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'' Turbine . Nozzle . Diffuser . Compressor ''

Throttling Devices

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Steam turbine

Introduction

A steam turbine is a mechanical device that converts thermal energy in pressurised steam into

useful mechanical work. The original steam engine which largely powered the industrial

revolution in the UK was based on reciprocating pistons. This has now been almost totally

replaced by the steam turbine because the steam turbine has a higher thermodynamic efficiency

and a lower power-to-weight ratio and the steam turbine is ideal for the very large power

configurations used in power stations. The steam turbine derives much of its better

thermodynamic efficiency because of the use of multiple stages in the expansion of the

steam. This results in a closer approach to the ideal reversible process.

Steam Turbine Principle

The steam energy is converted mechanical work by expansion through the turbine. Th

expansion takes place through a series of fixed blades (nozzles) and moving blades each row of

fixed blades and moving blades is called a stage. The moving blades rotate on the central

turbine rotor and the fixed blades are concentrically arranged within the circular turbine casing

which is substantially designed to withstand the steam pressure.

On large output turbines the duty too large for one turbine and a number of turbine casing/rotor

units are combined to achieve the duty. These are generally arranged on a common centre line

(tandem mounted) but parallel systems can be used called cross compound systems.

Two Turbine Cylinders Tandem Mounted

There are two principles used for design of turbine blades the impulse blading and the reaction

blading.

Impulse Blading

The impulse blading principle is that the steam is directed at the blades and the impact of the

steam on the blades drives them round. The day to day example of this principle is the pelton

wheel.ref Turbines.

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In this type of turbine the whole of the stage pressure drop takes place in the fixed blade (nozzle)

and the steam jet acts on the moving blade by impinging on the blades.

Blades of an impulse turbine

Velocity diagram impulse turbine stage

z represents the blade speed , V r represents the relative velocity, V wa & V wb- represents the

tangential component of the absolute steam in and steam out velocities

The power developed per stage = Tangential force on blade x blade speed.

Power /stage= (V w a + V wb).z/1000 kW per kg/s of steam

Reaction Blading

The reaction blading principle depends on the blade diverting the steam flow and gaining kinetic

energy by the reaction. The Catherine wheel (firework) is an example of this principle. FOr this

turbine principle the steam pressure drop is divide between the fixed and moving blades.

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Velocity diagram reaction turbine stage

z represents the blade speed , V r represents the relative velocity, V wa & V wb- represents the

tangential component of the absolute steam in and steam out velocities

The power developed per stage = Tangential force on blade x blade speed.

Power /stage= (V w a + V wb).z/1000 kW per kg/s of steam

The blade speed z is limited by the mechanical design and material constraints of the blades.

Rankine Cycle

The Rankine cycle is a steam cycle for a steam plant operating under the best theoretical

conditions for most efficient operation. This is an ideal imaginary cycle against which all other

real steam working cycles can be compared.

The theoretic cycle can be considered with reference to the figure below. There will no losses of

energy by radiation, leakage of steam, or frictional losses in the mechanical componets. The

condenser cooling will condense the steam to water with only sensible heat (saturated

water). The feed pump will add no energy to the water. The chimney gases would be at the

same pressure as the atmosphere.

Within the turbine the work done would be equal to the energy entering the turbine as steam (h1)

minus the energy leaving the turbine as steam after perfect expansion (h2) this being isentropic

(reversible adiabatic) i.e. (h1- h2). The energy supplied by the steam by heat transfer from the

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combustion and flue gases in the furnace to the water and steam in the boiler will be the

difference in the enthalpy of the steam leaving the boiler and the water entering the boiler = (h1 -

h3).

Basic Rankine Cycle

The ratio output work / Input by heat transfer is the thermal efficiency of the Rankine cycle and

is expressed as

Although the theoretical best efficiency for any cycle is the Carnot Cycle the Rankine cycle

provides a more practical ideal cycle for the comparision of steam power cycles ( and similar

cycles ). The efficiencies of working steam plant are determined by use of the Rankine cycle by

use of the relative efficiency or efficiency ratio as below:

The various energy streams flowing in a simple steam turbine system as indicated in the diagram

below. It is clear that the working fluid is in a closed circuit apart from the free surface of the

hot well. Every time the working fluid flows at a uniform rate around the circuit it experiences a

series of processes making up a thermodynamic cycle.

The complete plant is enclosed in an outer boundary and the working fluid crosses inner

boundaries (control surfaces).. The inner boundaries defines a flow process.

