ASHRAE HVAC APPLICATIONS IP CH 49-2015 WATER TREATMENT DEPOSITION CORROSION AND BIOLOGICAL CONTROL.pdf

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1、49.1CHAPTER 49WATER TREATMENT: DEPOSITION, CORROSION, AND BIOLOGICAL CONTROLWater Characteristics 49.1Alternative Water Sources 49.2Deposition, Scale, and Suspended-Solids Control . 49.3Corrosion and Corrosion Control 49.6Biological Growth Control. 49.11Cooling Tower Systems Start-Up and Shutdown 49

2、.14Heating and Steam-Generating Systems 49.15Selection of Water Treatment 49.18Terminology 49.21HIS chapter covers the fundamentals of water treatment. ItTprovides guidance on the treatment of water and water-basedfluids used in heating, air-conditioning, refrigeration, and processsystems, with a fo

3、cus on the control of corrosion, scale, fouling, andbiological growth. Proper treatment improves the performance andenergy efficiency of these systems while helping to protect humanhealth and safety. Water treatment also extends the life of equipmentin both open- and closed-loop systems. In addition

4、, water treatmentcan help conserve water resources as well as enable the use of alter-native sources of water. All of these benefits help to promote ahealthier, more sustainable environment.1. WATER CHARACTERISTICSAlthough water is a common material, it has unique propertiesthat make it ideal for he

5、ating, cooling, and steam generating pro-cesses. Water is the only common substance that exists in all threestates of matter solid (ice), liquid (water), and gas (steam) at nor-mal earth temperatures. Water absorbs more heat for a given temper-ature rise than any other common inorganic substance. Wa

6、terexpands 1600 times as it evaporates to form steam at atmosphericpressure. The steam is capable of carrying large quantities of heat.These unique properties of water make it an ideal material for heat-ing, cooling, and power-generating processes.All water contains varying amounts of impurities tha

7、t can causescale, corrosion, and other problems in industrial equipment. Ittakes on some of the characteristics of its surroundings as it dis-solves minerals and picks up impurities from the air, soil, and veg-etation or other materials it contacts. For that reason, water is oftenreferred to as the

8、universal solvent. When rain falls, it dissolvescarbon dioxide and oxygen in the atmosphere. The carbon dioxidemixes with the water to form carbonic acid (H2CO3). When car-bonic acid contacts soil that contains limestone (CaCO3), it dis-solves the calcium to form calcium bicarbonate. Calcium carbona

9、tein water used in heating or air-conditioning applications can even-tually become scale, which can increase energy costs, maintenancetime, and equipment shutdowns, and can eventually lead to equip-ment replacement.Waters composition changes as it is transported in pipelines,heated to make steam, or

10、 evaporated for cooling or other heatexchange processes. Impurities in water may reach a solubility limitand be deposited along the way, or water may corrode the metalequipment containing it. The actions of water in HVAC systemsdepend on the impurities in it and the temperatures and pressures towhic

11、h it is subjected, as well as aspects of the systems where it isused. This provides a basis for many stability index calculations thatare done on water to predict its corrosive and/or scaling tendencyin water systems. The Langelier Saturation Index and Ryznar Stabil-ity Index, referred to as LSI and

12、 RSI respectively, are examples ofcalculated indices often used in water treatment. For a proper evalu-ation, it is necessary to determine the impurities water contains, whatproblems these impurities may cause, and how they can be mechan-ically or chemically reduced, removed, or treated.The followin

13、g paragraphs present the more important chemicaland physical properties or characteristics of water relevant to watertreatment and that affect its use in HVAC applications.Hardness refers to the amount of calcium and magnesium (typ-ically expressed in ppm, as CaCO3) in the water. It is a historicalt

14、erm referring to hard calcium and magnesium carbonate scales,such as those found in improperly treated boilers and cooling-watersystems. Hardness contributes to scale formation, because its pres-ence encourages deposition of calcium carbonate, or lime scale.Because the solubility of most calcium and

15、 magnesium saltsdecreases with an increase in temperature, these salts tend to formscale on heat transfer surfaces where the metal skin temperature isgreater than the bulk water temperature. The amount of calcium andmagnesium equivalent to the alkalinity in a solution is called tem-porary hardness.

