Gas Turbine Engine Types, Jet Turbine Engines, Auxiliary Power Units
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Almost all electrical power on Earth is produced with a turbine engine of
some type. Today, very high efficiency gas turbines harness about 40% of
the thermal energy, with the rest exhausted as waste heat.
A gas turbine engine extracts energy from a flow of hot gas which is produced by combustion of gas, oil fuel or any other fuel producing gas, in a stream of compressed air. It has an upstream air compressor (radial or axial flow) mechanically coupled to a downstream turbine and a combustion chamber in between. "Gas turbine" may also refer to just the turbine element.
This machine has a single-stage centrifugal compressor and turbine, a recuperator, and foil bearings.
Energy is released when compressed air is mixed with fuel and ignited in the combustor.
The resulting gases are directed over the turbine's blades, spinning the turbine,
and mechanically powering the compressor.
Finally, the gases are passed through a nozzle, generating additional thrust by accelerating the hot exhaust gases by expansion back to atmospheric pressure.
Energy is extracted in the form of shaft power, compressed air and thrust, in any combination, and used to power aircraft, trains, ships, electrical generators, and even tanks.
Jet Turbine Engines
Airbreathing Jet Turbine Engines are gas turbine engines optimized to produce thrust from the exhaust gases, or from ducted fans connected to the gas turbine engines . Jet engines that produce thrust primarily from the direct impulse of exhaust gases are often called turbojets, whereas those that generate most of their thrust from the action of a ducted fan are often called turbofans or (rarely) fanjets.
A Jet Turbine Engine is an engine that discharges a fast moving jet of fluid
to generate thrust in accordance with Newton's third law of motion.
This broad definition of jet engines includes turbojets, turbofans, rockets, ramjets, pulse jets and pump-jets.
But in common usage, the term generally refers to a gas turbine Brayton cycle engine, an engine with a rotary compressor powered by a turbine, with the leftover power providing thrust.
Simulation of the jet engine airflow.
Jet Turbine Engines are so familiar to the modern world that gas turbine engines
are sometimes mistakenly referred to as a particular application of a jet engine,
rather than the other way around. Most jet engines are internal combustion
engines but non combusting forms exist also.
Jet Turbine Engines can be dated back to the first century AD, when Hero of Alexandria invented the Aeolipile. This used steam power directed through two jet nozzles so as to cause a sphere to spin rapidly on its axis. So far as is known, it was little used for supplying mechanical power, and the potential practical applications of Hero's invention of the jet engine were not recognized. It was simply considered a curiosity.
Jet propulsion only literally and figuratively took off with the invention of the rocket by the Chinese in the 11th century. Rocket exhaust was initially used in a modest way for fireworks but gradually progressed to propel formidable weaponry; and there the technology stalled for hundreds of years.
In Ottoman Turkey in 1633 Lagari Hasan Çelebi took off with what was described to be a cone shaped rocket and then glided with wings into a successful landing winning a position in the Ottoman army. However, this was essentially a stunt.
The problem was that rockets are simply too inefficient at low speeds to be useful for general aviation. Instead, by the 1930s, the piston engine in its many different forms (rotary and static radial, aircooled and liquid-cooled inline) was the only type of powerplant available to aircraft designers. This was acceptable as long as only low performance aircraft were required, and indeed all that were available.
However, engineers were beginning to realize that the piston engine was self-limiting in terms of the maximum performance which could be attained; the limit was essentially one of propeller efficiency. This seemed to peak as blade tips approached the speed of sound. If engine, and thus aircraft, performance were ever to increase beyond such a barrier, a way would have to be found to radically improve the design of the piston engine, or a wholly new type of powerplant would have to be developed. This was the motivation behind the development of the gas turbine engine, commonly called a "jet" engine, which would become almost as revolutionary to aviation as the Wright brothers' first flight.
The earliest attempts at jet engines were hybrid designs in which an external power source first compressed air, which was then mixed with fuel and burned for jet thrust. In one such system, called a thermojet by Secondo Campini but more commonly, motorjet, the air was compressed by a fan driven by a conventional piston engine.
Examples of this type of design were Henri Coandă's Coandă-1910 aircraft, and the much later Campini Caproni CC.2, and the Japanese Tsu-11 engine intended to power Ohka kamikaze planes towards the end of World War II. None were entirely successful and the CC.2 ended up being slower than the same design with a traditional engine and propeller combination.
