150 SECTIONAL ADDRESSES 
pits, for it works in conjunction with the supply company of the area, 
delivering its excess power into the mains, and relying on the mains for 
unusual overloads or possible breakdowns. 
CONDENSING WATER. 
An argument that has frequently been brought against the pit-head 
station is that there is little likelihood of a sufficiency of cooling water 
for the condensation of exhaust steam, in order to produce the high 
vacuum that the turbine can make use of. The cooling tower provides 
water that is still a little warm, and the condenser pressure is 1°5 lb. 
instead of 0-5 lb. But the gain in efficiency due to the high vacuum 
is often exaggerated by failure to apply comparable conditions and to 
take recent improvements into account. For a given turbine taking 
a given amount of steam and suitably modified in the final stage, a 
reduction in back pressure adds a definite amount of power. Also a 
rise in the initial pressure, again with suitable design, gives a definite 
increase of power for the same steam. Hence the effect of the improved 
vacuum is large if the initial pressure is low, but it becomes less and 
less as the boiler pressure is raised, and with 350 lb. initial pressure the 
actual loss of power due to a back pressure of 1-5 lb. instead of 0-5 lb. 
is theoretically only 5 per cent., and in practice the full expansion of the 
whole of the steam to 0-5 Ib. is not economical, so that the actual saving 
in fuel is barely 4 per cent. This is certainly not sufficient to condemn 
a plan which can offer other advantages. The case of Hams Hall station, 
in Birmingham, is of interest on this point. It has 30,000 kw. generating 
sets, working at a load factor of 0-32, and consumes the equivalent of 
1-35 lb. of good steam coal per unit delivered, attaining an overall thermal 
efficiency of 23-34 per cent. on the units generated. Though it works 
entirely on cooling towers, and the turbines are not of the largest size, 
its economy can hardly be improved upon. It may be claimed that 
the absence of cooling water can be definitely disregarded as a disability 
in the use of pit-head stations. 
INDUSTRIAL STEAM. 
Another source of cheap power may be found in the proper utilisation 
of industrial steam. Many industries need low-pressure steam in their 
processes, and use boilers working at a pressure of 50 lb. or less. There 
is no difficulty in producing steam at 350 lb., superheating it, and passing 
it through steam turbines, to exhaust at the required low pressure, and 
the steam so delivered is in all respects as good as that produced directly 
from boilers, as it does not come into contact with lubricating oil. The 
thermal efficiency of the turbine is 100 per cent., less the small radiation 
losses and bearing friction, for the rejected heat of the exhaust steam is 
used for the other purposes, and all steam friction loss is retained as heat 
in the steam. As compared with the coal used in the boilers to produce 
the low-pressure steam, taking into account the cooling and running 
losses of the turbine set, the extra boiler losses due to the higher 
temperature, and the higher pressure of the feed pumps, the additional 
coal works out at 0-4 lb. per unit delivered. The additional capital 
charges are also low, for there is no condensing plant, the turbines are 
