OLAZ S than an average of 0.75 per cent. Liquid fuel is absorbed in asbestos or pumice 22. stone and burns freely, these substances promoting the contact of the oxygen. A more elaborate and accurate instrument is the Mahler modification of the Berthelot bomb (fig. 79). Combustion takes place in a steel bomb B with a tightly screwed cover. It is made of mild steel of a tenacity of 55 kilos. per mm. and 22 per cent. elongation. The shell is 8 mm. thick and its capacity 654 c.c. It is nickel-plated outside and enamelled inside. The top cover is traversed by a well-insulated electrode for ignition and a valve to admit oxygen. Ignition is effected by a little bit of spiral iron wire which is heated by the electric current. A helicoidal agitator is worked by the lever L. A small bichromate battery of 2 amps. × 12 volts provides ignition. Two thermometers and a gauge to show 150 atmospheres are provided. A bottle of oxygen containing 1,200 litres (free) at 120 atmospheres suffices for 100 tests. The fuel being placed in the crucible and the bomb screwed up with the ignition details in place, oxygen is admitted till 25 atmospheres are recorded on the gauge, when the oxygen is shut off. The fuel is not too finely pulverized and oxygen is admitted very gently. The bomb is now immersed, the water agitated and the temperature carefully noted every minute for five minutes before ignition. Ignition being now effected, the temperature is noted half a minute after and one minute after, and then at intervals until it shows signs of going back. Readings are then made for five minutes, the agitator working all the time. Correction for loss of heat during test is thus found by Newton's law of cooling. 1. After maximum temperature has been read the descent of temperature represents the loss of heat from the calorimeter before the attainment of the maximum, and for one minute on the condition that during this minute the mean temperature does not vary more than 1°C. from maximum. 2. If the difference is over 1°C. and under 2°C. the law of descent of temperature at the maximum moment diminished by 0.005 gives the correction. After completing observations, the bomb is washed out to recover acidulous liquor formed during combustion. The hydric-nitrate (N,O,H,O) is tested for acid. The definition of the thermic value of the fuel is now determined as follows A difference for corrected temperature. W = weight of water. W’water equivalent of apparatus. n-weight of hydric nitrate found. f=weight of iron ignition spiral. 0.23=heat of formation of 1 gr. of dilute nitric acid. 1.6 heat of combustion of 1 gr. of iron. x=calorific power of combustible. 5 2 Then x A (W+W') - (0·23n+1.6 f) Any SO, may be usually neglected, but if necessary to allow for it, 2 grams fuel should be used under a pressure of 30 atmospheres. The formation of SO,H,O disengages 0.73 calories per gram. sof The following temperatures were noted in a particular test of oil The temperature variation has been 13.84-10-25-3-59°C. The corrections. are as follows During the two minutes 7-8 and 6-7 the thermic loss has been 0.012 × 2=0·024° for maximum cooling. In the half minute 5-6 the loss has been (0.012−0.005) 40-0035, and during the half-minute 5-5 a gain of whence the relative loss in the minute 5-6 will be 0-0035-0-0020-0-0015°. Thus during the experiment the thermic loss is 0.024 +0.0015=0.0255 to be added to 3.59° =3-615°. Consequently the heat value absorbed by the instrument and water is where W-2200 and W, 481 grams, the weight of water and the equivalence of the instrument. (2,200+481) × 3.615-9-69181 calories. from which, say, 0.13 grams of nitric acid formed will require 0·13 × 0·23=0·0299 calories to be subtracted, and for the iron spiral of 0.025 grams a further subtraction of 0.025 x 1.6=0.04. The final result is 9-6918-(0.0299 +0.0400)=9-6219 cals. or 9,621-9 per kilogram of oil 17,319-4 B.Th.U. Mahler's original paper, from which these figures were taken, was read before the Société d'Encouragement de Paris in June, 1892. The water equivalent of W' of a calorimeter may be determined by burning in it 1 gram of naphthalene of well known value, first, say, with 2,300 grams of water and then 2,100 grams. of water with 0.8 grams. of naphthalene, the discrepancy from the known value of naphthalene being obviously the instrumental effect. Draught. The draught due to a chimney arises from the difference of pressure of two columns of gas of the height between the grate surface and the chimney-top. The column inside the chimney is hot because of the furnace through which it has passed. That outside the chimney has