Of a power not exceeding 1,100 kW

84.11 - Turbo-jets, turbo-propellers and other gas turbines (+). 8411.11 - - Of a thrust not exceeding 25 kN 8411.12 - - Of a thrust exceeding 25 kN 8411.21 - - Of a power not exceeding 1,100 kW 8411.22 - - Of a power exceeding 1,100 kW - Other gas turbines : 8411.81 - - Of a power not exceeding 5,000 kW 8411.82 - - Of a power exceeding 5,000 k W - Parts : 8411.9 1 - - Of turbo-j ets or turbo-propellers 8411.99 - - Other This heading covers turbo-jets, turbo-propellers and other gas turbines. The turbines of this heading are, in general, internal combustion engines which do not usually require any external source of heat as does, for example, a steam turbine. (A) TURBOJETS A turbo-jet consists of a compressor, a combustion system, a turbine and a nozzle, which is a convergent duct placed in the exhaust pipe. The hot pressurised gas exiting from the turbine is converted to a hi h velocity gas stream by the nozzle. The reaction of this gas stream actin m the engine provi es the motive force which may be used to ower aircraft. In its sim lest o On the compressor and turbine are accommodated on a single s aft. In more corn lex esigns the compressor is made in two parts (a two spool compressor) in which the spoo of each part is driven by its own turbine through concentric shafting. Another variation is to add a ducted fan usually at the inlet to the corn ressor and drive this either by a thrd turbine or connect it to the first compressor spool. The an acts in the nature of a ducted propeller, most of its ou ut bypassing the compressor and turbine and joining the exhaust jet to provide extra thrust. $is version is sometimes called a " bypass fan jet ". d R P B B F So-called " after-burning " ap liances are auxiliary units for mounting in series with certain turbo-jet engines in order to oost their power output for short periods. These appliances have their own he1 supply and utilise the excess oxygen in the gases issuing from the turbo-jet. E (B) TURBO-PROPELLERS Such engines are similar to turbo-jets, but have a further turbine downstream of the compressor turbine, which is coupled to a conventional propeller such as is used on piston en ined aircraft. This latter turbine is sometimes referred to as a " free turbine ", meaning t%a t it is not mechanically coupled to the compressor and compressor turbine shaft. Thus most of the hot pressurised gas leaving the compressor turbine is converted into shaft ower by the free turbine Instead of being expanded in a nozzle as is the case in turbo-jets. some cases, the gases leaving the £tee turbine may be expanded in a nozzle to provide auxiliary jet power and assist the propeller. R (C) OTIIER GAS TURBINES This group includes industrial gas-turbine units which are either specifically designed for industrial use or adapt turbo-jets or turbo-propeller units for uses other than providing motive power for aircraft. There are two types of cycles : (1) The simple cycle, in which air is ingested and compressed by the compressor, heated in the combustion system and passed through the turbine, finally exhausting to the atmosphere. and passed through the air and is then passed to the The d g a s mixture hot gas side of the regenerator and (2) The re enerative cycle, in pipes ogf a regenerator. The combustion s stem where asses tbroud the turbine Enally to the atmosphere. There are two types of designs : (a) The single-shaft gas turbine unit, in which the compressor and turbine are built on a single shaft, the turbine providing power to rotate the corn ressor and to drive rotating machinery through a coupling. This type of drive is most e ective for constant speed applications such as electrical power generation. d (b) The two-shaft gas turbine unit, in which the compressor, combustion system and compressor turbine are accommodated in one unit generally called a as generator, whilst a second turbine on a se arate shaft receives the heated and pressurise gas fiom the exhaust of the gas generator. & i s second turbine known as the power turbine is coupled to a driven unit, such as a compressor or pump. Two-shaft gas turbines are normally applied where load demand variations require a range of power and rotational speed fiom the gas turbine. f These gas turbines are used for marine craft and locomotives, for electrical power generation, and for mechanical drives in the oil and gas, pipeline and petrochemical industries. This grou also includes other gas turbines without a combustion chamber, comprising simply a stator ancf rotor and which use energy from gases provided by other machines or appliances (e.g., gas r r a t o r s , diesel engines, fiee-plston generators) and compressed air or other compresse gas turbines. PARTS Sub'ect to the general provisions regardin the classification of parts (see the General Exp anatory Note to Sechon XVI),parts of t e en ines and motors of this heading are also classified here (e.g., gas turbine rotors, combustion &ambers and vents forjet engines, parts of turbo-jet engines (stator rings, with or without blades, rotor discs or wheels, w t h or without fins, blades and &), fuel feed regulators, fuel nozzles). 8,