423

Development of a novel gas turbine driven centrifugal compressor A B Turner1*, S J Davies1, P R N Childs1, C G Harvey1 and J A Millward2 1 School of Engineering, University of Sussex, Brighton, UK 2 Rolls-Royce plc, Derby, UK

Abstract: The purpose of the paper is to provide design and operational information for the conversion of small turboprop engines into land-based shaft power drives. These can provide economical solutions to individual power requirements such as standby electrical generators, combined heat and power units, fan drives or centrifugal compressor drives. Considerable numbers of ex-military engines are currently becoming available at economical prices. One such engine is the Rolls–Royce Dart turboprop gas turbine which, although there are over 2000 still in civil and military service worldwide, is now being phased out of Royal Air Force operation. This paper describes the modification of a Dart RDa 12 engine to drive a centrifugal compressor from a smaller RDa 7, Mk 552, engine for a compressed air facility on a university campus. The modified engine has a power of over 2400 hp (1790 kW) at 15 000 r/min and the compressor was driven directly from the engine output shaft supplying 10.5 kg/s of air at a pressure ratio of 3.3:1. To achieve this, the turboprop’s main reduction gearbox was removed, the teeth on the engine drive pinion ground down and a flexible coupling fitted to it to drive the RDa 7 compressor which was flange mounted on the engine gearbox-air intake casing. The removal of the main gearbox removed the drives to the lubricating oil pumps the fuel pump and the engine control system. The manner in which these problems were overcome and the method of operating the system are described. One of the most important considerations in starting such a direct drive system is the complete closing off of the air intake to the driven compressor and the design of an annular valve to perform this duty is described. The operating parameters including the flow, pressure ratio and engine turbine entry temperatures for this system are given. Keywords: gas turbine, centrifugal compressor, turboprop engines, shaft power drives

1 INTRODUCTION This paper describes the modifications made to a Dart RDa 12 turboprop to drive a two-stage centrifugal compressor to supply air to a suite of experimental gas turbine test rigs. The driven compressor was taken from the smaller RDa 7 version of the Dart engine. Although the Rolls–Royce Dart is no longer in production, with the Andover (HS748) transport aircraft being taken out of service by the Royal Air Force (RAF), the Royal Australian Air Force, Nikon and other airlines worldwide, a large number of Dart engines are becoming available in the market-place. There is therefore considerable interest in their use for a variety of other applications. These The MS was received on 20 August 1999 and was accepted after revision for publication on 30 September 1999. *Corresponding author: TFMRC, School of Engineering, University of Sussex, Falmer, Brighton, East Sussex BN1 9QT, UK. A08499  IMechE 2000

engines have a shaft power of between 2000 and 3000 hp (between 1492 and 2238 kW) and can provide an economical power source for emergency generators, compressors and wind tunnel fans, for example. Dart aeroengines have considerable useful life as shaft power units still left when taken out of airline service, especially military service, since component life requirements for military aircraft are more restrictive than those for civil aircraft and very much more than those for industrial power. For example, the high-pressure (HP) turbine rotor blades are restricted to 5000 h in military service but can have a life of 25 000 h in civil use. For a land-based power unit depending on the continuous power requirement this could be extended to over 50 000 h. The reason for the development of the compressor system described in this paper was the necessity to upgrade the research facilities supplying compressed air to the experimental gas turbine rigs of the Thermo-Fluid Proc Instn Mech Engrs Vol 214 Part A

424

A B TURNER, S J DAVIES, P R N CHILDS, C G HARVEY AND J A MILLWARD

Mechanics Research Centre at the University of Sussex. Experimental work on gas turbine secondary, internal air systems such as rotor–stator discs and cavities has been conducted at the University of Sussex for over 30 years [1, 2] and in the early 1990s it became clear that computational fluid dynamics was reaching a level of sophistication which required experimental validation close to engine representative conditions. Aircraft engines and large power gas turbines are expensive test vehicles and are limited in the data that they can produce and rig facilities operating at representative speeds and Reynolds numbers offer a more economical and widerranging solution. Consideration was given to an electric motor to drive the RDa 7 compressor but at a power of 2.7 MW this would have to be a 6 kV or even an 11 kV machine and would have required a gearbox. This, together with its starter, would have been expensive both in capital outlay and in operating costs. The two-stage centrifugal compressor used for the efficiency uprating tests of the Dart RDa 7 in the late 1980s was available together with an ex-RAF RDa 12 and the two were simply flange mounted together to form the system described in this paper. The Dart offered a cheap, compact and selfcontained solution and meets its design targets. A short history of the Dart series of engines has been provided by Heathcote [3].

