Internal Gearbox

The location of the internal gearbox within the core of an engine is dictated by the difficulties of bringing the driveshaft radially outwards and the space available within the engine core.

Thermal fatigue and a reduction in engine performance, due to the radial drive shaft disturbing the gas flow, creates greater problems within the turbine area than the compressor area. For any given engine, which incorporates an axial-flow compressor, the turbine area is smaller than that containing the compressor and therefore makes it physically easier to mount the gearbox within the compressor section. Centrifugal compressor engines can have limited available space, so the internal gearbox is located within a static nose cone or, in the case of a turbo-propeller engine, behind the propeller reduction gear as shown in Figure 8–1.

On multi-shaft engines, the choice of which compressor shaft is used to drive the internal gearbox is primarily dependent upon the ease of engine starting. This is achieved by rotating the compressor shaft, usually via an input torque from the external gearbox. In practice the high pressure system is invariably rotated in order to generate an airflow through the engine and the high pressure compressor shaft is therefore coupled to the internal gearbox.

To minimize unwanted movement between the compressor shaft bevel gear and radial driveshaft bevel gear, caused by axial movement of the compressor shaft, the drive is taken by one of three basic methods (Figure 8–2). The least number of components is used when the compressor shaft bevel gear is mounted as close to the compressor shaft location bearing as possible, but a small amount of movement has to be accommodated within the meshing of the bevel gears. Alternatively, the compressor shaft bevel gear may be mounted on a stub shaft that has its own location bearing. The stub shaft is splined onto the compressor shaft that allows axial movement without affecting the bevel gear mesh. A more complex system utilizes an idler gear that meshes with the compressor shaft via straight spur gears, accommodating the axial movement, and drives the radial driveshaft via a bevel gear arrangement. The latter method was widely employed on early engines to overcome gear engagement difficulties at high speed.

To spread the load of driving accessory units, some engines take a second drive from the slower rotating low pressure shaft to a second external gearbox (Figure 8–1). This also has the advantage of locating the accessory units in two groups, thus overcoming the possibility of limited external space on the engine. When this method is used, an attempt is made to group the accessory units specific to the engine onto the high pressure system, since that is the first shaft to rotate, and the aircraft accessory units are driven by the low pressure system. A typical internal gearbox showing how both drives are taken is shown in Figure 8–3.

Radial Driveshaft

The purpose of a radial driveshaft is to transmit the drive from the internal gearbox to an accessory unit or the external gearbox. It also serves to transmit the high torque from the starter to rotate the high-pressure system for engine starting purposes. The driveshaft may be direct drive or via an intermediate gearbox.

To minimize the effect of the driveshaft passing through the compressor duct and disrupting the airflow, it is housed within the compressor support structure. On by-pass engines, the driveshaft is either housed in the outlet guide vanes or in a hollow streamlined radial fairing across the low-pressure compressor duct.

To reduce airflow disruption it is desirable to have the smallest driveshaft diameter as possible. The smaller the diameter, the faster the shaft must rotate to provide the same power. However, this raises the internal stress and gives greater dynamic problems, which result in vibration. A long radial driveshaft usually requires a roller bearing situated halfway along its length to give smooth running. This allows a rotational speed of approximately 25,000 rpm to be achieved with a shaft diameter of less than 1.5 inch without encountering serious vibration problems.

Direct Drive

In some early engines, a radial driveshaft was used to drive each, or in some instances a pair of, accessory units. Although this allowed each accessory unit to be located in any desirable location around the engine and decreased the power transmitted through individual gears, it necessitated a large internal gearbox. Additionally, numerous radial driveshafts had to be incorporated within the design. This led to an excessive amount of time required for disassembly and assembly of the engine for maintenance purposes.

In some instances the direct drive method may be used in conjunction with the external gearbox system when it is impractical to take a drive from a particular area of the engine to the external gearbox. For example, Figure 8–1 shows a turbo-propeller engine that requires accessories specific to the propeller reduction drive, but has the external gearbox located away from this area to receive the drive from the compressor shaft.

Gear Train Drive

When space permits, the drive may be taken to the external gearbox via a gear train (Figure 8–1). This involves the use of spur gears, sometimes incorporating a centrifugal breather. However, it is rare to find this type of drive system in current use.

Intermediate Gearbox

Intermediate gearboxes are employed when it is not possible to directly align the radial driveshaft with the external gearbox. To overcome this problem an intermediate gearbox is mounted on the high-pressure compressor case and redirects the drive, through bevel gears, to the external gearbox. An example of this layout is shown in Figure 8–1.

External Gearbox

The external gearbox contains the drives for the accessories, the drive from the starter, and provides a mounting face for each accessory unit. Provision is also made for hand turning the engine, via the gearbox, for maintenance purposes. Figure 8–4 shows the accessory units that are typically found on an external gearbox.

The overall layout of an external gearbox is dictated by a number of factors. To reduce drag while the aircraft is flying it is important to present a low frontal area to the airflow. Therefore the gearbox is “wrapped” around the engine and may look, from the front, similar to a banana in shape. For maintenance purposes the gearbox is generally located on the underside of the engine to allow ground crew to gain access. However, helicopter installation design usually requires the gearbox to be located on the top of the engine for ease of access.

The starter/driven gearshaft (Figure 8–4) roughly divides the external gearbox into two sections. One section provides the drive for the accessories that require low power while the other drives the high power accessories. This allows the small and large gears to be grouped together independently and is an efficient method of distributing the drive for the minimum weight.

If any accessory unit fails, and is prevented from rotating, it could cause further failure in the external gearbox by shearing the teeth of the gear train. To prevent secondary failure from occurring, a weak section is machined into the driveshafts, known as a “shear-neck,” which is designed to fail and thus protect the other drives. This feature is not included for primary engine accessory units, such as the oil pumps, because these units are vital to the running of the engine and any failure would necessitate immediate shutdown of the engine.

Since the starter provides the highest torque that the drive system encounters, it is the basis of design. The starter is usually positioned to give the shortest drive line to the engine core. This eliminates the necessity of strengthening the entire gear train, which would increase the gearbox weight. However, when an auxiliary gearbox is fitted the starter is moved along the gear train to allow the heavily loaded auxiliary gearbox drive to pass through the external gearbox. This requires the spur gears between the starter and starter/driven gearshaft to have a larger face width to carry the load applied by the starter (Figure 8–5).

When a drive is taken from two compressor shafts, as discussed previously, two separate gearboxes are required. These are mounted on either side of the compressor case and are generally known as the “low speed” and “high speed” external gearboxes.

Auxiliary Gearbox

An auxiliary gearbox is a convenient method of providing additional accessory drives when the configuration of an engine and airframe does not allow enough space to mount all of the accessory units on a single external gearbox.

A drive is taken from the external gearbox (Figure 8–5) to power the auxiliary gearbox, which distributes the appropriate gear ratio drive to the accessories in the same manner as the external gearbox.

Gears

The spur gears of the external or auxiliary gearbox gear train (Figures 8–4 and 8–5) are mounted between bearings supported by the front and rear casings that are bolted together. They transmit the drive to each accessory unit, which is normally between 5000 and 6000 rpm for the accessory units and approximately 20,000 rpm for the centrifugal breather.

All gear meshes are designed with “hunting tooth” ratios that ensure that each tooth of a gear does not engage between the same set of opposing teeth on each revolution. This spreads any wear evenly across all teeth.

Spiral bevel gears are used for the connection of shafts whose axes are at an angle to one another but in the same plane. The majority of gears within a gear train are of the straight spur gear type; those with the widest face carry the greatest loads. For smoother running, helical gears are used but the resultant end thrust caused by this gear tooth pattern must be catered for within the mounting of the gear.