The key to a practical jet engine was the gas turbine, used to extract energy from the engine itself to drive the compressor. The gas turbine was not an idea developed in the 1930s: the patent for a stationary turbine was granted to John Barber in England in 1791. The first gas turbine to successfully run self-sustaining was built in 1903 by Norwegian engineer Ægidius Elling. The first patents for jet propulsion were issued in 1917.
Limitations in design and practical engineering and metallurgy prevented such engines reaching manufacture. The main problems were safety, reliability, weight and, especially, sustained operation. In 1923, Edgar Buckingham of the US National Bureau of Standard published a report expressing skeptism that jet engines would be economically competitive with prop driven aircraft at low altitude and the airspeeds of the period.
In 1929, Aircraft apprentice Frank Whittle formally submitted his ideas for a turbo-jet to his superiors. On 16 January 1930 in England, Whittle submitted his first patent (granted in 1932). The patent showed a two-stage axial compressor feeding a single-sided centrifugal compressor. Whittle would later concentrate on the simpler centrifugal compressor only, for a variety of practical reasons. Whittle had his first engine running in April 1937.
It was liquid-fuelled, and included a self-contained fuel pump. Whittle's team experienced near-panic when the engine would not stop, even after the fuel was switched off. It turned out that fuel had leaked into the engine and accumulated in pools. So the engine would not stop until all the leaked fuel had burned off. Whittle was unable to interest the government in his invention, and development continued at a slow pace.
In 1935 Hans von Ohain started work on a similar design in Germany, unaware of Whittle's work. His first engine was strictly experimental and could only run under external power, but he was able to demonstrate the basic concept. Ohain was then introduced to Ernst Heinkel, one of the larger aircraft industrialists of the day, who immediately saw the promise of the design.
Heinkel had recently purchased the Hirth engine company, and Ohain and his master machinist Max Hahn were set up there as a new division of the Hirth company. They had their first HeS 1 engine running by September 1937. Unlike Whittle's design, Ohain used hydrogen as fuel, supplied under external pressure.
Their subsequent designs culminated in the gasoline-fuelled HeS 3 of 1,100 lbf (5 kN), which was fitted to Heinkel's simple and compact He 178 airframe and flown by Erich Warsitz in the early morning of August 27, 1939, from Marienehe aerodrome, an impressively short time for development. The He 178 was the world's first jet plane.
Auxiliary Power Units (APU)
Auxiliary power units (APUs) are small gas turbines designed for auxiliary
power of larger machines, such as those inside an aircraft. They supply compressed
air for aircraft ventilation (with an appropriate compressor design), start-up power
for larger jet engines, and electrical and hydraulic power.
These are not to be confused with the auxiliary propulsion units, also abbreviated APUs, aboard the gas-turbine-powered Oliver Hazard Perry-class guided-missile frigates. The Perrys' APUs are large electric motors that provide maneuvering help in close waters, or emergency backup if the gas turbines are not working.
An Auxiliary power unit (APU) is a device on a vehicle whose purpose is to
provide energy for functions other than propulsion.
Different types of APU are found on aircraft, as well as some large ground vehicles.
An aircraft APU is a relatively small gas turbine used to produce shaft power to start the jet engines, usually with compressed air, and to provide electricity, hydraulic pressure and air conditioning while the aircraft is on the ground.
The APU exhaust at the tail end of an Airbus A380.
In many aircraft, the APU can also provide electrical power in the air through a
generator installed on the gearbox.
A gasoline-petrol piston engine APU was first used on the Pemberton-Billing P.B.31 Nighthawk Scout aircraft in 1916. The Boeing 727 in 1963 was the first jetliner to feature a gas turbine APU, allowing it to operate at smaller, regional airports, independent from ground facilities.
Although APUs have been installed in many locations on various military and commercial aircraft, they are usually mounted at the rear of modern jet airliners. The APU exhaust can be seen on most modern airliners as a small pipe exiting at the aircraft tail.
In most cases the APU is powered by a small gas turbine engine that provides compressed air from within or drives an air compressor (load compressor). Recent designs have started to explore the use of the Wankel engine in this role. The Wankel offers power-to-weight ratios better than normal piston engines and better fuel economy than a turbine.
