INDEPARTMENT In order to receive attention, all inquiries must be addressed to the Consulting Department, and must be accompanied with the full name and address of the sender. In view of the fact that the inquiries already received have far exceeded the available space, the Editors reserve the right to select for publication only those inquiries likely to be of greatest interest to the readers of the mag azine in general. Other inquiries will be answered by letter. Compressed Air Question 1: If air at 32° F. be compressed to 75 pounds (gauge), what will be the temperature at the latter pressure, providing no heat is given or conducted during compression? Question 2: How many pounds of steam per I. H. P. per hour are consumed by the theoretical engine, the boiler pressure being 100 pounds (absolute) and exhausting into the atmosphere? Question 3: At what temperature at atmospheric pressure does air become liquid, and how many B. T. U. are given off during liquefaction ?-1. A. H. Answer 1: The temperature of air of air when compressed with no heat loss, is oss is determined by means of the formula : clearance of 7 per cent, however, has been assumed. The above, then, you see, indicates what the possibilities are of steam at 100 pounds' absolute pressure (85 pounds' gauge pressure). If the steam is not allowed to expand, but is taken in at full pressure throughout the entire stroke, the economy is much lower. Under these conditions, about 34 pounds are required per horse-power per hour. Answer 3: Air has not yet been liquefied at atmospheric pressure. There is a certain "critical” temperature for all a certain gases, above gases, above which they cannot be liquefied by pressure alone. This temperature for air is given as — 220° F., and at this temperature it takes about 39 atmospheres to liquefy air. Its latent heat, or the number of B. T. U. given off during its change from a gaseous to a liquid state, is not known with certainty. It is, however, supposed to be about 123 B. T. U. T, being the absolute temperature of the intake air, and Pı being the absolute pressure of the same. T, and P2 are the corresponding final temperature and pressure. From this we see that the final temperature is 370° F. Answer 2: About 13 pounds is the quantity of steam consumed per horsepower of actual work done per hour. This is on the assumption that there is no back pressure of exhaust or of compression, and that there is no loss of steam by cylinder condensation or leakage. A Explosion of Gasoline Engines Question: Why do so many gasoline and naphtha engines explode?-J. W. H. Answer: If the operation of explosive motors have one or several mis-fires, the cylinder becomes overcharged with com CONSULTING DEPARTMENT-- (Continued) bustible mixture, and the fire is liable to of curve 2 is zero, and the ordinates of produce an excessive explosion in the the other two curves are equal in value, motor. The common cause for danger- but opposite in sign. At A B, the sum of ous explosions, however, is in having the two positive ordinates of the curves fire come in contact with the supply tank I and 2 is equal to the negative ordinate through some accident. of curve 3, because A E= Sine 30o = 12 ; and B E= Sine 90° = 1. If the system is balanced, each phase Direction and Energy of Polyphase will be carrying the same amount of curCurrents rent at corresponding points in the cycle; Question: In what directions, on the three but since, from the nature of a threewires of a three-phase high-tension alternat phase current, the phases are displaced ing-current system do the currents flow-that by 120°, these corresponding points are is, for three successive alternations? Does each of the three wires carry the same amount not reached at the same instant, but sucof energy as the others at any given instant, cessively for the three different phases. or at the maximum current of each alterna The above discussion refers to instantion ?-C. R. taneous values. The virtual or effective Answer: This matter can best be current of one phase is the vector sum of shown by means of the accompanying the currents in the other two phases. sketch, which shows the current curves of Operation of Cream Separator Question: How does a cream separator work?-IV. B. T 180 7270 360 Answer: Centrifugal force is used to separate the cream from the milk. The receptacle for the milk is arranged with a spindle passing through the casing, so that it can be revolved at the rate of Current CURVES OF 3-Phase BALANCED System. from 1,000 to 1,500 revolutions per mina three-phase balanced system. Now, ute. The centrifugal force throws the the direction and magnitude of current heavier milk near the outer part of the in each one of the three phases may be receptacle, where it is drawn off through determined at any given instant by draw- a pipe for the purpose. The lighter ing a line such as A B at any point, such cream, remaining near the center, is as for instance 150°. Let us consider that drawn out through another pipe. There the current in one direction is positive, as is also a supply pipe so that the original represented by the curves above the line; milk can be run in while the machine is and in the opposite direction negative, as in operation. represented by the curves below the line. Then, at A B, the currents in phases I and 2 are in a positive direction and Manufacture of Citric Acid equal, while the current in phase 3 is in the negative direction. Again, at CD, Question: How is citric acid made?