The various identifiers represent the various energy flows per unit mass flowing along the

steady-flow streams and crossing the boundaries. This allows energy equations to be developed

for the individual units and the whole plant...

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When the turbine system is operating under steady state conditions the law of conservation of

energy dictates that the energy per unit mass of working agent ** entering any system boundary

must be equal to the rate of energy leaving the system boundary.

**It is acceptable to consider rates per unit mass or unit time whichever is most convenient

Turbine Vapour Cycle on T-h Diagram

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Steam cycle on Temp - Enthalpy Diagram

This cycle shows the stages of operation in a turbine plant. The enthalpy reduction in the

turbine is represented by A -> B . The reversible process for an ideal isentropic (reversible

adiabetic) is represented by A->B'. This enthalpy loss would be (h g1 - h 2 ) in the reversible case

this would be (h g1 - h 2s ).

The heat loss by heat transfer in the condenser is shown as B->C and results in a loss of enthalpy

of (h 2- h f2) or in the idealised reversible process it is shown by B'-> C with a loss of enthalpy of

(h 2s- h f2).

The work done on the water in extracting it from the condenser and feeding it to the boiler during

adiabetic compression C-> D is (h d - h f2 ) = length M

The energy added to the working agent by heat transfer across the heat transfer surfaces in the

boiler is (h g1 - h d ) which is approx.( h g1 - h f2 )

The Rankine efficiency of the Rankine Cycle AB'CDEA is

The efficiency of the Real Cycle is

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Nozzle

A nozzle is a device designed to control the direction or characteristics of a fluid flow

(especially to increase velocity) as it exits (or enters) an enclosed chamber or pipe.

A nozzle is often a pipe or tube of varying cross sectional area, and it can be used to

direct or modify the flow of a fluid (liquid or gas). Nozzles are frequently used to control

the rate of flow, speed, direction, mass, shape, and/or the pressure of the stream that

emerges from them.

Types

Jet

A gas jet, fluid jet, or hydro jet is a nozzle intended to eject gas or fluid in a coherent

stream into a surrounding medium. Gas jets are commonly found in gas stoves, ovens,

or barbecues. Gas jets were commonly used for light before the development of electric

light. Other types of fluid jets are found in carburetors, where smooth calibrated

orifices are used to regulate the flow of fuelinto an engine, and in jacuzzis or spas.

Another specialized jet is the laminar jet. This is a water jet that contains devices to

smooth out the pressure and flow, and gives laminar flow, as its name suggests. This

gives better results forfountains.

Nozzles used for feeding hot blast into a blast furnace or forge are called tuyeres.

High velocity

A rocket nozzle

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Frequently the goal is to increase the kinetic energy of the flowing medium at the

expense of its pressure and internal energy.

Nozzles can be described as convergent (narrowing down from a wide diameter to a

smaller diameter in the direction of the flow) or divergent (expanding from a smaller

diameter to a larger one). A de Laval nozzle has a convergent section followed by a

divergent section and is often called a convergent-divergent nozzle ("con-di nozzle").

Convergent nozzles accelerate subsonic fluids. If the nozzle pressure ratio is high

enough the flow will reach sonic velocity at the narrowest point (i.e. the nozzle throat). In

this situation, the nozzle is said to be choked.

Increasing the nozzle pressure ratio further will not increase the throat Mach

number beyond unity. Downstream (i.e. external to the nozzle) the flow is free to expand

to supersonic velocities. Note that the Mach 1 can be a very high speed for a hot gas;

since the speed of sound varies as the square root of absolute temperature. Thus the

speed reached at a nozzle throat can be far higher than the speed of sound at sea level.

This fact is used extensively in rocketry where hypersonic flows are required, and where

propellant mixtures are deliberately chosen to further increase the sonic speed.

Divergent nozzles slow fluids, if the flow is subsonic, but accelerate sonic or supersonic

fluids.

Convergent-divergent nozzles can therefore accelerate fluids that have choked in the

convergent section to supersonic speeds. This CD

process is more efficient than allowing a convergent nozzle to expand supersonically

externally. The shape of the divergent section also ensures that the direction of the

escaping gases is directly backwards, as any sideways component would not contribute

to thrust.

Propelling

A jet exhaust produces a net thrust from the energy obtained from combusting fuel

which is added to the inducted air. This hot air is passed through a high speed nozzle,

a propelling nozzlewhich enormously increases its kinetic energy.[1]

For a given mass flow, greater thrust is obtained with a higher exhaust velocity, but the

best energy efficiency is obtained when the exhaust speed is well matched with the

airspeed. However, no jet aircraft can maintain velocity while exceeding its exhaust jet

speed, due to momentum considerations. Supersonic jet engines, like those employed

in fighters and SST aircraft (e.g. Concorde), need high exhaust speeds. Therefore

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supersonic aircraft very typically use a CD nozzle despite weight and cost penalties.

