Air separation units generally use water from rivers, lakes, or groundwater as cooling water. Such water usually contains suspended solids (silt and other contaminants) as well as calcium and magnesium bicarbonates [Ca(HCO₃)₂ and Mg(HCO₃)₂], and is referred to as "hard water." When suspended solids are excessive, they can easily block the passages of coolers, filter screens, and valves. Calcium and magnesium bicarbonates tend to form precipitates of calcium carbonate (CaCO₃) and magnesium carbonate (MgCO₃) when the water temperature rises, producing what is commonly called scale. Scale generally begins to form at water temperatures above 45 °C, and the higher the temperature, the easier scaling becomes. Scale adheres to the tube walls of coolers, and to the packing, spray nozzles, or sieve holes of the nitrogen-water precooler. It not only impairs heat transfer and reduces cooling efficiency, but also obstructs the flow of cooling water or air; in severe cases it can cause equipment failures-for example, water carryover in the nitrogen-water precooler can freeze the regenerator (or switching heat exchanger). Scale is quite hard and difficult to remove once attached to equipment walls. Therefore, cooling water should preferably be softened. Softening treatment using a magnetic water conditioner is relatively simple and gives satisfactory results. A sedimentation basin should be provided to remove suspended solids. Recycling the cooling water is beneficial to water softening, but requires more land area and a higher capital investment.
For compressor cooling water, the inlet temperature is generally required to be no higher than 28 °C, and the outlet temperature below 40 °C. Water quality requirements are as follows:
pH value: 6.5 – 8
Suspended solids: not more than 50 mg/L
Temporary hardness: not more than 17 °dH (German degrees)
Oil content: less than 5 mg/L
Chloride ion (Cl⁻) [mass fraction]: less than 50 × 10⁻⁶
Sulfate (SO₄²⁻) [mass fraction]: less than 50 × 10⁻⁶
The water supply and drainage of the nitrogen-water precooling system form an independent circulating loop. Because the cooling water temperature rise in the tower is large and the outlet water temperature is high, scaling is severe; therefore, the make-up water for this system should, as far as possible, be low-hardness water or soft water, with temporary hardness generally below 8.5 °dH. All other requirements are the same as for compressor cooling water. The lubricating water for the cylinders of high-pressure oxygen compressors used for cylinder filling shall be distilled water or soft water.
