If the bend is sharp,
or if there is a right-angle, an allowance should be made if it is
desired to put in pipes of the smallest permissible dimensions. In the
case of the most usual sizes of pipes employed for acetylene mains or
services, it will suffice to reckon that each round or square elbow is
equivalent in the resistance it offers to the flow of gas to a length of
5 feet of pipe of the same diameter. Hence if 5 feet is added to the
actual length of pipe to be laid for every bond or elbow which will occur
in it, and the figure so obtained is taken as the value of _l_ in
formulae (i), (ii), or (iii), the values then found for Q, _d_, or
_h_ will be trustworthy for all practical purposes.
It may now be useful to give an example of the manner of using the
foregoing formulae when the tables of sizes of pipes are not available.
Let it be supposed that an institution is being equipped for acetylene
lighting; that 50 burners consuming 0.70 cubic foot, and 50 consuming
1.00 cubic foot of acetylene per hour may be required in use
simultaneously; that a pressure of at least 2-1/2 inches is required at
all the burners; that for sufficient reasons it is considered undesirable
to use a higher distributing pressure than 4 inches at the gasholder,
outlet of the purifiers, or initial governor (whichever comes last in the
train of apparatus); that the gasholder is located 100 feet from the main
building of the institution, and that the trunk supply-pipe through the
latter must be 250 feet in length, and the supplies to the burners,
either singly or in groups, be taken from this trunk pipe through short
lengths of tubing of ample size.
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