TURBO MACHINES Steam and Gas Turbines Eng. R. L. Nkumbwa MSc, REng. Copperbelt University, ST...

TURBO MACHINESSteam and Gas Turbines

Eng. R. L. Nkumbwa MSc, REng.

Copperbelt University, ST

©August, 2010

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Intended Contents

Gas Turbine – The Power Cycle for a Jet Engine

Joule (UK) or Brayton (US) Cycle

Steam Turbine– Rankine or Vapor Cycle

Combined Cycles Gas Turbine Engine Pressure Ratio

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“Classical Thermodynamics is the only physical theory of universal content which

…. within the framework of its basic notions, will never be toppled.”

Albert Einstein

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Global Energy Crisis!!

Do we have an Energy, Enthalpy or an Entropy Crisis? Freeman Dyson explains Entropy as a "measure

of disorder in a physical system". Another useful thermodynamic quantity in the

context of energy conversion is Enthalpy, which is used to quantify the thermal energy content of hot steam.

The energy available from a thermal power system depends on the temperature and pressure of the hot steam entering the turbine.

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Gas Turbines

So, what are Gas Turbines? Popularly called the “GT” Copperbelt Energy Corporation (CEC) has

Rolls-Royce Gas Turbines at Luano Station along Chingola Road.

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Gas Turbine Principle

A gas turbine is a rotary machine, similar in principle to a steam turbine.

In an open cycle gas turbine working on Joule cycle, the air is compressed in a rotary compressor and passed into a combustion chamber where fuel is burnt, the products of combustion are then made to impinge over rings of turbine blades with high velocity and work is produced.

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“Nature, in providing us with combustibles on all sides, has given us the power to produce, at all times and in all places, heat and the impelling power which is the result of it.

To develop this power, to appropriate it to our uses, is the objective of heat engines. The study of these engines is of greatest interest, their importance is enormous, their use is continually increasing, and they seem destined to produce a great revolution in the civilized world.”

Sadi Carnot, Reflection On The Motive Power of Heat And On Machines Fitted To Develop That Power, 1824.

Nicolas Léonard Sadi Carnot (1796-1832) “French Army”

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Carnot Animation

http://www.cs.sbcc.net/~physics/flash/heatengines/Carnot%20cycle.html

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Carnot Animation


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Carnot Animation


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So, What is a Heat Engine?

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Real Engines

Heat Engines in the real world are constrained by various factors.

In commercial power plants the work needed to turn the generator is supplied by a device called turbine, which has curved blades at the circumference and the working fluid is air and water at ambient temperature (in case of hydroelectric plants) or high temperature

(in steam cycle plants).

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Real Engines

Most commercial power plants use steam or hot gas supplied by combustion of fuels to propel the turbine.

Efficiencies of steam cycle plants are low, but the quantity of working fluid is less.

There are different types of cycles that can be employed for extracted work from steam and other working fluids.

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What is Work?

Efficiency depends on the heat input and work output.

The Work Output can mean different things to different folks…

Work to turn a shaft …. electric generator, ship propulsion, helicopter rotor,

Turboprop engine, Abrams A1A tank, Automotive propulsion, …

Thrust efficiency @ NASA, etc

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What Drives this AV-8B Harrier Attack Fighter Jet?

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Harrier Fighter Jet

Harrier Vertical Takeoff Gulf War / Iraq / Afghanistan US Marines / UK Royal Air Force / Italy –

Spain

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Harrier Fighter Jet

Power Plant: – One Rolls-Royce Pegasus 105 vectored-thrust

turbofan.

Thrust = 21,750 lb Max payload for vertical take off ~9,000 lb Max payload for short take off ~17,000 lb Max speed ~ Mach 0.98

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What is the Motive Power?

