Birney C. Batcheller

“ It happens that we are living in an atmosphere of about fifteen pounds pressure per square inch, and it is very convenient oftentimes in our computations to take the pressure of the atmosphere as our zero, and reckon all other pressures above and below this. If all our pressure scales read from absolute zero, the pressure of a perfect vacuum, then all this confusion would be avoided. We have not used the absolute zero in our diagram, because all our gauges are graduated with their zero at atmospheric pressure, and it is customary to speak of pressures above and below the atmospheric. ”

“ In regard to the question of relative economy of the two systems, we will say that when long tubes are used, requiring high pressures, or, more strictly speaking, a large difference of pressure, to maintain the desired velocity of air-current, there seems to be some advantage in using an exhaust system. The reason is this: the friction of the air in the tube, which absorbs most of the power, increases as the air becomes heavier and more dense. ”

“ Efficiency.—It will be noted that most of the power is not used primarily in moving the carriers, but to move the air through the tube. Very nearly as much power is used to keep the air flowing in the tube when no carriers are in it as when carriers are being despatched. If we should define the efficiency of a pneumatic tube as the ratio of the power consumed in moving the carriers to the power consumed in moving the carriers and the air, we should find this so-called efficiency to be very low. ”

“ If a larger than eight-inch tube is to be used for mail service, it should be not less than thirty-six inches. Carriers larger than eight inches cannot be handled: they are too heavy. They are also too heavy to slide through the tube, hence, must be mounted upon wheels. It is not practical to make a carrier on wheels less than eighteen or twenty-four inches, and the carrier must be at least twenty-four inches to contain a large mail-pouch. Now, if we are going to despatch mail-pouches through a pneumatic tube we must send more than one in a carrier, otherwise the service will be too slow. ”

“ Now we will draw a smooth curve through the tops of all our vertical lines, and we have a curve, E, G, I, K, L, representing the pressure in the tube at every point. It falls gradually from ten pounds to zero, but it does not fall in exact proportion to the distance from the tank. Such a fall of pressure would be represented by the straight dash-line, E, L. The reason why the true pressure-curve is not a straight line, and lies above a straight line, is because air is an elastic fluid and expands, becoming larger in volume as the pressure diminishes. ”

“ The two systems are sometimes distinguished by calling one a pressure system and the other an exhaust system. These terms are very misleading, for an exhaust system is a pressure system. The current of air is kept flowing in a tube by maintaining a difference of pressure at the two ends, and the result is the same whether we raise the pressure at one end above the atmospheric or lower it at the other below the atmospheric. ”

“ Horse-Power.—Having shown how the quantity of air can be computed we are now in a position to estimate the horse-power of the air-compressor necessary to operate the tube. I say estimate, because we have to take into consideration the efficiency of the air-compressor, and that is not an absolutely fixed quantity: it varies with different types of machines and with their construction. When working with pressures of less than ten pounds, the friction of the machine is an important factor. ”

“ This moisture is brought into the tube with the air, and is deposited upon the walls of the tube when their temperature is sufficiently below that of the atmosphere. The atmosphere always contains more or less moisture in the state of vapor. The capacity of air for water-vapor depends upon its temperature, being greater the higher the temperature, but it is a fixed and definite quantity at any given temperature. When the air contains all the water-vapor it can hold at a certain temperature, it is said to be saturated. ”

“ Where the ground is firm, no other support is needed for the tubes than to tamp the earth solidly about them. In order to economize space in the streets, it is customary to lay the tubes one above the other; and it is very convenient, although not necessary, to separate them by cast-iron saddle brackets. Such an arrangement has to be frequently departed from in order to overcome obstructions in the streets and to get through narrow passages. At all low points in a tube line, traps are provided to catch any moisture that may accumulate. ”

“ In operating large tubes, that is to say, tubes six or eight inches in diameter, there is an advantage in using compressed, rather than exhausted air, in the construction of the sending and receiving apparatus, especially when the tubes are very long. With an ample supply of compressed air always at hand, the air-cushions can be made shorter and more effective in bringing the carriers quickly to rest.140 With exhausted air the cushions are ineffective, and consequently must be made very long in order to stop the carrier before it strikes the closed end of the tube. ”

“ If ten carriers per minute are despatched from each office, and each carrier contains two hundred letters, the tube has a carrying capacity of two hundred and forty thousand letters per hour, which is far beyond the requirements of this office. About five hundred carriers a day are despatched from each office. This varies considerably on different days and at different seasons of the year. Experience has taught that a certain period of time should elapse between the despatching of carriers, in order that they may not come in contact with each other, and that the receivers may have time to act. ”

“ Aim and Object of the Company.—When the Pneumatic Transit Company was formed, it was the aim and object of its promoters to construct an extensive system of underground tubes in the City of Philadelphia which would serve, first, for the rapid transmission of mail, second, for the quick delivery of merchandise from the large retail stores, third, for the transmission of telegrams or messages within the city limits, and, fourth, to conduct a general local express business with greater speed than can be done in any other manner. ”

