536 THE COLLIERY GUARDIAN. March 15, 1918. lowered the pressure by a certain amount and was known as a presssure stage. Combined Impulse and Reaction Turbine. Owing to the drop in pressure through the moving blades of the reaction turbine, steam had to be admitted around the entire circumference, which necessitated very short blades in the high pressure stages, and, as already stated, the clearance over the ends of the blades had to be very small, the percentage of leakage across the high pressure stages being also naturally higher' than for the low pressure end. This, together with the fact that the heat drop per stage was less, whilst the pressure drop was greater, resulted in the high-pressure end being the least efficient portion. The impulse turbine, however, was found to be more efficient at the high-pressure end, than in the later stages where water was present. These considerations resulted in a type of machine being developed which utilised the advantages of both and eliminated the least efficient portions. Steam was & x £ 5 5 rBa Conde nse □ a 8 * Fig. 3. admitted through nozzles, and expanded to about atmospheric pressure before passing through a multi- stage impulse wheel, the rest of the expansion taking place in impulse-reaction blading, as indicated in fig. 4, which shows a portion of blading of both types. The difference will be readily recognised. Governing. It may be just mentioned that the governing of steam to the turbine blading or nozzles can be effected in a number of ways. In some impulse reaction machines, the steam is admitted in puffs at regular intervals, the length of each puff or gust of steam being controlled by a mechanical governor. On the other hand, with some impulse turbines, steam is regulated by the governor operating valves leading to the nozzles, so that the number of nozzles supplying steam depended on the load. Having described the principles on which most colliery turbo plant operate, attention will now be paid to conditions which affect the operation of turbines generally, Superheated Steam. Owing to the fine clearances necessary in all turbine machinery, the steam must be kept as free from water as possible. With this object, it is now the practice to raise the steam temperature some 50 degs. to 100 degs. Fahr, above that of the steam of the boilers. It is found that for every 10 degs. Fahr, of superheat a gain in economy of about 1 per cent, is obtained, up to about 150 degs. to 180 degs. Fahr. In some collieries the benefits of superheating may not all be obtained at the turbine stop valve, as very often there is a considerable amount of steam piping between the boilers and the turbine. The net result, however, is that condensation throughout the whole system is minimised, and if sufficient superheat is obtainable to allow some loss in transmission and still retain the desired amount at the turbine, so much the better. Vacuum. The fact that places the steam turbine in a better position than the reciprocating engine as regards steam consumption is its ability to work on a higher vacuum. The reciprocating engine is somewhat at a disadvan- tage here, owing to the wide temperature differences in the low-pressure cylinder when using a high vacuum, and also because the cylinder would have to be abnormally large in order to deal with the increased volume of steam. The general practice is to use vacuums of 26 in. to 27 in., whereas most turbine practice is based on one of 28 in. to 28J in. Ther e is no limit to the degree of vacuum which may be applied to a turbine, except the size of air pumps and water circulating pumps. This is, however, a point above which the extra vacuum obtained is offset by the amount of power necessary to produce it, and these considerations have led to the present practice. Heat Drop. The steam engine in any form is naturally a heat engine, and therefore the type which utilises the maximum heat content of the steam will be the most efficient. Since a turbine can work on a higher vacuum than the steam engine, it is capable of utilising more of the heat energy in the steam, and is therefore more efficient. A reference to the curve given in fig. 5 shows that the heat drop on which the available energy depends is greatest at the lower pressures—i.e., from atmospheric pressure downwards. Without going into the matter of exhaust steam utilisation, it may be mentioned that the curve in fig. 5 also explains why a turbine is able to get so much energy from exhaust steam. Condenser. The type of condenser which is almost standard practice to use is the surface condenser, especially at collieries where feed water has to be treated. As the condensed steam does not come in contact with the condensing water it may be returned direct to the boilers, being free from oil or impurities. Furthermore, with the surface condenser there is no possibility of the circulating water getting back by any means to the turbine. As far as practicable the circulating water for surface condensers should be pure, since otherwise the tubes corrode, with the result that, although a good head of circulating water is maintained, it is prevented from carrying away the heat from the condenser, owing to the low heat-conducting properties of the incrustation. R R R Tie action Jlladnijr Si k Fig. 4. Condensers should be cleaned periodically, as other- wise the scale often becomes so hard as to require chemical treatment for its removal. During week-ends or periods of considerable length when the load is small, the circulating water can usually be reduced con- siderably, there being no necessity when the turbine is only partially loaded to keep a quantity of water circu- lating sufficient to maintain the required vacuum at full load. If this could be effected by some automatic Absolute so 3o 6 //o Z3O /Jo '40 'So '6o />» z