accumulator defined in 1939 yearaccumulator - ACCUMULATOR;
accumulator - Below are described the various types of accumulators. Related information should be sought in articles such as Battery; Cell; Electricity; Hydraulics; Motor-car; and Water Power.
An accumulator is a device by means of which energy can be stored for future use. Accumulators are of various types, which differ according to the method in which the power to be stored is generated and applied. For example, a water-spring may be said to constitute an accumulator in that, when wound up, it conserves and imparts power for operating mechanism, but the term is usually applied to devices for the storage of power in other forms. In engineering, the principal types of accumulator for the storage of energy on a large scale are hydraulic, steam, pneumatic, and electric.
hydraulic accumulatorWater being for all practical purposes inelastic, it follows that pressure imparted to it must be maintained without fluctuation, if the water is used for driving hydraulic machinery. To secure a constant pressure of 1,000 Ib. per sq. in. requires a natural head of water of more than 2,200 ft. As this is unobtainable, an artificial head of water, giving any desired pressure, is created by the hydraulic accumulator, invented by Sir W. G. Armstrong. Of the several varieties of this, the ordinary weight type (Fig. 1) consists of a long vertical cylinder in which a ram works up and down. The ram is heavily loaded with weights which always tend to force it downwards. Piping connected to the cylinder is laid to the various hydraulic machines to be served, To charge the accumulator, water is pumped into the cylinder at the lower end, lifting the loaded ram to the top of its stroke, when, the cylinder being full,the water supply is automatically cut off. The pressure in lb. per sq. in. exerted by the ram upon the column of water is W/A where W is the weight of ram plus its load in lb., and A is the cross-sectional area of the ram in sq. ins. By varying the weights on the ram, pressure on the water may be increased or diminished.
When, by starting any hydraulic machine, water is drawn from the cylinder, the ram descending maintains a uniform pressure on the water. As soon as water is drawn off, the pumps again come automatically into action and maintain the necessary supply in the cylinder. By this means the pumps, which could not directly maintain a constant pressure supply to meet the fluctuating demands of the machines, are enabled to perform their duty satisfactorily. Working pressures obtained in this manner usually range from 700 lb. to 1,500 lb. per sq. in., although much greater pressures can be obtained. With this type of accumulator the only auxiliary power required is for the water pumps, which can be driven by any power available and sufficient.
The main principle of the differential accumulator (Fig.2) is the same, but in this case a second and larger cylinder is superimposed on the first, and the top end of the ram is fitted with a larger piston working Binder. A third and and a fourth still may be added. By the necessary arrangement of valves and piping any one of these pistons may be used, thus giving a choice of working pressures.
The invention of the hydraulic accumulator by Sir Wm. Armstrong opened a great field for the use of water as a medium for the transmission of energy for operating machinery. Under the pressure imparted by an hydraulic accumulator water may be regarded in the light of a flexible piston rod, for, being almost as incompressible, it instantly communicates pressure imparted at one end throughout the length of a pipe.
In shipbuilding and armament works it enables pressures of many thousands of tons to be obtained from a single hydraulic press. On slipways hydraulic rams haul vessels of 5,000 tons each up the incline. In spite of the increasing use of electricity, which in many cases has superseded water power, the hydraulic accumulator is still almost indispensable for heavy work where, as in forging presses, great pressures are required.
steam accumulator(Fig. 3). Modified form of hydraulic accumulator in which steam pressure is substituted for the load upon the ram. A steam cylinder is fixed above the hydraulic cylinder, in which works a piston,, secured to the upper end of the ram. Steam, admitted to this upper cylinder, which is of much larger diameter than the hydraulic cylinder, acting upon the larger piston area, exerts a constant pressure upon the ram. By proportioning the areas of the steam piston and the ram, a comparatively low steam pressure can be made to exert a far greater pressure per sq. in. on the ram.