have been possible and would have had the advantage of keeping the engine oil and fuel systems intact but it would have produced a very long inelegant system of lower power. It was therefore decided to remove the RDa 12 main gearbox and to drive the RDa 7 compressor directly. The design concept is illustrated in Fig. 2, in which the RDa 7 compressor is driven directly from the RDa 12 engine shaft with the compressor which was flange mounted on the engine air intake–reduction gearbox casing. The overall length of the system is 3.5 m from the inlet throttle valve to the engine nozzle exit. The teeth on the engine high-speed pinion gear were ground down and the end of a flexible coupling shrink fitted to it (Fig. 3). The coupling used was a Turboflex HS8-525 lowwindage coupling capable of transmitting 9000 kW at 23 000 r/min. This therefore had considerable reserve to cope with the effects of surging in the driven compressor (which occurred frequently during definition of the surge line in the commissioning stage). The removal of the RDa 12 main reduction gear train also removed the original drive to the fuel pump, the fuel control system and the oil feed and scavenge pumps. All these were therefore provided electrically, external and independent from the engine. The interface between the engine and the driven compressor, together with the air intake, the starter motor and the externally driven accessories positioned below, can be seen in Fig. 4.

2 DESIGN OF THE MODIFIED GAS TURBINE– COMPRESSOR SYSTEM

3 THE OIL LUBRICATION SYSTEM

Although several gas turbine-driven compressors have been developed in the past, to the present authors’ knowledge all have used a gas turbine with a free-power turbine. A typical system is that of ABB-Alstom in Lincoln [4] in which a 5000 hp (3730 kW) industrial gas turbine is used to drive a multi-stage compressor for various compressor test rigs. The free power turbine arrangement considerably simplifies the starting and operation of a centrifugal compressor system since the gas generator can be started even with the driven compressor locked stationary. One of the problems in converting a turboprop engine for direct drive is the low speed of the output prop shaft. Most electrical generators of 2 MW or so run at 1500 r/min so that conversion of any engine in the Dart family will require a gearbox modification. The design engine speed of the RDa 12, Mk 201, is 15 000 r/min, which is reduced to 1162 r/min at the prop shaft by an epicyclic gearbox (the smaller RDa 7 prop shaft speed is 1395 r/min). One option for the proposed system was to use the RDa 12 engine (Fig. 1), complete with reduction gearbox, and to drive the RDa 7 compressor (whose turbine design speed is the same as the RDa 12) through another similar gearbox running in reverse. This would

One of the most interesting features of the original Dart engine design was the incorporation of the oil tank into the hollow air intake casing with the oil cooler mounted above and the oil pumps and pressure filter mounted below. This system kept the compressor intake free from icing. In the present system the removal of the main reduction gearbox removed the drive to the oil pumps (one feed and four scavenge) and separate electric motordriven systems had to be provided. This is shown schematically in Fig. 5. The original oil pump was used, together with three additional scavenge pumps, with two spring-loaded relief valves which maintained a minimum pressure of 1.5 bar on the supply rail. All the engine and compressor bearings, with the exception of the main high-speed pinion support bearing, were fed directly through their original feed points with the oil scavenge returns fed to a water-cooled heat exchanger which discharged directly into a separate 40 l tank. The oil in the tank was forced to flow under a weir and then up and over a de-aerator tray that spread it thinly to release air bubbles. This system cured a severe frothing problem that occurred because the air–oil mixture returned from the bearings with an air–oil mixture ratio of up to 4. In the production Dart engines the bearing and shaft

