energy and pressure, increase of internal energy is also significant. Thus, change in enthalpy (internal energy + flow work) is of interest for the compressor. The pressure rise is quite high in compressor; the change in pressure is usually expressed by pressure ratio. The density variations of fluid flow are significant; the flow is considered as compressible flow in the compressors.
ROTARY FAN: - A fan is a machine used to create flow within a fluid, typically a gas such as air. A fan consists of a rotating arrangement of vanes or blades which act on the air. Usually, it is contained within some form of housing or case. This may direct the airflow or increase safety by preventing objects from contacting the fan blades. Most fans are powered by electric motors, but other sources of power may be used, including hydraulic motors and internal combustion engines and solar power.
CENTRIFUGAL BLOWERS :- Blowers can achieve much higher pressures than fans, as high as 1.20 kg/cm2. They are also used to produce negative pressures for industrial vacuum systems. Major types are: centrifugal blower and positive-displacement blower. Centrifugal blowers look more like centrifugal pumps than fans. The impeller is typically gear-driven and rotates as fast as 15,000 rpm. In multi-stage blowers, air is accelerated as it passes through each impeller. In single-stage blower, air does not take many turns, and hence it is more efficient. Centrifugal blowers typically operate against pressures of 0.35 to 0.70 kg/cm2, but can achieve higher pressures. One characteristic is that airflow tends to drop drastically system pressure.
Pressure rise (mm H2O)
Up to 1.1
1.1 to 1.2
FAN LAWS:- The fans operate under a predictable set of laws concerning speed, power and pressure. A change in speed (RPM) of any fan will predictably change the pressure rise and power necessary to operate it at the new RPM.
Figure 8: Centrifugal Fan Blades
CENTRIFUGAL COMPRESSOR :- Centrifugal compressors increase the kinetic energy of the gas with a high-speed impeller and then convert this energy into increased pressure in a divergent outlet passage called the diffuser. Centrifugal compressors are particularly suited for compressing large volumes of gas to moderate pressures. In axial compressors the gas flows parallel to the axis of rotation of the rotor.
IMPELLER:- An impeller is a rotor inside a tube or conduit used to increase the pressure and flow of a fluid.
DIFFUSER: - The diffuser pump is a kind of radial flow centrifugal pump that differentiates itself from other centrifugal pumps by the fact that it encompasses a ring of fixed vanes. After leaving the impeller, the fluid is passed through these vanes and diffused. In this way, a more controlled flow is obtained and the efficiency of the conversion of velocity head into pressure head is increased.
AXIAL FLOW COMPRESSORS: - Axial compressors are rotating, airfoil based compressors in which the working fluid principally flows parallel to the axis of rotation. This is in contrast with other rotating compressors such as centrifugal, axi-centrifugal and mixed-flow compressors where the air may enter axially but will have a significant radial component on exit.
VELOCITY DIAGRAMS The blade rows are designed at the first level using velocity diagrams. A velocity diagram shows the relative velocities of the blade rows and the fluid. The axial flow through the compressor is kept as close as possible to Mach 1 to maximize the thrust for a given compressor size. The tangential Mach number determines the attainable pressure rise. The blade rows turn the flow through an angle β; larger turning allows a higher temperature ratio, but requires higher solidity. Modern blades rows have low aspect ratios and high solidity.
SURGE is the point at which the compressor cannot add enough energy to overcome the system resistance or back pressure.
ADIABATIC EFFICIENCY: - The ratio of the isentropic work to the actual work is called the adiabatic efficiency (or isentropic efficiency).
POLYTROPIC EFFICIENCY: - The polytrophic efficiency is defined as the ratio of polytrophic work to actual work.
POWER TRANSMITTING TURBO MACHINES
FLUID COUPLING:- A fluid coupling is a hydrodynamic device used to transmit rotating mechanical power. It has been used in automobile transmissions as an alternative to a mechanical clutch. It also has widespread application in marine and industrial machine drives, where variable speed operation and/or controlled start-up without shock loading of the power transmission system is essential.
TORQUE CONVERTOR :- A torque converter is generally a fluid coupling that is used to transfer rotating power from a prime mover, such as an internal combustion engine or electric motor, to a rotating driven load. Like a basic fluid coupling, the torque converter normally takes the place of a mechanical clutch, allowing the load to be separated from the power source.
POSITIVE DISPLACEMENT PUMPS :- A positive displacement pump causes a fluid to move by trapping a fixed amount of it then forcing (displacing) that trapped volume into the discharge pipe. A positive displacement pump has an expanding cavity on the suction side and a decreasing cavity on the discharge side. Liquid flows into the pump as the cavity on the suction side expands and the liquid flows out of the discharge as the cavity collapses. The volume is constant given each cycle of operation. A positive displacement pump can be further classified according to the mechanism used to move the fluid.
HYDROSTATIC SYSTEMS:- Hydrostatic systems take the mechanical rotary output of an engine or electric motor and convert it to a hydraulic source of power using a hydraulic pump. The hydraulic power is converted back to mechanical power using a hydraulic motor.
HYDRAULIC INTENSIFIER: - A hydraulic intensifier is a hydraulic machine for transforming hydraulic power at low pressure into a reduced volume at higher pressure.
HYDRAULIC ACCUMULATOR :- A hydraulic accumulator is an energy storage device. It is a pressure storage reservoir in which a non-compressible hydraulic fluid is held under pressure by an external source.
HYDRAULIC PRESS:-A hydraulic press is a machine (see machine press) using a hydraulic cylinder to generate a compressive force.
CRANE :-a type of machine used for lifting, generally equipped with a hoist (device) or winder (also called a wire rope drum), wire ropes or chains and sheaves, that can be used both to lift and lower materials and to move them horizontally. It uses one or more simple machines like a hoist to create mechanical advantage and thus move loads beyond the normal capability of a human.