The advantages of this type are that, by varying the steam pressure, any desired pressure, within the limits of the machine, may be obtained, and that steam, while maintaining a steady pressure, provides a cushioning effect as compared with a dead weight. It necessitates the use of a boiler for generating the steam required.
pneumatic accumulatorThis is a modified form of hydraulic accumulator. It is similar in action to the steam accumulator, but compressed air is employed in the upper cylinder in lieu of steam, the advantages being common to both. In this type an air compressor is an essential adjunct. Another form of pneumatic accumulator is known as a compressed air-receiver.
electric accumulatorThis is a secondary cell, storage cell, or "reversible cell," in which electrical energy, passed into the cell as an electric current, is converted into electro-chemical energy and remains inactive until reconverted into an electric current by the discharge of the cell, which takes place when the terminals of the positive and negative electrodes are connected outside the cell. An assemblage of such cells is termed a battery. A secondary cell, when charged, resembles a primary cell in that it is capable of generating current, but with the difference that, when exhausted, it can be recharged again and again by passing an electric current through it in the opposite direction.
description of cellThe first satisfactory secondary cell was invented by Plante in 1860, and the principle of this applies to the cell of to-day. It is a lead-sulphuric-acid cell, and consists of a jar or case containing a solution of sulphuric acid and water called the electrolyte; in this two or more lead plates are immersed, known as the electrodes, or as the positive and negative plates, or anode and cathode. The capacity of the cell depends upon the superficial area of plate exposed to the solution; consequently the greater the capacity required the larger the cell and the greater the number of plates employed,the positive and negative plates being arranged alternately.
preparation of platesBefore use the lead plates have to be formed or prepared. The Plante process was a tedious one, and involved immersing the plates in dilute sulphuric acid and repeatedly passing an electric current through them, first in. one direction and then in the opposite, until, as the result of electrochemical action, the positive plate became coated with peroxide of lead and the negative plate was reduced to a spongy condition of pure lead. This process was simplified in 1880 by Faure, who, by coating the plates with red lead, obtained the desired result much more rapidly. Another difficulty was that a permanent coating of lead sulphate was gradually deposited on flat plates. This led to the substitution of lead grids, into numerous interstices of which a paste, consisting of sulphuric acid and Pb304 (red lead) for positive plates and PbO (litharge) for negative ones, is firmly pressed, and which is rapidly reduced by the current to peroxide of lead on the positive, and to a soft porous mass of lead on the negative plate, presenting a greatly increased effective surface in contact with the electrolyte, for a given size and weight of plate. This is known as the E.P.S. cell.
chemical actionThis is of a complex nature, but may be explained as follows. Supposing the cell to be fully charged; during discharge, by the electrolytic action of the current, oxygen is freed and conveyed to the negative plate, the surface of which, attacked by the acid, tends to become converted into sulphate of lead, while hydrogen deposited on the positive plate is oxidised by the peroxide and the resultant oxide, attacked by the acid, tends to produce sulphate or lead on the positive plate also, a portion of the acid in the solution being abstracted in the process. When the cell is almost discharged, the temporary loss of acid in the solution will have reduced the specific gravity of the latter to 1 '18. This may be checked by means of an hydrometer, and forms a valuable means of ascertaining the condition of a cell. When the battery is recharged, the chemical process is reversed. Hydrogen is deposited at the negative plate, converting it again into pure lead; the positive plate is converted by oxygen into peroxide of lead again, while the sulphuric acid originally abstracted in the formation of the lead sulphate is freed and returned to the solution, raising its specific gravity to 1.2 again when the cell is fully charged.
electromotive forceE.M.F. This for a single cell averages two volts, commencing, with cell fully charged, at 2.2v. and falling finally to l.8v. when the cell is nearly discharged. If discharge be continued beyond this point, the plates become permanently sulphatcd and the efficiency of the cell seriously impaired. The energy efficiency in watt hours under favourable conditions may amount to 70 p.c., but, under unskilful supervision, it may be anything less.
capacity. Capacity is measured in ampere hours, and depends upon requirements, from the small two-cell four-volt accumulator up to large station batteries. The capacity, size, number, and arrangement of individual cells are proportioned to the number of ampere hours and voltage required. Cells may be connected either in series or in parallel. To connect them in series the positive terminal of one cell is connected to the negative of the next, and so on according to the voltage required. Each cell, irrespective of size, gives two volts; variation in size simply affects the number of ampere hours output. Thus the output of 24 cells of 20 ampere hours each in series will be 20 ampere hours at 48 volts. However many cells are connected in series only, the output in ampere hours will be that of one cell. Only cells of the same capacity should be coupled together. To connect cells in parallel all the positive terminals are joined together and all the negatives together; by this means the ampere hours output is increased while the voltage remains at two. Thus if one cell lias a capacity of say 20 ampere hours, 24 cells in parallel would givo 480 ampere hours but only at 2 volts. But if the 24 cells be arranged in two parallel rows, each having 12 cells in series, we should get 40 ampere hours at 24 volts.