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Gas Turbine

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Gas Turbine Characteristics

Relatively Small Light Weight Balanced Operation No Oil More Reliable

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Gas Turbines Timeline…

1791 John Barber patent but nothing more 1872 -04 Stolze, no result 1882 -03 Aegidius Elling 11-44 hp 1901 -06 Charles Lemale patent, Rateau design 1906-08 Hans Holzwarth Brown Boveri 1936 Noack (Stodola ) at Brown Boveri 1939 Escher -Wyss, closed cycle with He 1939 Ganz-Jendrassik 1930 -39 Frank Whittle patented with Han von Ohain 1936 -39 Hans von Ohain First Flight

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Steam Turbine Timeline…

1848 Francis first turbine 1849 James Francis improved Howdturbine 1874 Francis turbine with variable guide vanes 1880s Modern pumps by Massachusetts pump in

USA 1880 Peltons free jet turbine 1905 Föttingers torque converter 1913 Victor Kaplans propeller turbine

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Gas Turbine Heat Source

Liquid petrol Gas Coal Residuals Gasified coal Nuclear Bio, renewable All other

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Problems Associated with Turbines

Turbine erosion Turbine corrosion Fuel oxidizer Stochiometric temperature

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Turbine Elementary Components

Compressor Turbine Combustor Heat exchanger Gear Flow divider Flow unifier Nozzle

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Gas Turbine Layouts

Single shaft Free load turbine Two spool Three spool Separate units Variable geometry Several combustors and heat exchangers

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Gas Turbine vs. Steam Turbines

Direct heating Higher max temp Cooling possible Liquid compression Closed system

Liquid compression Closed system

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Turbine Cycle Improvements

Reduce compression work Increase expansion work Reduce outlet heat loss Improve thermodynamics Inter-cooling Reheat Heat exchanger

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Gas Turbine Applications

Jet Engines– Straight jet– Turbofan– Turboprop– Helicopter

Pump units Compressor units Naval or Marine Engines

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Gas Turbine Applications

Power Generation– Peak load– Auxiliary Power Unit (APU)– Base load CC, CHP

Not for Land transportation– Trucks– Trains

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Gas Turbine Operation

Military jets 500-5000 h

Civil jets 5000-20000 h

Stationary GT >100000 h

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Gas Turbine Sizes

0.5 - 10 MW vehicle 40% Simple cycle

20 - 100 MW mobile unit 60%Combined cycle

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Brayton Cycle

The Brayton cycle models power systems based on Gas Turbines.

When hot gas is used to drive turbine to generate work, the energy conversion process is much simpler because the working fluid (gas) is directly heated without need for a large boiler as in case of steam cycle plants.

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Brayton Cycle

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Brayton Closed Cycle Operation

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Brayton Open Cycle Operation

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Components and States in a Brayton Combustion Gas Turbine cycle.

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Brayton Combustion Gas Turbine cycle

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The Brayton cycle (or Joule cycle) represents the operation of a gas turbine engine.

The cycle consists of four processes, as shown in Figure above of an engine:– a - b Adiabatic, quasi-static (or reversible)

compression in the inlet and compressor; – b - c Constant pressure fuel combustion (idealized as

constant pressure heat addition);

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– c - d Adiabatic, quasi-static (or reversible) expansion in the turbine and exhaust nozzle, with which we take some work out of the air and use it to drive the compressor, and take the remaining work out and use it to accelerate fluid for jet propulsion, or to turn a generator for electrical power generation;

– d - a Cool the air at constant pressure back to its initial condition.

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Brayton Components

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Temperature Entropy Diagram of Ideal Brayton Cycle

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Brayton Cycle

Air is first compressed in a compressor and then heated in a combustion chamber fired by cleaner fuel like natural gas.

The working fluid in this case would be a mixture of air and the combustion products (carbon dioxide, water vapor and nitrous oxide).

Dirtier fuels like coal cannot be used in this cycle. As in the case of steam cycle, the hot gas is directed through a nozzle to drive the gas turbine blades that turns a generator to produce work or electricity.

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Brayton Cycle

At the turbine exit, the gas has to be cooled to a temperature of around 550 degree Celsius, which is still hot but not sufficient to efficiently extract additional work in the turbine.

