ASHRAE Journal - June 2022 - 24

ENGINEER'S NOTEBOOK
COLUMN
Water Treatment 102
BY JEFF BOLDT, P.E., HBDP, FELLOW ASHRAE; JOSHUA INCE, P.ENG., MEMBER ASHRAE
In HVAC systems the most common goals of water treatment are to maintain peak
performance, reduce water consumption and minimize corrosion, scale, and fouling
while controlling microbial growth. In " Water Treatment 101 " in the March ASHRAE
Journal, we discussed closed water systems, glycol systems, condenser water and other
open water systems, and steam systems, as well as related topics. This month we
concentrate more on domestic water, sprinkler systems, and pure water.
Domestic Water and Sprinkler Systems
Water Softening
In general softening water increases its corrosiveness
and reduces its tendency to scale pipes. The decision
about whether to soften, and what percentage of the
water to soften, depends on the inlet water chemistry
and the temperature at which it will be circulated.
Softeners are available in simplex (one resin tank),
Figure 1, and duplex (two resin tanks for continuous
operation), Figures 2 and Figure 3. Essentially softeners
replace each 2+ molecule (e.g., calcium) with two 1+
molecule (e.g. sodium).
Water Analyses Indices
Jeff and Josh created a workbook that can be used to
calculate the Langelier Saturation Index (LSI),* Ryznar
Saturation Index (RSI),† and Puckorius Scaling Index
(PSI).‡ See Reference 1 for more detailed information
about each of these indices. LSI is the oldest index and
was developed to predict whether city water mains would
scale or corrode. RSI is an updated index that is generally
more accurate and more applicable to a wider range of
temperatures. PSI is intended more for condenser water.
Often service (domestic) hot water will be softened and
cold water not, depending on the incoming water.
*Langelier Saturation Index: LSI is an index that estimates the propensity for water to either dissolve or deposit calcium carbonate. LSI is
often used as an indicator of the corrosivity of water. It considers total alkalinity (M-alkalinity), calcium concentration, dissolved solids, and
pH at a specific temperature.
†Ryznar Stability Index: RSI is an index that was developed from empirical evidence. Water with RSI <6.5 tends to form scale. Water with
RSI >8 tends to dissolve CaCO3 (lime). It is intended to be an improvement over LSI, which was developed mainly for municipal water
mains.
‡Puckorius Scaling Index (PSI): PSI is most often used for cooling towers and other evaporative heat rejection components. PSI attempts
to quantify the relationship between saturation state and scale formation by incorporating an estimate of buffering capacity that is not in
the LSI or RSI indices. Water high in calcium, but low in alkalinity and buffering capacity can have a high calcite saturation level. The high
calcium level increases the ion activity product. A plot of ion activity product versus precipitate for the water would show a rapid decrease
in pH as calcium precipitated due to the low buffering capacity. Even small decreases in carbonate concentration in the water would drastically
decrease the ion activity product due to the small quantity present prior to the initiation of precipitation. Such water might have a
high tendency to form scale due to the driving force, but scale formed might be of such a small quantity as to be unobservable. The water
has the driving force but not the capacity and ability to maintain pH as precipitate matter forms. The PSI index is calculated in a manner
like the Ryznar stability index. Puckorius uses an equilibrium pH rather than the actual system pH to account for the buffering effects.
Jeff Boldt, P.E., HBDP, is managing principal at IMEG Corp. He is SSPC 90.1 mechanical subcommittee chair. Joshua Ince, P.Eng., is a regional sales manager at DuBois Chemicals.
He is vice chair of TC 3.6, Water Treatment.
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