APUs are also fitted to some tanks to provide electrical power when stationary, without the high fuel consumption and large Infrared signature caused by running the main engine.
Some commercial vehicles now mount Auxiliary power units of their own. A typical APU for a commercial truck is a small diesel engine with its own cooling system, heating system, generator or alternator system, and air conditioning compressor, mounted to one of the frame rails of a semi-truck along with an internally mounted inverter in some cases. This unit is used to provide climate control and electrical power for the truck's sleeper cab and engine block heater during downtime on the road.
In the United States, federal Department of Transportation regulations require 10 hours of rest for every 11 hours of driving. During these times, truck drivers often idle their engines to provide heat, light, and power for various comfort items. Although diesel engines are very efficient when idling, it is still financially and environmentally costly to idle them like this, from a fuel consumption and an engine wear perspective.
The APU is designed to eliminate these long idles. Since the generator engine is a fraction of the main engine's displacement, it uses a fraction of the fuel; some models can run for eight hours on a US gallon (≈ 4 litres) of diesel.
The generator also powers the main engine's block and fuel system heaters, so the main engine can be started easily right before departure if the APU is allowed to run for a period beforehand. An APU can save up to 20 gallons (Cat 600 - 10 hours downtime @ 2 gallons per hour idling) (≈ 76 litres) of fuel a day, and can extend the useful life of the main engine by around 100,000 miles (≈ 160,000 kilometres), by reducing non-productive run time.
Some vehicle APUs can also use an external shore power connection for their heating and cooling functions, thus eliminating fuel consumption during rest periods altogether. Many truck stops already provide shore power connections in their parking areas. On some older diesel engines an APU was used instead of an electric motor to start the main engine.
These were primarily used on large pieces of construction equipment. As an alternative to the diesel units, APUs using an auxiliary battery system or hydrogen fuel cells as a source of power have also been designed. Freightliner has shown a demonstration model of a fuel cell APU, run on a tank of liquid hydrogen mounted to the truck, on one of their Century Class S/T road tractors.
Gas Turbines for Electrical Power Production
Industrial gas turbines range in size from truck-mounted mobile plants to enormous,
They can be particularly efficient——up to 60%——when waste heat from the gas turbine is recovered by a heat recovery steam generator to power a conventional steam turbine in a combined cycle configuration.
They can also be run in a cogeneration configuration: the exhaust is used for space or water heating, or drives an absorption chiller for cooling or refrigeration. A cogeneration configuration can be over 90% efficient.
GE H series power generation gas turbine. This 480-megawatt unit has a rated thermal efficiency of 60% in combined cycle configurations.
The power turbines in the largest industrial gas turbines operate at 3,000 or 3,600
rpm to match the AC power grid frequency and to avoid the need for a reduction gearbox.
Such engines require a dedicated enclosure.
Simple cycle gas turbines in the power industry require smaller capital investment than either coal or nuclear power plants and can be scaled to generate small or large amounts of power. Also, the actual construction process can take as little as several weeks to a few months, compared to years for base load power plants. Their other main advantage is the ability to be turned on and off within minutes, supplying power during peak demand.
Since they are less efficient than combined cycle plants, they are usually used as peaking power plants, which operate anywhere from several hours per day to a couple dozen hours per year, depending on the electricity demand and the generating capacity of the region.
In areas with a shortage of base load and load following power plant capacity, a gas turbine power plant may regularly operate during most hours of the day and even into the evening.
A typical large simple cycle gas turbine may produce 100 to 300 megawatts of power and have 35–40% thermal efficiency. The most efficient turbines have reached 46% efficiency.
Turboshaft Engines are often used to drive compression trains (for example in gas pumping stations or natural gas liquefaction plants)and are used to power almost all modern helicopters. The first shaft bears the compressor and the high speed turbine (often referred to as "Gas Generator" or "N1"), while the second shaft bears the low speed turbine (or "Power Turbine" or "N2"). This arrangement is used to increase speed and power output flexibility.
A Turboshaft Engine is a form of gas turbine which is optimized to produce
shaft power, rather than jet thrust.
In principle a turboshaft engine is similar to a turbojet, except the former features additional turbine expansion to extract heat energy from the exhaust and convert it into output shaft power.
Ideally there should be little residual thrust energy in the exhaust and the power turbine should be free to run at whatever speed the load demands.