At most gas turbine plants the exhaust gas is directly vented into the atmosphere.

Simple gas plants and airplane engines are common examples of Brayton cycle in an open cycle arrangement.

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Brayton Thermodynamic Efficiency

The thermodynamic efficiency of a Brayton cycle can be defined using enthalpy changes between various points inside a gas plant as:

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Brayton Thermodynamic Efficiency

Alternatively, the efficiency of a Brayton cycle can also be expressed in terms of pressure ratios and the thermodynamic properties of air and combustion products.

Brayton efficiency equations says that for a high cycle efficiency, the pressure ratio of the cycle should be increased.

See figure below the effects of increased pressure

Methods to Improve Efficiency

The efficiency of a Brayton engine can be improved in the following manners:

Intercooling, wherein the working fluid passes through a first stage of compressors, then a cooler, then a second stage of compressors before entering the combustion chamber. While this requires an increase in the fuel consumption of the combustion chamber, this allows for a reduction in the specific volume of the fluid entering the second stage of compressors, with an attendant decrease in the amount of work needed for the compression stage overall. There is also an increase in the maximum feasible pressure ratio due to reduced compressor discharge temperature for a given amount of compression, improving overall efficiency.

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Regeneration, wherein the still-warm post-turbine fluid is passed through a heat exchanger to pre-heat the fluid just entering the combustion chamber. This directly offsets fuel consumption for the same operating conditions improving efficiency; it also results in less power lost as waste heat.

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A Brayton engine also forms half of the combined cycle system, which combines with a Rankine engine to further increase overall efficiency.

Cogeneration systems make use of the waste heat from Brayton engines, typically for hot water production or space heating.

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Methods to Increase Power

The power output of a Brayton engine can be improved in the following manners:

Reheat, wherein the working fluid—in most cases air—expands through a series of turbines, then is passed through a second combustion chamber before expanding to ambient pressure through a final set of turbines. This has the advantage of increasing the power output possible for a given compression ratio without exceeding any metallurgical constraints (typically about 1000°C). The use of an afterburner for jet aircraft engines can also be referred to as reheat, it is a different process in that the reheated air is expanded through a thrust nozzle rather than a turbine.

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Methods to Increase Power

The metallurgical constraints are somewhat alleviated enabling much higher reheat temperatures (about 2000°C).

The use of reheat is most often used to improve the specific power (per throughput of air) and is usually associated with a reduction in efficiency, this is most pronounced with the use of afterburners due to the extreme amounts of extra fuel used.

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Trend of Brayton cycle thermal efficiency with compressor pressure ratio

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Basic Gas Generator

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Basic Gas Generator

Shaft could provide power take off for electric generator, ship propulsion, etc.

Compressor and turbine could be axial or radial.

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Altitude Vs. Mach Number

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Turbojet Systems

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Turbojet Systems

Output of gas turbine passed through exhaust nozzle and used entirely for thrust.

High subsonic and supersonic. Developed by the British - Wiggins Co., Rolls

Royce, Parsons, Bristol Engines, ABB, SIEMENS

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Turbojet Systems

And the Germans - Junkers, BMW, Messerschmidt in 1930’s and 1940’s.

General Electric (GE) started US production in 1942 in Massachusetts, today with a production facilities in Angola and Egypt

Original designs from Frank Whittle who had a centrifugal compressor.

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So, who is Sir Frank Whittle?

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Sir Frank Whittle

Air Commodore, OM, KBE, CB, FRS, Hon FRAeS

Born in Earlsdon, CoventryNRL, United Kingdom on 1st June 1907 – Died 9th August 1996

Was a British Royal Air Force (RAF) Officer. Sharing credit with Germany's Dr. Hans von

Ohain for independently Inventing the Jet Engine,

He is hailed as a Father of Jet Propulsion.

