To determine the scale on which a central station plant should be designed is frequently a difficult matter. The rate of growth of the expected demand for the power is an important factor, but it has been clearly established that the reduction of working expenses resulting from the increase of size of an undertaking proceeds in a diminishing ratio. Increase in output is in fact sometimes accompanied by more than a proportionate increase of expenses. During recent years there have been causes at work which have raised considerably the price of labour, fuel, other items of expense, and the law of the "diminishing ratio" has been masked
On the diagram (fig. 3) of the costs of the London undertaking and the amount of power supplied, have been plotted points marking the total expenses of each year in relation to the output of power. These points for the years 1884-1899, and for output of from 50 to 700 million gallons followed approximately a straight line. Since 1899, however, though the output has increased from 708 millions to 1040 million gallons, the costs per unit of output have been always considerably above the preceding periods. The details of the London supply given in table 1 partly explain this by the relatively high price of fuel, but an equally important factor has been the rise in the local rates, which in the period 1899-11909 have risen from 2d. up to 3d. per 1000 gallons. If the cost of fuel, rates and wages had remained constant the plotting of expenses in relation to output would have been approximately along the extension of the line AB. This line cuts the vertical axis at A above the origin O, and the line OA indicates the minimum amount of the expenses, and by implication the initial size of the first central station erected in London. The curve in this diagram gives the cost per 1000 gallons.
Whether it is more economical to have several smaller stations in any particular system of power transmission, or a single centre of supply, is mainly governed by the cost of the mains and the facilities for laying them in the area served. No general rule can, however, be formulated, for it is a question of balance of advantages, and the
Fig. 3.
solution must be obtained by consideration of the special circumstances of each case. It has been found desirable as the demand for the power and the area within which it is supplied has enlarged, not only to increase the number of central stations but also their capacity. The first pumping station erected was installed with 4 pumping engines of 200 h.p. each. The pumping capacity of this station has been increased to 7 units. The station at Rotherhithe completed in 1904 has 8 units together 160O h.p., and the plant at the new station at Grosvenor Road has 8 units equalling 2400 h.p. The pumping stations are situated about 3 m. apart and concurrently; with the increase in their size it has been found desirable to introduce a system of feeder mains (see below).
There are in all five central stations at work in connexion with the public supply of hydraulic power in London, having an aggregate of 7000 i.h.p. All the stations and mains are connected together and worked as one system. There are 14 accumulators with a total capacity of 4000 gallons, most of them having rams 20 in. diameter by 23 ft. stroke. The pumping engines are able together to deliver 11,000 gallons per minute. Details of the London supply are given in fig. 3 and in table 1.
Table I.
Year. Gallons Pumped. Annual Load-Factors. Maximum 24 hours Load-factors. Cost of Fuel per 1000 gallons. Price of Fuel per ton in Bunkers. Number of Machines at work. Miles of Mains.
d. s. d.
1889 163,883,000 0.328 0.524 3.11 10 9 1022 38
1894 400,516,000 0.338 0.553 1.96 10 0 2204 73
1898 620,662,000 0.340 0.483 1.98 11 3 3515 109
1903 888,925,000 0.361 0.491 2.7 14 3¾ 5337 146
1909 1,027,147,000 0.354 0.495 2.78 15 1 6504 168
The load-factors are calculated on the actual recorded maximum output, and not on the estimated capacity of the plant running or installed. The daily periods of maximum output are shown in fig. 2. The table shows that the load-factors have not been much affected either by the increase of the area of supply or by the increased consumption of power. The coal used has been principally Durham small. The capital cost of the London undertaking has been about £950,000. In the central station at Wapping, erected in 1891, there are six sets of triple-expansion, surface-condensing vertical pumping engines of 200 i.h.p. each; six boilers with a working pressure of 150 lb per square inch, and two accumulators with rams 20 in. diameter by 23 ft. stroke loaded up to 800 lb per square inch. The engines run at a maximum piston speed of 250 ft. per minute, and the pumps are single-acting, driven directly from the piston rods. The supply given from this station in 1909 was approximately 6,800,000 gallons per week, and the cost for fuel, wages, superintendence, lighting, repairs and sundry station expenses 4.28d. per 1000 gallons, the value of the coal used being 14s. I1-3d. per ton in bunkers. The capital cost of the station, including the land, was £70,000. The load-factor at this station for 1909 was .49, and the supply was maintained for 168 hours per week. The conditions are exceptionally favourable, and the figures represent the best result that has hitherto been obtained in hydraulic power central station work, having regard to the high price of fuel.