Gearbox Sealing

Sealing of the accessory drive system is primarily concerned with preventing oil loss. The internal gearbox has labyrinth seals where the static casing mates with the rotating compressor shaft. For some of the accessories mounted on the external gearbox, an air-blown pressurized labyrinth seal is employed. This prevents oil from the gearbox entering the accessory unit and also prevents contamination of the gearbox, and hence engine, in the event of an accessory failure. The use of an air-blown seal results in a gearbox pressure of about 3 lbs. per sq. in. above atmospheric pressure. To supplement a labyrinth seal, an “oil thrower ring” may be used. This involves the leakage oil running down the driving shaft and being flung outwards by a flange on the rotating shaft. The oil is then collected and returned to the gearbox.

Materials

To reduce weight, the lightest materials possible are used. The internal gearbox casing is cast from aluminum, but the low environmental temperatures that an external gearbox is subjected to allow the use of magnesium castings, which are lighter still. The gears are manufactured from non-corrosion resistant steels for strength and toughness. They are case hardened to give a very hard wear resistant skin and feature accurately ground teeth for smooth gear meshing.

Electric

The electric starter is usually a direct current (D.C.) electric motor coupled to the engine through a reduction gear and ratchet mechanism, or clutch, which automatically disengages after the engine has reached a self-sustaining speed (Figure 8–7).

The electrical supply may be of a high or low voltage and is passed through a system of relays and resistances to allow the full voltage to be progressively built up as the starter gains speed. It also provides the power for the operation of the ignition system. The electrical supply is automatically cancelled when the starter load is reduced after the engine has satisfactorily started or when the time cycle is completed. A typical electrical starting system is shown in Figure 8–8.

Cartridge

Cartridge starting is sometimes used on military engines and provides a quick independent method of starting. The starter motor is basically a small impulse-type turbine that is driven by high velocity gases from a burning cartridge. The power output of the turbine is passed through a reduction gear and an automatic disconnect mechanism to rotate the engine. An electrically fired detonator initiates the burning of the cartridge charge. As a cordite charge provides the power supply for this type of starter, the size of the charge required may well limit the use of the cartridge starters. A triple-breech starter is illustrated in Figure 8–9.

Iso-Propyl-Nitrate

This type of starter provides a high power output and gives rapid starting characteristics. It has a turbine that transmits power through a reduction gear to the engine. In this instance, the turbine is rotated by high-pressure gases resulting from the combustion of iso-propyl-nitrate. This fuel is sprayed into a combustion chamber, which forms part of the starter, where it is electrically ignited by a high-energy ignition system. A pump supplies the fuel to the combustion chamber from a storage tank and an air pump scavenges the starter combustion chamber of fumes before each start. Operation of the fuel and air pumps, ignition systems, and cycle cancellation is electrically controlled by relays and time switches. An iso-propyl-nitrate starting system is shown in Figure 8–10.

Air

Air starting is used on most commercial and some military jet engines. It has many advantages over other starting systems, and is comparatively light, simple, and economical to operate.

An air starter motor transmits power through a reduction gear and clutch to the starter output shaft, which is connected to the engine. A typical air starter motor is shown in Figure 8–11.

The starter turbine is rotated by air taken from an external ground supply, an auxiliary power unit (A.P.U.) or as a cross-feed from a running engine. The air supply to the starter is controlled by an electrically operated control and pressure-reducing valve that is opened when an engine start is selected and is automatically closed at a predetermined starter speed. The clutch also automatically disengages as the engine accelerates up to idling rpm and the rotation of the starter ceases. A typical air starting system is shown in Figure 8–12.

A combustor starter is sometimes fitted to an engine incorporating an air starter and is used to supply power to the starter when an external supply of air is not available. The starter unit has a small combustion chamber into which high pressure air, from an aircraft-mounted storage bottle, and fuel, from the engine fuel system, are introduced. Control valves regulate the air supply that pressurizes a fuel accumulator to give sufficient fuel pressure for atomization and also activates the continuous ignition system. The fuel/air mixture is ignited in the combustion chamber and the resultant gas is directed onto the turbine of the air starter. An electrical circuit is provided to shut off the air supply, which in turn terminates the fuel and ignition systems on completion of the starting cycle.

Some turbo-jet engines are not fitted with starter motors, but use air impingement onto the turbine blades as a means of rotating the engine. The air is obtained from an external source, or from an engine that is running, and is directed through non-return valves and nozzles onto the turbine blades. A typical method of air impingement starting is shown in Figure 8–13.

Gas Turbine

A gas turbine starter is used for some jet engines and is completely self-contained. It has its own fuel and ignition system, starting system (usually electric or hydraulic), and self-contained oil system. This type of starter is economical to operate and provides a high power output for a comparatively low weight.

The starter consists of a small, compact gas turbine engine, usually featuring a turbine-driven centrifugal compressor, a reverse flow combustion system, and a mechanically independent free-power turbine. The free-power turbine is connected to the main engine via a two-stage epicyclic reduction gear, automatic clutch and output shaft. A typical gas turbine starter is shown in Figure 8–14.

On initiation of the starting cycle, the gas turbine starter is rotated by its own starter motor until it reaches self-sustaining speed, when the starting and ignition systems are automatically switched off. Acceleration then continues up to a controlled speed of approximately 60,000 rpm. At the same time as the gas turbine starter engine is accelerating, the exhaust gas is being directed, via nozzle guide vanes, onto the free-power turbine to provide the drive to the main engine. Once the main engine reaches self-sustaining speed, a cut-out switch operates and shuts down the gas turbine starter. As the starter runs down, the clutch automatically disengages from the output shaft and the main engine accelerates up to idling rpm under its own power.

Hydraulic

Hydraulic starting is used for starting some small jet engines. In most applications, one of the engine-mounted hydraulic pumps is utilized and is known as a pump/starter, although other applications may use a separate hydraulic motor. Methods of transmitting the torque to the engine may vary, but a typical system would include a reduction gear and clutch assembly. Power to rotate the pump/starter is provided by hydraulic pressure from a ground supply unit and is transmitted to the engine through the reduction gear and clutch. The starting system is controlled by an electrical circuit that also operates hydraulic valves so that on completion of the starting cycle the pump/starter functions as a normal hydraulic pump.

Ignition

High-energy (H.E.) ignition is used for starting all jet engines and a dual system is always fitted. Each system has an ignition unit connected to its own igniter plug, the two plugs being situated in different positions in the combustion system.

Each H.E. ignition unit receives a low voltage supply, controlled by the starting system electrical circuit, from the aircraft electrical system. The electrical energy is stored in the unit until, at a predetermined value, the energy is dissipated as a high voltage, high amperage discharge across the igniter plug.

Ignition units are rated in joules (one joule equals one watt per second). They are designed to give outputs that may vary according to requirements. A high value output (e.g., 12 joule) is necessary to ensure that the engine will obtain a satisfactory relight at high altitudes and is sometimes necessary for starting. However, under certain flight conditions, such as icing or takeoff in heavy rain or snow, it may be necessary to have the ignition system continuously operating to give an automatic relight should flame extinction occur. For this condition, a low value output (e.g., 3–6 joule) is preferred because it results in a longer life of the igniter plug and ignition unit. Consequently, to suit all engine operating conditions, a combined system giving a high and low value output is favored. Such a system would consist of one unit emitting a high output to one igniter plug, and a second unit giving a low output to a second igniter plug. However, some ignition units are capable of supplying both high and low outputs, the value being pre-selected as required.

An ignition unit may be supplied with direct current (D.C.) and operated by a trembler mechanism or a transistor chopper circuit, or supplied with alternating current (A.C.) and operated by a transformer. The operation of each type of unit is described in the subsequent paragraphs.

The ignition unit shown in Figure 8–15 is a typical D.C. trembler-operated unit. An induction coil, operated by the trembler mechanism, charges the reservoir capacitor (condenser) through a high voltage rectifier. When the voltage in the capacitor is equal to the breakdown value of a sealed discharge gap, the energy is discharged across the face of the igniter plug. A choke is fitted to extend the duration of the discharge and a discharge resistor is fitted to ensure that any residual stored energy in the capacitor is dissipated within 1 minute of the system being switched off. A safety resistor is fitted to enable the unit to operate safely, even when the high tension lead is disconnected and isolated.

