5.10 Water quality and material selection
Indian water ranges from soft rainwater to borewell water with TDS above 1,500 mg/L. Hardness (calcium and magnesium) forms scale on hot surfaces. Scale is a thermal blanket: a 1 mm layer can cut heat transfer noticeably and raise condensing pressure.
Key terms:
- TDS (total dissolved solids): all dissolved salts, mg/L (ppm).
- Hardness: calcium + magnesium, expressed as mg/L CaCO₃.
- Chlorides: attack stainless steel (pitting), especially when hot.
- pH: acidity; below about 6.5 corrodes copper, above about 9 encourages scale.
Typical guide values for water passing through a brazed plate or coaxial condenser (check manufacturer):
| Parameter | Typical acceptable | Comment |
|---|---|---|
| pH | 7.0–9.0 | |
| Total hardness | < 150–200 mg/L as CaCO₃ | Soften above this |
| TDS | < 500 mg/L (preferred), up to ~1,000 with care | |
| Chlorides | < 100–200 mg/L for SS304 | SS316 or titanium above |
| Free chlorine | < 0.5–1 mg/L | Pools need care |
| Iron | < 0.2 mg/L | Fouling |
Material selection:
| Water type | Condenser material |
|---|---|
| Soft / RO municipal | Copper-brazed SS316 plate or copper coaxial |
| Hard borewell | Indirect loop + softener; SS316 PHE on user side |
| Swimming pool (chlorinated) | Titanium coil |
| Sea / brackish water | Titanium |
| Process with ammonia contact | Nickel-brazed or fully welded SS (no copper) |
Worked example — scale penalty
A unit in Jaipur on borewell water (hardness 450 mg/L) builds scale. Condensing temperature rises 4 K. Rule of thumb: COP falls roughly 2–3 % per K of extra lift (typical). COP drops from 3.5 to about 3.5 × (1 − 0.025 × 4) = 3.15. On 200 kWh heat/day: electricity rises from 57.1 to 63.5 kWh/day = 6.4 kWh × ₹9 = ₹58/day ≈ ₹21,000/year, before the cost of a failed condenser.
Remember
- Test the water before selecting: pH, hardness, TDS, chlorides, iron.
- Pools and sea water need titanium.
- High hardness → softener and/or indirect loop.
- Plan CIP (clean-in-place) connections on PHEs.
Bad water kills heat exchangers faster than anything else, so match materials and treatment to the site water.
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