16、The excess calcium and magnesium, if any, iscalled permanent hardness. Boiler and cooling-water treatmentprograms must control the deposition of hardness salts using pre-treatment removal (such as with boiler makeup or feedwater soft-ening) or internal conditioning to solubilize and remove orotherwi

17、se control deposition.Alkalinity is a measure of the capacity of water to neutralizestrong acids. It is the measured carbonate and bicarbonate minerals(calculated as calcium carbonate, CaCO3), and refers to the primaryalkaline earth mineral contributing to alkalinity. Alkalinity is alsomeasured and

18、calculated as the hydroxide ion (OH), when present.All natural waters contain some quantity of alkalinity. The presenceof alkalinity contributes to scale formation, because it encouragesdeposition of calcium carbonate. In natural waters, the alkalinityalmost always consists of bicarbonate, although

19、some carbonatealkalinity may also be present. Borate, hydroxide, phosphate, andother constituents, if present, are included in the alkalinity measure-ment in treated waters.Alkalinity is measured using two different end-point indicators.The phenolphthalein alkalinity (P alkalinity) measures the stro

20、ngalkali present, and the methyl orange alkalinity (M alkalinity), ortotal alkalinity, measures the total alkalinity in the water. Note thatthe total alkalinity includes the phenolphthalein alkalinity. For mostnatural waters, the actual chemical species present can be estimatedfrom the two alkalinit

21、y measurements. Treated waters include thehydroxide alkalinity contributed by OH(Table 1).pH is a measure of the concentration of hydrogen ions, or the acidstrength, of a solution. It is the negative logarithm of the hydrogenion concentration (pH 1, very acidic; pH 14, very basic; pH 7, neutralat am

22、bient temperature.) The pH concept is fundamental to anunderstanding of water chemistry and to control of pretreatment sys-tems, boilers, and cooling towers. All of these systems depend oneither precise pH control or on maintaining the pH above a specifiedThe preparation of this chapter is assigned

23、to TC 3.6, Water Treatment.49.2 2015 ASHRAE HandbookHVAC Applicationsminimum level. Unexpected changes in pH are usually a warning ofproblems that must be corrected.Chlorides are the total of dissolved chloride salts of sodium,potassium, calcium, and magnesium present in water. Sodium chlo-ride (com

24、mon salt, NaCl), and calcium chloride (CaCl2) are themost common of the chloride salts found in water. Chlorides do notordinarily contribute to scale, because they are very soluble. How-ever, chlorides are corrosive and cause excessive corrosion whenpresent in high concentration, as in seawater, bec

25、ause of their con-ductivity and because the small size of the chloride ion permits thecontinuous flow of corrosion current when surface films are porous.The amount of chlorides in water is a useful measuring tool in evap-orative systems for determining cycles of concentration. Most waterconstituents

26、 change (increase or decrease) when common treatmentchemicals are added or because of chemical changes that take placein normal operation. With few exceptions, only evaporation affectschloride concentrations, so the ratio of chlorides in a water samplefrom an operating system to those of the makeup

27、water provides ameasure of how much the water has been concentrated. (Note: Chlo-ride levels will change if the system is chlorinated.)Conductivity or specific conductance measures the ability ofwater to conduct an electrical current electricity or be an electrolyte.Conductivity increases with incre

28、asing total dissolved solids (ions)present in the water. Specific conductance can be used to estimatetotal dissolved solids.Dissolved solids consist of salts and other materials that com-bine with water as a solution. They can affect the formation of cor-rosion and scale. Low-solids waters are gener

29、ally corrosive becausethey have less tendency to deposit protective scale. If a high-solidswater is nonscaling, it tends to produce more intensive corrosionbecause of its high conductivity. Total dissolved solids are referredto as TDS.Suspended solids include both organic and inorganic solids sus-pe

30、nded in water (particularly unpurified water from surface sourcesor those that have been circulating in open equipment). Organicmatter in surface supplies may be colloidal. Naturally occurringcompounds such as lignins and tannins are often colloidal. At highvelocities, hard suspended particles can a

31、brade equipment. Settledsuspended matter of all types can contribute to corrosion. Total sus-pended solids are referred to as TSS.Silica is dissolved sand or silica-bearing rock (such as quartz)through which the water flows. Silica is the cause of very hard andtenacious scales that can form in heat

32、transfer systems. These depos-its can be particularly hard to remove if allowed to concentrate. For-tunately, silicate deposition is less likely than other deposits.Soluble iron in water can originate from metal corrosion inwater systems or as a contaminant in the makeup water supply. Theiron can fo

33、rm heat-insulating deposits by precipitation as ironhydroxide or iron phosphate in the presence of phosphate-basedwater treatment or phosphate in the makeup water.Sulfates are the dissolved sulfate salts of sodium, potassium,calcium, and magnesium in the water. They are present because ofthe dissolu

34、tion of sulfate-bearing rock, such as gypsum. Calciumand magnesium sulfate scale is very hard and difficult to remove andgreatly interferes with heat transfer. Sulfates also contribute to scaleformation in high-calcium waters. Calcium sulfate scale, however,forms at much higher concentrations than t

35、he more commoncalcium carbonate scale. High sulfates also contribute to increasedcorrosion because of their high conductivity and support of micro-biologically influenced corrosion (MIC).Turbidity can be interpreted as a lack of clarity or brilliance in awater. It should not be confused with color.