Operation of the transistorized ignition unit is similar to that of the D.C. trembler-operated unit, except that the trembler-unit is replaced by a transistor chopper circuit. A typical transistorized unit is shown in Figure 8–16: such a unit has many advantages over the trembler-operated unit because it has no moving parts and gives a much longer operating life. The size of the transistorized unit is reduced and its weight is less than that of the trembler-operated unit.

The A.C. ignition unit, shown in Figure 8–17, receives an alternating current that is passed through a transformer and rectifier to charge a capacitor. When the voltage in the capacitor is equal to the breakdown value of a sealed discharge gap, the capacitor discharges the energy across the face of the igniter plug. Safety and discharge resistors are fitted as in the trembler-operated unit.

There are two basic types of igniter plug; the constricted or constrained air gap type and the shunted surface discharge type. The air gap type is similar in operation to the conventional reciprocating engine spark plug, but has a larger air gap between the electrode and body for the spark to cross. A potential difference of approximately 25,000 volts is required to ionize the gap before a spark will occur. This high voltage requires very good insulation throughout the circuit. The surface discharge igniter plug (Figure 8–18) has the end of the insulator formed by a semi-conducting pellet that permits an electrical leakage from the central high-tension electrode to the body. This ionizes the surface of the pellet to provide a low resistance path for the energy stored in the capacitor. The discharge takes the form of a high intensity flashover from the electrode to the body and only requires a potential difference of approximately 2000 volts for operation.

The normal spark rate of a typical ignition system is between 60 and 100 sparks per minute. Periodic replacement of the igniter plug is necessary due to the progressive erosion of the igniter electrodes caused by each discharge.

The igniter plug tip protrudes approximately 0.1 inch into the flame tube. During operation the spark penetrates a further 0.75 inch. The fuel mixture is ignited in the relatively stable boundary layer, which then propagates throughout the combustion system.

Relighting

The jet engine requires facilities for relighting should the flame in the combustion system be extinguished during flight. However, the ability of the engine to relight will vary according to the altitude and forward speed of the aircraft. A typical relight envelope, showing the flight conditions under which an engine will obtain a satisfactory relight, is shown in Figure 8–19. Within the limits of the envelope, the airflow through the engine will rotate the compressor at a speed satisfactory for relighting; all that is required therefore, provided that a fuel supply is available, is the operation of the ignition system. This is provided for by a separate switch that operates only the ignition system.

Principles of Operation

There are several methods of obtaining reverse thrust on turbo-jet engines; three of these are shown in Figure 8–34 and explained in the following paragraphs.

One method uses clamshell-type deflector doors to reverse the exhaust gas stream and a second uses a target system with external type doors to do the same thing. The third method used on fan engines utilizes blocker doors to reverse the cold stream airflow.

Methods of reverse thrust selection and the safety features incorporated in each system described are basically the same. A reverse thrust lever in the crew compartment is used to select reverse thrust; the lever cannot be moved to the reverse thrust position unless the engine is running at a low power setting, and the engine cannot be opened up to a high power setting if the reverser fails to move into the full reverse thrust position. Should the operating pressure fall or fail, a mechanical lock holds the reverser in the forward thrust position; this lock cannot be removed until the pressure is restored. Operation of the thrust reverser system is indicated in the crew compartment by a series of lights.

Clamshell Door System

The clamshell door system is a pneumatically operated system, shown in detail in Figure 8–35. Normal engine operation is not affected by the system, because the ducts through which the exhaust gases are deflected remain closed by the doors until reverse thrust is selected by the pilot.

On the selection of reverse thrust, the doors rotate to uncover the ducts and close the normal gas stream exit. Cascade vanes then direct the gas stream in a forward direction so that the jet thrust opposes the aircraft motion.

The clamshell doors are operated by pneumatic rams through levers that give the maximum load to the doors in the forward thrust position; this ensures effective sealing at the door edges, so preventing gas leakage. The door bearings and operating linkage operate without lubrication at temperatures of up to 600°C.

Cold Stream Reverser System

The cold stream reverser system (Figure 8–36) can be actuated by an air motor, the output of which is converted to mechanical movement by a series of flexible drives, gearboxes and screw jacks, or by a system incorporating hydraulic rams.

When the engine is operating in forward thrust, the cold stream final nozzle is “open” because the cascade vanes are internally covered by the blocker doors (flaps) and externally by the movable (translating) cowl; the latter item also serves to reduce drag.

On selection of reverse thrust, the actuation system moves the translating cowl rearwards and at the same time folds the blocker doors to blank off the cold stream final nozzle, thus diverting the airflow through the cascade vanes.

Turbo-Propeller Reverse Pitch System

As mentioned earlier, reverse thrust action is affected on turbo-propeller powered aircraft by changing the pitch of the propeller blades through a hydro-mechanical pitch control system (Figure 8–37). Movement of the throttle or power control lever directs oil from the control system to the propeller mechanism to reduce the blade angle to zero, and then through to negative (reverse) pitch. During throttle lever movement, the fuel to the engine is trimmed by the throttle valve, which is interconnected to the pitch control unit, so that engine power and blade angle are coordinated to obtain the desired amount of reverse thrust. Reverse thrust action may also be used to maneuver a turbo-propeller aircraft backwards after it has been brought to rest.

Several safety factors are incorporated in the propeller control system for use in the event of propeller malfunction, and these devices are usually hydro-mechanical pitch locking devices or stops.

Construction and Materials

The clamshell and bucket target doors (Figure 8–38) described earlier form part of the jet pipe. The reverser casing is connected to the aircraft structure or directly to the engine. The casing supports the two reverser doors, the operating mechanism and, in the case of the clamshell door system, the outlet ducts that contain the cascade vanes. The angle and area of the gas stream are controlled by the number of vanes in each outlet duct.

The clamshell and bucket target doors lie flush with the casing during forward thrust operation and are hinged along the center line of the jet pipe. They are, therefore, in line with the main gas load and this ensures that the minimum force is required to move the doors.

Both the clamshell door system and the bucket target system are subjected to high temperatures and to high gas loads. The components of both systems, especially the doors, are therefore constructed from heat-resisting materials and are of particularly robust construction.

The cold stream thrust reverser casing (Figure 8–39) is fitted between the low-pressure compressor casing and the cold stream final nozzle. Cascade vane assemblies are arranged in segments around the circumference of the thrust reverser casing.

Blocker doors are internally mounted and are connected by linkages to the external movable (translating) cowl, which is mounted on rollers and tracks. Because the thrust reverser is not subjected to high temperatures, the casing, blocker doors, and cowl are constructed mainly of aluminum alloys or composite materials. The cowl is double-skinned, with the space between the skins containing noise absorbent material.

Operation of Afterburning

The gas stream from the engine turbine enters the jet pipe at a velocity of 750–1200 feet per second, but as this velocity is far too high for a stable flame to be maintained, the flow is diffused before it enters the afterburner combustion zone, i.e., the flow velocity is reduced and the pressure is increased. However, as the speed of burning kerosene at normal mixture ratios is only a few feet per second, any fuel lit even in the diffused air stream would be blown away. A form of flame stabilizer (vapor gutter) is, therefore, located downstream of the fuel burners to provide a region in which turbulent eddies are formed to assist combustion and where the local gas velocity is further reduced to a figure at which flame stabilization occurs while combustion is in operation.

An atomized fuel spray is fed into the jet pipe through a number of burners, which are so arranged as to distribute the fuel evenly over the flame area. Combustion is then initiated by a catalytic igniter, which creates a flame as a result of the chemical reaction of the fuel/air mixture being sprayed on to a platinum-based element, by an igniter plug adjacent to the burner, or by a hot streak of flame that originates in the engine combustion chamber (Figure 8–42); this latter method is known as “hot-shot” ignition. Once combustion is initiated, the gas temperature increases and the expanding gases accelerate through the enlarged area propelling nozzle to provide the additional thrust.

