In the case of factories the exhaust steam from the engines used for driving the working machinery is made use of and forms the most economical method of heating possible.
The system is economical in fuel, but needs skilled attendance to keep the appliances and fittings in order.
In the choice of a boiler of this description it should be remembered that rapid heating, economical combustion of fuel, and facilities for cleaning, are requisites, the absence of any of which considerably lowers the efficiency of the apparatus.
About 1879 he began experiments in the economical utilization of fuel, and his efforts led him to the system of making producer-gas, known by his name (see GAS: II.
The returns from the copper fields in the state are at present a little over half a million sterling per annum, and would be still greater if it were not for the lack of suitable fuel for smelting purposes, which renders the economical treatment of the ore difficult; the development of the mines is also retarded by the want of easy and cheaper communication with the coast.