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FIGURE 1381 Pressure and velocity changes in an impulse turbine having one pressure stage and two velocity stages
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FIGURE 1382 Pressure and velocity changes in a reaction turbine
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70 Multicylinder or Compound Turbines The increase in capacity of steam-
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turbine units has led to the development of compound machines having two or three cylinders After partial expansion of the steam in the high-pressure cylinder, the steam passes to the next cylinder, where the process is continued
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71 Back-Pressure, Low-Pressure, and Mixed-Pressure Turbines Turbines
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operated noncondensing, in connection with building heating or industrial processes, are frequently called back-pressure turbines Since the heat in the exhaust can be usefully applied, the power from the machine is obtained at very low cost Low-pressure turbines are supplied only with exhaust steam from other apparatus and should exhaust into high vacuum Mixed-pressure turbines are designed to operate on low-pressure steam, supplemented by high-pressure steam when the lowpressure supply becomes insuf cient to carry the load
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72 Extraction or Bleeder Turbines Extraction or bleeder turbines are supplied
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with high-pressure steam and operated condensing, but they have the distinctive feature of permitting the withdrawal of steam from the machine at one or more intermediate points between supply and exhaust, for use in building heating, feedwater heating, or manufacturing processes
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73 Turbine Performance The best measure of comparative performance of
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steam turbines is the engine ef ciency; brake engine ef ciency is the ratio of the brake thermal ef ciency of the turbine-generator to the thermal ef ciency of the corresponding ideal engine (Rankine engine if there is no reheating or regenerative feedwater heating) Typical values of the brake engine ef ciency of steam turbines are given in Table 134 These values are not exact, and manufacturers guarantees should be obtained for new units
CONDENSING EQUIPMENT
74 General The object in applying condensing equipment to steam prime mov-
ers is to improve their economy and increase their capacity by lowering the back pressure against which they exhaust For a given condition of steam supply, a
CHAPTER THIRTEEN
TABLE 134 Brake Engine Ef ciencies of Steam Turbines
Brake hp 100 500 1,000 5,000 10,000 20,000 100,000
kW 746 3730 7460 37300 74600 14,920 74,600
Brake engine ef ciency, % 50 58 63 72 76 80 83
reduction in back pressure increases the available energy per pound (kg) of steam When the back pressure is reduced to 1 in Hg (254 cm Hg) [1 29-in (737-cm) vacuum referred to a 30-in (762 cm) barometer], the temperature of the exhaust steam is only about 80 F (267 C) Condensers are of two general types: (1) surface and (2) direct contact Both types use cooling water to change the exhaust vapor to a liquid of much smaller volume, but the surface condenser keeps the two uids separate while the directcontact condenser brings them together in a mixture The selection of type of condenser depends upon: (1) water quality and abundance, (2) xed charges on condenser and auxiliaries, (3) power cost for condenser pumps, (4) maintenance expense, and (5) space requirements
75 Surface Condensers This type of condenser consists of a shell surrounding
a compact nest of tubes Cooling water is circulated through the tubes, and exhaust steam lls the shell surrounding them Tube sizes are usually 3 4 in (19 cm), 7 8 in (22 cm), or 1 in (25 cm), and 0049 in (012 cm) (No 18 BWG) thick; 5 8-in (159-cm) tubes are sometimes used in small condensers The overall coef cient of heat transfer in surface condensers depends primarily upon the water velocity, cleanliness of the tubes, and the amount of noncondensible gases in the steam Economic water velocities are usually between 6 and 8 ft / s (18 and 24 m / s) The heat balance for a surface condenser is given as follows: Ws(hs where Ws hs hc Ww t2 t1 U A
Ww(t2
(1354)
weight of steam condensed, in lb / h (kg / h) enthalpy of exhaust steam entering the condenser, in Btu / lb (kJ / kg) enthalpy of condensate leaving condenser, in Btu / lb (kJ / kg) weight of cooling water owing through condenser, in lb / h (kg / h) temperature of exit cooling water, in F ( C) temperature of entering cooling water, in F ( C) overall coef cient of heat transfer, in Btu / h ft2 F (W / m2 C) (for rough approximations, a value of 350 may be used for USCS) area of tube surface, in ft2 (m2) mean temperature difference between steam and water, F ( C)
With single-pass condensers, the condensing surface provided for turbine installations is usually between 06 and 10 ft2 / kW (006 and 009 m2 / kW), while for two-pass condensers between 07 and 14 ft2 / kW (007 and 013 m2 / kW) is common; the larger gure in each case is for small turbines The water circulated is commonly between 10 and 20 gal / min ft2 (07 and 14 L / s m2)
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