In view of the high temperature of the gases entering the jet pipe from the turbine, it might be assumed that the mixture would ignite spontaneously. This is not so, for although cool flames form at temperatures up to 700°C, combustion will not take place below 800°C. If, however, the conditions were such that spontaneous ignition could be effected at sea level it is unlikely that it could be effected at altitude where the atmospheric pressure is low. The spark or flame that initiates combustion must be of such intensity that a light-up can be obtained at considerable altitudes.

For smooth functioning of the system, a stable flame that will burn steadily over a wide range of mixture strengths and gas flows is required. The mixture must also be easy to ignite under all conditions of flight and combustion must be maintained with the minimum loss of pressure.

Construction

Burners

The burner system consists of several circular concentric fuel manifolds supported by struts inside the jet pipe. Fuel is supplied to the manifolds by feed pipes in the support struts and sprayed into the flame area, between the flame stabilizers, from holes in the downstream edge of the manifolds. The flame stabilizers are blunt nosed V-section annular rings located downstream of the fuel burners. An alternative system includes an additional segmented fuel manifold mounted within the flame stabilizers. The typical burner and flame stabilizer shown in Figure 8–43 is based on the latter system.

Control System

It is apparent that two functions, fuel flow and propelling nozzle area, must be coordinated for satisfactory operation of the afterburner system. These functions are related by making the nozzle area dependent upon the fuel flow at the burners or vice-versa. The pilot controls the afterburner fuel flow or the nozzle area in conjunction with a compressor delivery/jet pipe pressure sensing device (a pressure ratio control unit). When the afterburner fuel flow is increased, the nozzle area increases; when the afterburner fuel flow decreases, the nozzle area is reduced. The pressure ratio control unit ensures the pressure ratio across the turbine remains unchanged and that the engine is unaffected by the operation of afterburning, regardless of the nozzle area and fuel flow.

Since large fuel flows are required for afterburning, an additional fuel pump is used. This pump is usually of the centrifugal flow or gear type and is energized automatically when afterburning is selected. The system is fully automatic and incorporates “fail safe” features in the event of an afterburner malfunction. The interconnection between the control system and afterburner jet pipe is shown diagrammatically in Figure 8–44.

When afterburning is selected, a signal is relayed to the afterburner fuel control unit. The unit determines the total fuel delivery of the pump and controls the distribution of fuel flow to the burner assembly. Fuel from the burners is ignited, resulting in an increase in jet pipe pressure (P₆). This alters the pressure ratio across the turbine (P₃/P₆), and the exit area of the jet pipe nozzle is automatically increased until the correct P₃/P₆ ratio has been restored. With a further increase in the degree of afterburning, the nozzle area is progressively increased to maintain a satisfactory P₃/P₆ ratio. Figure 8–45 illustrates a typical afterburner fuel control system.

To operate the propelling nozzle against the large “drag” loads imposed by the gas stream, a pump and either hydraulically or pneumatically operated rams are incorporated in the control system. The system shown in Figure 8–46 uses oil as the hydraulic medium, but some systems use fuel. Nozzle movement is achieved by the hydraulic operating rams that are pressurized by an oil pump, pump output being controlled by a linkage from the pressure ratio control unit. When an increase in afterburning is selected, the afterburner fuel control unit schedules an increase in fuel pump output. The jet pipe pressure (P₆) increases, altering the pressure ratio across the turbine (P₃/P₆). The pressure ratio control unit alters oil pump output, causing an out-of-balance condition between the hydraulic ram load and the gas load on the nozzle flaps. The gas load opens the nozzle to increase its exit area and, as the nozzle opens, the increase in nozzle area restores the P₃/P₆ ratio and the pressure ratio control unit alters oil pump output until balance is restored between the hydraulic rams and the gas loading on the nozzle flaps.

Thrust Increase

The increase in thrust due to afterburning depends solely upon the ratio of the absolute jet pipe temperatures before and after the extra fuel is burned. For example, neglecting small losses due to the afterburner equipment and gas flow momentum changes, the thrust increase may be calculated as follows.

Assuming a gas temperature before afterburning of 640°C (913°K) and with afterburning of 1269°C (1542°K), then the temperature ratio = 1542/913 = 1.69. The velocity of the jet stream increases as the square root of the temperature ratio. Therefore, the jet velocity = √1.69 = 1.3. Thus, the jet stream velocity is increased by 30%, and the increase in static thrust, in this instance, is also 30% (Figure 8–47).

Static thrust increases of up to 70% are obtainable from low by-pass engines fitted with afterburning equipment and, at high forward speeds, several times this amount of thrust boost can be obtained. High thrust boosts can be achieved on low by-pass engines because of the large amount of oxygen in the exhaust gas stream and the low initial temperature of the exhaust gases.

It is not possible to go on increasing the amount of fuel that is burned in the jet pipe so that all the available oxygen is used because the jet pipe would not withstand the high temperatures that would be incurred and complete combustion cannot be assured.

Fuel Consumption

Afterburning always incurs an increase in specific fuel consumption and is, therefore, generally limited to periods of short duration. Additional fuel must be added to the gas stream to obtain the required temperature ratio. Since the temperature rise does not occur at the peak of compression, the fuel is not burned as efficiently as in the engine combustion chamber and a higher specific fuel consumption must result. For example, assuming a specific fuel consumption without afterburning of 1.15 lb./hr./lb. thrust at sea level and a speed of Mach 0.9 as shown in Figure 8–48, then with 70% afterburning under the same conditions of flight, the consumption will be increased to approximately 2.53 lb./hr./lb. thrust. With an increase in height to 35,000 feet this latter figure of 2.53 lb./hr./lb. thrust will fall slightly to about 2.34 lb./hr./lb. thrust due to the reduced intake temperature. When this additional fuel consumption is combined with the improved rate of takeoff and climb (Figure 8–49), it is found that the amount of fuel required to reduce the time taken to reach operation height is not excessive.

Lift/Propulsion Engines

The lift/propulsion engine is capable of providing thrust for both normal wing-borne flight and for lift. This is achieved by changing the direction of the thrust either by a deflector system consisting of one, two, or four swiveling nozzles or by a device known as a switch-in deflector that redirects the exhaust gases from a rearward facing propulsion nozzle to one or two downward facing lift nozzles (Figure 8–53).

Thrust deflection on a single nozzle is accomplished by connecting together sections of the jet pipe, the joint faces of which are so angled that, when the sections are counter-rotated, the nozzle moves from the horizontal to the vertical position (Figure 8–54). To avoid either a side component of thrust or a thrust line offset from the engine axis during the movement of the nozzle it is necessary that the first joint face is perpendicular to the axis of the jet pipe. If it is desired that the nozzle does not rotate, as may be the case if it is a variable area nozzle, a third joint face that is perpendicular to the axis of the nozzle is required.

The two and four nozzle deflector systems use side mounted nozzles (Figure 8–55) that can rotate on simple bearings through an angle of well over 90° so that reverse thrust can be provided if required. A simple drive system, for example, a sprocket and chain, can be used and by mechanical connections all the nozzles can be made to deflect simultaneously. For forward flight, to avoid a high performance loss and consequent increase in fuel consumption, careful design of the exhaust unit and nozzle aerodynamic passages are essential to minimize the pressure losses due to turning the exhaust flow through two close coupled bends (Figure 8–56).

The switch-in deflector consists of one or a pair of heavily reinforced doors, which form part of the jet pipe wall when the engine is operating in the forward thrust condition. To select lift thrust, the doors are moved to blank off the conventional propelling nozzle and direct the exhaust flow into a lift nozzle (Figure 8–57). The lift nozzles may be designed so that they can be mechanically rotated to vary the angle of the thrust and permit intermediate lift/thrust positions to be selected.

