Strongly Basic Material. If a strongly basic material is used in
the exchanger, acids are absorbed by the anion resin. In reality, no chemical
exchange takes place. The resin picks up (absorbs) the siliceous acid but, in
return, does not give up anything to the makeup water, except water. No
solids are added to the makeup water. By similar reactions, the
demineralizing anion unit absorbs carbonic acid, sulfuric acid, and
hydrochloric acid which are the end products of the hydrogen cation process.
A degasifier or vacuum deaerator should be installed after the cation
exchanger to reduce the carbon dioxide load on the anion exchanger. A
demineralizing unit of this type produces top quality makeup water for high
Mixed-Bed Demineralizer. A mixed-bed demineralizer is a single
vessel containing an intimate mixture of cation and anion exchanger material.
The mixed bed is equivalent to an almost infinite number of two-step
demineralizers in series. Mixed-bed treated water is the closest approach to
pure water. It is recommended in preference to the multi-bed unit where first
cost is important and operating cost is secondary.
Demineralizer versus Evaporators. A demineralizing system is able
to produce high quality water without heat. An evaporator requires heat.
This fact is important in starting up a boiler plant. The evaporator is tied
in with a fixed plant heat cycle, while the demineralizer is independent. An
economic study to select one should include equipment cost, owning costs,
value of evaporator heat loss or energy degradation, chemical operating cost,
and labor maintenance charges.
for sodium cation exchangers. Raw water flow and total dissolved solids
affect the overall operating and owning costs. Continuous boiler blowdown is
Other External Treatment
Filters. Pressure filters may be used to remove suspended matter
by straining, screening, and frictional resistance; see Figure 35.
Media. Generally, use sand as the filtering material for cold
water; for above 125 degrees F (79.75 degrees C), anthracite coal should be
substituted to avoid silica dissolving in the hot water.
Size. Size the filter for a maximum flow rate of 3 gpm per square
foot (0.002 cubic meters per second per square meter) of filter area.
Backwash. The rate of backwash should be 12 to 15 gpm per square
foot (0.149 to 0.186 cubic meters per second per square meter) of filtering
area for sand, and 8 to 10 gpm per square foot (0.0005 to 0.0007 cubic meters
per second per square meter) for anthracite coal. The rate should be
accurately regulated by an orifice, a rate-of-flow controller, or other means.