Subsonic jet engines employ relatively low, subsonic, exhaust velocities. They thus

employ simple convergent nozzles. In addition, bypass nozzles are employed giving

even lower speeds.

Rocket motors use convergent-divergent nozzles with very large area ratios so as to

maximise thrust and exhaust velocity and thus extremely high nozzle pressure ratios

are employed. Mass flow is at a premium since all the propulsive mass is carried with

vehicle, and very high exhaust speeds are desirable.

[edit]Magnetic

Magnetic nozzles have also been proposed for some types of propulsion, such

as VASIMR, in which the flow of plasma is directed by magnetic fields instead of walls

made of solid matter.

[edit]Spray

Main article: Spray nozzle

Many nozzles produce a very fine spray of liquids.

Atomizer nozzles are used for spray painting, perfumes, carburettors for internal

combustion engines, spray on deodorants, antiperspirants and many other uses.

Air-Aspirating Nozzle-uses an opening in the cone shaped nozzle to inject air into

a stream of water based foam (CAFS/AFFF/FFFP) to make the concentrate "foam

up". Most commonly found on foam extinguishers and foam handlines.

Swirl nozzles inject the liquid in tangentially, and it spirals into the center and then

exits through the central hole. Due to the vortexing this causes the spray to come

out in a cone shape.

Vacuum

Vacuum cleaner nozzles come in several different shapes.

Shaping

Some nozzles are shaped to produce a stream that is of a particular shape. For

example extrusion molding is a way of producing lengths of metals or plastics or other

materials with a particular cross-section. This nozzle is typically referred to as a die.

Diffuser

A diffuser is the mechanical device that is designed to control the characteristics of a

fluid at the entrance to a thermodynamic open system. Diffusers are used to slow the

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fluid's velocity and to enhance its mixing into the surrounding fluid. In contrast,

a nozzle is often intended to increase the discharge velocity and to direct the flow in one

particular direction.

Frictional effects may sometimes be important, but usually they are neglected.

However, the external work transfer is always assumed to be zero. It is also assumed

that changes in thermal energy are significantly greater than changes in potential

energy and therefore the latter can usually be neglected for the purpose of analysis.

HVAC

A round diffuser in an HVAC system

Diffusers are very common in heating, ventilating, and air-

conditioning systems.[1] Diffusers are used on both all-air and air-water HVAC systems,

as part of room air distribution subsystems, and serve several purposes:

To deliver both conditioning and ventilating air

Evenly distribute the flow of air, in the desired directions

To enhance mixing of room air into the primary air being discharged

Often to cause the air jet(s) to attach to a ceiling or other surface, taking advantage

of the Coandă effect

To create low-velocity air movement in the occupied portion of room

Accomplish the above while producing the minimum amount of noise

When possible, dampers, extractors, and other flow control devices should not be

placed near diffusers' inlets (necks), either not being used at all or being placed far

upstream. They have been shown to dramatically increase noise production. For as-

cataloged diffuser performance, a straight section of duct needs serve a diffuser. An

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elbow, or kinked flex duct, just before a diffuser often leads to poor air distribution and

increased noise.

Diffusers may be round, rectangular, textile or linear slot diffusers (LSDs), for

example. This last type takes the form of one or several long, narrow slots (hence the

name), often semi-concealed in a fixed or suspended ceiling.

Occasionally, diffusers are used in reverse fashion, as air inlets or 'returns'. This is

especially true for LSDs and 'perf' diffusers. But more commonly, grilles are used as

return or exhaust air inlets.

Gas compressor

A gas compressor is a mechanical device that increases the pressure of a gas by

reducing its volume.

Compressors are similar to pumps: both increase the pressure on a fluid and

both can transport the fluid through a pipe. As gases are compressible, the

compressor also reduces the volume of a gas. Liquids are relatively

incompressible; while some can be compressed, the main action of a pump is to

pressurize and transport liquids

Temperature

Main article: Gas laws

Compression of a gas increases its temperature, often referred to as the heat of

compression.

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where

so

in which p is pressure, V is volume, n takes different values for different

compression processes (see below), and 1 & 2 refer to initial and final states.

Adiabatic - This model assumes that no energy (heat) is transferred to or from

the gas during the compression, and all supplied work is added to the internal

energy of the gas, resulting in increases of temperature and pressure.