A second type of switch-in deflector system is used on the tandem fan or hybrid fan vectored thrust engine (Figure 8–58). In this case the deflector system is situated between the stages of the fan of a mixed flow turbo-fan engine. In normal flight the valve is positioned so that the engine operates in the same manner as a mixed flow turbo-fan and for lift thrust the valve is switched so that the exhaust flow from the front part of the fan exhausts through downward facing lift nozzles and a secondary inlet is opened to provide the required airflow to the rear part of the fan and the main engine. On a purely subsonic V/STOL aircraft where fuel consumption is important the valve may be dispensed with and the engine operated permanently in the latter high by-pass mode described above.

Thrust deflecting nozzles will create an upstream pressure distortion that may excite vibration of the fan or low-pressure turbine blades if the nozzle system is close to these components. Snubbers may be used on the fan blades to resist vibration. On the low-pressure turbine, shrouds at the blade tips or wire lacing may be used to achieve the same result.

Lift Engines

The lift engine is designed to produce vertical thrust during the takeoff and landing phases of V/STOL aircraft. Because the engine is not used in normal flight it must be light and have a small volume to avoid causing a large penalty on the aircraft. The lift engine may be a turbo-jet, which for a given thrust gives the lowest weight and volume. Should a low jet velocity be necessary a lift-fan may be employed.

Pure lift-jet engines have been developed with thrust/weight ratios of about 20:1 and still higher values are projected for the future. Weight is reduced by keeping the engine design simple and also by extensive use of composite materials (Figure 8–59). Because the engine is operated for only limited periods during specific flight conditions, i.e., during takeoff and landing, the fuel system can be simplified and a total loss oil system, in which the used lubricating oil is ejected overboard, can be used.

Lift engines can be designed to operate in the vertical or horizontal position and a thrust-deflecting nozzle fitted to provide some of the advantages of thrust vectoring. Alternatively, the engine may be mounted so that it can swivel through a large angle to provide thrust vectoring. The lift-jet engine will have an extremely hot, high velocity jet exhaust and, to reduce ground erosion by the jet, the normal exhaust nozzle may be replaced by a multi-lobe nozzle to increase the rate of mixing with the surrounding air.

The lift-fan engine is designed to reduce the jet exhaust velocity, to reduce ground erosion, and allow operation from unprepared ground surfaces. It also reduces the jet noise significantly. A range of design options have been considered for this type of engine and some are shown in Figure 8–60.

Remote Lift Systems

Direct lift remote systems duct the by-pass air or engine exhaust air to downward facing lift nozzles remote from the engine. These nozzles may be in the front fuselage of the aircraft or in the wings. The engine duct is blocked by means of a diverter similar to that described earlier.

The remote lift-fan (Figure 8–61) is mounted in the aircraft wing or fuselage, and is driven mechanically or by air or gas ducted into a tip turbine. The drive system is provided by the main propulsion power plant or by a separate engine.

The advantage of the remote lift system is that it gives some freedom to the aircraft to position the propulsion system to the best advantage while still maintaining the resultant thrust near the aircraft center of gravity in the jet lift mode. This freedom is achieved at a cost of increased volume, particularly with the gas driven systems, due to the size of the ducts to feed the gas to the remote lift system. Although the mechanically driven remote lift-fan eliminates the need for these large gas ducts, it is done at the expense of long shafts and high power gearboxes and clutch systems.

Swiveling Engines

This method consists of having propulsion engines that can be mechanically swiveled through at least 90° to provide thrust vectoring (Figure 8–62). In addition to these propulsion engines, one or more lift engines may be installed to provide supplementary lift during the takeoff and landing phase of flight.

The swiveling engine system can only be used with two or more engines. This then introduces the problem of safety in the event of an engine failure. So, although there is only a small weight penalty and no increase in fuel consumption, safety considerations tend to offset these advantages compared to some of the other powered lift systems. The normal method of providing aircraft control at low speeds is by differential throttling and vectoring of the engines that simplifies the basic engine design but makes the control system more complex.

Bleed Air for STOL

Figure 8–63 shows one method how STOL can be achieved with a form of “flap blowing” The turbo-fan engine has a geared variable pitch fan and an oversized low pressure (L.P.) compressor from the exit of which air is bled and ducted to the flap system in the wing trailing edge. The variable pitch fan enables high L.P. compressor speed and thus high bleed pressure to be maintained over a wide range of thrusts. This gives excellent control at greatly different aircraft flight conditions.

Special Engine Ratings

Experience has shown that an engine rating structure can be devised that provides high thrust levels for short periods of time without reducing engine life. Operation in ground effect and the takeoff and landing maneuvers require maximum thrust for less than 15 seconds so that use of a short lift rating for that time is feasible. Figure 8–64 shows an example of thrust permissible with a 15 second short lift rating compared to that with a 2.5 minute normal lift rating.

At high ambient temperatures, the engine may run into a turbine temperature limit before reaching its maximum rpm and suffer a thrust loss as a result. Restoration of the thrust can be achieved by means of water injection into the combustion chamber, which allows operation at a higher turbine gas temperature for a given turbine blade temperature. If desired, water injection can also be used to increase the thrust at low ambient temperatures.

Lift Burning Systems

The thrust of the four-nozzle lift/propulsion engine may be boosted by burning fuel in the by-pass flow in the duct or plenum chamber supplying the front nozzles. This is called plenum chamber burning (P.C.B.) (Figure 8–65) and thrust of the by-pass air may be doubled by this process. This thrust capability is available for normal flight as well as takeoff and landing and so can be used to increase maneuverability and give supersonic flight.

The thrust of a remote lift jet can also be augmented by burning fuel in a combustion chamber just upstream of the lift nozzle (Figure 8–66). This system is commonly known as a remote augmented lift system (R.A.L.S.). The thrust boost available from the burner reduces the amount of airflow to be supplied to it and therefore reduces the size of the ducting needed to direct the air from the engine to the remote lift nozzle.

Ejectors

The principle of the ejector is that a small, high energy jet entrains large quantities of ambient air by viscous mixing and an increase in thrust over that of the high energy jet results. A number of projected V/STOL aircraft have incorporated this concept using either all the engine exhaust air or just the by-pass flow.

Reaction Controls

This system bleeds air from the engine and ducts it through nozzles at the four extremities of the aircraft (Figure 8–67). The air supply to the nozzles is automatically cut off when the main engine swiveling propulsion nozzles are turned for normal flight or when the lift engines are shut down. The thrust of the control nozzles is varied by changing their area that varies the amount of airflow passed.

Differential Engine Throttling

This method of control is used on multi-engined aircraft with the engines positioned in a suitable configuration. A rapid response rate is essential to enable the engines to be used for aircraft stability and control. It is usually necessary to combine differential throttling with differential thrust vectoring to give aircraft control in all areas.

Automatic Control Systems

Although it is possible for the pilot to control a V/STOL aircraft manually, some form of automation can be of benefit and in particular will reduce the pilot workload. The pilot’s control column is electronically connected to a computer or stabilizer that receives signals from the control column, compares them with signals from the sensors that measure the attitude of the aircraft, and automatically adjusts the reaction controls, differential throttling, or thrust vectoring controls to maintain stability.

Thrust on Compressor Casing

To obtain the thrust on the compressor casing it is necessary to calculate the conditions at the inlet to the compressor and the conditions at the outlet from the compressor. Since the pressure and the velocity at the inlet to the compressor are zero, it is only necessary to consider the force at the outlet from the compressor. Therefore, given that the compressor

Diffuser Duct

The conditions at the diffuser duct inlet are the same as the conditions at the compressor outlet, i.e., 19,049 lb.

Therefore, given that the diffuser

OUTLET Area (A) = 205 sq. in.

Pressure (P) = 95 lb. per sq. in. (gauge)

Velocity (v_j) = 368 ft. per sec.

Mass flow (W) = 153 lb. per sec.

The thrust

= (205 × 95) + (153 × 368)/32 – 19,049

= 21,235 – 19,049

= 2186 lb. of thrust in a forward direction.

Combustion Chambers

The conditions at the combustion chamber inlet are the same as the conditions at the diffuser outlet, i.e., 21,235 lb. Therefore, given that the combustion chamber

OUTLET Area (A) = 580 sq. in.

