Page:EB1911 - Volume 22.djvu/662

 removing the air or other gas from a vessel, whilst a compression pump compresses the air. The simplest forms of pumps employed for forcing liquids are “plunger pumps,” consisting essentially of a piston moving in a cylinder, provided with inlet and outlet pipes, together with certain valves. The disposition of these valves divides this type of pump into suction pumps and force pumps.

Fig. 1. shows the arrangement in a suction pump. A is the cylinder within which the piston B is moved up and down by the rod C. D is the inlet pipe (the lower extremity of which is placed beneath the surface of the liquid to be removed), and G is the outlet pipe. E is a valve in the inlet pipe opening into the cylinder; and the piston is perforate by one or more holes, each fitted with valves opening outwards on its upper surface. On raising the piston, the valve F remains closed and a vacuum tends to be created in the cylinder, but the pressure of the atmosphere forces the liquid up the tube D and it raises the valve E and passes into the cylinder. On reversing the motion the valve E closes and the liquid is forced through the valve F to the upper part of the cylinder. On again raising the piston, more liquid enters the lower part of the cylinder, whilst the previously raised liquid is ejected from the delivery pipe. Obviously the action is intermittent. Moreover, the height of the lift is conditioned by the atmospheric pressure, for this is the driving force; and since this equals 34 ft. of water, the lift cannot be theoretically more than this distance when water is being pumped. In practice it may be considerably less, owing to leakage at the valves and between the piston and cylinder.

In the force pump (fig. 2) there is no such limitation to the lift. In this case the piston is solid, and the outlet pipe, G which is placed at the bottom of the cylinder, has a valve F opening outwards, the inlet pipe and valve are the same as before. On raising the piston the liquid rises in the cylinder, the valve E opening and F remaining shut. On reversing the motion the valve E opens admitting more liquid whilst F remains closed. It is seen that the action is intermittent, liquid only being discharged during a down stroke, but since the driving force is that which is supplied to the piston rod, the lift is only conditioned by the power available and by the strength of the pump. A continuous supply can be obtained by leading the delivery pipe into the base of an air chamber H, which is fitted with a discharge pipe J of such a diameter that the liquid cannot escape from it as fast as it is pumped in during a down stroke. The air inside is compressed in consequence and during an upstroke of the piston this air tends to regain its original volume and so expels the water, thus bringing about a continuous supply. For a description of modern pumps, see.

. 2.

Air-pumps.—Pumps for evacuating vessels may be divided into three classes: (1) mechanical, (2) mercurial, and (3) jet pumps; the last named are treated in. The invention of the mechanical air-pump is generally attributed to Otto von Guericke, consul of Magdeburg, who exhibited his instrument in 1654; it was first described in 1657 by Gaspar Schott, professor of mathematics at Württemberg, in his Mechanica hydraulic-pneumatic, and afterwards (in 1672) by Guericke in his Experimenta nova Magdeburgica de vacus spatia. It consisted of a spherical glass vessel opening below by means of a stop-cock and narrow nozzle into the cylinder of an “exhausting syringe,” which inclined upwards from the extremity of the nozzle. The cylinder, in which a well-fitting piston worked, was provided at its lower end with two valves. One of these opened from the nozzle into the cylinder, the other from the cylinder into the outside air. During the down-stroke of the piston the former was pressed home, so that no air entered the nozzle and vessel, while the latter was forced open by the air which so escaped from the cylinder. During the return stroke the latter was kept closed in virtue of the partial vacuum formed within the cylinder, while at the same time the former was forced open by the pressure of the denser air in the vessel and nozzle. Thus, at every complete stroke of the piston, the air in the vessel or receiver was diminished by that fraction of itself which is expressed by the ratio of the volume of the available cylindrical space above the outward opening valve to the whole volume of receiver, nozzle and cylinder. The action is essentially that of the common suction pump. The construction was subsequently improved by many experimenters, notably by Boyle, Hawksbee, Smeaton and others; and more recently two pump barrels were employed, so obtaining the same degree of exhaustion much more rapidly. This type of pump is, however, not very efficient, for there is not only leakage about the valves and between the piston and cylinder, but at a certain degree of exhaust the air within the vessel is insufficient to raise the inlet valve; this last defect has been met in some measure by using an extension of the piston to open and close the valve.

The so-called oil air-pumps are much more efficient; the valve difficulty is avoided, and the risk of leakage minimized; whilst in addition there is no air clearance between the piston and the base of the cylinder as in the older mechanical forms. The Fleuss pump may be taken as an example. The piston, provided with a valve opening upwards, is packed in the cylinder by a leather cup which is securely pressed against the sides of the cylinder by the atmospheric pressure. The piston rod passes through a valve in the upper part of the cylinder which is held to its seat by a spring. The inlet pipe enters an elliptical vessel which communicates with the cylinder a little way up from its base, whilst at the base there is a relief tube leading into the elliptical vessel already mentioned. Oil is placed both above the upper valve seating, and also in the cylinder up to the height of the lower edge of the inlet pipe. The action is as follows: On raising the piston it cuts off communication with the inlet pipe and then compresses the air above, forcing it through the upper valve and oil into the atmosphere. Some of the oil is also driven out, but as the valve does not close until the piston has descended a short distance, a certain amount of oil returns. On lowering the piston its valve opens and air passes in from the vessel to be exhausted; this is further rarefied on the next stroke and so on. The Max Kohl pumps are based on the same principle, but are constructed with more elaborate detail, leading to a greater efficiency, an exhaust of 0·0008 mm. being claimed as readily obtainable.

The invention of the barometer and Torricelli’s explanation of the vacuity above the mercury column placed before the members of the Florentine academy a ready method of obtaining vacua; for to exhaust a vessel it was only necessary to join, by means of a tube provided with stopcocks, the vessel to a barometer tube, fill the compound vessel with mercury and then to invert it in a basin containing this liquid, whereupon the mercury column fell, leaving a Torricellian vacuum in the vessel, which could be removed after shutting off the stop-cocks. This was the only method known until the invention of the mechanical air-pumps; it was subsequently employed by Count Rumford, and as late as 1845, Edward A. King patented filament electric lamps exhausted by the same methods. Although modern mercurial pumps have assumed a multiplicity of forms, their actions can be reduced to two principles, one statical, the other hydrodynamical-at the same time instruments have been devised utilizing both these principles.

Statical Pumps.—The earliest mercurial pump, devised by Swedenborg and described in his Miscellanea observata circa res naturales (1722), was statical in action, consisting essentially in replacing the solid piston of the mechanical pump by a column of mercury, which by being alternately raised and lowered gradually exhausted a vessel. A more complicated pump, but of much the same principle, was devised in 1784 by Joseph Baader, to be improved by C. F. Hindenburg in 1787, by A. N. Edelcrantz in 1804 and by J. H. Patten in 1824; whilst in 1881 Rankine Kennedy resuscitated the idea for the purpose of exhausting filament electric lamps. The pump devised by