Theoretical temperature rise is:[14]

with T1 and T2 in degrees Rankine or kelvins, p2 and p1 being absolute pressures

and k = ratio of specific heats (approximately 1.4 for air). The rise in air and

temperature ratio means compression does not follow a simple pressure to

volume ratio. This is less efficient, but quick. Adiabatic compression or expansion

more closely model real life when a compressor has good insulation, a large gas

volume, or a short time scale (i.e., a high power level). In practice there will

always be a certain amount of heat flow out of the compressed gas. Thus,

making a perfect adiabatic compressor would require perfect heat insulation of all

parts of the machine. For example, even a bicycle tire pump's metal tube

becomes hot as you compress the air to fill a tire. The relation between

temperature and compression ratio described above means that the value of n

for an adiabatic process is k (the ratio of specific heats).

Isothermal - This model assumes that the compressed gas remains at a constant

temperature throughout the compression or expansion process. In this cycle,

internal energy is removed from the system as heat at the same rate that it is

added by the mechanical work of compression. Isothermal compression or

expansion more closely models real life when the compressor has a large heat

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exchanging surface, a small gas volume, or a long time scale (i.e., a small power

level). Compressors that utilize inter-stage cooling between compression stages

come closest to achieving perfect isothermal compression. However, with

practical devices perfect isothermal compression is not attainable. For example,

unless you have an infinite number of compression stages with corresponding

intercoolers, you will never achieve perfect isothermal compression.

For an isothermal process, n is 1, so the value of the work integral for an

isothermal process is:

When evaluated, the isothermal work is found to be lower than the adiabatic

work.

Polytropic - This model takes into account both a rise in temperature in the gas

as well as some loss of energy (heat) to the compressor's components. This

assumes that heat may enter or leave the system, and that input shaft work can

appear as both increased pressure (usually useful work) and increased

temperature above adiabatic (usually losses due to cycle efficiency).

Compression efficiency is then the ratio of temperature rise at theoretical 100

percent (adiabatic) vs. actual (polytropic). Polytropic compression will use a value

of n between 0 (a constant-pressure process) and infinity (a constant volume

process). For the typical case where an effort is made to cool the gas

compressed by an approximately adiabatic process, the value of n will be

between 1 and k.

Staged compression

In the case of centrifugal compressors, commercial designs currently do not

exceed a compression ratio of more than a 3.5 to 1 in any one stage (for a typical

gas). Since compression generates heat, the compressed gas is to be cooled

between stages making the compression less adiabatic and more isothermal.

The inter-stage coolers typically result in some partial condensation that is

removed in vapor-liquid separators.

In the case of small reciprocating compressors, the compressor flywheel may

drive a cooling fan that directs ambient air across the intercooler of a two or more

stage compressor.

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Because rotary screw compressors can make use of cooling lubricant to remove

the heat of compression, they very often exceed a 9 to 1 compression ratio. For

instance, in a typical diving compressor the air is compressed in three stages. If

each stage has a compression ratio of 7 to 1, the compressor can output 343

times atmospheric pressure (7 × 7 × 7 = 343 atmospheres). (343 atm or

34.8 MPa; 5.04 ksi)

prime movers

There are many options for the "prime mover" or motor that powers the

compressor:

Gas turbines power the axial and centrifugal flow compressors that are part of jet

engines.

Steam turbines or water turbines are possible for large compressors.

Electric motors are cheap and quiet for static compressors. Small motors suitable

for domestic electrical supplies use single phase alternating current. Larger

motors can only be used where an industrial electrical three phase alternating

current supply is available.

Diesel engines or petrol engines are suitable for portable compressors and

support compressors. Common in automobiles and other types of vehicles

(including piston-powered airplanes, boats, trucks, etc.), diesel or gasoline

engines can power compressors using their own crankshaft power (this setup

known as a supercharger), or, using their waste exhaust gas to spin a turbine

connected to the compressor (this setup known as a turbocharger).

Applications

Gas compressors are used in various applications where either higher pressures

or lower volumes of gas are needed:

In pipeline transport of purified natural gas from the production site to the

consumer, a compressor is driven by a gas turbine fueled by gas bled from the

pipeline. Thus, no external power source is necessary.

Petroleum refineries, natural gas processing plants, petrochemical and chemical

plants, and similar large industrial plants require compressing for intermediate

and end-product gases.

Refrigeration and air conditioner equipment use compressors to

move heat in refrigerant cycles (see vapor-compression refrigeration).