Proc Instn Mech Engrs Vol 214 Part A

A08499  IMechE 2000

A08499  IMechE 2000

425

Proc Instn Mech Engrs Vol 214 Part A

DEVELOPMENT OF A NOVEL GAS TURBINE DRIVEN CENTRIFUGAL COMPRESSOR

The Rolls–Royce Dart RDa 12, Mk 201 (the Military Version of the RDa 10) Fig. 1

426

A B TURNER, S J DAVIES, P R N CHILDS, C G HARVEY AND J A MILLWARD

Fig. 2 The system concept, showing the RDa 7 compressor, which was flange mounted on the front of the RDa 12 engine air intake casing and driven from the engine pinion

splines immediately aft of the main engine drive pinion (Fig. 1) are lubricated by means of two separate oil jets from the epicyclic gearbox stationary planet wheel carrier feeding two separate weirs inside the hollow drive shaft. The front weir feeds the highly loaded pinion bearing from underneath, through holes in the inner race, and the rear weir feeds the bevel splines with both discharging into the gearbox casing. Clearly this spray jet system could not be used with the flexible coupling in place and accordingly the pinion bearing and splines were lubricated from a single spray jet positioned at the front of the RDa 7 compressor, which fed two consecutive oil centrifuge tubes extending through the length of the central hollow drive shaft and the flexible coupling. The second oil centrifuge tube was fitted with an oil flow splitter that fed oil to the two separate weirs inside the drive pinion shaft. The simplicity of this system, which was tested in a high-speed lathe, is shown in Fig. 6. The oil centrifuge tubes were supported inside the hollow shafts by solid rings and these inadvertently created a severe oil leakage problem from the RDa 7 bearings into the compressed air. In the Dart engine the hollow high-speed pinion shaft and compressor shafts are used as breathers through the compressor–turbine coupling and out to the auxiliary gearbox drive housing. The first gear of this gearbox drive carries a centrifugal breather, and the air released passes to the atmosphere through the top of the compressor casing. The solid rings supporting the oil centrifuge tubes effectively blocked this air breather escape route and pressurized the bearing chambers. The solution adopted was to vent the scavenge returns to atmosphere although an alternative would be to replace the solid support rings with slotted rings to allow the vent air to pass to the breather as original. This would Proc Instn Mech Engrs Vol 214 Part A

require a major strip of the facility and would not be a significant improvement over the solution adopted.

4 THE FUEL SYSTEM In the production aero-engine the fuel control system has to compensate automatically for various changes in engine conditions and air density as the power demand changes and as the aircraft changes height. In the Dart the fuel is controlled by a hydromechanical system which governs the engine in response to the throttle control to provide a set engine speed. Such fuel systems are necessarily complex and the Dart controls, and others, have been described in detail in The Jet Engine [5]. A complex aero-engine type of fuel control system was not necessary for the present development, and the fuel pump drive and thus the fuel control unit links had been removed anyway together with the main reduction gearbox. Because the compressed air delivery system of the present development was complex, with its system of inlet throttle valve control and delivery blow-off or bypass control, the development of an automatic fuel control system could not be justified. Also since the engine is only run for short periods for rig testing, a manual control system was used, protected by a fastacting overspeed trip. The fuel system, illustrated in Fig. 7, used the original main Dart fuel pump driven by a variable-speed 7 kW TASC electric motor. The TASC system uses a constant a.c. speed motor to drive the output shaft through an electromagnetic induction link. The speed control is precise by varying the current through the link. The main A08499  IMechE 2000

A08499  IMechE 2000

427

Proc Instn Mech Engrs Vol 214 Part A

DEVELOPMENT OF A NOVEL GAS TURBINE DRIVEN CENTRIFUGAL COMPRESSOR

System cross-section showing the connection between engine and the directly driven centrifugal compressor Fig. 3

