Reaction turbines

An impulse-type turbine experiences its entire enthalphy drop in the nozzle, thus naving a very high velocity entering the rotor. The velocity entering the rotor is about twice the velocity of the wheel. The reaction type turbine divides the enthalphy drop in the nozzle and in the rotor. Thus, for example, a 50 percent reaction turbine has a velocity leaving the nozzle equal to the wheel speed and produces about V2 the work of a similar size impulse turbine at about 2-3 percentage points higher efficiency than the impulse turbine (0 percent reaction turbine). The effect on the efficiency and ratio of the wheel speed to inlet velocity is shown in Fig. 29-27 for an impiilse turbine and 50 percent reaction turbine. 

FIG. 29-29 Velocity and pressure distribution in a three-stage reaction turbine. 

It appheslower axial thrust to the shaft than a single-stage axial reaction turbine. 

The reaction turbine, shown schematically in Figure 2-2, is generally more efficient. In its primary (stationary) nozzles only half the pressure energy of the gas stream is converted to velocity. The rotor with a blade speed matching the full-jetted stream velocity receives this jetted gas stream. In the rotor blades the other half of the pressure energy is used to jet the gas backward out of the rotor and, hence, to exhaust. Because half the pressure drop is taken across the rotor, a seat must be created around the periphery of the rotor to contain this pressure. Also, the pressure difference across the rotor acts on the full rotor area and creates a large thrust load on the shaft. 

Figure 2-3. Radial reaction turbine. (Source Kuehnie, Kipp, and Kausch.)...

The two types of turbines—axial-flow and radial-inflow turbines—can be divided further into impulse or reaction type units. Impulse turbines take their entire enthalpy drop through the nozzles, while the reaction turbine takes a partial drop through both the nozzles and the impeller blades. 

The two conditions that vary the most in a turbine are the inlet pressure and temperature. Two diagrams are needed to show their characteristics. Figure 3-12 is a performance map that shows the effect of turbine inlet temperature and pressure, while power is dependent on the efficiency of the unit, the flow rate, and the available energy (turbine inlet temperature). The effect of efficiency with speed is shown in Figure 3-13. Figure 3-13 also shows the difference between an impulse and a 50% reaction turbine. An impulse turbine is a zero-reaction turbine. 

In most designs, the reaetion of the turbine varies from hub to shroud. The impulse turbine is a reaetion turbine with a reaetion of zero (R = 0). The utilization factor for a fixed nozzle angle will increase as the reaction approaches 100%. For = 1, the utilization factor does not reach unity but reaches some maximum finite value. The 100% reaction turbine is not practical because of the high rotor speed necessary for a good utilization factor. For reaction less than zero, the rotor has a diffusing action. Diffusing action in the rotor is undesirable, since it leads to flow losses. 

The 50% reaction turbine has been used widely and has special significance. The velocity diagram for a 50% reaction is symmetrical and, for the maximum utilization factor, the exit velocity (V4) must be axial. Figure 9-11 shows a velocity diagram of a 50% reaction turbine and the effect on the utilization factor. From the diagram IV = V4, the angles of both the stationary and rotating blades are identical. Therefore, for maximum utilization. 

The 50% reaction turbine has the highest efficiency of all the various types of turbines. Equation (9-15) shows the effect on efficiency is relatively small for a wide range of blade speed ratios (0.6-1.3). 

Figure 14-17B. Wheel arrangements and performance of (a) impulse turbines and (b) reaction turbines. (Used by permission Breseler, S. A. Chemical Engineering, p. 124, May 23, 1966. McGraw-Hill, Inc. All rights reserved.)...

Gas pressure drops in both moving and Stationary bladine of reaction turbine... 

Figure 15.38 The radial-flow double-motion reaction turbine (Ljungstrom)...

Reaction turbine design also makes use of steam jets (which are produced by steam flowing across static vanes), although the turbine has rotor discs that incorporate movable blades rather than buckets. The design utilizes the reactive force produced by steam accelerating through a nozzle (created by the combination of a stationary vane and a moving blade) to rotate the shaft. 

Another advantage of the radial reaction turbine is that it can be designed to accept condensation in any amount without efficiency deterioration or erosion." This is possible because there are two forces acting on suspended fog particles, the deceleration force and the centrifugal force, and these two forces can be balanced against each other to prevent the droplets from impinging on specially shaped blades. The process is explained as follows ... 

Hydraulic turbines are used for recovering energy from high-pressure liquid streams. A common process application is an absorber-stripper combination. In this apphcation, a gas is absorbed in a solvent at a high pressure, where absorption is favored. Then, the solvent is stripped of the absorbed components at a low pressure, where stripping is favored, to recover the solvent. Thus, the energy of the high-pressure solvent stream from an absorber can be partially recovered by a hydraulic turbine. There are three types of hydraulic turbines, the Pelton-wheel turbine, the Francis turbine, and the propeller reaction turbine, an axial type turbine. The propeller reaction turbine is used in hydroelectric applications and will not be considered further. The Pelton-wheel and Francis tur-... 

The best values of are from 1 to 4 for tangential and from 30 to 60 for reaction turbines. Allowable values for the former are up to 8 (per nozzle) and for the latter from 10 to 100, but extreme values lead to low efficiencies. If the available head and required power and speed lead to abnormal values of some adjustment is possible by varying the number of units. Thus if 2,550 hp. were required at 750 r.p.m. with 100 ft. head, n, would be 120, a value not permissible. By using two wheels (or runners) of 1,275 hp. each, an allowable value results. In some cases the speed could be changed. The equation for specific speed leads to the following forms ... 

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