The usefulness of water as a heat transfer medium is largely due to the ready availability of water in the liquid state at low cost. Unfortunately, the demands placed on water in heat transfer applications can lead to operating problems associated with the various impurities found in most natural water supplies. Hardness (calcium and magnesium salt) scale encrustation, corrosion and microbiological fouling all stem from failure to properly manage the water impurities and all can lead to inefficiency or premature failure of heat transfer equipment. (See also Corrosion and Fouling.) The preparation of water for use as a heat transfer medium may utilize various physical and chemical processes to remove or modify the impurities. The process, or combination of processes needed, will depend upon both the condition of the supply water and the nature of the heat transfer process under consideration.
The impurities which have accumulated in water drawn directly from oceans, lakes, rivers or underground aquifers will reflect the history of the water in the period since it last existed as atmospheric, water vapor. They may be divided into four main categories. Particulate solids (including micro-organisms) in colloidal or suspended form, dissolved ions of atmospheric or mineral salt origin, dissolved organic matter from decaying vegetation and dissolved gasses (principally oxygen and nitrogen). The total dissolved solids (TDS) content of the water may vary from as little as 0.03 kg/m3 in highland catchment waters, to over 30 kg/m3 in seawater and other brines.
The standard of purity demanded for heat transfer processes can vary widely. High temperature, high heat transfer rates and evaporative concentration all tend to elevate the level of purity required of water for use as the heat transfer medium. High pressure steam boilers demand feed water with less than 0.001 kg/m3 dissolved solids whilst recirculatory chilled water systems may require brine concentrations of 100 kg/m3 or more, in order to prevent freezing. (See also Desalination.)
Purification by physical separation includes coarse screening, settlement, filtration through fixed elements or particulate media beds to remove particulate solids, as well as membrane techniques such as electrodialysis and reverse osmosis, which can remove dissolved ions. Physical separation is often preceded by dosage of coagulants and flocculants to increase particle size of finely divided and colloidal solids thereby facilitating separation. Clarification and removal of particulate solids is commonly the responsibility of the water supplier. (See also Liquid-Solid Separation, Membrane Processes.)
Purification by precipitative or ion exchange processes removes undesirable mineral salts, or exchanges them for less harmful species. Precipitative (lime-soda) softening converts undesirable calcium and magnesium ions into solid precipitates which may be removed by filtration. Base exchange softening utilizes immobilized anions within a bed of resin beads to exchange the undesirable calcium and magnesium ions for sodium ions, which form soluble nonscaling salts. Other ion exchange processes utilize the same principle to replace all the mineral ions with hydroxyl or hydrogen ions, thus achieving their complete removal. (See also Ion Exchange.)
Distillation can remove water from all associated nonvolatile impurities in a single step and millions of gallons of water are purified daily by this means, wherever fresh water is scarce and cheap energy is plentiful. Evaporative processes are also important in the preparation of waste water for re-use whilst minimizing the effluent volume. (See also Desalination and Distillation.)
Typical percentages of the different classes of influent water impurity remaining after the various processes are represented in Figure 1:
It must be noted that the prepared water may require the addition of chemical additives such as antiscalents, corrosion inhibitors and microbicides as the final stage of conditioning for successful use as a heat transfer medium.