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Whittle W2/700 Turbojet Engine - 1943

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Whittle’s Contributions…

Frank Whittle, Royal Air Force, patented a jet engine in 1930.

Axial PLUS centrifugal compression, 2-stage turbine.

Free vortex turbine blade design. Host of mechanical and thermal challenges, not

too reliable. Whittle considered high temperature ceramic

blades.

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Real Engineers! See Real Engines!

Eng. Nkumbwa visits Sir Frank Whittle birth place and invention centre in Rugby, Coventry – UK.

See Living evidence below…

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Sir Frank Whittle

MIT Video Lecture on Sir Frank Whittle Jet Engine Invention in Coventry, England

Engineer Nkumbwa visits Sir Frank Whittle birth place in Rugby – Coventry, England 2006.

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Turbojet Operation

Output of gas turbine passed through exhaust nozzle and used entirely for thrust.

High subsonic and supersonic. Developed by the British (Wiggins Co., Rolls

Royce, Parsons, Bristol Engines) And the Germans (Junkers, BMW,

Messerschmidt) in 1930’s and 1940’s. GE started US production in 1942 in

Massachusetts.

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T-s Block Diagram for a TBJET

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Turbofan

Exhaust passes through additional turbine stage to power a ducted fan that accelerates

A large stream of air passing around the core. Typically flow through the outer part of the fan is

5-6 times that flowing through the engine core (compressor).

Thrust from both hot gases leaving the nozzle and from the cold by-pass flow.

Bypass ratio = flow through fan / Flow through engine Type values: 0 -10

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Turbofan Operation

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Turbofan Operation

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T-s Diagram for a Turbofan

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Turboprop Operation

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Turboprop Operation

Actually developed prior to the turbofan. Rate of airflow through the prop may be 25-30

times that through the engine. Gear box necessary for both lower prop speed

and higher turbine speed. At higher speeds, tip losses considerable. Also called a Turbo shaft engine to power

helicopter or marine.

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Ramjet Engine

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Ramjet Engines

Air-breathing engine similar to a turbojet but without the mechanical compressor or turbine.

Compression is accomplished entirely by ram air and it is thus sensitive to vehicle forward speed. No thrust at rest.

Subsonic combustion, interior air slowed to subsonic speed for combustion.

Mach number: 3~6

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Scramjet Engines

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Scramjet Engines

Supersonic combustion ramjet. The flow through the combustor is still supersonic.

“Strange and tricky” combustion dynamics. Typically hydrogen fuels.

Mach number >5+

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Combined Ramjet and Scramjet Engine

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Rocket Engine

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Internal forces to Create Thrust

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Thrust Force Analysis

The thrust from a jet engine is not as easy to assess as a rocket.

Thrust and drag forces occur throughout the engine.

Del-P across compressor stages helps push engine forward.

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Internal Pressure & Thrust Distribution

Single Shaft Turbo Engine

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Power to Weight Ratio

Look at the following Case Study for the “Proflight Zambia Airlines” which operates between Lusaka and Copperbelt

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Gas Turbines Engine Manufacturers

General Electric (GE) - Ecomagination Rolls-Royce Platt & Whitney (PW) ABB SIEMENS

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Gas Turbine Engine Pressure Ratios

See the next slide

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Gas Turbines Advantages

They operate at high temperatures. They can and are capable of meeting peak

load demands. They are compact and easy to operate, and

take advantage of aerospace propulsion applications.

They operate at relatively low pressures.

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Gas Turbines Advantages…

Many installations burn natural gas, or in dual fuel mode burning NG and/or oil.

They do not handle wet gases, and are not as vulnerable to corrosion as steam turbines.

Combustion Gas Turbines do not require heat transfer equipment on the low-temperature side, and no coolant either

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Gas Turbines Limitations…

They have relatively low efficiency since their maximum temperature is limited by material.

Their efficiency is low because of the high compressor work, and low efficiency of compressors.

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Gas Turbines Limitations

Open cycle turbines are limited by the high exhaust temperature, which limits the turbine work.

