180 CAUSE AND EFFECT OF INCRUSTATION.
therein, the barometer indicating 30 inchesof mercury.
Common sea water 213 • 2° 1*029
#2 214-4 1-058^215-5 1-087
Up to this point nodeposit will be
formed . . . 216-7 1*116
3^ 217-9 1-145
219-1 1-174
sV 220-3 1•203
221-5 1-232
A 222-7 1-261if 223-8 1-290
-H 225-0 1-319
it 226-1 1.348 sa ^rated
solution.
“ As a general rule, the atmospheric boilingpoint of the water should never be allowed toexceed 216°. The temperature must he as-certained by drawing off a small quantity ofthe brine, and boiling it in a deep coppervessel in the engine room, a correction beingmade, if necessary, for the state of the baro-meter.
“ The following Table shows the height ofthe boiling point in Fahrenheit’s scale atdifferent heights of the barometer.
Barometer. Boiling point. Barometer. Boiling point.
27 inches. 206-96° 29| inches. 211-20°
271 207-84° 30 212°
28 208-69° 30^ 212-79°
28J 209-55° 31 213-57°
29 210-38°
“ It will be seen that if we would preservethe water of the boiler at a degree of satura-tion indicated by of salt, we must blow off
one-half of the feed water; if at then
one-third must be blown of; at yU, one-fourth,and so on. We have said that is the
highest degree of saturation that should bepermitted. The degree of saturation of thewater should be tested at least once everyhour.”
It will be seen that Mr. Murray advocatesa continual loss of one-third of the feed water,to prevent incrustation.
In a paper read by him at the “ Institutionof Naval Architects,” in 1865, he states :—
“ A good deal of misapprehension exists asto the loss entailed by the process of blowingoff, but I think the following rough calculationwill show that it cannot be very great. Thefeed water enters the boiler at, say 110°, andis blown off at 220°. The fuel required forraising this quantity of water by 110° is thusthrown away. Now the steam which is evapo-rated had absorbed altogether about 1,210° ofheat sensible and latent, and we know that thewater blown off bears to the water evaporateda proportion of, say one-half, therefore theheat thrown away in the brine will bear theproportion of one-half of 110 (or 55) to 1,210,which is one twenty-second part. We maytherefore infer that 5 per cent, only of thewhole quantity of heat utilized in the boilerhas been lost by blowing off.
Professor Rankine, when alluding to theproperties of sea water, says in his work, towhich prior allusion has been made :—■
“ Ordinary sea water contains aboutits weight of salt. The brine in the boilershould never be allowed to rise above treblethat strength ; and for that purpose the volumeof brine discharged should be equal to halfthe volume of the net feed-water. But is it