428

A B TURNER, S J DAVIES, P R N CHILDS, C G HARVEY AND J A MILLWARD

Fig. 4 Photograph of the system centre section showing the Dart RDa 12 engine on the left and the driven Dart RDa 7, Mk 552, compressor on the right with the fuel pump, the oil pumps and the fuel solenoids positioned underneath the engine air intake

fuel pump, a Lucas-type GB 244-9BV, is a swash-plate or cam-plate pump and in the aero-engine can vary the fuel delivery either by changing the speed or by varying the angle of the swash-plate using servo-pressure from the main delivery pressure. In the present system the swashplate was set to a fixed angle close to the maximum, and the fuel pressure and thus the engine speed were controlled simply by changing the fuel pump speed. The burners in the Dart RDa 7, Mk 552, are of the single nozzle sprayer type but those in the larger Dart RDa 12, Mk 201, have two nozzles: a primary sprayer for better lighting and which operates continuously, and a main sprayer which is opened by a spring-loaded ‘pressurizing’ valve at a pressure of about 25 bar. The original RDa 12 burner ‘pressurizing’ valve was incorporated into the present fuel system and ensures that ignition and acceleration to idle occurs with the primary sprayer (at a constant fuel pressure of 21.4 bar, a flowrate of 0.055 l/s using aviation kerosene). Typical operating fuel pressures for an RDa 12 when delivering 2400 hp (1790 kW) (dry) at 15 000 r/min are a fuel pump pressure of 65 bar, a primary burner pressure of 55 bar and a main burner pressure of 37 bar. The RDa 12 fuel flow at full power is 1090 l/h. This limited operation in the present application to just over 4 h at full power as the capacity of the fuel tank available was 4560 l. A low-pressure (LP) fuel pump backed by an LP fuel pressure switch ensured that the (expensive) main fuel pump received a continuous fuel supply. A system of two Proc Instn Mech Engrs Vol 214 Part A

solenoid valves were used to provide a full-spill recirculation of fuel through a pre-set throttle valve which could be adjusted to give the correct starting burner pressure (21.4 bar) before the engine starter was engaged. One solenoid was a ‘power on to close’ while the other, the main actuator, was a ‘power on to open’. The overspeed trip operated digitally, taking its signal from an inductive probe monitoring the pulses from one of the gears in the auxiliary gearbox, and could trip the power to the two solenoids virtually instantaneously. The operating time of the solenoid valves (Hale–Hamilton) was estimated to be 150 ms to close fully, which was about half the time that the engine would take to accelerate from 15 000 r/min to the estimated burst speed of 17 000 r/min if the flexible coupling failed at full power. This time was estimated from calculated values of the engine output torque and moment of inertia.

5 THE STARTING SYSTEM It was realized very early on that the driven compressor inlet would have to be completely closed during starting to prevent excessive windage–pumping drag and a purpose-designed valve was built for this duty. The original RDa 12 starter motor was used, powered by a transformer–rectifier system, 415 V, three phase a.c. to 28 V d.c. rated at 200 A continuous and 1500 A intermittently (Fig. 8). The starter motor was a Rotax type A08499  IMechE 2000

429

Fig. 5 Oil lubrication system (PRV, pressure relief valve; F, filter)

DEVELOPMENT OF A NOVEL GAS TURBINE DRIVEN CENTRIFUGAL COMPRESSOR

A08499  IMechE 2000

Proc Instn Mech Engrs Vol 214 Part A

430

A B TURNER, S J DAVIES, P R N CHILDS, C G HARVEY AND J A MILLWARD

Fig. 6 The Turboflex1 engine–compressor coupling showing the shrink fit connection to the ‘ground-down’ engine pinion and the pinion bearing housing. The rear oil centrifuge tube with two rows of splitter holes is shown below

C4204 rated at 17.5 bhp at 6000 r/min on 28 V d.c., 650 A. The starting torque is transmitted to the engine through a multi-plate clutch and bevel gear to a pawl and ratchet mechanism immediately behind the main shaft high-speed pinion. This remained untouched in the modifications. Some concern was initially expressed that the starter motor would have to be uprated to be able to drive both the original engine (minus the gearbox and feathered prop) and the RDa 7 compressor up to the starting–ignition minimum speed of 1500 r/min. These concerns proved to be unfounded. With the two multiple-strand copper cables of 17 mm diameter and each 6 m long originally specified for this duty the voltage drop between the transformer–rectifier pack and the starter motor was excessive and the engine– compressor speed barely reached 1100 r/min. The cables were therefore doubled up, two to each terminal (Fig. 4)

