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
• 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
3
)
• D : depends on the HCO
3
-
concentration or M-alkalinity
The values of A, B, C and D can be found in below table:
Total
dissolved
solids
(mg/l)
A
Temperature
(°C)
B
Ca hardness
(ppm CaCO
3
)
C
M-Alkalinity
(ppm CaCO
3
)
D
< 30
0.1
0 - 1
2.3
9 - 11
0.6
10 - 11
1.0
30 - 320
0.2
2 - 6
2.2
12 - 14
0.7
12 - 14
1.1
> 320
0.3
7 - 11
2.1
15 - 17
0.8
15 - 17
1.2
12 - 16
2.0
18 - 22
0.9
18 - 22
1.3
17 - 22
1.9
23 - 28
1.0
23 - 28
1.4
23 - 27
1.8
29 - 35
1.1
29 - 35
1.5
28 - 32
1.7
36 - 44
1.2
36 - 44
1.6
33 - 38
1.6
45 - 56
1.3
45 - 56
1.7
39 - 43
1.5
57 - 70
1.4
57 - 70
1.8
44 - 49
1.4
71 - 89
1.5
71 - 89
1.9
50 - 55
1.3
90 - 112
1.6
90 - 112
2.0
56 - 61
1.2
113 - 141
1.7
113 - 141
2.1
62 - 67
1.1
142 - 177
1.8
142 - 177
2.2
68 - 73
1.0
178 - 223
1.9
178 - 223
2.3
74 - 79
0.9
224 - 281
2.0
224 - 281
2.4
80 - 85
0.8
282 - 355
2.1
282 - 355
2.5
Instruction book
72
2920 1796 01
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