Pressure (P) = 93 lb. per sq. in. (gauge)

Velocity (v_j) = 309 ft. per sec.

Mass flow (W) = 153 lb. per sec.

The thrust

= (A × P) + Wv_j/g – 21,235 lb.

= (580 × 93) + (153 × 309)/32 – 21,235 lb.

= 55,417 – 21,235

= 34,182 lb. of thrust in a forward direction.

Turbine Assembly

The conditions at the turbine inlet are the same as the conditions at the combustion chamber outlet, i.e., 55,417 lb.

Therefore given that the turbine

OUTLET Area (A) = 480 sq. in.

Pressure (P) = 21 lb. per sq. in. (gauge)

Velocity (v_j) = 888 ft. per sec.

Mass flow (W) = 153 lb. per sec.

The thrust

= (A × P) + Wv_j/g – 55,417

= (480 × 213) + (153 × 888)/32 – 55,417

= 14,326 – 55,417

= –41,091.

This negative value means a force acting in a rearward direction.

Exhaust Unit and Jet Pipe

The conditions at the inlet to the exhaust unit are the same as the conditions at the turbine outlet, i.e., ,14,326 lb.

Therefore, given that the jet pipe

OUTLET Area (A) = 651 sq. in.

Pressure (P) = 21 lb. per sq. in. (gauge)

Velocity (v_j) = 643 ft. per sec.

Mass flow (W) = 153 lb. per sec.

The thrust

= (A × P) + Wv_j/g – 14,326 lb.

= (651 × 21) + (153 × 643)/32 – 14,326

= 16,745 – 14,326

= 2419 lb. of thrust in a forward direction.

Propelling Nozzle

The conditions at the inlet to the propelling nozzle are the same as the conditions at the jet pipe outlet, i.e., 16,745 lb.

Therefore, given that the propelling nozzle

OUTLET Area (A) = 332 sq. in.

Pressure (P) = 6 lb. per sq. in. (gauge)

Velocity (v_j) = 1917 ft. per sec.

Mass flow (W) = 153 lb. per sec.

The thrust

= (A × P) + Wv_j/g – 16,745 lb.

= (332 × 6) + (153 × 1917)/32 – 16,745

= 11,158–16,745

= 5587 lb, acting in a rearward direction.

It is emphasized that these are basic calculations and such factors as the effect of air offtakes have been ignored.

Based on the individual calculations, the sum of the forward or positive loads is 57,836 lb., and the sum of the

rearward or negative loads is 46,678 lb. Thus, the resultant (gross or total) thrust is 11,158 lb.

Engine

It will be of interest to calculate the thrust of the engine by considering the engine as a whole, as the resultant thrust should be equal to the sum of the individual gas loads previously calculated.

Although the momentum change of the gas stream produces most of the thrust developed by the engine (momentum thrust = Wv_j/g), an additional thrust is produced when the engine operates with the propelling nozzle in a “choked” condition. This thrust results from the aerodynamic forces that are created by the gas stream and exert a pressure across the exit area of the propelling nozzle (pressure thrust). Algebraically, this force is expressed as (P – P₀)A.

where

A = Area of propelling nozzle in sq. in.

P = Pressure in lb. per sq. in.

P₀ = Atmospheric pressure in lb. per sq. in.

Therefore, assuming values of mass flow, pressure, and area to be the same as in the previous calculations, i.e.,

Area of propelling nozzle (A) = 332 sq. in.

Pressure (P) = 6 lb. per sq. in. (gauge)

Atmospheric pressure (P₀) = 0 lb. per sq. in. (gauge)

Mass flow (W) = 153 lb. per sec.

Velocity (v_j) = 1917 ft. per sec.

The thrust = (P – P₀)A + Wv_j 0

= (6 – 0) 332 + (153 × 1917)/32 – 0

= 1992 + 9166

= 11,158 lb., the same as previously calculated by combining the gas loads on the individual engine locations.

On engines that operate with a non-choked nozzle, the (P – P₀)A function does not apply and the thrust results only from the gas stream momentum change.

Inclined Combustion Chambers

In the previous example the flow through the combustion chamber is axial; however, if the combustion chamber is inclined towards the axis of the engine, then the axial thrust will be less than for an axial flow chamber. This thrust can be obtained by multiplying the sum of the outlet thrust by the cosine of the angle (see Figure 8–69). The cosine = base/hypotenuse and for a given angle is obtained by consulting a table of cosines. It should be emphasized that if the inlet and outlet are at different angles to the engine axis, it is necessary to multiply the inlet and outlet thrusts separately by the cosine of their respective angles.

Thrust due to Afterburning

When the engine is fitted with an afterburner, the gases passing through the exhaust system are reheated to provide additional thrust. The effect of afterburning is to increase the volume of the exhaust gases, thus producing a higher exit velocity at the propelling nozzle.

Assuming that an afterburner jet pipe and propelling nozzle are fitted to the engine used in the previous calculations, and the new conditions at the propelling nozzle are as follows:

Therefore, compared with the previous calculation for a propelling nozzle, it will be seen that the negative thrust is reduced from –5587 lb. to –2676 lb.; the overall positive

thrust is thus increased by 2911 lb., which is equivalent to a thrust increase of more than 25%.

To arrive at the total thrust of the engine with afterburning the calculations should use the above figures.

Standard Design

The modular design of the turbogenerator permits the selection of a standard version with either an open or closed cooling system and static or brushless excitation systems.

The generators satisfy the requirement of IEC-34 and other relevant standards such as SEN, ANSI, NEMA, CSA, etc. This means that the generators can be used in countries in which these or comparable standards apply. Generators can be custom built to satisfy other standards.

Configuration

The turbogenerators are two-pole, air-cooled synchronous generators with cylindrical rotor and direct air-cooled rotor winding. They are intended for both basic and peak-load operations and designed to withstand high short-circuiting stresses. The configuration with journal bearings in the end shields permits delivery of the machines as a single unit, which considerably simplifies installation and commissioning (see Figure 8–70).

Degrees of Protection and Methods of Cooling

The design of the generators provides for the following degrees of protection and methods of cooling in accordance with IEC 34-5 and 34-6, 1983.

These combinations of degrees of protection and methods of cooling are standard for this information source.

The different cooling methods require two fans mounted on the generator rotor.

The use of the IC 01 and IC 21 methods of cooling presupposes that the cooling air supplied is cleaned by thorough filtration. For this reason, IC 01 and IC 21 should be avoided when the cooling air available contains corrosive gases or large quantities of other pollution.

Configuration/Arrangement Forms

The machines can be supplied in the following versions:

Excitation System

The generators can be delivered with one of two alternative excitation systems:

Accessories

The following accessories can be supplied on request:

The following information and requirements should be provided by the end user to the OEM with any request for a bid:

Technical Data on Typical Available Generators

Design Description

Stator Frame

The stator frame is of welded steel construction (see Figure 8–71).

The side plates are dimensioned to bear the weight of the complete generator during lifting. Openings are provided in the top or sides for cooling air supply and exhaust. Longitudinal foot plates are provided at the bottom of the long sides of the stator frame for fixing the stator to the foundation.

Stator Terminals

The connections to the busbars are located outside the generator casing (see Figure 8–72). The terminal bushings are connected to the terminal connections in the stator winding with clamps with generously dimensioned contact areas. The clamps are easily accessible for removing the stator terminals for, e.g., transport.

Bearing End Shields

The bearing end shields consist of a very stiff lower half with bearing housing (see Figure 8–73). The upper half of the bearing housing is easily removed to facilitate inspection of the bearing. Holes for supply and drainage of oil are drilled in the lower half. The bearing housing is provided with connections for air extraction.

The upper half of the bearing end shield consists of a sound insulating screen plate divided into three parts to simplify removal for inspection.

Bearings

Each bearing housing contains a radial bearing (see Figure 8–74) for the generator rotor. The radial bearings are pressure-lubricated slide bearings with white-metal linings divided into staggered upper and lower segments. Lubricating oil is supplied to the bearing through drilled channels.

