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12.40 CHAPTER TWELVE

1. Subsurface backwash. Subsurface backwash is performed at the normal backwash
rate for a reduced period of time compared to other designs, by bringing water in
through the regenerant collector and out the top of the tank.
2. Drain to bed level. Normally air is brought in through the vent and is used to
pressure-drain the water to the level of the regenerant collector.
3. Chemical (regenerant) injection. Chemical injection is employed at the same flow
rate, concentration, and time as in other designs. The limitation of upward regenerant
flow rate is about the same as for other upwardly regenerated units (approximately 2
to 3 gpm/ft 2 maximum).
4. Displacement or slow rinse. Displacement is normally employed at the dilution wa-
ter rate, although it is sometimes advantageous to displace at a higher rate. The re-
quirement is at least three bed volumes.
5. Refill. Refill is generally performed from the top through the rinse water valve.
6. Optional rinse to waste. Rinse to waste depends on the water quality requirements.
The rinse to waste is approximately three to five bed volumes.
7. Rinse recycle. Rinse recycle is 6 to 10 bed volumes for the cation and 10 to t5 bed
volumes for the anion, normally performed at the service rate. The volume depends
on the endpoint desired.

Packed Bed Design
Another factor in the choice of demineralizer is the size required. The packed bed design
offers the very smallest possible size. The coflow design offers the widest potential vari-
ety of sizes and does not require that the size be fitted exactly to the resin volume. One
of the potential problems with all countercurrently regenerated units is that the resin vol-
ume is fixed by the vessel size and must be very close to an exact volume (Figure 12.10).
Resins expand and contract during service and exhaustion. Resins are shipped by weight

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FIGURE 12.10 Packed bed exchanger.