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It will be noted that the spacing pieces at the roots of each row of blades project above the surfaces of the rotor and cylinder, as the case may be, and form a continuous barrier. There is a thin brass shrouding strip rivetted to the free ends of the blades, which pro- jects over the side of the blades and is almost in contact with the barrier of the next row. The space between the projecting edge of the shrouding strip and the adjacent barrier forms the actual working clearance which can be adjusted to any desired amount while the machine is running. The radial clearances between the shrouding and the surfaces of the rotor and casing are never less than tV in., so that contact is out of the question. Considerably greater economy is said to be obtainable with this type of blading.
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The modern type of blading is illustrated in fig. 3. The blades, made of drawn brass strip, are assembled in units, complete with spacing pieces and shrouding. The blades and spacing pieces are brazed solidly together at the roots, and circumferential serrations are then cut in the solid part as shown.
The following table gives the results actually obtained with certain large turbo alternators of the Parsons Company :
Installation
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Chicago
Carville
Lots Road
Dunstan
Date of test
Nov. 1918
Nov. 1919
June 1916
Mar. 1921
K.W. Economical
rating
20,000
10,000
15,000
12,000
K.W. Output .
21,227
9-991
15-047
11,967
Speed, r.p.m.
75"
2,442
1,000
2,400
Gauge pressure at
stop valve, Ib.
202-4
251
176-5
175-7
Temperature at stop
valve, deg. F.
548-7
687
524-3
607-1
Superheat, deg. F. .
159-8
281
146-1
229-2
Vacuum, ins. Hg.
1-13
0-96
0-97
1-02
Heat drop, B.Th.U.
per Ib. .
408
459-4
402-8
421-2
Steam consump. per
K.W.H. .
10-84
10-04
11-82
10-80
Efficiency ratio .
77-4
73-98
71-80
75-20
Thermodynamic effi-
ciency
25-31
25-7
23-35
24-70
Length of time tur-
bine had been work-
ing . . . .
5 years.
2 1 years.
3 years.
4 years.
In all cases the instruments used were calibrated before and aftci the tests, and the steam consumption was obtained by weighing th( condensate, so that a very high degree of accuracy was obtained The tests go to show, incidentally, that reaction machines fall off in efficiency after several years' operation under commercia conditions of service.
FIG. 3
The Impulse Steam Turbine. The Rateau steam turbine is a, typical modern multistage impulse turbine. Fig. 4 shows a longitudinal section through a machine of this type constructed in 1919; by the Metropolitan Vickers Electrical Co. for the Dalmarnock; power station, the machine in question having a maximum continuous rating of 18,750 K.W. at a speed of 1,500 revs, per minute. The shaft carries altogether 15 wheels keyed upon it, each wheel running in a separate compartment. The diaphragms dividing in compartments from each other are fitted with nozzles, in which the steam undergoes successive partial expansions in its progress through the turbine, and from which it emerges with a velocity due to the drop in pressure which it has undergone. This velocity is abstracted; by the action of the blading which the steam enters after issuing from each set of nozzles, the steam being brought more or less to rest and the energy due to its partial expansion appearing as useful mechanical work on the shaft.
In all large machines of this type, especially when they are work ing with a high vacuum, the volume of the steam %t the low-pressure end becomes so great that the length of the turbine blades at this part tends to become excessive. In the machine in question a part of the steam, after having passed through 10 wheels, being then at a pressure of about 4 Ib. abs. is passed out of the casing and used to heat the boiler feed water, the feed heater for this purpose being shown in section in the illustration. This practice diminishes,
