Page 182 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

WATER TREATMENT AND PURIFICATION

4.18 CHAPTER FOUR

pressure. There are two types of deaerators, steam and vacuum. When heated water is
needed, for example, for boilers, the steam type is preferred. When cold water is required,
the vacuum type is used.
Steam deaerators break up water into a spray or thin film, then sweep the steam
across and through it to force out the dissolved gases. Using this method, oxygen can
be reduced to near the limit of detectability. Designs fall broadly into spray, tray, and
combination units. Typical deaerators have a heating and deaeration section and a storage
section for hot, deaerated water. Often, a separate tank is provided to hold an additional
10-mm supply of deaerated water. The direction of steam may be crossflow, downflow,
or countercurrent. The majority of the steam condenses in the first section of the unit.
The remaining mixture of noncondensable gases is discharged to the atmosphere through
a vent condenser.
Vacuum deaerators use a steam jet or mechanical vacuum pumps to develop the required
vacuum used to draw off the unwanted gases. The vessel has a packing material inside, and
the inlet water is introduced to the top of the unit and passed down through this packing.
Deaerated water is stored at the bottom of this vessel. The steam or vapor vacuum located
at the top of the unit discharges the unwanted gases. The vacuum unit is far less efficient
than the steam one, and is most often used in demineralizer systems to reduce the chemical
operating cost and the demineralizer size, and to protect anion exchange resins from pos-
sible oxidation damage.

DECARBONATION

Decarbonation is rarely used but should be considered if the bicarbonate level in the feed-
water is in the range of 14 to 20 mg/L or higher. Decarbonation is usually accomplished
by the use of filtered air counterflowing through the water stream and stripping out the
carbon dioxide.

DEALKALIZING

Dealkalizing is a process that reduces the alkalinity of feedwater. This can be done either
with or without the use of acid regenerant. Without the acid, the regenerant used is salt, and
the process is often called salt splitting. The salt-splitting process exchanges all bicarbon-
ate, sulfate, and nitrate anions for chloride anions. For best results, it is recommended that
deionized water be used as feedwater. Where hard, alkaline, low sodium water is available,
the use of a weak acid resin should be considered.
The entire regeneration cycle is similar to that previously described for a water soft-
ener. It is not uncommon to use the same salt and regenerant piping to accommodate
both the softener and dealkalizer. Some caustic soda may be added (1 part caustic soda
to 9 parts salt) to reduce leakage of alkalinity and carbon dioxide. Since this can cause
hardness leakage from the dealkalizer, a filter downstream of the processed water is
necessary.
A weak acid resin can also be used. The process transfers the alkaline salts of cal-
cium and magnesium to the weak acid resin. This process should include degasification
if required by the product water. The weak acid process operates at a very high utilization
factor, near the theoretically required amount. Hydrochloric acid is preferred for regenera-
tion rather than sulfuric acid. This process is very sensitive to the flow rate, temperature,
and contaminant level of the feedwater. These changing conditions must be considered by
the manufacturer in the design of the process. The entire regeneration cycle is similar to
that of cation ion exchange column.

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