Water Wheels

This type is still to be seen working in this country (old water mills), having once been practically the only type. It is as efficient as any in operation, provided it is properly designed and made, it should be able to give 80 percent, efficiency; is best used on heads of from 10 to 25 feet, with up to 1,500 cubic feet of water per minute, thus giving at say, 12 feet head, and 1,000 cubic feet per minute, at 80 per cent, efficiency, a brake horse power of about 24.

Now in this country the above amount of water flowing under ideal conditions is not common. To realise this it should be pointed out that 1,000 cubic feet of water per minute is about the amount that would be represented by water flowing over a rectangular notch 12 inches deep by 5 feet wide. These water wheels or " over-shot" wheels have, however, the great disadvantage of slow speed of rotation, so that a loss occurs in gearing up to the load, whatever it may be, also in this country they are displaced very much by turbines of one design or another, so that it would now be difficult to obtain an " over-shot" wheel as quickly as some form of turbine, moreover, the size overall and the weight of the former will be considerably above that of the turbine. The Francis type of turbine is more generally used here when suitable, than any other, and in this the turbine is frequently sunk into such a position that the water has to pass through it on its way downstream. The water in passing through the turbine between the blades causes them to revolve, this action giving rise to the name " Pressure or Reaction type " turbine. On heads of say 50 feet and over, a Pelton Wheel (which is a form of Impulse Turbine) would be generally used ; this type, working under a large head of water, receives it from a jet or nozzle on a series of small bucketlike blades, which are forced round as they come under the jet. This means that a pipe line must be constructed to bring the water down the whole length of travel corresponding to the head of water. At the higher end of the pipe provision must be made to stop any other thing than water going down the pipe, as an obstinate blockage would be awkward to get at and clear. The pipe line itself must be securely fixed or anchored to the ground, to guard against strains, etc.

In the majority of cases the power obtained from a small fall or stream turbine would have to be taken further to be used, and this would probably introduce a dynamo and motor, etc., with additional unavoidable losses, and the first cost is more than likely to be higher with the water plant than it would be if some sort of internal combustion engine was installed to do the work direct. The position quite alters when large heads and volumes of water are available, which is not often in this country.