HEATING VALUE OF WOOD The heating value of perfectly dry wood is very nearly the same for the various specimens. Each contains approximately 50% of carbon. Hardwood gives a little less heat per pound than soft wood.
Newly felled wood contains from 30% to 5o% of moisture, a fair average being 40%, and after eight to ten months ordinary drying in the air, this per centage of moisture is reduced to approximately 20% to 25%.
The average heating value of perfectly dry wood is about 85oo B.T.U. per pound, which will be reduced to about 6000 B.T.U. per pound for wood containing 25% moisture and to about 460o B.T.U. per pound for 40% moisture, the last two heating values allowing for the evaporation and superheating of the moisture from 70 deg. F. to 50o deg. F.
The following table gives the average weight per cord of 128 cu. ft. for various woods, thoroughly air-dried, and the corresponding equivalent pounds of oil per cord of dry, 25% moisture and 40 % moisture wood. The computations are based upon 45% boiler and grate efficiency when burning wood, and oil with an evaporative power of 15 lbs. of water from and at 212 deg. F. per pound of oil. The oil weighs 314 lbs. per barrel.
Dry wood at 45% boiler and grate efficiency will 0o x .45 evaporate 85 = 3.94. lbs. of water from and at 970.4 212 deg. F. per lb. of wood.
25% moisture wood at 45% boiler and grate oo effi ciency will evaporate — 2.78 lbs. of water 970.4 from and at 212 deg. F. per lb. of wood.
4o% moisture wood at 45% boiler and grate effi ciency will evaporate X .45 = 2.13 lbs. of water 970.4 from and at 212 deg. F. per lb. of wood.
A general rule is that, when burned in boiler fur naces, 2 % lbs. of dry wood are equal to one pound of an average quality of soft coal. Hence, if we assume a coal with an evaporative power of 8 lbs. of water from and at 212 deg. F. per lb. of coal, the dry wood will have a similar evaporative power equal to 3.55 lbs. of water from and at 212 degrees
F., per i lb. of wood, or a boiler and grate efficiency of 3.55 x 970.4 = 40.5%.
85oo In a small publication of this kind, a detailed de scription of the methods to be followed in conducting an exhaustive boiler test would be out of place, but it is desirable on the part of operating engineers to know what efficiency they are obtaining with their equipment, and it is thought that a general outline of the data necessary to compute the efficiency of the boiler and grate would be of value.
Obtain the amount and temperature of the feed water entering the boiler for a period of eight or ten hours under average boiler room conditions. If extremely accurate results are desired, this water should be weighed, but for most purposes a water meter known to be fairly accurate or the gauge of a water tank may be used.
Record the steam pressure frequently during the same period. Readings should be taken every 20 minutes if the pressure varies considerably, but if it is fairly constant, every hour will be sufficient.
Record by tank gauge or otherwise the amount of oil used for the eight or ten hours, as the case may be, and reduce the result to pounds.
Compute the factor of evaporation for the average steam pressure and feed water temperature (see page and determine the equivalent evaporation per hour from and at F.; divide this result by the pounds of oil, consumed per hour, and if the result is in the neighborhood of 15 pounds of water per one pound of oil, the boiler and grate efficiency will be in the neighborhood of 75%, which is fairly good.
The quantity of steam used to vaporize the oil in per centage of the total evaporation of the boiler may be determined as described on page 12o, and should not exceed 5%.