Schematic diagram showing the operation of a simplified turboshaft engine. The compressor spool is shown in green and the free / power spool is in blue.
The general layout of a Turboshaft Engine is similar to that of a turboprop,
the main difference being the latter produces some residual propulsion thrust to
supplement that produced by the shaft driven propeller.
Another difference is that with a turboshaft the main gearbox is part of the vehicle (e.g. helicopter rotor reduction gearbox), not the engine. Virtually all turboshafts have a "free" power turbine, although this is also generally true for modern turboprop engines. At a given power output, compared to the equivalent piston engine, a turboshaft is extremely compact and, consequently, lightweight.
The name turboshaft is most commonly applied to engines driving ships, helicopters, tanks, locomotives and hovercraft or those used as stationary power sources.
The first true Turboshaft Engine was built by the French engine firm Turbomeca, led by the founder, Joseph Szydlowski. In 1948 they built the first French-designed turbine engine, the 100shp 782. In 1950 this work was used to develop the larger 280shp Artouste, which was widely used on the Aérospatiale Alouette II and other helicopters. The distinct whine of the Artouste is familiar to all those who have watched a 1967 UK television series The Prisoner, since an Alouette was featured in many of the episodes. Note that Artouste is also the name of an unrelated English design, the Blackburn Artouste.
Major efforts were underway in the United States and the United Kingdom to build similar engines. In the US Anselm Franz followed the same principles of simplicity that he used to develop the Jumo 004 in Germany, producing the T53 engine at Lycoming in 1953, and following this with the larger T55. General Electric beat his design into operation with their T58 series.
Today almost all engines are built so that power-take-off is independent of engine speed, using the free turbine stage. This has two advantages:
1. It allows a helicopter rotor or propeller to spin at any speed instead of being geared directly to the compressor turbine.
2. It allows the engine to be split into two sections, the "hot section" containing the majority of the engine, and the separate power-take-off, allowing the hot-section to be removed for easier maintenance.
This leads to slightly larger engines—compare the Pratt & Whitney PT-6 and similar models from Garrett Systems, for instance—but for the speed ranges served by these engines it is considered to be unimportant. Today practically all smaller turbine engines come in both turboprop and turboshaft versions, differing primarily in their accessory systems.
Some vehicle APUs can also use an external shore power connection for their heating and cooling functions, thus eliminating fuel consumption during rest periods altogether. Many truck stops already provide shore power connections in their parking areas. On some older diesel engines an APU was used instead of an electric motor to start the main engine. These were primarily used on large pieces of construction equipment. As an alternative to the diesel units, APUs using an auxiliary battery system or hydrogen fuel cells as a source of power have also been designed. Freightliner has shown a demonstration model of a fuel cell APU, run on a tank of liquid hydrogen mounted to the truck, on one of their Century Class S/T road tractors.
Radial Gas Turbines
In 1963, Norway, Jan Mowill initiated the development of the Radial Gas Turbine
at Kongsberg Våpenfabrikk. The turbine had a unique, all radial configuration, originally
rated at 1,200 kW. The turbine proved very successful and was generally sold in
electric generating packages. The major markets for the units were in the maritime,
offshore oil and gas and communications industries.
During the following years, more than a thousand units were delivered world wide. Kongsberg Våpenfabrikk was privatized, split up and sold off in the late nineteen eighties and development of the original turbine business was discontinued under the new ownership. As a result, Jan and Hiroko Mowill founded OPRA in Hengelo in 1991.
Consequently the first 1.6 MW OP16 was designed as a single shaft, all-radial machine. NOx emissions were developed to a very low level for both diesel fuel and natural gas. This was achieved with a unique, patented fuel and air pre-mixer in connection with an annular combustor.
The current production model, OP16-3 features both single and dual fuel operation as well as low emissions on natural gas. For improved maintenance and serviceability, a four can combustion systems was favored rather than the annular combustor used on the prototype.
For a single stage radial turbine the pressure ratio of 6.7: 1 is relatively high, which entails a high turbine impeller tip speed of 700 m/s (equal to the velocity of a rifle bullet).
Since this is nearly the same as the velocity of the gas entering the impeller tip from the nozzle guide vanes, an "impact" between the hot gas and the turbine impeller is avoided.
