Atlas Copco GA 11+ Instruction Book page 90

Type of cooling system
Closed loop
The values in bold are rejection limits.
When the system contains Zn or Al, the pH must be < 8.5.
2. Total dissolved solids (TDS) and conductivity
The conductivity is expressed in μS/cm, the TDS in ppm.
Both parameters are related with each other. The conductivity is convenient for quick
monitoring of general water quality, but the TDS is required for calculating the RSI. If only
one of both parameters is measured, an estimation can be obtained by using a theoretical
conversion factor (0.67):
TDS = conductivity x 0.67
3. Hardness
Different types of hardness are in relation with each other and together with the pH and the
alkalinity of the water they determine the equilibrium situation of the water, determined and
specified by the RSI.
In addition, the calcium hardness must be limited to:
Type of cooling system
Single pass
Recirculating (with tower)
Closed loop
4. The Ryznar Stability Index (RSI)
The Ryznar Stability Index is a parameter for predicting whether water will tend to dissolve
or precipitate calcium carbonate. The adhesion of scaling depositions and their effect are
different on different materials, but the equilibrium of the water (scaling or corrosive) is only
determined by its actual pH value and by the saturation pH value (pH
value is determined by the relationship between the calcium hardness, the total alkalinity,
the total solids concentration and the temperature.
The Ryznar Stability Index is calculated as follows:
RSI = 2*pH
in which
• pH = measured pH (at room temp) of the water sample
• pH = pH at saturation
s
pH is calculated from:
s
pH = (9.3 + A + B) - (C + D),
s
in which
• A : depends on the total solids concentration
• B : depends on the water temperature at the outlet of the heat exchanger
• C : depends on the calcium hardness (CaCO
• D : depends on the HCO
The values of A, B, C and D can be found in below table:
88
Materials
Containing copper
Stainless steel with carbon
steel and / or cast iron
Stainless steel only
Standard
< 500
< 500
< 1000
- pH,
s
-
concentration or M-alkalinity
3
pH
Standard
7.5 - 9.3
7.5 - 9.3
6 - 9.3
Ca (ppm Ca CO )
3
Energy recovery
< 2
not applicable
< 50
). The saturation pH
s
)
3
Instruction book
Energy recovery
7.5 - 9.3
7.5 - 9.3
6 - 9.3
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