392. Q.—­What is the temperature at which sea water boils in a steam boiler?

A.—­Sea water contains about 1/33rd its weight of salt, and in the open air it boils at the temperature of 213.2 deg.; if the proportion of salt be increased to 2/33rds of the weight of the water, the boiling point will rise to 214.4 deg.; with 3/33rds of salt the boiling point will be 215.5 deg.; 4/33rds, 216.7 deg.; 5/33rds, 217.9 deg.; 6/33rds, 219 deg.; 7/33rds, 220.2 deg.; 8/33rds, 221.4 deg.; 9/33rds, 222.5 deg.; 10/33rds, 223.7 deg.; 11/33rds, 224.9 deg.; and 12/33rds, which is the point of saturation, 226 deg..  In a steam boiler the boiling points of water containing these proportions of salt must be higher, as the elevation of temperature due to the pressure of the steam has to be added to that due to the saltness of the water; the temperature of steam at the atmospheric pressure being 212 deg., its temperature, at a pressure of 15 lbs. per square inch above the atmosphere, will be 250 deg., and adding to this 4.7 deg. as the increased temperature due to the saltness of the water when it contains 4/33rds of salt, we have 254.7 deg. as the temperature of the water in the boiler, when it contains 4/33rds of salt and the pressure of the steam is 15 lbs. on the square inch.

393. Q.—­What degree of concentration of the salt water may be safely permitted in a boiler?

A.—­It is found by experience that when the concentration of the salt water in a boiler is prevented from exceeding that point at which it contains 2/33rds its weight of salt, no injurious incrustation will take place, and as sea water contains only 1/33rd of its weight of salt, it is clear that it must be reduced by evaporation to one half of its bulk before it can contain 2/33rds of salt; or, in other words, a boiler must blow out into the sea one half of the water it receives as feed, in order to prevent the water from rising above 2/33rds of concentration, or 8 ounces of salt to the gallon.

394. Q.—­How do you determine 8 ounces to the gallon to be equivalent to twice the density of salt water, or “two salt waters” as it is sometimes called?

A.—­The density of the water of different seas varies somewhat.  A gallon of fresh water weighs 10 lbs.; a gallon of salt water from the Baltic weighs 10.15 lbs.; a gallon of salt water from the Irish Channel weighs 10.28 lbs.; and a gallon of salt water from the Mediterranean 10.29 lbs.  If we take an average saltness represented by a weight of 10.25 lbs., then a gallon of water concentrated to twice this saltness will weigh 10.5 lbs., or the salt in it will weigh .5 lbs or 8 oz., which is the proportion of 8 oz. to the gallon.  However, the proportion of 2/33rds gives a greater proportion than 8 oz. to the gallon, for 2/33 = 1/16 nearly, and 1/16 of 10 lbs. = 10 oz.  By keeping the density of the water in a marine boiler at the proportion of 8 or 10 oz. to the gallon, no inconvenient amount of scale will be deposited on the flues or tubes.  The bulk of water, it may be remarked, is not increased by putting salt in it up to the point of saturation, but only its density is increased.