Where the bearings must take up axial forces, one is provided with axial thrust-bearing pads. The thrust bearing is double-acting, i.e., it takes up forces in both directions. One of the bearings is insulated from the bearing end shield to prevent the development of damaging bearing currents.

When the generator is to be driven from both ends, both bearings are insulated. The support bearing is always insulated. The shaft seals prevent oil leakage from the bearing housing and consist of a divided seal of oil-resistant insulation material. An air extraction system is connected to the bearing space to prevent oil leakage through the external seals. The internal shaft seals are provided with sealing caps and the intermediate space is connected to blocking air from the pressure side of the fan.

Rotor

The rotor body (see Figure 8–75) is manufactured from a cylindrical forging of high-alloy steel with suitable magnetic properties. A hole is drilled into the shaft extension for terminal conductors. The terminal conductors carry excitation current from the exciter to the rotor winding.

A number of axial winding slots for the excitation windings of the rotor are milled on both sides of the pole sections.

Rotor Terminals

The rotor terminals (see Figure 8–76) are located in the shaft extension and consist of two copper conductors, enclosed in a tube of insulating material and insulated from each other. The connections from the terminal conductor and radially outward to the winding consist of contact screws. The design of the contact screws eliminates the risk of fatigue failure in the connection details caused by the differential movement of the rotor winding and the shaft.

All joints are provided with gold-plated spring-contact elements to ensure effective current conduction between the contact surfaces.

Rotor Cooling

The rotor winding (see Figure 8–77) is directly air-cooled by ventilation channels in direct contact with the winding slots. Cooling air is drawn in between the support ring and the shaft extension by the centrifugal fan effect of the rotor.

The air cooling the coil ends passes out radially through holes in the coil end insulation to the retaining ring and axially in channels under the retaining ring through holes in the support ring.

The cooling air in the winding slots is drawn in under the coil ends and into the axial cooling air channels between the sides of the slots and the conductor package. The rotor slot wedges and the beams between the winding slots are provided with a number of exhaust openings, connected in parallel, for the cooling air.

Cooling Systems

The generator is provided with two axial fans. The main task of the fans is to ventilate the stator (see Figure 8–78); the rotor itself functions as a centrifugal fan. The generator can be provided with an open or closed cooling system. The air circulation in the different cooling systems is shown in Figures 8–78 through 8–80.

In closed cooling systems (see Figure 8–79), the cooler housings are mounted, with the air/water coolers, on the long sides of the generator. The cooler housings are welded steel constructions that lead cooling air from the coolers to the axial fans. The air/water coolers are of the lamellar tube type. The materials chosen for the tubes and water chamber are dependent on the quality of the cooling water available.

Compensation air is drawn in through a filter to replace air leakage from the machine.

With an open cooling system (see Figure 8–80), the incoming air is to be filtered.

The choice of filter is determined by the site conditions. Recommendations for filter selection are based on the information regarding the environment provided with the request to tender.

Figure 8–81 illustrates a generator assembly that has not been installed.

Brushless Excitation

Main Exciter

The main exciter (see Figure 8–82) is a multiple synchronous generator with salient poles in the stator.

The stator and rotor winding insulation satisfies the requirements for temperature class F (155°C). The windings are impregnated for protection against damp and vibration.

Surface Treatment

In its standard version, the generator is painted with a lacquer of two-component type based on ethoxylized chlorine polymer. The generator is primed inside and outside and then finished externally in a neutral blue color. The paint is resistant to corrosive, tropical, and other aggressive atmospheres.

Static Excitation

When the rotor winding of the generator is supplied with current from a static rectifier unit, the rotor is provided with a slipring shaft and supplied via brush gear.

Slipring Shaft

The slipring shaft (see Figures 8–83 and 8–84) consists of shaft extension, insulations, sliprings, contact screws, and terminal conductors.

The shaft extension consists of a steel forging with flange for connection to the shaft of the generator rotor. The center of the shaft extension is drilled for the terminal conductors.

The sliprings are manufactured of steel and have generously dimensioned contact surfaces for the carbon brushes. The spiral-machined contact surface is carefully ground and polished. This prevents current concentrations and reduces brush and slipring wear. The sliprings are shrunk on the shaft extension on a cylinder of insulating material. The radial connections from the sliprings to the terminal conductors consist of insulated contact screws through holes in the shaft extension.

The terminal conductors in the slipring shaft and the rotor shaft are connected with contact screws.

Inspection and Testing

General basic inspection and testing points performed during the fabrication of the generators are included in a check plan. Each manufacturing operation is subject to extensive checking.

Control and Protection

Operating Characteristics

Noise Reduction

When there are special acoustic requirements, the generator can be installed in a sound-absorbing enclosure consisting of a steel frame with panels of perforated steel sheets with sound-absorbing mineral wool in-fill.

The sealing against water leakage between the panels and the supporting structure consists of a self-adhesive rubber strip and silicon-rubber caulking.

The roof and walls of the enclosure are provided with service openings.

Methods of Cleaning Fouled Gas Turbine Compressors

Online Cleaning Methods

However, with the popular resurgence of the gas turbine as an industrial prime mover over the past decades, serious interest in the problem of lost performance and increased fuel consumption caused by fouling has led to the development of so-called “online” or “fired-wash” compressor cleaning systems. The objective of these systems being to chemically clean the compressor while the engine remains in operation at up to full speed and load in order to extend the output for longer and avoid increase in heat rate and subsequent increases in fuel consumption.

In reality, the number of companies and individuals that have been seriously involved in the development of fired-wash systems over the years are few and far between. However, since the process has, of late, gained the official blessing of some major gas turbine manufacturers, there has been a sudden proliferation of system suppliers and even more running-wash chemical suppliers who, in many cases, may have scant knowledge or experience of the fired-wash process and gas turbines to which it is being applied.

If done properly, fired-washing can be a very safe and successful method of keeping gas turbines running more efficiently. The process is being constantly improved and perfected and is, without doubt, here to stay as more and more gas turbine manufacturers offer running-wash systems as a standard fit or recommended option. However, it can also be a dangerous process if injection systems or chemicals are incorrectly designed, fitted, or used and operators should be cautious when selecting any online cleaning system.

About the Injection System

Since online washing has rather suddenly come into vogue even though deep routed knowledge and experience of the process is known to relatively few system suppliers and operators it is hoped that this section will be of sound practical help to those operators who would like to adopt online chemical washing procedures but who are unfamiliar with the concept.

Can Online Washing Really Be Cost Effective and Is It Really Necessary—or Just Another Fad?

Due to the current Middle East crisis the cost of fuel has increased by roughly 70% over the past 3 months. For example, No. 2 diesel was, as of October 1, 1990, being quoted on the Rotterdam market at $300/ton whereas 3 months prior it was priced in the region of $180/ton.

Even if the fuel drops back to pre-crisis levels, in due time the fuel costs for operating any gas turbine are still substantial even when the machine is kept in perfect operating condition and at peak efficiency. A fouled compressor can easily increase fuel consumption by 5% or more and, in real terms, a 5% increase for the operator of a typical 25 MW heavy industrial unit running base load for say 8000 hours per year would add over $0.5 million/year to the fuel bill at pre-Gulf crisis fuel prices and close to $1 million extra at current prices.

Advantages and Disadvantages of Offline Compressor Washing

Why Not Try to Avoid Fouling Altogether Instead of Spending Time and Money on Compressor Washing Techniques?

The simple answer—at least at this time—is that it is virtually impossible even with the finest multi-stage, high efficiency filtration systems to completely avoid compressor fouling.

There are many sources of compressor fouling but the worst and the most common is the ingestion of oily vapors that can readily pass through filtration systems.

Typical Causes of Gas Turbine Compressor Fouling

Typically these oily vapors are the catalyst to compressor fouling in most cases. After restarting the gas turbine with a clean, dry compressor, a film of oily/greasy deposit can build up rapidly on the compressor airfoils (roughly in the first third of the compressor) and this forms a perfect “flytrap” to catch and absorb dry particulate matter that would otherwise have passed harmlessly through the compressor.

