FEED-WATER HEATERS are employed to pre-heat the feed water supplied to steam boilers, in order to save fuel, increase the steaming capacity of the boiler, protect the boiler against temperature strains and to elim inate scale-forming and corrosive impurities. For each 11° F. that the temperature of the water is raised, about 1 per cent less fuel will be required or if the same amount of fuel be burned, the steaming capacity will he increased about 1 per cent. Pre-heating of the water prevents chilling of plates, tubes and other metal parts of the boiler and thus avoids injurious contraction strains about riveted joints, tube ends, etc. Certain scale-forming impurities in natural water supplies are precipitated to some extent by heating of the water and for that reason are known as "temporary hardness" in distinction from substances which are not affected by ordinary pre-heating, known as per raanent hardness. The precipitation of calcium carbonate by heat is a phenomenon familiar to most users of water from the ordinary kitchen range heater. Nearly all water supplies also have in solution oxygen from the air and other gases, which have a corrosive effect upon iron pipes and boilers. Inasmuch as the solubility of these gases is reduced to zero in water heated to atmospheric boiling point, they may be driven off by heating in the open type of heater de Scribed below. There are three classes of feed water heaters in general use: so-called closed or tubular heaters, in which the boiler feed water is passed through metal pipes surrounded by steam, usually steam exhausted from pumps, engines or other steam using appliances; open heaters, in which the water is sprayed over trays in contact with the steam in an open chamber or box; and fuel economizers, in which the water is pumped through cast iron or other pipes, over which are passed the flue gases on the way from the boilers to the chimney. The advantage of the closed heater is that inasmuch as the water does not come into actual contact with the steam there is no danger of the water becoming contaminated by the cylinder oil which is present in the steam exhausted by reciprocating engines and pumps. Its disad vantages are that the heat must be transmitted through a metal wall and that the water cannot therefore be raised to the full temperature of the steam. A' temperature of 160° to 180° F. is ordinarily all that is obtained with closed heaters. The exhaust steam in giving up its heat to the water is condensed and, unless an oil separator is used in the steam line, contains grease and oil and must be thrown away, al though except for the oil, it is pure distilled water. The' closed heater receives cold water at temperatures varying from 32° to 100° F., and delivers water at much higher temperatures. It is therefore apt .to be subjected to severe temperature strains, causing loosening of the joints and leakage.• The heating of the water also causes the deposit of scale-forming matter upon one side of the heating surface, while oil and grease may be deposited upon the other, both interfering with the transfer of heat and requiring frequent cleaning. In open heaters, the danger of contamination from cylinder oil has been averted by the use of oil separators attached to and forming a part of the heater; and because of the higher temperature which such heaters give, also the fact that the exhaust steam condensed in heating the water becomes a part of the boiler feed supply, thus saving the heating of an equal amount of raw cold Water, also due to the fact that the water is purified by the driving off of air and gases, the open heater has become practically standard in the United States. The open heater is or dinarily provided with attachments performing functions additional to that of heating the water. It is ordinarily used as a hot well or receiver for water discharged from steam traps, water condensed in heating coils, jackets, etc.. and for the condensate from the condenser, by. means of which a vacuum is maintained on the engine or turbine exhaust. The additional amount of water, ordinarily known as make-up yiater, which may be required for the boiler feed pumps, is admitted through a valve con trolled by a float in the water storage space of the heater, which thus takes the place of an automatic make-up water regulator. Where
more exhaust steam is available than can he used for heating the boiler feed water, the surplus is often applied to heating buildings or to industrial operations and for such use should be purified of oil, so that the condensate may be brought back to the heater and used for boiler feeding. In such cases the oil separator attached to the heater is made large enough to purify all of the exhaust steam, the surplus not required in the heater passing through a steam-stack attached to the separator, directly to the heating system or other apparatus. Where the raw water supply used for boiler feeding contains troublesome amounts of scale-forming and corrosive substances, such as sulphates of lime and magnesia, which are not precipitated by heat, provision is made for feeding a suitable amount of some transforming reagents, such as hydrate of lime and sodium carbonate, in which case the heater is known as a hot process softener, or softener heater. The reagents em ployed may be the same as those used in the ordinary cold process softener, but the action of heat is greatly to accelerate the chemical transformations and to form coarser and larger precipitates, which settle out more quickly in the settling chamber.. The latter is made larger than in the ordinary open heater, but requires to be only one-sixth to one-third as large as would be necessary with a cold process softener. The reaction in hot water is also more com plete and less reagent is required to give a given degree of removal of the scale-forming substances. The effect of heat in accelerating chemical reactions is a familiar phenomenon, as for example the action of soap upon grease and dirt in the washing of clothes and dishes. Recently a new form of feed water heater has been widely introduced, in which the water is passed over a weir having the form of a V-notch, in order to measure it before it is pumped to the boilers. An automatic clock driven mechanism constantly records the height of the water on the up-stream side of the weir, thus preserving an accurate record of the amount passed. By comparing the amount of water evaporated by the boiler with the amount of fuel used, a measure is obtained of the effi ciency of the boiler• and improvements in effi ciency, as by the use of different grades of coal; improved methods of managing the fires, repairing of leaks in the boiler setting, cleaning of scale and soot off the heating sur face of the boiler, etc., are readily recognized. The record supplied by the meter thus becomes a tell-tale of the efficiency of the plant and of its operation. Heaters having this attachment or modification are known as metering heaters.
i Fuel economizers, in which the water is heated by the exhaust gases, have the advantages that they utilize waste heat not otherwise applicable, except possibly for pre-heating the air supplied to the boiler furnace and that they are able to raise the temperature considerably above atmospheric boiling point, as to 250° to 350° F. The disadvantages are that a large amount of costly heating surface is required and that it must be kept free of soot on the outside and of scale on the inside. Nevertheless recent practice has favored the use of less boiler heating surface, hence the discharge of the gases from the boiler at higher temperatures, the heat being recovered more advantageously by the economizer, since the latter contains water at a temperature lower than that of the water in the boiler. The economizer should in no case receive water at a temperature lower than about 120° F., as otherwise sweating and rapid external corrosion of the tubes will occur. The water may advantageously be pre-heated in an exhaust steam feed water heater, receiv ing exhaust steam from steam plant auxiliaries such as boiler feed pumps, stoker and fan engines, condenser, circulating and air pumps, etc. So long as all of the exhaust steam from the latter is condensed in the feed water heater, the fuel cost of their operation is practically nil, all of the heat in the exhaust steam being returned to the boiler.