Getting Technical
Supply water usually contains amounts of dissolved solids
other than calcium and magnesium carbonates, which do not
contribute to water hardness but are included in the total amount
of all dissolved solids (TDS), which is commonly used to measure
and control C/Cs, mainly because it is easily determined by
sensing electrical conductivity of both the supply make-up water
and the circulating water. Anti-scaling chemical dosing levels,
however, must be calculated on the hardness of the supply water
and the chemicals dosed in proportion to supply water demand.
There is a simple empirical formula used to calculate how
much bleed-off is needed to run an evaporative water circuit at
any particular C/C:
Bleed–off rate = Evaporation rate
C/C – 1
Water usage is evaporation plus bleed–off; therefore, minimum
water demand will be when bleed–off is minimised which, in turn,
will depend on the maximum C/C at which the circuit can be run
without problems arising due to scaling and/or other types of
fouling. In practice, most commercial and industrial evaporative
cooling water circuits run at between 6 and 8 C/C, rising up to
10–12 C/C in areas where water is supplied at lower hardness
levels. Inherent operating costs therefore include amount of
bleed-off water running to waste and chemicals.
It must also be borne in mind that insoluble solids and
substances carried by circulating water flows become more
concentrated in accordance with increasing C/Cs and tend to
drop down out of suspension in low-flow areas such as cooling
tower and evaporative condenser sumps. Increasing amounts of
these deposits create sludge, which progressively blocks suction
screens and spray nozzles. Suspended solids also tend to reduce
the effectiveness of the majority of scale control chemicals that
are anionic.
Figure 1: Cycles of concentration and water usage.
Figure 1 depicts how water usage reduces at increasing C/C and
shows clearly how running at higher C/C results in progressively
diminishing reductions in water usage. In practice, most
evaporative circuits do not exceed 9 C/C, as the increased risks of
fouling leading to costly unscheduled shutdowns do not justify
the additional small reductions in water usage. At 9 C/C, the
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amount of bleed-off water calculated by the formula given above
is 12.5% of evaporation.
As an illustration, the evaporative condensers of a
refrigeration plant at a supermarket chain’s area cold storage
facility in Midrand use an average of 27m 3 /day of supply water.
At 9 C/C, this is 24m 3 evaporation and 3m 3 bleed-off/day. Taking
into account water drained from condenser sumps at regular
monthly maintenance services, the total amount of water going
to waste exceeds 1 100m 3 /year, which means additional costs
as water prices increase, and also looming problems in ensuring
compliance with continually stricter pollution regulations.
On several occasions, Getting Technical has commented
that no fundamentally new water treatment chemicals have
been commercialised since the mid-1970s; a period of more
than 40 years. Non-chemical technologies using magnetic and
pulsing electromagnetic fields have demonstrated effective
anti-scaling capabilities, but only within limited specifications
of potentially scaling supply water. Removing scaling potential
by water softening has been increasingly adopted as an option
but is relatively costly, as well as using additional water for
regenerations. Now there is the option of replacing chemical
dosing for scale control by electrolysis, which fundamentally
alters how and where precipitation of hardness occurs in
evaporative cooling water circuits.
CHEMICAL REACTIONS DURING EL ECTROLYSIS
In water itself the reactions are:
At the anode: 2H 2 O – 4e → O 2 + 4H +1
At the cathode: 4H 2 O + 4e - → 2H 2 + 4OH - 1
When the water is potentially scale-forming, it contains dissolved
bicarbonates causing further reactions:
At the anode: H +1 + HCO 3-1 + H 2 CO 3 → CO 2 ↑ + H 2 O
At the cathode: Ca +2 + CO 3-2 → CaCO 3 ↓
These four equations demonstrate how electrolysis applied to
potentially scale-forming hard water creates conditions at the
anode producing carbon dioxide gas, which rises in bubbles
to the water surface and then disperses into the atmosphere.
At the cathode, precipitation occurs of calcium carbonate
crystals, which adhere to and build up on the surface area of
the cathode. Any system or mechanism that reduces dissolved
calcium carbonate lowers the scaling potential. Therefore,
if capture of calcium carbonate by the cathode is equal to
or more than the amounts introduced by the supply water,
then the scaling potential in circulating water is theoretically
reduced to zero.
Following several years of recent developments from
laboratory tests as well as rapidly increasing numbers of site
installations, electrolysis units are normally installed in side-
RACA Journal I January 2019
59