Gasoline gas, or carbureted air, sometimes called air gas, is a mixture of gasoline vapor with air, and, when pure, is so rich in carbon that special burners must be employed for lighting purposes. The quantity required to produce 1,000 cu. ft. of gas of from 14 to 16 candlepower is about 4 gal. of the best grade, and more if the gasoline is of a lower grade. The specific gravity of gasoline is about .74 that of water. The temperature of a gasoline-gas machine should range between 40° to 80°. The highest grade of gasoline, that is, the grade that will evaporate most freely at ordinary temperatures, should be used for winter service. The generators should be located outside the building, in a sheltered place. An air pump is used to pump fresh air through the generator, and a mixing device is commonly employed for mixing air with the gas before it reaches the burners. The pump and mixer are usually located in the cellar of the building. PRESSURE OF GAS. If a gas is lighter than air, at the same temperature, the pressure will be greatest at the top of the chamber containing the gas; if heavier, the greatest pressure will be at the bottom of the chamber. The upward pressure of gas having a less density than air is caused by its deficiency in weight, and consequent inability to balance the pressure of the atmosphere. For illustration, consider a column of gas 1 ft. square and 100 ft. high, having a density of .5, or one-half that of air, the temperature being the same as that of the atmos. phere-say 60°. Air at 60° weighs .0764 lb. per cu. ft., and as the column contains 100 cu. ft., it will weigh .0764 X 100 7.64 lb. The gas having a density of 5 will weigh only half as much, or 3.82 lb., and is, therefore, unable to balance an equal volume of air. Consequently, it is pressed upwards with a force of 7.64 -3.823.82 lb. against the top of the chamber which contains it. Whatever the actual pressure of the gas may be at the bottom of the column, it will, in this case, be increased at the top to the extent of 3.82 lb. per sq. ft. The increase of pressure in each 10 ft. of rise in the pipes, with gas of various densities, is shown in the following table: EXAMPLE. The pressure in the basement, at the meter, is 1.2 in. of water; what will be the pressure on the sixth story, 70 ft. above, the density of the gas being .4? SOLUTION.-The table shows that the increase will be .088 for each 10 ft. of rise; therefore, .088 X 7 .616 increase. Then pressure at sixth story 1.2+.616 = 1.816 in. = = Pressures measured in inches of water or mercury may be translated into pounds per square inch or square foot, by multiplying the reading by the following figures: 1 in. of water at 62° F. 5.2020 lb. per sq. ft. 1 in. of water at 62° F. in. Pressure per square inch or square foot may be converted into inches or feet of = MEASUREMENT OF PRESSURE AND FLOW. Pressure. The pressure of gas is measured by the common water gauge, which is shown in Fig. 1. The tubes b and c are glass, and are filled with water up to the zero of the scale, which is graduated in inches and fifths or tenths of an inch. The tube c is opened to the air at the top. When pressure is admitted to a, the water will sink in the tube b, and will rise in c. The difference in the height of the water in the two tubes, measured in inches, is the measure of the pressure exerted in inches of water. For measuring heavier pressures, mercury is used instead of water. .5 .4 .3 FIG. 1. water, or inches of mercury, by multiplying the pressures by the following figures: 1 lb. per sq. ft. 1 lb. per sq. .1923 in. of water at 62° F. Flow. The volume of gas, passing through a pipe in a given time, is computed by multiplying the velocity by the area of the pipe. The velocity may be measured by a Pitot tube, as shown in Fig. 2. This consists of two tubes, a and b, inserted in a plug c, the lower end of a being square, and that of b curved to face the current; the upper ends are connected to a water gauge d. Gas entering through b depresses the water column as shown; the velocity corresponding to the reading is found from tables which are gen FIG. 2. erally furnished with the instrument. The actual quantity of the gas is computed by correcting the volume for temperature and pressure, reducing it to a volume at standard temperature of 32° F. and standard pressure of 1 in. of water. The correction for temperature may be made as follows: Rule 1.-Multiply the measured volume by 492 and divide the product by 460 plus the actual temperature. The quotient will be the volume at 32° F. The correction for pressure may be made as follows: Rule 2.-Multiply the volume at 32° F. by the pressure in inches of water plus 407, and divide the product by 408. The quotient will be the volume at 1 inch pressure, and at 32° F. EXAMPLE.-A pipe passes 1,000 cu. ft. of gas per hour, under a pressure of 8 in. of water and at a temperature of 60°. What will the volume be when the pressure is reduced to 1 in., and the temperature to 32°? SOLUTION.-By the first rule, the volume at 32° is 1,000 X 492 946.1 cu. ft. By rule 2, the volume under 1 in. pressure and at 32° is 946.1 X (8+407) 962.3 cu. ft. 408 If the quantity of gas delivered through a pipe of given length is known, that supplied through a longer or shorter pipe is to the known volume as the square root of the given length is to the square root of the required length. With pipes of the same length and diameter, the volume delivered at any proposed pressure is to that supplied at any other pressure as the square root of the proposed pressure is to the square root of the given pressure. GAS METERS AND PRESSURE REGULATORS. Meters. For ordinary purposes, the volume of gas passing through a pipe is measured by an apparatus called a gas meter. A gas meter measures the volume only, and its indications are not affected by any change that may occur in the pressure of the gas. The difficulty thus encountered in correctly measuring the volume of gas actually delivered under varying pressures is overcome by using a governor between the meter and the street main, or service pipe. The governor is a species of reducing valve which will receive gas at any pressure, whether steady or variable, and will discharge it at a steady low pressure. The illustration shows an ordinary meter. To read such a meter, note the lesser of the two figures between which each hand points, or the figure to which it points, beginning at the left-hand dial; then add two ciphers to the right of the three figures, and the number so obtained will be the amount of gas in cubic feet which the meter has measured. Thus, the pointers in the diagram indicate that 14,200 cu. ft. of gas have passed through the meter. The dial marked two feet may be used to ascertain the quantity of gas consumed per hour by a burner, by noting the time required for the pointer to make a revolution. Thus, the hand will make 2 revolutions per hour if 5 cu. ft. pass through the meter in that time. This dial may also be used in testing for leaks. Pressure Regulators.-The objects sought in the use of pressure regulators or governors are economy in the consumption of gas, steadiness of the lights, and most effective operation of the burners. It is of great importance that both volume and pressure at the burners should be closely regulated. The amount of gas wasted by overpressure is much greater than is generally believed. A good new lava-tip burner consuming 5 cu. ft. per hr. at .5 in. pressure, will consume about .5 of a cu. ft. more for each increase of .1 in. in the pressure. Thus, an overpressure of .1 in. will increase the gas bill about 10 per cent. The variation, in even the best-regulated systems, is usually much greater than in., and is frequently 18 or more. The two systems of regulation in use are the pressure and the volumetric regulation. In the first system, a governor is attached to the service pipe at the meter, and the house distributing pipes are maintained at constant pressure; in the second system, each burner is supplied with a governor, the pressure in the pipes not being controlled. The proper place for a pressure regulator, if used, is between the meter and the main. The pressure required at the burners, to secure the best results, varies greatly in different forms of apparatus. The following are the pressures generally used: Argand burners. .2 in. of water. .5 in. of water. Common batswing burners .5 to 1 in. or more. 1.0 in. or more. ......... |