They cannot be used with “dirty” fuels, such as coal, since sulfur oxides can damage their blades.

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Steam or Rankine Cycle

Steam, or Vapor Rankine Cycles overcome some of these limitations, and hence have been very popular in electric power generation.

All steam cycle plants are modeled using what is known as Rankine cycle.

Steam cycle can be constructed using closed and open cycles.

Open cycle require dumping exit steam into environment and so is restricted to 100 degree Celsius.

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Rankine Cycle Plants

Modern steam cycle plants consist of a boiler which generates steam using the energy provided by coal, oil, gas or nuclear fuels.

In addition they have a feed water pump that pumps the fully condensed steam exiting the condenser to the boiler at high pressure.

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Rankine Efficiency

Efficiencies are very low in such arrangement. Modern steam plants are based on a closed

cycle arrangement that uses a condenser to cool the waste steam, an innovation that was originally conceived and designed by James Watt.

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Rankine Efficiency

Here overall efficiency is the electrical output from the turbine generator minus the energy needed to operate the feed water pump and frictional losses in the turbine generator.

The steam cycle efficiency in this case is defined by the following equation:

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Rankine Efficiency

Modern steam plants achieve steam cycle efficiencies of 30-45%.

It is possible to raise efficiencies by maximizing the inlet steam enthalpy, but these are limited due to engineering constraints.

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Components of a Rankine Cycle

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T-s Diagram of Open Rankine Cycle

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Rankine Power Cycles Operation

A schematic of the components of a Rankine cycle is shown in Figure below.

The cycle is shown on P-v, T-s, and h-s coordinates.

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Rankine p-v Coordinates

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Rankine T-s Coordinates

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Rankine h-s Coordinates

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Rankine Cycle Process

The processes in the Rankine cycle are as follows:– a b: Heat added at constant temperature T2 (constant

pressure), with transition of liquid to vapor– c d: Liquid-vapor mixture condensed at temperature

T1 by extracting heat.– d e: Cold liquid at initial temperatureT1 is pressurized

reversibly to a high pressure by a pump. In this, the volume changes slightly.

– e a: Reversible constant pressure heating in a boiler to temperature T2

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In the Rankine cycle, the mean temperature at which heat is supplied is less than the maximum temperature, T2 , so that the efficiency is less than that of a Carnot cycle working between the same maximum and minimum temperatures.

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The heat absorption takes place at constant pressure over eab, but only the part ab is isothermal.

The heat rejected occurs over cd; this is at both constant temperature and pressure.

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Rankine Cycles Advantages…

Fuel flexible, works well with coal (closed cycle).

High efficiency, low pumping power. Lower flow rate (latent enthalpy). Run at low T (works with geothermal and

solar), but high p.

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Rankine Cycles Advantages

Works well with nuclear energy: – Pressurized : T = 350 C – Boiling : T = 400-500 C – Gas Cooled R: T = 600-800 C – High Temperature GR T > 800 C

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Rankine Cycle Disadvantages

High inertia, good for base load but not for load following.

Require cooling, big condensers, .. Water … Find bellow the Efficiency levels of Current

Power plants

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Combined Cycles

Recent years have seen a growth of plants employing Rankine and Brayton cycles.

Such plants are called combined cycle plants. Rather than venting the hot exhaust gas from the

Brayton cycle into the atmosphere there are plant designs that use this heat for other purposes using heat recovery steam generators.

This has resulted in overall efficiencies reaching as high as 50 to 55%.

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Combined Cycle

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Combined cycles take advantage of high T gas turbine exhaust. Combined cycle efficiency:

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Wrap up…

Any more worries… God help u…

TURBO MACHINES Steam and Gas Turbines Eng. R. L. Nkumbwa MSc, REng. Copperbelt University, ST...

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Transcript of TURBO MACHINES Steam and Gas Turbines Eng. R. L. Nkumbwa MSc, REng. Copperbelt University, ST...