into steam, makes a total of (320 +845) 1165 heat units which must be added to each pound of water entering the boiler to make one pound of steam.

If instead of entering the boiler at 60 degrees, the feed water were heated to 200 degrees Fahrenheit, only (380 - 200) 180 heat units would have to be added to bring it to the boiling point instead of 320 as before, and the total heat added per pound of steam would be (180 + 845) 1025 instead of 1165 heat units. In other words, to each pound of water converted into steam the boiler would now have to add only 88 per cent. of the amount of heat it did before, and 12 per cent. of the coal might be saved, or, providing the same amount of coal was burned on the grates, it would make nearly 14 per cent. more steam than it did with feed water at 60 degrees. The table on page 97 shows the saving that may be expected by heating feed water various amounts.

Another very convincing way of looking at this matter is from the view of engine efficiency. The best engine yet designed, with all the modern improvements of high steam pressure, multiple expansion, condensers, etc., cannot possibly use more than one-fifth of the heat contained in the steam, because of the latent heat necessarily discharged in the exhaust. How very much more wasteful then must be the pumps, blower engines, and other auxiliary machinery on board ship, even if, as is often the case, they exhaust into the condenser.

It is the general impression that auxiliaries will take much less steam if the exhaust is turned into the condenser, thereby reducing the back pressure. As a matter of fact, vacuum is rarely registered on an indicator card taken on auxiliary cylinders unless the exhaust connection is short and without bends, long pipes and many angles vitiating the effect of the condenser. On the other hand, if the exhaust steam in the auxiliaries can be used for heating the feed water, all the latent heat of this steam, except what is lost by radiation, goes back to the boiler and is saved instead of being thrown away in the condensing water or wasted with the free exhaust. Taking the whole plant into consideration, this makes the auxiliary machinery more efficient than the main engine.

For illustration, take the first of the series of tests of the steamship "Pennsylvania," as found on page 145. The total amount of steam furnished per hour was 20,407 pounds, of which 17,252 pounds were used in the main engine and 3155 in the auxiliaries, i.e., the auxiliaries required 15.46 per cent. of the total steam. Of the 3155 pounds of auxiliary steam, 139 pounds. were used by the stoker engines and exhausted into the ash pits, leaving 3016 pounds that exhausted into the heater.

The feed water was taken from the hot well at a temperature of 99.3 degrees Fahrenheit and pumped through a closed feed-water heater, where it was heated to 222 degrees Fahrenheit by means of the exhaust steam from the auxiliary machinery. From this heater it passed to the boilers and

H. M. ARMORED CRUISER "MINOTAUR"

BABCOCK & WILCOX BOILERS, 27.000 INDICATED HORSE-POWER

was converted into steam at a pressure of 242 pounds. The auxiliaries exhausted into the heater at about 3 pounds back pressure.

By referring to the steam tables, it will be found that the 3016 pounds of steam supplied to the auxiliary machinery contained 3,624,930 British thermal units (1201.9 X 3016). At 3 pounds back pressure the same amount of steam consumed would contain 3,480,162 British thermal units. The difference between these amounts-166,483 British thermal units-is all that is available for doing useful work, and as no engine can use all of this without waste, it will be seen that the proportion of heat that is converted into work is very small indeed.

If the exhaust steam from the auxiliary machinery had been turned into the condenser, it is true that not quite so many pounds would have been required each hour, but all the latent heat would have been thrown away in the condensing water, while as a matter of fact, by sending it into the feed-water heater, over three-quarters of the entire 3,480,162 British thermal units were saved. This is shown by the heat units absorbed by the feed water which was heated from 99.3 degrees to 222 degrees, a difference of 122.7 degrees Fahrenheit. This multiplied by the number of pounds heated gives (20,407 X 122.7) 2,503,939 British thermal units as the actual amount of heat taken from the exhaust steam of the auxiliaries each hour and returned to the boiler. Of the remaining 976,123 British thermal units, part is lost in radiation, condensation in the pipes, etc., and part, amounting to nearly 600,000 British thermal units, is wasted in the drips from the heater, on account of the impossibility of cooling the condensed steam much below 222 degrees Fahrenheit.

It may be noted, further, that each pound of coal burned contained 11,790 British thermal units, of which 75.7 per cent., or 8923 British thermal units were utilized in making steam. If, therefore, 2,503,939 heat units had not been saved by heating the feed water, it would have been necessary to have heated the same by an additional expenditure of 280 pounds of coal per hour, thereby increasing the total coal burned in the plant, per indicated horse-power, to 2.15 pounds instead of 1.92 pounds, as shown by the test.

There is another reason for heating feed water, aside from the obvious saving of heat units, and that is the fact that the boiler steams more economically when using hot feed water than when using cold. This was demonstrated experimentally by Kirkaldy. of England, and the theory advanced by M. Normand seems very plausible, namely, that cold water checks the circulation in the boiler, and in re-establishing this a certain amount of heat disappears in mechanical work, with a consequent loss in evaporation.

Water-tube boilers with their rapid and uniform circulation, are not liable to injury by the use of cold feed water, but the above points make it clear that cold water should never be used by the engineer who wishes to obtain the highest economy from his plant.

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BABCOCK & WILCOX SUPERHEATER

HE illustration on the opposite page shows a cross-section of a Babcock & Wilcox Marine Boiler fitted with a superheater. From this it will be seen that the superheater is composed of a series of tubes bent into U-shape and expanded into forgedsteel headers which run across the boiler at right angles to the tubes. The length of the headers and the number of tubes depends upon the degree of superheat required.

The superheater is placed in a box which is arranged to form a continuation of the first and second passes for the gases of combustion as they pass around the tubes of the boiler, so that the superheater is located where there is a great difference of temperature between the hot gases and the steam, and not, like the old-fashioned ones, in the uptake, where this difference was smaller.

In order that the steam as it passes through the superheater may be thoroughly exposed to the hot gases, removable baffles or division plates are put in the headers of the superheater, two in the upper header at one quarter of the length from each end and one in the lower header at midlength. The result of this location of the baffles is to force the steam as it goes through the superheater tubes to pass through the hot gases eight times, thereby giving ample opportunity for the elevation of temperature to the desired extent.

Superheater tubes are 2 inches in diameter and are arranged in groups of four, accessible from a single handhole just as are the tubes in the boiler headers, thus affording ready access to any tube for expanding or renewal. The plates for these handholes are interchangeable with those on the boiler headers.

1200 HORSE-POWER