It could be said that this phenomenon constitutes "dynamic" cooling gaining about 100°C compared to a temperature increase in an axial turbine. OPRA's radial turbine is able to take this high tip speed due to it's "Eiffel Tower" shape with a strong base and a thinner blade tip region with low stresses. Having low stresses in the hot tip region and higher stresses in the cold, "fat" hub region makes OPRA work with nature rather than against it.
The OPRA radial turbine stage has an advanced aerodynamic design with an efficiency of 90% from the inlet of the guide vanes to the exhaust diffuser exit.
The efficient centrifugal compressor has a very good "match" with the turbine as their optimal running speeds are similar.
Since both compressor and turbine are close coupled via a Hirth-type teeth connection, an overhung rotor suspension is possible. This system provides balance integrity despite the differential thermal expansions between the compressor and turbine.
A ball bearing is placed in the front of the rotor support housing taking the combined thrust- and radial load. The rear, tilting pad bearing takes the main radial load. The cantilever, or overhung suspension of the rotor places the bearings in the cold section of the engine, avoiding oil supply to hot bearings. This system has considerable positive impact on engine reliability and maintenance.
A flexible coupling connects the turbine to the two stage planetary gearbox, reducing the turbine speed from 26000 to 1500 or 1800 rpm, depending on generator speed requirements.
The OP16-3 has an ISO rating of 1.9 MW. The engine efficiency of nominally 27% is at the highest level in the below 2 MW power range. Past competitors (no longer active) in this range have been at the 23–25% level.
Utilising proven radial gas turbine technology, the OP16 gas turbine is a compact, efficient and reliable industrial gas turbine designed for supplying power generation applications to both the Oil and Gas and Industrial markets.
The OP16 generator sets can be provided in a variety of configurations to meet customer specific requirements. The engineering design, component selection and maintenance accessibility of the generator sets enhance high reliability and long product life. The generator sets can be provided with low emission and dual and multifuel capabilities.
Single or multiple OP16 units can effectively cover installations from 1.5 to 10 MW electric power demand.
OPRA provides gas turbine generating sets for customers world-wide within the oil & gas and industrial sectors. OPRA's 2 MW class OP16 gas turbine is of an industrial, all-radial design which provides robustness, reliability and class leading efficiency and emissions. Dual fuel and off-specification fuel options are also available. Complete gas turbine generating sets are engineered to meet customer specific requirements both for land based and offshore applications.
Scale Jet Engines
Also known as miniature gas turbines or micro-jets.
Many model engineers relish the challenge of re-creating the grand engineering feats of today as tiny working models. Naturally, the idea of re-creating a powerful engine such as the jet, fascinated hobbyists since the very first full size engines were powered up by Hans von Ohain and Frank Whittle back in the 1930s.
Recreating machines such as engines to a different scale is not easy. Because of the square-cube law, the behaviour of many machines does not always scale up or down at the same rate as the machine's size (and often not even in a linear way), usually at best causing a dramatic loss of power or efficiency, and at worst causing them not to work at all.
An automobile engine, for example, will not work if reproduced in the same shape at the size of a human hand.
With this in mind the pioneer of modern Micro-Jets, Kurt Schreckling, produced one of the world's first Micro-Turbines, the FD3/67. This engine can produce up to 22 newtons of thrust, and can be built by most mechanically minded people with basic engineering tools, such as a metal lathe.
Its radial compressor, which is cold, is small and the hot axial turbine is large experiencing more centrifugal forces, meaning that this design is limited by Mach number. Guiding vanes are used to hold the starter, after the compressor and before the turbine, but not after that. No bypass within the engine is used.
Also known as:
A micro turbine designed for DARPA by M-Dot.
Microturbine Engines are becoming wide spread for distributed power and combined
heat and power applications. They range from hand held units producing less than
a kilowatt to commercial sized systems that produce tens or hundreds of kilowatts.
Part of their success is due to advances in electronics, which allows unattended operation and interfacing with the commercial power grid. Electronic power switching technology eliminates the need for the generator to be synchronized with the power grid. This allows the generator to be integrated with the turbine shaft, and to double as the starter motor.
Microturbine Engine systems have many advantages over reciprocating engine generators, such as higher power density (with respect to footprint and weight), extremely low emissions and few, or just one, moving part. Those designed with foil bearings and air-cooling operate without oil, coolants or other hazardous materials.
