"To facilitate entrance into the tubes at the back end, a step, which may be of brick or cast-iron, should be built into the side wall, as зhown in the plan and also in Fig. 69, at a height of 2 feet from the flue bottom. Some engineers prefer to keep the floor of the side flues at the level of the boiler bottom, and for such a general arrangement, as the figures indicate,

this might be done without inconvenience. The lower level is, however, for the reasons stated, preferable, though the seating blocks may thus be rendered large and heavy. The breadth of the seating block at the point of contact with the boiler is 3 inches. This narrow width is to ensure that any corrosion which may exist upon the seating may not be

hidden from inspection, as in the case of the old-fashioned seatings, which were from 6 to 12 inches broad. A narrow seating allows a greater extent of heating surface to be exposed, and is, therefore, more economical. The thickness of the brickwork above the side flue crown is also kept down to 9 inches. In cases where the width of the bottom flue, as found by this method of construction lines, brings the seating blocks under the longitudinal seams of the boiler, this width should be altered so that the seams may be quite clear of the seating, but in altering the width for this purpose it should not be allowed to exceed 4 feet in a 7 feet boiler, and so in proportion for other sizes. Special care should always be taken to arrange the longitudinal seams so as to be clear of all brickwork. In constructing the brickwork of a boiler seating all joints should be thin and carefully bedded, and no lime-mortar should be suffered to come into contact with the boiler plates, as its action in causing corrosion is most prejudicial, especially in the presence of moisture. Fire clay only should be used for the joints near the iron plates. The damper frames, if in pairs, are built into the brickwork, about a foot behind the boiler, but instead of a damper to each side flue a single damper may, if more convenient, be placed in the flue formed by the junction of the two side flues."

THE EVAPORATIVE POWER OF STEAM BOILERS.

If the evaporative power of a steam boiler is obtained from the amount of steam that leaves the boiler under a pressure above that of the atmosphere, then if the boiler primes and water carried away with the steam is credited to the boiler as steam, unless, indeed, the quantity of water so carried off is uniform throughout the test, and its quantity is ascertained and taken account of in estimating the evaporative efficiency of the boiler. Again, the temperature of the feed water must be taken into account in determining the evaporative efficiency, and in order to provide a common basis upon which all boilers may be tested or compared, it is customary to express the boiler's evaporative power in pounds of water evaporated per pound of coal, the feed water being supposed to be at a temperature of 212° (or boiling point under atmospheric pressure), and the steam escaping at the same temperature (212°). This is usually stated as " from and at 212°," the "from" meaning the temperature of the feed water, and the "at" meaning the temperature of the escaping steam. If the average

gauge pressure at which the steam escapes is taken from the boiler, the average temperature of the feed water, and the number of pounds of water the boiler evaporates are known, we may find the equivalent of the evaporative power of the boiler in pounds of water from and at 212° by means of the following table:

TABLE FOR REDUCING TO STANDARD THE OBSERVED EVAPORATION OF A BOILER.

Multiply the Actual Evaporation by the Proper Figures from the Table; the Result will be the Evaporation from and at 212°.

Temperature of

Feed.

32°

40°

50°

60°

70°

80°

95°

100°

110°

120°

130°

140°

150°

160°

170°

180° 190° 200° 210°

Gauge Pressure, Pounds per Square Inch.

30 40 50 60 70 80 90 100 110 120 130 140 150

1.207 1.211 1.214 1.217 1.220 1.223 1.225 1.227 1.229 1.231 1.233 1.234 1.236 1.199 1.203 1.206 1.209 1.212 1.214 1.217 1.219 1.221 1.223 1.224 1.226 1.228 1.188 1.192 1.196 1.199 1.201 1.204 1.206 1.208 1.210 1.212 1.214 1.216 1.217 1.178 1.182 1.185 1.188 1.191 1.194 1.196 1.198 1.200 1.202 1.204 1.205 1.207 1.167 1.171 1.175 1.178 1.181 1.183 1.185 1.188 1.190 1.191 1.193 1.195 1.196 1.157 1.161 1.165 1.168 1.170 1.173 1.175 1.177 1.179 1.181 1.183 1.185 1.186 1.147 1.151 1.154 1.157 1.160 1.162 1.165 1.167 1.169 1.171 1.172 1.174 1.176 1.136 1.140 1.144 1.147 1.150 1.152 1.154 1.156 1.158 1.160 1.162 1.164 1.165 1.126 1.130 1.133 1.136 1.139 1.142 1.144 1.146 1.148 1.150 1.152 1.153 1.155 1.116 1.120 1.123 1.126 1.129 1.131 1.134 1.136 1.138 1.140 1.141 1.143 1.145 1.105 1.109 1.113 1.116 1.118 1.121 1.123 1.125 1.127 1.129 1.131 1.133 1.134 1.095 1.099 1.102 1.105 1.108 1.110 1.113 1.115 1.117 1.119 1.120 1.122 1.124 1.085 1.088 1.092 1.095 1.098 1.100 1.102 1.104 1.106 1.108 1.110 1.1121.113 1.074 1.078 1.081 1.084 1.087 1.090 1.092 1.094 1.096 1.098 1.100 1.101 1.103 1.064 1.067 1.071 1.074 1.077 1.079 1.081,1.084 1.086 1.087 1.089 1.091 1.092 1.053 1.057 1.060 1.064 1.066 1.069 1.071 1.073 1.075 1.077 1.079 1.080 1.082 1.043 1.047 1.050 1.053 1.056 1.058 1.061 1.063 1.065 1.067 1.068 1.070 1.072 1.032 1.036 1.040 1.043 1.045 1.048 1.050 1.052 1.054 1.056 1.058 1.059 1.061 1.022 1.026 1.029 1.032 1.035 1.037 1.040 1.042 1.044 1.046 1.047 1.049 1.051