In more than 10 years of designing and developing compressor cleaning systems the writer has observed that classic fouling—particularly in polluted industrial environments—is basically a three-phase affair.

Firstly, there is the laying down of the oily film and the rapid absorption and entrapment of dry particulates. This can occur within days or even hours after restarting a cleaned engine and there can be a substantial reduction in output in a relatively short period of time.

Next, there is a slowing down of performance loss as the surface deposit gradually dries and the rate of fouling decreases.

Finally, there is a relatively long phase of slow decline over a period of weeks or even months followed by a further sharp decline as the aerodynamic tolerance of the compressor decreases and/or the surface deposit once more becomes sticky with more oily residues.

A fingernail scratch into the apparent dry, carbonaceous surface deposit on the bell mouth struts of many a compressor is almost sure to reveal the initial oily substrate that was laid down shortly after the cleaned engine was put back into service.

In general, one could say with confidence that compressor fouling would be much diminished if all traces of oil and grease could be prevented from entering the compressor and in this regard close attention to the complete integrity of the air inlet plenum after the filters is strongly recommended since even small breaches can allow inordinately large quantities of unfiltered vapors and particulates to pass freely into the compressor.

Because it is almost impossible to prevent some degree of compressor fouling the logical approach is to try to find a way of controlling and reducing the rate of fouling and ultimately to control it to a point that it does not affect performance at all.

This means regular compressor cleaning on a sufficiently frequent basis to prevent any meaningful amounts of deposit from building up between each wash and that could mean once a day or once per week depending on operating circumstances.

Advantages and Disadvantages of Online Chemical Cleaning of Compressor

Other Online Compressor Cleaning Methods

The question is, is online chemical washing the only online cleaning method?

The answer is no but of the other available choices it may be the most efficient, safe and practical online procedure.

The first alternative—and probably as old as the gas turbine itself—is abrasive cleaning using a variety of crushed nutshells or carbon-based pellets but in general this method is impractical for most modern gas turbines because, among other problems and drawbacks, the abrasive medium can damage coatings and plug turbine cooling holes with possible catastrophic results.

Advantages and Disadvantages of Abrasive Compressor Cleaning

Possible Disadvantages

The second alternative to online chemical cleaning is online plain water washing, which has been advocated by some as a cheap solution with the apparent simplistic argument that water must be inexpensive and chemical must be expensive.

This would, of course, be the perfect solution if all compressor fouling was water-soluble and the water used was virtually free of all dissolved and suspended solids so as to avoid corroding the engine in the process of trying to clean it.

Unfortunately, compressor fouling is rarely, if ever, of a water-soluble nature and good quality pure water is not always easy to come by. Many a gas turbine has been badly corroded by poor attention to water quality particularly when applied to a hot engine.

Water washing should not be confused with the use of so-called water-based or aqueous chemicals. Generally, water-based chemicals are formulated from various types of surfactants (surface acting agents) and corrosion inhibitors with water being in the majority (between 80 and 98% depending on the brand) carrying agent.

Some of the new formula water-based materials (some using very pure food-grade surfactants) are quite effective at breaking down stubborn oily/greasy deposits and are proving quite successful even for fired-washing but are particularly attractive for offline washing because of their high biodegradability factor, which makes the disposal of the waste chemical a lot easier.

Advantages and Disadvantages of Cleaning Compressors with Plain Water

Types of Expansion Joints

Unrestrained Assemblies

• Single expansion joint assemblies are the simplest type of expansion joint consisting of a single bellows element welded to end fittings, either flange or pipe ends.

• Universal expansion joint assemblies consist of two bellows connected by a center spool piece with flange or pipe ends. The universal arrangement allows greater axial, lateral, and angular movements than a single bellows assembly.

• In externally pressurized expansion joints, line pressure acts externally on the bellows by means of a pressure chamber. This allows a greater number of convolutions to be used for large axial movements, without fear of bellows instability. Externally pressurized expansion joints have the added benefit of self-draining convolutions if standing media are a concern. Anchors and guides are an essential part of a good installation.

Externally Pressurized Expansion Joints

Restrained Assemblies

• Tied single bellows assemblies add tied rods to a single bellows assembly to increase design flexibility in a piping system. The tie rods are attached to the pipe or flange with lugs that carry the pressure thrust of the system, eliminating the need for main anchors.

• Tied universal assemblies are similar in construction to a universal assembly except that tie rods absorb pressure thrust and limit movements to lateral offset and angulation only.

• Hinged bellows assemblies limit movement to angulation in one plane. Hinged assemblies are normally used in sets of two or three to absorb large amounts of expansion in high pressure piping systems.

• Gimbal bellows assemblies are designed to absorb system pressure thrust and torsional twist while allowing angulation in any plane. Gimbal assemblies, when used in pairs or with a single hinged unit, have the advantage of absorbing movements in multi-planer piping systems.

Accessories for Special Service

Bellows Manufacture

The bellows element is the most important component of an expansion joint. This thin-walled, corrugated membrane allows flexibility in a piping system while containing the pressure and media. When complete, each bellows has a unique working pressure, spring rate, and cycle life that are entirely dependent on its geometry and material.

Bellows are manufactured using an expanding mandrel (punch forming) method followed by a finish rolling. A rectangular sheet is sheared and rolled into a tube. The tube is welded using an auto flat-bed welder with no filler metal added. The longitudinal seam weld is then “plannished” back down to the parent material thickness. Any dye penetrant, X-ray, or air testing is performed at this time. When testing is complete, the convolutions are punched individually drawing material from the top and bottom of the tube. This drawing process eliminates any thinning in the bellows material.

At this point, the convolutions have more of a “V” shape than the final “U” shape that is required. This is easily solved with the final re-rolling of the bellows between a series of aluminum bronze rolls. After trimming the bellows attachment ends (skirts), the bellows is complete and ready to have attachment ends installed.

Shipping

Every expansion joint that leaves the factory is provided with installation instructions. These instructions describe the simple, straightforward requirements that must be followed to ensure a trouble-free installation.

Shipping Bars

These are temporary attachments that “hold” the expansion joint at its correct installed length during shipping and installation. Angle iron or channel section is used and is always painted bright yellow. Shipping bars must never be removed until after the unit has been correctly welded or bolted into the piping system. Caution: tie rods or limit rods are sometimes mistaken for shipping bars.

Never Tamper with These Attachments

Note: Great care must be taken when removing the shipping bars. If a welding or burning torch is used, always protect the bellows element from burn splatter with a flame-retardant cloth or other shielding material.

Installation Guidelines

Unrestrained Expansion Joints

Unrestrained expansion joints are not provided with attachments such as tie rods or hinges to restrain pressure thrust. Therefore, they can be used only in a piping system that incorporates correctly designed anchors and pipe alignment guides. These components prevent the bellows from overextension and damage due to distortion under operating conditions.

Restrained Expansion Joints

Tied expansion joints can be of the single or universal type provided with restraints such as tie rods, hinges or gimbals. Tie rods and gimbals allow the expansion joint to move in all planes. Hinges allow movement in a single plane only. These restraints are designed to absorb the pressure thrust and other external loads like pipe dead weight. For restraints to remain effective, the expansion joint can absorb only lateral offset or angulation in directional changes in the piping system, such as “Z” bends, “U” bends, or “S” bends. Tied units are used where the equipment or adjacent structures cannot accommodate pressure thrust.

Specific Applications

Special product lines include a variety of rectangular joints, PenSeals (boiler seals), silencer bellows seals, expansion joints for fluid catalytic cracking units (FCCU), gas turbine exhaust (GTX) systems, aerospace, heat exchangers, and many other heavy industrial applications.

Expansion joints are made in both metal and fabric designs from 3˝ nominal diameter to 200˝ with process temperatures from –325–2500°F. Pressures for these designs range from full vacuum to 2000 psig. Single-ply, multi-ply, root ring, equalizer ring, and toroid bellows designs are also available.