2819. If exhaust steam is to be had for heating purposes, a double line of pipes, running under the shelves on which the cells rest, will be beneficial, as the heat will cause a circulation of the electrolyte, and will decrease its resistance. Such an arrangement is very necessary for cells of the Waddell-Entz type (see Art. 2783), but for ordinary lead accumulators the advantage is not sufficient to warrant any great outlay for such heating.
2820. The space to be allowed for a battery depends on the make and type, and may usually be found in the catalogues of the manufacturers. Lead accumulators of ordinary size will usually have a capacity of 2.5 to 4 ampere-hours per square inch of floor space that they occupy; for large accumulators in lead-lined boxes, such as are installed in central stations, this value may be increased to 5. A good average figure for cells of 200 to 500 ampere-hours capacity is 3 ampere-hours per square inch of floor space occupied.
The output per cubic inch of volume is rather more constant, being about .25 to .3 ampere-hour. The weight of a battery of accumulators is considerable, and the shelves or other supports intended to hold them should be made amply strong, for if they sag or bend, the glass jars are liable to be broken.
2821. The electrolyte of an accumulator will not freeze until exposed to a temperature of about — 11° C. (about 13° F.); freezing should be avoided, as it is very liable to break the jars.
2822. The number of cells required for any given installation depends upon the E. M. F. desired; ordinary lighting plants are usually designed for an E. M. F. of 50 to 55 or 100 to 120 volts. The number of accumulators required may be found by dividing the E. M. F. required by the average E. M. F. of the cell during discharge, which is usually taken (for lead accumulators) as 1.9 volts; a 55-volt installation would then require 551.9 = 29 cells (obviously, fractional cells are an impossibility).
