MIL-HDBK-1003/6
Control. Control of the softening and regeneration cycles may be
7.5.2.1
either automatic, semi-automatic (manual initiation), or manual.
a) Manual systems -- Provide a valve nest or a multiport valve to
control the steps in regeneration. A water meter shall indicate water flow
through the unit.
b) Automatic systems -- Provide a meter on the soft water effluent
line to actuate the multiport control valve after a predetermined quantity of
water has been softened. An automatic valve nest may be provided in lieu of
the multiport.
The heart of regeneration control, automatic and manual operation,
is the multiport valve, which opens and closes the ports for the various
operations of regeneration. The multiport valve shall have a manual operator
for emergency use. Additional control is provided by rate-of-flow controllers
for wash and rinse water. For the cold water ion-exchange process, provide
either float-operated butterfly valves, in sumps, or automatic-throttling
backwash outlet valves of the multiport type by properly located orifice
plates.
For the hot ion-exchange process, a valve nest is used, the
backwash rate must be controlled to avoid thermal shock and resin carryover.
Brine may be transferred from the brine measuring tank to the ion-exchange
tank either by pump or hydraulic eductor.
Computation. Calculation of the size of equipment should be done
7.5.2.2
in accordance with the following example.
a) Assume 100 gpm continuous makeup and total hardness of 171 ppm
(i.e. 171 mg/L or 75 grains/cubic foot as calcium carbonate).
b) The amount of daily makeup at 100 gpm (0.379 cubic meters) is
144,000 gallons (545.5 cubic meters). In the double unit system each softener
handles half this amount, or 72,000 gallons.
c) If each softener is sized to regenerate daily, the unit
capacity must be 72,000 gallons times 10 grains per gallon, or 720,000 grains,
or 720 kilograins.
d) A styrene-based cation resin is rated at 25,000 grains per
cubic foot and is regenerated with 10 pounds of salt per cubic foot. The
amount of resin for 720,000 grains is found by dividing 720,000 grains by
25,000 grains per cubic foot; therefore 28.8 cubic feet of resin is required.
e) The maximum unit flow of 10 gallons per minute per square foot
requires 10 square feet (0.929 square meters) of area. The closest unit size
is 42 inches (1.07 m) in diameter. The exchanger bed depth is then about 3 ft
(0.9 m).
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