Cooling water chemistry and cycles

LSI, RSI, PSI and Larson-Skold for your tower water — and the cycles at which the first water limit is reached.

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How this calculator works

All dissolved salts are multiplied by the cycles. The saturation pH (pHs) uses the Marley correlation: pHs = (9.3 + A + B) − (C + D), with A from TDS, B from temperature, C from calcium and D from alkalinity. LSI = pH − pHs; RSI = 2·pHs − pH; PSI uses the equilibrium pH 1.465·log(alkalinity) + 4.54 (Puckorius and Brooke). The cycles at which each limit is reached are worked out, and the lowest one governs. The suggested operating cycles are 90 % of it.

The example is the PlantCalcKit standard water: at 4 cycles and 60 °C skin temperature, pHs 5.89, LSI 2.31, RSI 3.58; magnesium silicate limits the cycles at 4.8, suggested 4.3.

How to read the LSI and RSI. Bands follow GPSA Section 18, the Betz Handbook and SPX 2009; treatment programmes with scale inhibitors can run at LSI up to about +2.5.

What the indices cannot tell you. LSI, RSI and PSI show calcium carbonate tendency only. They are not corrosion rates and they do not prove that a number of cycles is safe. Real scaling depends on surface temperature, flow speed, inhibitors, microbes and metals. Use the result as a screening estimate and confirm it with your treatment programme and site results.

References: Marley, Water Cooling Towers (pHs correlation); SPX Cooling Tower Fundamentals Sect. I-G; Betz Handbook of Industrial Water Conditioning; GPSA Sec. 18; Puckorius & Brooke, Corrosion 47(4), 1991; Larson & Skold, Corrosion 14, 1958.

Learn the method step by step in Cooling Towers Made Simple, Chapters 10 to 12. The first three chapters are free.