pH
Type of cooling system Materials Standard Energy recovery
Single pass Containing copper 6.8 - 9.3 6.8 - 9.3
Stainless steel with carbon
steel and / or cast iron
6.8 - 9.3 6.8 - 9.3
Stainless steel only 6 - 9.3 6 - 9.3
Recirculating (with tower) Containing copper 6.8 - 9.3 not applicable
Stainless steel with carbon
steel and / or cast iron
6.8 - 9.3
Stainless steel only 6 - 9.3
Closed loop Containing copper 7.5 - 9.3 7.5 - 9.3
Stainless steel with carbon
steel and / or cast iron
7.5 - 9.3 7.5 - 9.3
Stainless steel only 6 - 9.3 6 - 9.3
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:
Ca (ppm Ca CO
3
)
Type of cooling system Standard Energy recovery
Single pass < 500 < 2
Recirculating (with tower) < 500 not applicable
Closed loop < 1000 < 50
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
s
). The saturation 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
s
- pH,
in which
• pH = measured pH (at room temp) of the water sample
• pH
s
= pH at saturation
pH
s
is calculated from:
pH
s
= (9.3 + A + B) - (C + D),
in which
Instruction book
2920 7109 60 155
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