Microturbine Engines also have the advantage of having the majority of their waste heat contained in their relatively high temperature exhaust, whereas the waste heat of recriprocating engines is split between its exhaust and cooling system. However, reciprocating engine generators are quicker to respond to changes in output power requirement and are usually slightly more efficient, although the efficiency of microturbines is increasing. Microturbines also lose more efficiency at low power levels than reciprocating engines.
They accept most commercial fuels, such as natural gas, propane, diesel and kerosene. They are also able to produce renewable energy when fueled with biogas from landfills and sewage treatment plants.
Microturbine Engine designs usually consist of a single stage radial compressor, a single stage radial turbine and a recuperator. Recuperators are difficult to design and manufacture because they operate under high pressure and temperature differentials. Exhaust heat can be used for water heating, drying processes or absorption chillers, which create cold for air conditioning from heat energy instead of electric energy.
Typical microturbine efficiencies are 25 to 35%. When in a combined heat and power cogeneration system, efficiencies of greater than 80% are commonly achieved.
MIT started its millimeter size turbine engine project in the middle of the 1990s when Professor of Aeronautics and Astronautics Alan H. Epstein considered the possibility of creating a personal turbine which will be able to meet all the demands of a modern person's electrical needs, just like a large turbine can meet the electricity demands of a small city. According to Professor Epstein current commercial Li-ion rechargeable batteries deliver about 120-150 Wh/kg. MIT's millimeter size turbine will deliver 500-700 Wh/kg in the near term, rising to 1200-1500 Wh/kg in the longer term.
Advances in Technology
Gas turbine technology has steadily advanced since its inception and continues
to evolve; research is active in producing ever smaller gas turbines. Computer design,
specifically CFD and finite element analysis along with material advances, has allowed
higher compression ratios and temperatures, more efficient combustion, better cooling
of engine parts and reduced emissions.
Computational fluid dynamics (CFD) is one of the branches of fluid mechanics that uses numerical methods and algorithms to solve and analyze problems that involve fluid flows. Computers are used to perform the millions of calculations required to simulate the interaction of fluids and gases with the complex surfaces used in engineering.
However, even with simplified equations and high-speed supercomputers, only approximate solutions can be achieved in many cases. More accurate software that can accurately and quickly simulate even complex scenarios such as transonic or turbulent flows are an ongoing area of research. Validation of such software is often performed using a wind tunnel.
On the emissions side, the challenge in technology is actually getting a catalytic combustor running properly in order to achieve single digit NOx emissions to cope with the latest regulations. Additionally, compliant foil bearings were commercially introduced to gas turbines in the 1990s. They can withstand over a hundred thousand start/stop cycles and eliminated the need for an oil system.
Foil bearings are a type of air bearing. A shaft is supported by a compliant, spring loaded foil journal lining. Once the shaft is spinning fast enough, the working fluid (usually air), pushes the foil away from the shaft so that there is then no contact. The shaft and foil are separated by the air's high pressure which is generated by the rotation which pulls gas into the bearing via viscosity effects. A high speed of the shaft with respect to the foil is required to initiate the air gap, and once this has been achieved, no wear occurs. Unlike aero or hydrostatic bearings, foil bearings require no external pressurisation system for the working fluid, so the hydrodynamic bearing is self-starting.
On another front, microelectronics and power switching technology have enabled commercially viable micro turbines for distributed and vehicle power.
Advantages and Disadvantages of Gas Turbine Engines
Advantages of Gas Turbine Engines
Very high power-to-weight ratio, compared to reciprocating engines (ie. most road
Smaller than most reciprocating engines of the same power rating;
Moves in one direction only, with far less vibration than a reciprocating engine,
so very reliable;
Low operating pressures;
High operation speeds;
- Low lubricating oil cost and consumption.
Disadvantages of Gas Turbine Engines
Cost is much greater than for a similar-sized reciprocating engine (very high-performance,
strong, heat-resistant materials needed);
- Use more fuel when idling compared to reciprocating engines - not so good unless in continual operation.
These disadvantages explain why road vehicles, which are smaller, cheaper and follow a less regular pattern of use than tanks, helicopters, large boats and so on, do not use gas turbine engines, regardless of the size and power advantages imminently available.
Some tips on how to avoid business failure:
Don't underestimate the capital you need to start up the business.
Understand and keep control of your finances - income earned is not the same as
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More volume does not automatically mean more profit - you need to get your pricing
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