G.—ENGINEERING 149 
grouped together, and it is becoming more the custom to bring the coal to 
a central point for cleaning, which will facilitate the use of the waste coal. 
If the figure of 10 per cent. is taken as a working hypothesis, then a 
station of 100,000 kw., working on a load factor of 0-4, will use per 
day some 700 tons of waste coal, and will require a total output from the 
cleaning plant of 9,000 tons per day over the working week. This is 
not an exceptional quantity, and any additional advantage in grouping 
will tend to increase the custom. 
The scheme will evidently provide an important amount of cheap fuel, 
and will permit of power stations of a size that ensures a low figure for 
cost of plant and running costs, so that the low price of the fuel is not 
offset by any increase in cost in other directions. It is true that the 
stations will not be placed in the towns, and to that extent distribution costs 
are increased ; but, on the other hand, land is cheaper, and it is being 
found that a station consuming many hundred tons of coal a day will 
compel the use of expensive remedies against sulphur and dust, so the 
advantages of an urban site will be sensibly diminished. Moreover, 
most of the large towns are not far from coal mines, and the cost of 
transmission will be very small. With pit-head stations of the 100,000 kw. 
size the economy is easily determined, for all working costs other than fuel 
will be practically the same as those of existing stations, if the latter 
were designed and built to-day. 
There will be doubtless a good many stations of smaller size, in which 
there will be some increase in the capital cost per kilowatt and in wages. 
But down to a size of 30,000 or even 20,000 kw. the influence will 
be slight. Coupled by the grid or other lines to neighbouring stations, 
they will not resemble the existing stations of this size, but will contain 
perhaps two generating sets of 10,000 kw. and boilers to correspond, 
so that the present figures of increase of cost per kw. with decreasing 
size will not apply. It will be economical to put all necessary spare plant 
into the large stations, and the equipment of these smaller stations can 
be simplified. Their cost of production will therefore be little different 
from that of the larger stations, and will be substantially lower than 
the best of present-day large stations. 
An actual example will show what can be done in a pit-head station 
equipped with efficient modern plant and run with economy on very 
low-grade fuel. It is only 4,000 kw. in two sets, working at a load factor 
of 0:7. The coal used contains 40 per cent. of ash and moisture, a 
very remnant of fuel, and is given in the colliery accounts a rather 
exaggerated value of 3s. per ton, corresponding in calorific value to a good 
steam coal at 4s. 6d. The consumption corresponds to 1-5 lb. of steam 
coal per unit delivered, notwithstanding the small size of the sets and 
the absence of a supply of water for condensing purposes, and the whole 
cost of fuel, wages, maintenance, and supervision, with interest and 
depreciation at 9-5 per cent., is not more than 0-137 pence per unit 
delivered. It will be shown below that the usual cost for the largest 
stations to-day, on the same charge for interest and depreciation but with 
normal coal, is at this load factor 0-185 pence, so that even small stations, 
suitably designed, can be usefully brought into the scheme. This 
particular station corresponds closely to what is proposed for isolated 
