At intervals along these mains are placed a number of
water reservoirs which receive the water injected into the mains; in
addition to these the direct flow of the air is interrupted by
numerous siphons, the stop valves to branches, etc. Investigation
showed that the presence of these reservoirs created considerable
resistance on account of an increased and subsequently reduced
section. The exact loss from this cause was, therefore, carefully
measured, as well as the losses existing in the mains not so
interrupted. The results show that the loss by expansion at one
reservoir, when the speed of the air flow was 23 ft. per second, was
equal to 0.15 atmosphere; with a speed of 29 ft. 6 in. per second, it
amounted to 0.2 atmosphere.
Therefore, the presence of five such reservoirs would cause a loss in
pressure equal to one atmosphere. This very undesirable arrangement is
not repeated in the new system, the sumphs being connected in such a
way as not to modify the section of the tube, nor consequently the
pressure of the air. The presence of the siphons and stop valves did
not seem to affect the pressure to any measurable extent. The
following table contains a list of the more important mains tested,
and it may be mentioned that the resistance, due to the reservoirs,
was at first partially included. The trials were carried out while the
mains were not being drawn upon by subscribers.
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