-H. K. the current in phase 2 is zero, while that Answer: Citric acid is made on a in phase 1 is positive and exactly equal large scale from the juice of the lime and to that in phase 3, which is negative. In the lemon, chiefly in the months of Nothe same way, by passing lines through vember and December. The juice is ferthe curves, similar to A B and C D, it is mented for some time, to free it from possible to investigate the direction and mucilage; then boiled, filtered, and neuvalue of currents at any desired point. tralized with powdered chalk and a little The algebraic sum of the ordinates of milk of lime. The precipitate of calcium the thiree curves is equal to zero at any citrate so obtained is decomposed with point. For instance, at CD, the ordinate dilute sulphuric acid ; and the resulting CONSULTING DEPARTMENT, (Continued) solution of citric acid is separated by lamps that can be run is limited by the filtration, evaporated to remove calcium capacity of the generator. If the capacsulphate, and then concentrated in ity of the generator is 100 amperes, you vacuum pans. can operate 200 lamps from it. It is not a matter of internal resistance of the ma chine, but rather of current-carrying caVelocity of Steam pacity of the armature conductors and of commutation without excessive sparkQuestion: Could you give me the velocity of steam at different pressures-say 100 lbs., lbs. ing. Of course, the mains should have 200 lbs., 300 lbs., etc., up to 750 pounds, per sufficient carrying capacity to handle the square inch; or the rule or way to figure it current without overheating. out? The sizes of the holes are 38 inch, 12 Answer 2: The heating effect upon a inch, and 58 inch in diameter.-A. H. conductor of an alternating current is Answer: The flow of steam of greater the same as that of a direct current ; that pressure into an atmosphere of less press- is, if a direct current of 100 amperes is ure increases as the difference of press- flowing in a wire, the heating effect will ure is increased, until the external press- be the same as that of an alternating ure becomes only about 4-7 of the inside current whose virtual or effective value is pressure. The flow is neither increased 100 amperes. This must be the case, benor diminished by a difference of press cause the virtual value of an alternating ure greater than this ratio. When steam current is the value of an unvarving diof varying initial pressures is discharged rect current which would produce the into the atmosphere, the atmosphere same heating effect. being not more than 4-7 of the initial pressure, the velocity of outflow at constant density is given by the formula V=3.595 V h, in which V=the veloc Clearance Loss in Ammonia Compressor ity of outflow, in feet per second, for Question 1: I should like to have you calsteam of the initial density; h = height, culate the loss in ammonia compressor from in feet, of the column of steam of the following data : following data:" Com Compressor double-acting; given absolute initial pressure of uniform size of cylinder, 15 inches by 23 inches' stroke; density, the weight of which is equal to high pressure, 210 pounds; back pressure, 28 · pounds; engine makes 60 r. p. m.; clearance, pressure on the unit of base. Applying 14 inch. This is very large, in fact too large the above formula, the velocity for steam a clearance, and the loss must be great. The at 100 pounds absolute pressure will be machine is rated at 125 tons' refrigeration in nearly 900 feet per second. If the orifices 24 hours. With this large clearance, how much would it reduce refrigeration in 24 are small, the velocity will not vary ex hours ? Explain fully the loss per stroke, and cept with changes in the pressure. per 24 hours. Question 2: Would a spherical piston 17 inches in diameter have the same pressure (total pressure) exerted upon it as a flat pisNumber of Lamps-Heating of Con ton? ductors Question 3: Give formula for finding area of a segment of a sphere 17 inches in diameter Question 1: What is the maximum num at base, height 9 5-16 inches.-R. H. K. ber of incandescent lamps (220-ohm) which may be operated from one machine of 110 Answer 1: We have made calculavolts ? Is the number limited only by internal tions on the compressor, and find that the resistance of machine and external resistance loss due to clearance is about 7 per cent. of mains ? Question 2: Is the heating effect upon a This is indeed excessive. Under the conductor, such as a trolley wire, greater or conditions of operation which you have less when carrying an alternating current than given, it requires 2.81 cubic feet of gas a direct current?—P. H. to be passed through the compressor per Answer 1: A 16-candle-power incan- minute, to produce one ton of refrigdescent lamp at 110 volts has a resistance eration in 24 hours. Having allowed for of about 220 ohms when hot, and takes a the clearance, the compressor handles 264 current of 12 ampere. The number of cubic feet per minute ; and consequently CONSULTING DEPARTMENT -(Continued) this equals 94 tons of refrigeration per 24 Substituting these two values for I and hours. This, you see, is considerably less L, we have: than the rating. The loss per stroke is E in the same proportion as the number of strokes per minute. The second formula is based on the Answer 2: The pressure on the spher fact that the resistance of one foot of ical piston of any diameter whatever is commercial copper wire one mil in dithe same as the pressure on the flat pis ameter at a temperature of 75° F., is ton of the same diameter. about 10.8 ohms. Then, since the resAnswer 3: The segment having but sistance varies directly as the length and one base, multiply half the height of the inversely as the area, we have: segment by the area of the base, and the D 10.8 x L cube of the height by .5236, and add the two products. in which R is the resistance; L, the length; and CM, the area in circular Drop in Voltage mils. Then, Question: Please explain how to figure the R. formula for the drop in voltage in electric wires for lighting. I know the formula, but but, as resistance equals electromotive do not know where you get it.