The service life of a heat exchanger depends heavily on water quality. Impurities enter the unit in three states — dissolved, dissolved gas and colloidal — and each causes a different failure mode.

1. Impurities dissolved as ions or molecules

a. Calcium salts. Water usually carries Ca(HCO₃)₂, CaCl₂, CaSO₄ and CaSiO₃. Calcium is the main source of scale. CaSO₄ forms a hard, finely crystallised deposit; other calcium salts form a looser sludge that separates from the water, stays mobile and can be removed from heating surfaces relatively easily.

b. Magnesium salts. Mg(HCO₃)₂, MgCl₂ and MgSO₄ decompose on heating to Mg(OH)₂, another sludge-type deposit. Below pH 7 the dissolved magnesium salts hydrolyse and cause acid corrosion of the metal wall.

c. Sodium salts. NaCl, Na₂SO₄ and NaHCO₃ do not themselves form scale, but free oxygen in the water accelerates corrosion of the metal wall. Excessive sodium salt content deposits on fittings downstream of the evaporator and endangers safe operation, while bicarbonate decomposes under temperature and pressure and can damage the metal grain structure.

2. The hazard of dissolved oxygen

Corrosion has many causes, but oxygen is by far the fastest and most severe. Iron sits above hydrogen in the electrochemical series: in oxygen-free neutral water the iron atoms at the metal surface lose electrons and become ferrous ions, which combine with hydroxyl ions and establish an equilibrium in the water.

When oxygen is present, Fe(OH)₂ is oxidised further into insoluble ferric hydroxide and precipitates out. Because Fe(OH)₂ is continuously removed from solution, more iron ions pass from the anode area into the water and corrosion accelerates.

Water and oxygen are therefore both necessary for corrosion: the anode is where metal is lost, the cathode is where the corrosion products accumulate. Where attack proceeds evenly over the whole surface the rate stays low and the damage is limited — this is general corrosion. Where attack concentrates at particular points it is localised corrosion, which is fast, easily perforates the wall, and is far more dangerous. Pitting is the most common form of localised corrosion in heat exchangers.

3. Impurities present as colloids

a. Iron compounds. Mainly Fe₂O₃, which forms iron scale; water with a high iron content usually looks yellow.

b. Micro-organisms. Cooling water temperature, dissolved oxygen and nutrients give micro-organisms ideal conditions to multiply. They come from soil and air; aeration in the cooling tower raises the oxygen content, phosphate treatment chemicals act as nutrients, and towers are usually outdoors where sunlight encourages algae. Biological growth blocks plate passages and pipework and also promotes corrosion.

c. Sludge. Dust from the air and suspended solids in make-up water settle out in the low-velocity parts of the exchanger.

d. Biological slime. Secretions of micro-organisms bind with silt, corrosion products and algae residues, adhere to the exchanger wall and generate organic acids that attack the metal.

Fluid quality is therefore critical. Operation and maintenance procedures should include the necessary anti-scale and anti-corrosion equipment to extend the service life of the unit.