Fig. 7 Fuel system

Fig. 8 Starter circuit Proc Instn Mech Engrs Vol 214 Part A

A08499  IMechE 2000

DEVELOPMENT OF A NOVEL GAS TURBINE DRIVEN CENTRIFUGAL COMPRESSOR

and shortened in length to 4 m, and this resulted in starting speeds of almost 2000 r/min. Typically the initial voltage at the starter motor terminals was 6.5 V rising to only 15.6 V as the engine accelerated to 1900 r/min. Nevertheless, this gave satisfactorily cool starting, again with the compressor inlet closed completely.

Fig. 9

431

6 THE DRIVEN COMPRESSOR INLET THROTTLE AND CONTROL VALVE The remotely controlled multiple-vane annular valve designed and built to control completely the compressor inlet flow from zero to full flow is shown in Figs 9a and b.

The RDa 7, Mk 552, compressor air inlet throttle and control valve: (a) internal mechanism; (b) valve shown partially open

A08499  IMechE 2000

Proc Instn Mech Engrs Vol 214 Part A

432

A B TURNER, S J DAVIES, P R N CHILDS, C G HARVEY AND J A MILLWARD

The valve vanes were capable of being driven by a geared electric motor through a worm and wheel in either direction from fully closed to fully open with the vanes lying in the direction of air flow. This valve formed one of the main controls over the compressor operating point by being able to change the mass flow and outlet pressure at any speed. In principle it could alter the inlet swirl ratio but in practice the large aerodynamic struts in the Dart annular inlet casing effectively prevented this. No measurable inlet swirl was produced. The inlet throttle valve was opened progressively as soon as the engine had reached idling speed (8000 r/min) to prevent excessive windage heating in the LP impeller and inlet casing. Trouble was experienced during commissioning with the use of a stepper motor driving the inlet throttle valve gears. At a speed close to 15 000 r/min the load on the partially open vanes caused the stepper motor torque to be completely overridden and the valve slammed shut, causing the system to accelerate rapidly. Following this experience a worm and wheel driven by a conventional d.c. motor was installed.

7 THE EXHAUST JET NOZZLE The performance rating of the Dart RDa 12 for maximum continuous power is 2400 shp (1790 kW) with a jet thrust from the exhaust nozzle of 670 lbf (2981 N). The jet thrust is not needed in most stationary land-based applications and is wasteful in that it reduces the turbine expansion ratio. It was established from the computer model deck of the Dart RDa 8 (no model of the RDa 12 was available) that a doubling of the final nozzle area would increase the output shaft power by 180 hp (134 kW) and reduce the turbine exit temperature by 12 K. The larger RDa 12 engine would be expected to have a greater increase in shaft power than this. It was also calculated that the thrust balance across the turbine bearing would be unaffected to any significant degree. The jet nozzle exhaust unit also carries the rear turbine stator shroud and forms the rear rotor–stator rim seal. It could not therefore simply be removed without drastically changing the pressures in the LP cooling air system. The nozzle unit was therefore retained but had two large holes cut in it to reduce the turbine back pressure and thus to prevent acceleration of the exhaust gas.

8 THE COMPRESSED AIR SYSTEM Operating normally, the RDa 7, Mk 552, original compressor power consumption is considerably more than the RDa 12, Mk 201, dry output power over the whole Proc Instn Mech Engrs Vol 214 Part A