The installation in Hull differs little from the numerous private plants at work on the docks and railways of the United Kingdom. The value of the experiment was chiefly commercial, and the large public hydraulic power works established since are to be directly attributed to the Hull undertaking. In Birmingham gas engines are employed to drive the pumps. In Liverpool there are two central stations. The working pressure is 850 lb per square inch. There are 27 m. of mains, and about 1100 machines at work. In Manchester and Glasgow the pressure adopted is 1100 lb per square inch. In Manchester this pressure was selected principally in view of the large number of hydraulic packing presses used in the city, and the result has been altogether satisfactory. The works were established by the corporation in 1894, the central station being designed for 1200 i.h.p. Another station has since been built of equal capacity, and nearly 5 million gallons per week are being supplied to work about 2100 machines. Twenty-three miles of mains are laid.
In Antwerp a regular system of high-pressure hydraulic power transmission was established in 1894 specially to provide electric light for the city. The scheme was due to von Ryssleburgh, an electrical engineer of Ghent, who came to the conclusion that the most economical way of installing the electric light was to have a central hydraulic station, and from it transmit the power through pipes to various sub-stations in the town, where it could be converted by means of turbines and dynamos into electric energy. The coal cost of the electricity supplied—0.88d. per kw. hour-compares favourably with most central electric supply stations, although the efficiency of the turbines and dynamos used for the conversion does not exceed 40%. Von Ryssleburgh argued that hydraulic pumping engines would be more economical than steam-engines and dynamos, and that the loss in transmission from the central station to the consumer would be less with hydraulic converters than if the current were distributed directly. The loss in conversion, however, proved to be twice as great as had been anticipated, owing largely to defective apparatus and to under-estimation of the expense 0% maintaining the converting stations; and the net result was commercially unsatisfactory. At Buenos Aires hydraulic mains are laid in the streets solely for drainage purposes. Each of the sumps, which are provided at intervals, contains two hydraulic pumps which automatically pump the sewage from a small section of the town into an outfall sewer at a higher level. The districts where this system is at work lie below the general drainage level of Buenos Aires. The average efficiency (pump h.p. to i.h.p.) is 41%, which is high, having regard to the low heads against which the pumps work. In this application all the conditions are favourable to hydraulic power transmission. The work is intermittent, there is direct action of the hydraulic pressure in the machines, and the load at each stroke of the pumps is constant. The same system has been adopted for the drainage of Woking and district, and a somewhat similar installation is in use at Margate.
Hydraulic power is supplied from the hydraulic mains on a sliding scale according to the quantity consumed. The minimum charge in London except for very large quantities is 1s. 6d. per 1000 gallons. In 1000 gallons at 750 lb per square inch there is an energy of 10,000x173033,000x60 = 8.74 h.p. hours, thus 1s. 6d. per 1000 gallons 2d. per h.p. hour nearly. This amount is made up approximately of 9d. per 1000 gallons for the cost of generation, distribution and general expenses including rates and 9d. for capital charges. The average rate charged to consumers in 1908 was about 2s. 4d. per 1000 gallons. Even under the most favourable circumstances it does not appear probable that hydraulic power at 750 lb per square inch can be supplied from central stations in towns on a commercial basis over any considerable areas at less than 1s. per 1000 gallons. Allowing