In station installations a few additional cells are included to overcome circuit resistance, etc., which causes a drop in the voltage. Thus, if a current of 50 volts is to ba provided, the number of cells required in series will be not 50/2 but 50/1.85 or 27 cells. Station cell eases (Fig. 4) are of glass, or lead or lead-lined boxes are employed. Small portable accumulator cases (Fig. 5) are made of celluloid or othersuitable material forlightness.
chargingWhen an accumulator is charged for the first time, the acid solution must not be placed in the cell until the charging current is ready to switch on. In mixing the solution acid must be added slowly to water - not water to acid - in the proportion of one part (by measurement) of sulphuric acid and five parts of distilled water. When cold, test the specific gravity, which should be 1-19. The rate of charging (direct current) should not exceed the accumulator rating. The voltage should not be less than the number of cells in series multiplied by 2'75. When fully charged, the specific gravity rises to nearly 1'21, the solution has a milkv appearance, and the positive plates are a dark chocolate colour. Other types of cell include that invented by Edison, who used iron and nickel peroxide in a solution of caustic potash. The mean E.M.F. of this cell is about 1.1 volts. Advantages claimed are great strength and capacity per pound of its own weight; rapid charging and discharging; and that it does not deteriorate if left uncharged.
Drumm accumulator, which is fitted with pencil-type positive plates and negative plates in the form of sheets of gauze, on which metal zinc is electrolytically deposited. A cell of this type is capable of high rates of discharge and can be charged at fairly high rates; generally its characteristics make it suitable for electric trains and it was used on the old Great Southern Railway of Ireland on trains from Dublin.
The nickel-cadmium cell employs nickel hydroxide and graphite as the active positive material and negative plates containing a mixture of cadmium and iron. Discharge characteristics are like those of the Edison nickel-iron cell.
care of accumulatorsThe efficiency and life of lead accumulators depend upon careful maintenance. A cell which might last 8 years may be rendered useless in as many months by mishandling or neglect. (1) The amperage of the charging current should not exceed that for which the accumulator is rated, or the plates become buckled and portions of the active materials fall out as the result of too rapid freeing of the gases. (2) Discharge must not proceed more rapidly than the rated capacity or beyond the point where the voltage drops to T85. (3) Accumulators should never be allowed to remain in a discharged condition, otherwise permanent sulphating of the plates occurs. They should not be allowed to remain unused, even when charged, for very long periods. It is better occasionally to discharge and recharge them. (4) The specific gravity of the solu. tion should be occasionally checked and evaporation made good with distilled water only. Too high a specific gravity results in the acid unduly attacking the lead while too low a specific gravity reduces efficiency. (5) In replenishing solution only brimstone sulphuric acid and distilled water should be used. (6) Individual cells, as well as the battery, should be tested once a week for voltage and, if incorrect, should at once be examined for short circuit or other defect. (7) Terminals should be kept clean and free from corrosion. (8) If a eel! or battery is not required for use for a considerable time it should first be fully charged, all the solution taken out, the terminals cleaned and greased, and in this condition it should be stored in a dry place or carefully packed for carriage. The exterior of cases should be kept clear of all moisture and dirt. Purity of materials and cleanliness are important. These precautions apply to all lead-sulphuric-acid accumulators.
The applications of electric accumulators are numerous. At power stations and hospitals they form a valuable stand-by in case of emergency, such as a temporary failure of the supply. In country-house and similar installations a battery is a necessity, since without it the plant would have to be run whenever a light was required. Small portable accumulators are used for radio sets.
Bibliography. H. G. Brown, Lead Storage Battory, 1938; W. S. Ibbetson, Accumulator Maintenance and Repair, 1942; G. W. Vinal. Storage Batteries, 1940.
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