running range. At 15 000 r/min the RDa 7 power requirement, neglecting bearing losses, is 3620 hp (2700 kW) when delivering 10.6 kg/s at a pressure ratio of 6.3 whereas the RDa 12 output power is 2400 hp (1790 kW) continuous and 2970 hp (2220 kW) on water– methanol. An inlet throttle to reduce the mass flow over the whole running range was therefore necessary if both LP and HP impellers were fitted to the RDa 7. Although it is possible for the system to be operated in this manner the inherent pressure ratio across the impellers would remain, producing a high temperature rise which was unnecessary for the current test rigs. The HP impeller (and diffuser) was therefore removed and replaced with a simple distance piece (tube) to produce a better match with the RDa 12 output power, especially at full speed, and this is the configuration that has been used throughout. From the seven outlets of the RDa 7 compressor the flow was taken through straight conical diffusers and into an annular collection chamber to the delivery pipe of 250 mm diameter. The diffusers can be seen in Fig. 4. The original RDa 7 Mk 552 compressor performance with both LP and HP impellers in place is shown in Fig. 10 and for the LP stage alone the characteristics are shown in Fig. 11. These tests are for standard inlet pressure and temperature: 1.013 bar and 15 °C. For the modified system there is no one single running line and operation can range over the full extent of the map since the compressor can be operated at HP or LP at any speed by using a combination of inlet throttle and discharge bypass. Operation at a high flowrate and a low delivery pressure, however, is not advisable since the compressor is very inefficient under these conditions and high engine turbine gas entry temperatures (TGTs) result. The discharge bypass was achieved by fitting a pipe of 150 mm diameter with a remote consolecontrolled valve to take excess flow from just after the annular collection chamber outlet to the exhaust stack (Fig. 2).

9 NOISE With the system installed on a university campus, care was exercised to reduce noise. A standard Callum detuner for this engine power (1.05 m in diameter and 4 m long) was mounted above the purpose-built test cell and took both the engine exhaust and the compressed air bypass. This system was extremely effective. The air intake silencer, consisting of seven ducts each 150 mm wide by 2.2 m high and 1.7 m long, was less effective than the exhaust detuner and most of the noise experienced emanated from the sound-proofed ducts. Further work is required to improve the air intake silencer. Overall, during the commissioning of the compressor unit and the test rigs together, most external disturbance was A08499  IMechE 2000

DEVELOPMENT OF A NOVEL GAS TURBINE DRIVEN CENTRIFUGAL COMPRESSOR

Fig. 10 A08499  IMechE 2000

433

Compressor performance of the Dart RDa 7, Mk 552, with both LP and HP impellers Proc Instn Mech Engrs Vol 214 Part A

434

A B TURNER, S J DAVIES, P R N CHILDS, C G HARVEY AND J A MILLWARD

Fig. 11

Proc Instn Mech Engrs Vol 214 Part A

Compressor performance of the Dart RDa 7, Mk 552, with LP impeller only

A08499  IMechE 2000

DEVELOPMENT OF A NOVEL GAS TURBINE DRIVEN CENTRIFUGAL COMPRESSOR

produced by the exhaust compressed air from the rigs and an extensive silencing system had to be built for this.

10 STARTING AND OPERATION OF THE SYSTEM It is during starting that the highest TGTs are experienced and the greatest care should be exercised to avoid overheating. With a directly connected gas turbine, as distinct from a free-power gas turbine, the torque–speed characteristic of the load is crucial during starting and the necessity of shutting off all the air flow to the driven centrifugal compressor has already been mentioned. The Dart turboprop engine is started at a constant fuel pressure of 21.4 bar, which usually gives a peak TGT less than the permitted maximum of 855 °C and usually around 800 °C. This procedure had to be modified and the following manual procedure was adopted in the starting of the present system: 1. With the LP fuel pump pressurizing the main fuel pump, the TASC drive a.c. motor can be started and the two solenoid valves set to recirculate fuel through the adjustable throttle valve at a pressure of 21.4 bar. 2. With the igniters on, the starter switch is then closed. At the highest engine speed reached on the starter

Fig. 12

435

motor (about 1950 r/min) the solenoid valves are switched to direct fuel to the burners. The engine fires and accelerates with the starter motor still assisting up to an engine speed of 2500 r/min. 3. During the ‘acceleration to idle’ phase (8000 r/min) it was found necessary to reduce the fuel pressure slightly to limit the TGT to just over 800 °C. As the idle speed is approached, the fuel pressure is reduced and the TGT falls rapidly as the compressor and turbine efficiencies improve with increasing speed. However, the inlet throttle to the driven RDa 7 compressor must be cracked open as soon as possible after the idle speed is reached to prevent the windage heating from creating excessive temperatures in the LP impeller and inlet air casing. Immediately this happens the fuel pressure has to be increased to maintain acceleration to 13 000 r/min or so. Operation below this speed, even with the inlet throttle valve only partly open, was found to produce TGTs above the recommended in-flight maximum for those speeds. These recommended maximum values over the operating range are compared with typical values for the present system in Fig. 12. The values of the engine speed, the TGT, the temperature and pressure after the RDa 7 inlet throttle valve and the air temperature in the flexible coupling space for a typical starting sequence are shown in Fig. 13.

