Atlas Copco GA 7 VSD+ Instruction Book page 93

Instruction book
Type of cooling system
Recirculating (with tower)
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
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
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Materials
Containing copper
Stainless steel with carbon
steel and / or cast iron
Stainless steel only
Containing copper
Stainless steel with carbon
steel and / or cast iron
Stainless steel only
Standard
< 500
< 500
< 1000
- pH,
s
Standard
6.8 - 9.3
6.8 - 9.3
6 - 9.3
7.5 - 9.3
7.5 - 9.3
6 - 9.3
Ca (ppm Ca CO
Energy recovery
< 2
not applicable
< 50
). The saturation pH value is determined by the
s
)
3
pH
Energy recovery
not applicable
7.5 - 9.3
7.5 - 9.3
6 - 9.3
)
3
91