“ The currents of air entering and leaving the tubes are not so strong as to cause any special annoyance. At the cashier’s desk some simple receiving and sending apparatus has to be used, but it is better to concentrate all of the apparatus at one point rather than have it distributed about the store, as would be the case if the double system were not used. In the London pneumatic telegraph both methods of operation have been in use. Double lines were laid, and the engines exhausted the air from one tube and forced it into the other. ”

“ In working by pressure it has been found that, notwithstanding a fair interval may be allowed, carriers are apt to overtake one another, for no two carriers travel in the same times, because of differences in fit, unless they are placed end to end. If signalling be neglected and a carrier happens to stick fast, being followed by several others, a block will ensue which it will be difficult to clear, while the single carrier could readily have been dislodged. ”

“ If poppet valves are used in the air-cylinders, and they are most common, the speed in revolutions per minute should not be high. Duplex are better than single cylinder compressors, because they deliver the air in a more steady stream,—the pulsations are less. For constant running, economy of steam is an important item; therefore some good type of cut-off valve should be provided. The air-cylinders should not be water-jacketed unless the pressure is above twenty-five pounds per square inch. It is better to use the air as warm as possible, for it will soon be cooled after entering the tube. ”

“ Usually the basement of a building would have to be utilized for the termination of such a tube. There are but few places in our large cities where the streets are so free from pipes, sewers, conduits, etc., that it would be practicable to build a thirty-six-inch pneumatic tube. When the service can be rendered by an eight-inch tube, the cost of installation favors its adoption. ”

“ In order to give some idea of the energy possessed by one of these carriers travelling at this speed, it may be said that if the end of the tube were left open and turned upward, an emerging carrier would rise about forty feet into the air. It is easy to imagine how apparatus, depending for its operation upon impact with a moving carrier, would be soon destroyed, as well as the carriers themselves. This is why receiving apparatus used with small tubes and light carriers cannot be applied to large tubes with heavy carriers. ”

“ While the Pneumatic Transit Company has ample field in the State of Pennsylvania to carry out the work which it has mapped out, a field broad enough to yield a good profit for the capital invested, there is no reason why the system should be limited to one State. So, in order to obtain a broader charter, covering all places where pneumatic service may be needed, a new corporation was formed and styled the Batcheller Pneumatic Tube Company.
It is impossible to lay down a rigid system equally well58 adapted to all places and purposes. We must accommodate ourselves somewhat to circumstances. ”

“ But the disadvantage is greatly overshadowed by the advantage of being able to connect several stations by one line, instead of having to run independent lines from each station to the central, especially when the business of the individual stations is not sufficient to occupy a separate tube all the time. It makes it possible to have stations where otherwise the business would not warrant the cost of installation and expense of operation. ”

“ Except in isolated cases, an independent steam plant will be erected to supply the air for a system of tubes. This plant should be designed with a view to obtaining the maximum economy in coal consumption, labor, water, cartage, and incidental expenses. We might say that the same general rules of economy which govern the design and construction of electric-lighting plants should be applied to the plans and construction of air-compressing plants. ”

“ When the air is exhausted from the tube, we are using a current of rarefied air, and this moves through the tube with less friction and, consequently, a higher velocity, for the same difference of pressure, than the more dense compressed air. But for short tubes that require only a small difference of pressure, this advantage becomes very small, and is overbalanced by other advantages of a compressed air system. So, taking141 everything into consideration, there is not so much to be said in favor of an exhaust system. ”

“ Some of the streets of our large cities are so filled with water- and gas-pipes, electrical conduits, sewers, steam-pipes, etc., etc., that it is almost impossible to find space for pneumatic tubes, especially of large diameter. Railway or water facilities have much to do with the location of a central pumping station, on account of the coal supply. ”

“ The carriers are light enough to be easily handled; they are not so large in capacity as to make it necessary to wait for an accumulation of mail to fill them; they can be delivered from the tube on to tables at any point in the building where the mail is wanted, for cancelling, distribution, or pouching, thus rendering a very rapid service; the mail is kept moving in an almost constant stream, keeping the postal employees more uniformly employed ”

“ At curves a guard-rail is placed upon the side wall, making it impossible for a carrier to leave the track. The carriers are made of hard wood with an iron frame, and are as light as consistent with the service required of them. They are open on top. Their outside dimensions are thirty-four inches by thirty-four inches by eight feet. The sending and receiving apparatus for these very large tubes have to be specially designed for each particular station, so no attempt will be made here to describe them. The air-pressure required depends upon the length of the line. ”

“ To insure the impossibility of having carriers despatched too rapidly, we place on the sending apparatus a time-lock that will automatically lock it for a determined length of time after each carrier is despatched, the time-lock being adjustable for any desired time. ”

“ The moving parts of the electro-magnetic valve—for such is the valve C, with the magnet A, Fig. 31—are made very light, in order that they may respond easily and quickly to the closing of the electric circuit.
It is a disadvantage to have stations too numerous upon the same line, especially if they do a large amount of business, for each station will delay the sending of carriers from the others more or less, and the interference will be greatest during the busiest hours of the day. ”