Turbo-blowers and Turbo-compressors

Transcription

1 Ing. Pavel Dostál, Ph.D., Ostrava University 1 Turbo-blowers and Turbo-compressors Turbo-blowers are rotational radial blade appliances without external intercoolers instrumental to the transport and compression of air with an operating overpressure above 10 4 Pa. Turbo-compressors are rotational radial blade appliances with external intercoolers or axial appliances instrumental to the transport and compression of air with operating overpressure above 10 4 Pa. It is evident from the definitions above that radial appliances without external intercoolers are called turbo-blowers and radial appliances with external inter-coolers and all axial appliances are called turbo-compressors. The turbo-blowers can be singlegrade (one-wheel) or multi-grade (multi-wheel). The turbo-compressors are always multi-grade (Fig. 2.8). a) b) c) d) Fig. 2.8: Scheme of turbo-blowers and turbo-compressors; a) a single-grade turbo-blower; b) a three-grade turbo-blower; c) a four-grade radial turbo-compressor with an inter-cooler between the second and third grade and aftercooler; d) axial turbo-compressor (5). These pneumatic-dynamic appliances operate with a two-fold conversion of energy. By means of rotor blades the mechanic energy of propulsion is converted into kinetic energy, part of which is converted in the stator into pressure energy. Reaching higher head pressures requires gradual compression. An ideal turbo-compressor would compress the air isoentropically, the real compression is polytropic (Fig. 2.9).

2 Ing. Pavel Dostál, Ph.D., Ostrava University 2 Fig. 2.9: T s diagram of the compression process; a) in a three-grade turboblower; b) in a four-grade turbo-compressor with an inter-cooler between the second and third grade and after-cooler (the solid line indicates ideal compression, the dotted line is the actual compression); p 1 suction pressure; p 2 head pressure, p m1 t to p m3 pressures behind particular grades (5). A comparison of working parameters, for which it is economical to use radial and axial turbo-compressors and piston compressors is in Fig Fig. 2.10: Area of economical use of piston compressors (A), radial turbocompressors (B) and axial turbo-compressors (C); ε - compression ratio; p 1 suction pressure; p 2 head pressure; Q v volumetric flow (5). 1 Cooling of turbo-blowers and turbo-compressors In contrast to the compression of liquids, a marked increase of temperature occurs during compression of air.

3 Ing. Pavel Dostál, Ph.D., Ostrava University 3 In these appliances we can distinguish between inner cooling, which is accomplished so that there is a system of channels in the stator case through which cooling water flows (mantle cooling) and outer cooling, when inter-grade coolers (inter-coolers) are connected outside the appliance (Fig. 2.11). Another possibility is cooling by injection of a suitable liquid into the compressed air. Quick evaporation of the injected liquid occurs. The required vaporization heat is taken from heated air, through which its temperature is decreased. (5). Fig. 2.11: Cross-section of a radial turbo-compressor with outer inter-cooler The use of inter-coolers will save some energy (Fig. 2.12), which enables achieving higher compression efficiency. The entire thermal mode of the appliance and conditions for lubrication of the machine improve and the output air temperature decreases. After-coolers are sometimes used to decrease the output air temperature. Fig. 2.12: Theoretical process of compression in a four-grade turbocompressor with an inter-cooler between the 2 nd and 3 rd grade compared to an uncooled appliance (the hatching indicates energy saved); T 5 output air temperature in the

4 Ing. Pavel Dostál, Ph.D., Ostrava University 4 appliance without an inter-cooler; T 4 in an appliance with an intercooler (5). 2 Propulsion of turbo-blowers and turbo-compressors The propulsion of turbo-blowers and turbo-compressors has to ensure high revolutions. That is why steam turbines, gas turbines or electromotors with acceleration gear are the most frequently used in large appliances (Fig. 213). Fig. 2.13: Large industrial turbo-blower assembly; electromotor on the left; gear and control box in the middle; turbo-blower on the right (8). 3 Radial turbo-blowers and turbo-compressors The basic construction parts are a rotor with impellers and a stator with diffusers (does not have to be used in lower overpressures), reversible channels and screw case. The air entering the appliance axially is accelerated by the blades of the impeller, from which it leaves radially (see Fig. 2.8). Through the blades of the diffuser in which some of the kinetic energy is converted into pressure, the air is returned through reverse channels to a further rotor grade (which, naturally does not happen in single-grade appliances) (Fig. 2.14). The function of a screw case is explained in Chapter

5 Ing. Pavel Dostál, Ph.D., Ostrava University 5 Fig. 2.14: Scheme of a working grade of a radial turbo-compressor or a turboblower; 1) reverse channel; 2) diffuser; 3) impeller (6). The impellers are usually riveted or the blades are welded to a bearing and covering disc or the impeller is made of a single piece (cast, forged or milled) (Fig. 2.15). The shaft has to satisfy strength conditions at combined stress of torsion + flexion. The case of multi-grade appliances is usually split horizontally. Both parts are usually connected with pre-stressed screws. The partitions are separated from one another by single pressure grades and contain blades of the diffuser, reverse channels and plugs. a) b) c) Fig. 2.15: Impellers of radial turbo-blowers and turbo-compressors; a) riveted wheel b) welded wheel; c) one-piece wheel without covering disc (5).

6 Ing. Pavel Dostál, Ph.D., Ostrava University 6 a) b) Fig. 2.16: a) single-grade turbo-blower powered by an electromotor (9); b) scheme of an eleven-grade radial turbo-compressor (4). 4 Axial turbo-compressors The rotor consists in a drum on which lines of screw-twisted blades are mounted. The shaft has to satisfy strength conditions at combined stress of torsion + flexion and its working revolutions must differ from the critical by at least 20%. The stator consists in a case with fixed blades (diffusers) so that the blades of the rotor are in the interspaces between the lines of the stator blades. A fixed ring of guidance blades is usually mounted in front of the first line of circulating blades. The case of the axial turbo-compressors is split horizontally, connected with pre-stressed screws. Because a relatively small pressure difference is obtained in one working grade, they have more grades; for higher pressures they are multi-grade (Fig. 2.17). Fig. 2.17: Axial turbo-compressor without the upper part of the case (10) SKOPAL, V., ADÁMEK, J., HOFÍREK, M Stavba a provoz strojů IV. 2. publication. Prague : SNTL, 1987, pp