Recommended Dart RDa 12 in-flight maximum temperatures compared with typical values for the modified engine driving the RDa 7 compressor, LP impeller only, with and without the jet nozzle fitted

A08499  IMechE 2000

Proc Instn Mech Engrs Vol 214 Part A

436

A B TURNER, S J DAVIES, P R N CHILDS, C G HARVEY AND J A MILLWARD

Fig. 13

Typical starting and run parameters for a short test

11 CONCLUSIONS The main purpose of the paper is to provide design and operational information for the conversion of small turboprop engines into land-based shaft power drives. These can provide economical solutions to individual Proc Instn Mech Engrs Vol 214 Part A

power requirements such as standby electrical generators, combined heat and power units, fan drives or centrifugal compressor drives such as that described. Considerable numbers of ex-military engines are currently coming available at very low prices. The paper has described the design and successful A08499  IMechE 2000

DEVELOPMENT OF A NOVEL GAS TURBINE DRIVEN CENTRIFUGAL COMPRESSOR

operation of a 10.5 kg/s, 3.3:1 pressure ratio centrifugal compressor directly driven from the turbine shaft of a Rolls–Royce Dart turboprop gas turbine having an output power of 2400 hp (1790 kW) at 15 000 r/min. How the problems associated with removing the turboprop’s main reduction gearbox and the drives to the lubricating oil pumps and fuel pumps were overcome have been described. One of the main difficulties of starting a gas turbine with a directly driven centrifugal compressor was found to be the high pumping and windage loads imposed on the gas turbine at all speeds until just below the design operating speed. When starting the system it was found necessary to shut off the air completely to the driven compressor and a multiple-vane valve purposely designed for this duty is described. In the system developed, a flexible coupling was fitted directly on to the output drive shaft pinion and a novel arrangement for the lubrication of the main pinion bearing is described. The system is currently manually controlled and this is unsatisfactory. An automatic control system for starting and operation is currently being designed.

ACKNOWLEDGEMENTS The authors would like to acknowledge the very considerable help received from the Dart Project team

A08499  IMechE 2000

437

at Rolls–Royce Limited, East Kilbride, and particularly Mr Brian McEwan. Also, the assistance given by Mr Brian Burrage of H and S Aviation, Portsmouth, was invaluable. The authors would also like to thank Turboflex Couplings Limited for the low-windage flexible coupling, Mr Barry Jackson for the digital over-speed trip, Mr John Ward for advice on the lubrication system and Mr David Sharp for much of the rig general assembly.

REFERENCES 1 Bayley, F. J., Long, C. A. and Turner, A. B. Discs and drums: the thermo-fluid dynamics of rotating surfaces. Proc. Instn Mech. Engrs, Part C, Journal of Mechanical Engineering Science, 1993, 207(C2), 73–81. 2 Turner, A. B., Long, C. A., Childs, P. R. N., Hills, N. J. and Millward, J. A. A review of some current problems in gas turbine internal air systems. ASME paper 97-GT-325, 1997. 3 Heathcote, R. The Rolls–Royce Dart-Pioneering Turboprop, Historical Series, No. 18, 1992 (Rolls–Royce Heritage Trust, Derby). 4 Norster, E. R. and De Pietro, S. M. Dry low emissions combustion systems for EGT small gas turbines. Publication 945, Institute of Diesel and Gas Turbine Engineers, March 1996. 5 Anon. The Jet Engine, 4th edition, 1986 (Rolls–Royce plc, Derby).

Proc Instn Mech Engrs Vol 214 Part A