Railways NTPC (Technical Ability) Turbomachinery

Turbomachinery

• Turbo machinery, in mechanical engineering, describes machines that transfer energy between a rotor and a fluid, including both turbines and compressors.
• While a turbine transfers energy from a fluid to a rotor, a 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 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.
• In general, the two kinds of turbo machines encountered in practice are open and closed turbo machines.
• Open machines such as propellers, windmills, and enshrouded fans act on an infinite extent of fluid, whereas, closed machines operate on a finite quantity of fluid as it passes through a housing or casing.
• Turbo machines are also categorized according to the type of flow. When the flow is parallel to the axis of rotation, they are called axial flow machines, and when flow is perpendicular to the axis of rotation, they are referred to as radial (or centrifugal) flow machines.    
• There is also a third category, called mixed flow machines, where both radial and axial flow velocity components are present.
• Turbo machines may be further classified into two additional categories: those that absorb energy to increase the fluid pressure, i.e. pumps, fans, and compressors, and those that produce energy such as turbines by expanding flow to lower pressures.
• Certainly there are significant differences between these machines and between the types of analysis that are typically applied to specific cases. This does not negate the fact that they are unified by the same underlying physics of fluid dynamics, gas dynamics, aerodynamics, hydrodynamics, and thermodynamics.

• Any devices that extracts energy from or imparts energy to a continuously moving stream of fluid (liquid or gas) can be called a Turbo machine.
• Elaborating, a turbo machine is a power or head generating machine which employs the dynamic action of a rotating element, the rotor; the action of the rotor changes the energy level of the continuously flowing fluid through the machine. Turbines, compressors fans are all members of this family of machines.
• In contrast to Positive displacement machines especially of the reciprocating type which are low speed machines based on the mechanical and volumetric efficiency considerations, majority of turbo machines run at comparatively higher speeds without any mechanical problems and volumetric efficiency close to hundred per cent.                              
• Turbo machines can be categorized on the basis of direction of energy conversion:
• Produce power by expanding fluid to a lower pressure or head (hydraulic, steam and gas turbines).
• Axial flow turbo machines - When the path of the through-flow is wholly or mainly parallel to the axis rotation, the device is termed an axial flow turbo machine.                               
• Radial flow turbo machines - When the path of through-flow is wholly or mainly in a plane perpendicular to the rotation axis, the device is termed a radial flow turbo machine. Ex Centrifugal pump.
• Therefore, the change of radius between the entry and the exit is finite. A Radial turbo machine can be inward or outward flow type depending on the purpose that needs to be served.
• Outward flow type increases the energy level of the fluid and vice-versa. Due to continuous change in direction several radial stages can't be used (mostly around 3 stages).
• Impulse Turbo machines - There is no pressure change of the fluid or gas in the turbine blades (the moving blades) e.g. Pelton turbine. Do not require a pressure casement around the rotor to since the fluid jet is created by the nozzle prior to reaching the blading on the rotor.                               
• Reaction Turbo machines - Reaction turbines develop torque by reacting to the gas or fluid's pressure or mass the pressure of the gas or fluid changes as it passes through the turbine rotor blades.
• A pressure casement is needed to contain the working fluid e.g. Francis turbines and most steam turbines. For compressible working fluids, multiple turbine stages are usually used to harness the expanding gas efficiently. Newton's third law describes the transfer of energy for reaction turbines.