36、A water may be dark incolor but still clear and not turbid. Turbidity is caused by suspendedmatter, in a finely divided state, that can scatter and deflect incominglight. Clay, silt, organic matter, microscopic organisms, and similarmaterials are contributing causes of turbidity. Although suspendedm

37、atter and turbidity are closely related, they are not synonymous.Suspended matter is the quantity of insoluble material in water thatcan be removed by filtration. Turbidity is the amount of opacitycaused by suspended matter. Turbidity of water used in HVAC sys-tems should be as low as possible. This

38、 is particularly true for boilerfeedwater. The turbidity can concentrate in the boiler and settle out assludge or mud and lead to deposition. It can also cause increasedboiler blowdown, plugging, overheating, priming, and foaming.Biological matter such as bacteria, algae, and fungi can be pres-ent i

39、n water and their growth in water systems can cause operating,maintenance, and health problems. Microbial growth can occur inmost water systems below 150F. Problems caused by biologicalmaterials range from green algae growth in cooling towers to bac-terial slime formations. This growth can plug equi

40、pment where flowis essential, drastically reduce heat exchanger (transfer) efficien-cies, and also cause microbial corrosion.2. ALTERNATIVE WATER SOURCESThis section is based on Browning and Weimar (2011).Facilities are under continuous pressure to reduce costs and oper-ate in a more environmentally

41、 responsible manner. Historically,energy conservation has overshadowed water conservation in theseefforts. However, in many locations, the reduced availability andincreased cost of water now make water conservation a much moreattractive target. More facilities are subject to limits on water con-sump

42、tion and the amount of wastewater that can be discharged.Although there are energy source alternatives to oil and gas, there isno such substitute for water. The economic and environmental pay-back on reducing water usage is greater than ever.As a large consumer of potable water, evaporative-cooling-

43、watersystems, including cooling towers, are an obvious target for waterconservation efforts. Because of supply and cost pressures, morefacilities are considering alternative makeup water sources, such asair handler condensate, rainwater, and reclaim water. In some cases,alternative water sources are

44、 being blended with potable water toimprove water use efficiency. Using one or more of these alternativewater sources for cooling tower makeup conserves fresh water forother uses and can provide significant cost savings.Evaluating an Alternative Water SourceIt is important to understand the impuriti

45、es in an alternativemakeup water source and how they will affect cooling tower oper-ation before substituting for potable water. Some water sources canbe successfully used without further treatment, whereas othersrequire additional treatment measures to control problems related tocorrosion, scale, a

46、nd microbiological growth. An engineering andeconomic analysis is necessary to determine both the feasibility andpotential cost savings. In general, the higher the cost and poorer thequality of the potable water source, the better the payback associ-ated with using an alternative source. As with any

47、 water conserva-tion product, water meters are essential to monitor water usage andbenchmark improvements.Table 1 Alkalinity Relationship Based on P and M TestsSituation HydroxylLevel of AlkalinityContributed by Carbonate BicarbonateP = M M 0 0P 1/2M 2P M 2 (M P) 0P = 1/2M 0 M 0P 6.0 indicating a te

48、ndency to dissolvecalcium carbonate, and values 0) and hydroxide alkalinity species present in the solution. Furtheracceleration of white rust formation can be caused by calcium hard-ness below 50 ppm (as CaCO3), requiring the evaluation of the useof water softeners on makeup water during passivatio

49、n.Todays cooling tower treatment programs generally involve theaddition of phosphate-polymer-based scale and corrosion inhibitorsand operating cooling-water systems at alkaline pH. Water chemis-try at these higher pH levels (8 to 9+) is naturally less corrosive tosteel and copper, but can create an environment where white rust ongalvanized steel can occur. Also, some scale prevention programssoften the water to reduce hardness, rather than use acid to reducealkalinity. Resulting soft waters with 8) during passivation is unde-sirable, and consistent monitoring and control are esse