=Cooling Towers.=--Where it is necessary to husband water for condensing ammonia or steam the use of cooling towers comes into play. These function by the cooling of water by evaporation, the evaporation abstracting the heat. There are various kinds--sprays over ponds, water dripping over spatter boards made in various ways, such as brush piled loosely; a checker work of wood slats; screen wire, etc. These usually depend upon the circulation of air in a natural way. There are various patented types, depending upon fans, to pass air through falling water. The efficiency is dependent upon the fineness of the water and the quantity of air passing through it, the relative humidity of the air controlling greatly the cooling effect. =Economizers.=--Where the utmost economy of fuel is striven for and on larger plants, these are installed. An economizer is a bank of cast iron flues placed in a position between the boilers and the smoke stack. Water is circulated through the tubes and the furnace gases pass about them. They are usually calculated upon to decrease the stack temperatures from 100 to 150 degrees--dependent upon its initial temperature and to heat the feed water one hundred degrees. This is a saving from a wasted source, since it is impossible to take up this heat in any boiler yet designed. =Superheaters.=--In an earlier chapter reference was made to superheated steam. The office of this apparatus is to heat the steam to a higher point than it rises to, under ordinary boiler conditions. For instance, steam as delivered from the nozzle of a boiler is spoken of as “saturated” steam, but, if before use this is subjected to a further heating before it is used, it is said to be “superheated.” Superheaters are of two types. They can be described as follows: A bank of coils located in the flame pass in the boilers, the steam within the coils and the flame on the outside; or a bank of coils in a separate furnace, the flame or heat generated in the furnace passing over the coils. Steam superheated beyond 450° F. total sensible temperature requires especial pipe fittings, type of valves, and engines for reasonable safe operation, and renders an economy that a small plant cannot afford, due to the expenditure, to take advantage of. =Steam Engines.=--Owing to the cheapness of fuel in the United States, users have been satisfied with equipment at a low initial cost and in consequence pay bills continually by using uneconomical prime movers, viz.: ordinary steam engines of the simple Corliss type, using from 26 lbs. to 35 lbs. of steam per horsepower hour, or compound condensing at 15 lbs. to 18 lbs. of steam per horsepower hour or high speed direct connected slide valve engine generator sets using from 35 to 50 lbs. steam per horsepower, depending upon load.