| Part 1. Cooling Water Systems |
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| Three systems normally used are: |
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| 1). Once through |
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| 2). Open evaporative recirculating |
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| 3). Closed non-evaporative recirculating |
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| 1. Once through systems |
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| Cooling water passes through the heat exchanger once. Once through systems can be used when plenty of cheap cool water is available and adequate facilities for disposal of warm water exist. |
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| Advantages: |
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| No cooling tower system; |
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| Disadvantages: |
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| Corrosion |
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| Fouling |
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| Waste of water |
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| Thermal pollution of river |
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| 2. Open evaporative recirculating systems |
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| Cooling water evaporate about 1% water. Water is reused after make up. |
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| Less water required |
| Enhanced corrosion control feasible |
| Disadvantages: |
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| Higher capital cost than once through; |
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| Large cooling towers may be unacceptable; |
| System purge may pose environmental problems |
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| 3. Closed nonevaporative recirculating systems |
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| Cooling water is cooled in a secondary (air) heat exchanger. No evaporate, no makeup. |
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| Water remains clear |
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| Cooling water temperature above 100oC is possible |
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| Disadvantages: |
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| High capital cost |
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| Limited by air temperature |
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| Open evaporative systems are usually used. |
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| Basic calculations for open evaporative recirculating cooling water systems |
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| (m3/hr) |
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| D T: temperature difference between feed and return water (oC) |
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| F: circulation rate (m3/hr) |
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| Windage loss, W: |
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| due to liquid entrainment normally specified by tower manufacturer |
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| 0.01% of circulation for modern units and 0.2% for old units |
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| Purge and Blowdown |
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| Liquid water loss other than windage loss is termed Total Purge (P). |
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| P = B + IL |
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| B: blowdown, to limit solid build up |
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| IL: leaks |
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| Make up |
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| Mm = E + W + P = E + W + B + IL |
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| Concentration factor (CF) |
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| Evaporation increases the concentration of solid in the circulation water. |
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| CF = (%X in circulating water) ¸ (%X in make up) |
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| Typically, "makers" for "X" are magnesium or chlorine ions. |
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| Calculation of make up and blowdown rates |
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| mass balance on the marker |
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| Mm Xin = (P + W)Xout = (Mm - E)Xout |
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| Hence |
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| Since |
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| Therefore, higher CF gives lower Mm and B. |
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| System half life |
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| This is the time taken for the concentration of a soluble component (e.g. additive to control corrosion) to halve its initial concentration. |
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| Part 2. Cooling Water Treatment |
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| Evaporation in the cooling tower causes a build up of suspended/dissolved solids which can inhibit heat transfer by building up on heat exchanger surfaces - usually mould steel. |
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| Two problems in cooling water system: |
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| 1). Fouling |
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| silting/sedimentation (particles in source water, e.g. sand) |
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| scaling (precipitation of salts) |
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| biological growth (heat, oxygen, phosphates promote biological growth) |
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| 2). Corrosion |
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| Cooling water treatment is required to overcome these problems. The purpose of water treatment is to control fouling and corrosion. |
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| Environmental considerations may restrict the disposal & choice of treatment chemicals, e.g. chromate treatments are widely applied in view of their corrosion protection. However, the discharge of chromate treated water is viewed with increasing concern. |
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| Inlet water quality must be first known: |
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| e.g. pH, total dissolved solids, suspended solids, Ca++, SO4--, |
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| Scale formation |
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| Precipitation of the least soluble salts may occur, e.g. CaCO3, CaSO3. |
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| Ca++ + 2(HCO3)-- ® CaCO3¯ + H2O + CO2 |
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| High concentration of Ca++ and SO4-- may also gives calcium sulphate scale (CaSO4). |
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| Scale impairs heat transfer efficiency and may increase pumping cost. With stainless steel, scaling may promote stress corrosion cracking. |
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| Factors affecting scaling |
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| 1. Concentration factor: unless acid is added, the alkalinity will increase in the circulating water in evaporative systems ® more CaCO3 scales |
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| 2. pH value: high pH changes CO2/HCO3-/CO3--, in favour of carbonate ® more CaCO3 scales |
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| 3. Temperature: dissociation of HCO3- to CO3--, CO2, and H2O is greater at higher temperature. Also CaCO3 solubility decreases, \ scaling increases with temperature. |
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| 4. Bacteria slime: can give sites for scale growth, e.g. on cooling tower timbers. |
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| 5. Corrosion: roughens metal surfaces and gives scaling sites. |
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| 6. Flow velocity: low values (< 1m/s) increase silting and associated scaling. |
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| 7. Retention time/circulating rate: long half lives gives longer time for the following equilibrium to be achieved |
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| Ca(HCO3)2 « CaCO3 + CO2 + H2O |
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| also with faster circulation there is more CO2 stripping in cooling tower. Hence both factors reinforce scaling tendency. |
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| Scale prevention |
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| 1. Higher system purge to reduce CF – at the expense of higher water/chemical costs. |
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| 2. Soften makeup water: using external ion exchangers. |
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| 3. Acid treatment to reduce [CO3--]: with water of medium to high CaCO3, i.e. > 800 mg/l, reducing the alkalinity to 20 - 40 mg/l will reduce CO3-- below the scaling level. H2SO4 or HCl are normally used. |
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| 4. Scale inhibitors: modify crystal scale growth |
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| inorganic: polyphosphates |
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| organic: phosphorous compounds |
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