the temperature of the outer atmosphere. At a temperature of 300°C. (572°F.) the inner column is just about double the abso lute temperature of the outer column, so that the relative density is one-half. The velocity of flow of a gas under any head is v=2gh where v is the velocity in feet per second, h is the head in feet and 2g=64.4 or 2 gravity. Expressed in metres values of v and h we have v√2 g h where g=9-81. Assuming that at ordinary temperatures 13 cubic feet of air weigh one pound. the atmospheric pressure of 2,115 pounds per square foot represents a column 27,495 feet in height, which would flow into a vacuum at a velocity of approximately 827,495 1,321 feet per second. The pressure to produce draught, however, is only measured by inches of water pressure. If a chimney has an internal absolute temperature double that of the external atmosphere, it will contain only one pound of gas for each 26 feet of a column of gas 1 foot square, or, what is the same thing, the external column is halfbalanced only. Thus if I be the height of the chimney H÷(2× 13) will give the pressure per square foot, producing draught. Thus a chimney of 104 feet will give an acting pressure of 4 pounds. As 1 inch of water gives a pressure of 0-036 pounds per square inch, the draught pressure of the above chimney would be Having found the pressure, the air column equivalent to this must be found. Water weighs 62.4 pounds per cubic foot. Air weighs 0-077 pounds. whence the equivalent air column in feet per inch of water column will be found The velocity of flow is then 8√67H or fully 64 ✅H where H is the pressure in inches shown by the actual water gauge. In coal-fired furnaces the reading of the draught gauge is much greater at the chimney base than in the flues, for the friction of the flues exerts considerable resistance. The simplest form of water gauge is a bent glass tube of Ụ form, one end being open to the atmosphere, the other connected by a piece of indiarubber tubing to a piece of pipe which enters the flues at the point where the draught intensity is sought. It is convenient to remember that where the velocity of flow due to head in feet is v√2gh, that due to a pressure as shown in inches of water is almost exactly v=2g√H. All these figures can only be approximate, because they will vary with the temperature. They are sufficiently accurate to base designs upon respect of providing sufficient openings for air to burn the oil. in The following table of velocities of air for a few pressures in inches of water will be useful— An ordinary U gauge is not capable of being finely read. It possesses a capillarity which is difficult to allow for and will not serve for accurate work. A better gauge consists of a glass-fronted box in two divisions partly filled with water (fig. 80). A hook gauge, reading on a scale, permits the water level in one division to be determined to 0.01 inch easily. A very sensitive gauge (fig. 81) consists of a U tube with a drum at the top of each leg. Half the height is filled with water and alcohol. A stop-cock at the bottom of the U is then closed and the upper parts are filled with olive oil to the middle of the top drums, which intercommunicate by a cross tube. The oil has a specific gravity of 1 to 2 per cent. less than the alcohol mixture. One tube is open to the air, the other to the pressure to be gauged. The lines of contact of the oil and spirit move up and down in the two tubes respectively, and they move through a distance as many times as the difference of specific gravity of the two liquids. Thus for 2 per cent. difference the reading is fifty times greater than that of a plain water tube. The two tubes are about 30 inches long, to provide length for reading, and there are two scales to read, respectively the up and down travel of the point of liquid contact. In Hoadley's design of this gauge he used tubes 0-4" diameter and drums at the top of each 4-25" diameter with glass fronts. The large area of these drums prevents any great variation in their level for quite a large range of movement in the tubes. Fig. 80. In coal firing, about three-fourths of the draught is swallowed up by grate and fuel friction, and much of the remainder in the flues and chimney itself. With oil firing alone and no grate friction there is usually ample velocity of the inflowing air. The chimney, in fact, ceases to possess so much importance, but must be large enough in area to carry off the waste gases. The weight of a cubic foot of air at 0°C. =32°F. being 0.08 lb., that at any other temperature will be 0.08 × 273 where to is expressed in degrees Centigrade |