-F. 2. W. force divided by current, Answer: There are two common 10.8 x L 10.8 X LXI. formulæ for figuring size of wires to sup CM = E Ε ply direct current to a given number of incandescent lamps at a given distance. The first formula is based on Ohm's law : Horse-Power of Rotary Blower Question: Please give rule for calculating the horse-power of a rotary blower. Is the Then, following correct?--R. S. D. C. ft. X 1,728 x P oz. xx H. P.= - in which R is the resistance of the cir 33,000 cuit : I. the current flowing; and E, the in which C. ft.=Cubic feet of blast per revoallowable drop in voltage. lution ; P oz.=Pressure of blast in ounces; and Or, if R is to equal the resistance per N=Revolutions per minute. foot, then we must multiply by L (the Answer: Your formula is incorrect, total length of circuit) to get the total since you have made no allowance for resistance. In other words, the work necessary to give the air its 33,000 + 2, 125,000 delivery pipe, in pounds per square foot ; Now a 16-candle-power 110-volt lamp () = Number of cubic feet of air distakes 12 an ampere; hence, if N equals charged per minute ; 1= Velocity of air the number of lights, then, in discharge pipe in feet per second ; and D= Density or weight of one cubic foot of air at the observed temperature and barometric pressure. The first term in And, if L equals the total length of cir- this formula agrees practically with the cuit, and F equals the distance one way, formula you have given ; that is, we have then, the factor of force P times the space per L = 2 F minute, which is Q, the number of cubic : RE 16 Then, CONSULTING DEPARTMENT-(Concluded) feet discharged per minute. The second Circuit for Extension Telephone term of the formula indicates that part Question: Will you please give me the outof the power necessary to give the veloc line of the circuits on an extension telephone, ity, the work being determined by the central-energy system, where there are but two formula 1/2 m va, m being the mass, and v wires running to the extension telephone? E. L. L. the velocity in feet per second. Now, m, Answer: In this case, as we underthe mass, is equal to Weight divided by stand it, you have a telephone located Gravity (g), so that it is equivalent to somewhere in the room, as upon the wall, W 7,2 equipped complete with transmitter, re28 ceiver, and bell ; and you wish to connect W = Number of cubic feet of air dis an extension telephone upon the line, ob a charged per minute times the weight of É taining your signals from the bell on the each cubic foot, or Q x D. The above formula applies in cases where the air is drawn into the fan directly from the atmosphere. If there is a long suction pipe, the difference of pressure at inlet and exit is employed in obtaining P; and the difference in the squares of the velocities must be employed, instead of 1'2. first-mentioned phone. The accompanying sketch shows the proper connections, where A is the plione located upon the wall, and B is your extension desk telephone. If you wish a bell connected on your extension phone, it would simply be bridged across the line as shown in the connections for A. Explosive Pressure of Gasoline Question: I should like to know the increase of volume or pressure of a cubic foot of a well-mixed, properly proportioned charge of gasoline and air, just after ignition. Will a charge of air in gasoline ignite as readily under low pressure as under high?-H. P. Ir. Answer: The constitution of the vapors of gasoline and other products of petroleum, is so complex and variable that the exact heating value of any certain vapor can be determined only by direct experiment. The heating value of a cubic foot of gasoline vapor is nearly 1,000 B. T. U., or about equal to the average natural gas. When mixed with the right proportion of air at atmospheric pressure, the maximum pressure due to explosion is about 100 pounds per square inch. The pressure attained under any circumstances, is only about one-half that which would be reached if the total heat of combustion were evolved at the instant of explosion. By compressing the mixture before explosion, much higher pressures can be reached. The maximum pressure is not reached immediately after ignition, but a fraction of a second afterwards, the exact time depending upon the mixture and compression. The charge will not ignite as readily under low pressure as under high. Breaking Gauge Glass Question: Will you please give me the cause and remedy for the following: At the last four boiler washings, I have taken out the drain nipple below the water glass, and swabbed out water glass with a small wad of waste on end of a thin pine stick. Now, in each case the glass burst. Last time, it cracked before steam was raised. The swab fits very loose, and I cannot assign any cause for it.H. B. H. Answer: The water glass is always a very frail piece of apparatus, and especially so when at a high temperature. Often, if the glasses are not lined up perfectly straight, they will break when slightly heated, owing to expansion and internal strains. Of course, if a glass is hot, nothing cold should be allowed to touch it. The only remedy is not to touch the glass unless this is absolutely necessary, and then to use care to see that the adjustment is not altered in any way. |