To use the table we find the pressure by steam gauge at which the steam is in the boiler, and look down the vertical column until we come to the figures or co-efficients for the temperature at which the feed water is fed to the boiler.

Suppose, for example, that the feed water has a temperature of 32°, that the boiler pressure averages 40 lbs. per square inch, and that 1 lb. of coal evaporates 9 lbs. of water, and to find what this evaporation is equal to from and at 212° we look along the 32° line in the table, and under the 40 we find 1.211, which multiplied by the pounds of water evaporated gives 10 lbs. as the evaporation from and at 212°.

899

1000

Again suppose the water evaporated per pound of coal is 8; the feed

water is at a temperature of 100° and the boiler pressure averages 70, and

beneath the 70 and on the line of the 100° in the table we find 1.150, which multiplied by the 8 gives us 9.200 lbs. of water as the equivalent evaporation from and at 212°.

Another use to which the table may be put is as follows: Suppose a boiler is guaranteed to evaporate 10 lbs. of water from and at 212° per 1 lb. of coal, and that it is to carry a pressure of 80 lbs. per square inch by steam gauge, the feed water being 150°, and we may find what its evaporation under these conditions must be in order to be equivalent in evaporative efficiency to 10 lbs. from and at 212°, by dividing the 10 by the 1.100 found in the table beneath the 80 and opposite the 150°, and thus get 9.09 as the pounds of water per pound of coal that the boiler must evaporate under its working conditions in order to fulfil its guarantee.

It is shown that in the use of a table of this kind the amount of fuel consumption need not be known, and that another standard, such as time, may be used.

899

Thus we take as an example a case in which a boiler evaporates say 9 lbs. of water per minute, the temperature of the feed being 32° and the boiler pressure 40; the equivalent evaporation from and at 212° will still be 10 lbs. as in the first example here given. It is more convenient, however, to take the coal consumption, because the economy of boilers is usually stated in pounds of water evaporated per pound of coal and per pound of combustible. If the weight only of the coal is taken, its quality is left out of the account, and no comparison of the boiler efficiency can be made unless with boilers using the same kind and quality of coal. If, however, we deduct from the weight of the coal used the weight of the clinkers, of the ashes and of the unconsumed coal we get the pounds of combustible or pounds of coal that have actually burned up.

This is sometimes considered as reducing the qualities of different coals to a common standard; but such is not actually the case, because it leaves out of account the fact that oue kind of coal requires more air to promote combustion than another, and this extra quantity of air has to be heated at the expense of the fuel. Again, clinkers, ashes, etc., fall into the ashpan at a high temperature, and thus heat is lost; hence in taking the pound of combustible, we get what the actual results in boiler evaporation are, rather than establishing a standard of quality for all coal.

The standard for the evaporative efficiency of a boiler recommended by the judges of boiler tests at the Centennial Exhibition of 1876 is as follows:

"The evaporation of 30 lbs. of water per hour from feed-water having a temperature of 100° Fahrenheit, into steam having a pressure of 70 lbs. per square inch above the atmosphere, is equal to one horse-power.

"This differs by only about one-thirtieth of one per cent. from an evaporation of 34 lbs. of water per hour from and at 212° Fahrenheit, which is the standard for one boiler horse-power recommended by a committee appointed by the American Society of Mechanical Engineers to consider the subject; hence the two standards are practically the same.

"But the feed temperature and steam pressures are constantly varying in practice, so that it becomes necessary to have recourse to calculation to compare results. To facilitate this comparison we have the following table, which gives the equivalent evaporation from feed at 100° into steam of 70 lbs. pressure, for various other pressures and temperatures occurring in practice. A single example will show the application of the table on page 188.

"Suppose we have a boiler which evaporates 2400 lbs. of water in one hour from feed at 70° into steam at 80 lbs. per square inch, what is the equivalent evaporation from 100° into steam of 70 lbs. ?

"Looking in the first column under 'Temperature of the Feed' we find 70; following along the horizontal line from this point until we reach the line of pressures having 80 at the top we find 1.029; multiplying this by 2400 we have 2469.6 for the equivalent evaporation from 100° at 70 lbs., and the nominal horse-power of the boiler would be, by the Centennial Committee's Standard, 2469.6÷30-82.3 horse-power, and similarly in any other case."

CENTENNIAL BOILER TESTS-ACTUAL RESULTS.

Water evaporated from and at 212° per pound of combustible.

Relative No.

The above are the results of the tests of stationary engine boilers

9.990

9.355