1、AN-04-8-3 Water Chemistry Issues in Geothermal Heat Pump Systems Kevin D. Rafferty, P.E. Associate Member ASHRAE ABSTRACT Calcium carbonate scale and iron related fouling are the most common water quality problems in the US. When hard water is supplied to an open-loop heat pump operating in the cool
2、ing mode, or to a desuperheater (in an open- or closed- loop system), scale can occur on the heat exchanger surfaces. This scale can reduce the performance of the heatpump and, in some cases, has rendered desuperheaters inoperable. This paper discusses water chemistry as it relates to scaling along
3、with how this issue impacts geothermal heat pumps. Condi- tions necessary for the occurrence oJ and design strategies io limit, the impact of iron precipitation are also discussed along with treatment methods jr iron bacteria. INTRODUCTION One of the most common problems in water systems is the deve
4、lopment of scale on heat exchange surfaces and its impact on equipment performance. In GSHP systems, the issue is of greater interest in open-loop systems-particularly those in which the groundwater is supplied directly to the main refrig- erant-to-water heat exchanger (residential systems and stand
5、- ing column systems), but closed-loop systems can be affected as well. In the course of domestic hot water heating, either via the desuperheater or in dedicated water-heating applications, the formation of scale can seriously reduce equipment perfor- mance. It is therefore important to identify the
6、 character of the water the system will be using such that appropriate design strategies can be employed and the owner can be advised, if necessary, of future maintenance requirements. In addition to scaling, iron precipitation with subsequent fouling is a common problem in water systems around the
7、country. Virtually all groundwater contains some iron and particularly those aquifers producing from igneous rocks or those with adjacent clay sequences. GSHP systems can be successfully operated with waters of very high iron content, provided the water is carefully isolated from exposure to air. Sp
8、ecific design strategies that limit exposure to air are key in applications where the water is characterized by high iron content. In the case of both scaling and iron precipitation, system design strategies can limit the extent to which the water chem- istry impacts maintenance requirements. In the
9、 case of iron bacteria, the system design has little impact on the occurrence of problems; however, maintenance procedures used to address iron bacteria can substantially increase the time between treatments if properly applied. WATER CHEMISTRY AND SCALING Depending upon its specific chemistry, wate
10、r can promote scaling, corrosion, or both. Scaling is the number one water quality problem in the U.S., arising from the fact that water sources classified as “hard“ are found in 85% of the country (Water Quality Association 2003). Scale can be formed from a variety of dissolved chemical species, bu
11、t calcium carbonate, the most common form of scale deposi- tion, is closely associated with elevated levels of hardness and alkalinity. Hardness is primarily a measure of the calcium and magnesium salts in water. In addition, other minor contribut- ing components to hardness can be aluminum, mangane
12、se, iron, and zinc (Carrier 1965). Two types of hardness are gener- ally recognized: carbonate (sometimes referred to as tempo- rary hardness) and noncarbonate hardness. Carbonate hardness, depending upon the nature of the water, is composed Kevin Rafferty is associate directodsenior engineer with t
13、he Geo-Heat Center, Klamath Falls, Ore. 550 02004 ASHRAE. of calcium or magnesium carbonates and bicarbonates. It is this form of hardness that contributes most to scale formation. Noncarbonate hardness is normally a small component of the total hardness and is characterized by much higher solubilit
14、y, and its role in scale formation is generally negligible (Carrier 1965). Water hardness is classified according to a somewhat subjective criterion that varies from reference to reference, and Table 1 provides a common interpretation. Scaling prob- lems typically occur above levels of 80 ppm hardne
15、ss. In order to evaluate a particular water sample for scaling, the following parameters, at minimum, must be established: calcium hardness (as ppm CaCO,), total or “M alkalinity (as pprn CaCO,), total dissolved solids (TDS), pH, and the temperature to which the water will be exposed. Temperature is
16、 a function of the design of the system, and the remaining parameters can be inexpensively determined by a water anal- ysis laboratory. Calcium hardness is a key parameter in evaluating scale formation. It generally constitutes 70% or more of the total hardness in water. For worst-case evaluations o
17、r in the absence of sufficient information, calcium hardness can be considered equal to total hardness. If a calcium ion value is available from a water chemistry analysis (typically expressed as ppm Ca), calcium hardness (as ppm Caco3) can be calculated by rnulti- plying the calcium ion value by 2.
18、5. The 2.5 arises from the atomic weights of the components+alcium (20) combines with carbonate (30) to form calcium carbonate (50); thus, 50/ 20 = 2.5. Alkalinity is a measure of waters ability to neutralize acid. Like hardness, it is usually expressed as ppm CaC3. In the range of normal groundwate
19、r chemistry, alkalinity is the result primarily of the bicarbonate content of the water. At pH values of greater than 8.3, carbonate and hydroxide can also contribute to alkalinity. Two measures of alkalinity are common: methyl orange (“M alkalinity or total alkalinity) and phenolphtalien (“P” alkal
20、inity). Since P alkalinity measures that portion of the alkalinity effective at very high pH, the M alkalinity is the value of interest in evaluating scale potential. Hardness (as ppmt Caco3) 4 5 15 to 50 The total dissolved solids content is a general indication of the quality of a water source. As
21、 TDS increases, water qual- ity problems are more likely to occur. Whether these problems are on the corrosion or scaling end of the spectrum is depen- dant upon other indicators. The pH value of groundwater varies widely but is usually in the range of 5.0 on the acid end ofthe spectrum to 9.0 on th
22、e alkaline end. Scaling problems are common at pH values above 7.5. Classification very soft soft PREDICTING SCALING Two indices commonly used in the water treatment indus- try to evaluate the nature of a water source are the Langelier Saturation Index (LSI or saturation index) and the Ryznar Stabil
23、ity Index (RSI or stability index). In both cases, these indices are based upon a calculated pH of saturation for calcium carbonate (PH,). ThepH, value is used in conjunction with the waters actual pH to calculate the value of the index as follows: LSI = pH -pH, RSI 2pH, - pH Evaluation of the satur
24、ation index is as indicated in Table 2. The stability index (Table 3) produces a slightly different value numerically but is interpreted in a similar fashion. It is important to point out that the accuracy of the RSI and LSI is much greater as a predictor of scaling than of corrosion. This results f
25、rom the fact that both methods are based upon the saturation of calcium carbonate. The assumption implicit in Table 1. Water Hardness Classification* 50 to 100 100 to 200 200 medium hard hard very hard LSI Index Value 2.0 0.5 Both alkalinity and hardness are most often expressed in uents are not bas
26、ed on calcium carbonate but are only Gamer 1965 convert gpg to ppm as Caco3, multiply by 17.1. ppm-parts per million, for water chemistry calculations considered equivalent to mg/i. Hardness is sometimes expressed in units of grains per gallon (gpg). To Ilnits Of carbonate These Indication scale for
27、ming but noncorrosive slight scale forming and corrosive expressed as their equivalent in units of calcium carbonate. The use of consistent units of calcium carbonate allows several different chemical constituents to be more conveniently added or subtracted in the course of water chemistry calculati
28、ons- such as the scaling index. A relationship between hardness and alkalinity exists as follows (Johnson-UOP 1975): If M alkalinity is greater than total hardness, all hard- ness is due to carbonates and bicarbonates. If M alkalinity is less than total hardness, carbonate hardness = M alkalinity, n
29、oncarbonate hardness = total hardness - M alkalinity. Table 2. Interpretation of the Langelier Saturation Index* I 0.0 I balanced but Ditting corrosion oossible I I I - I -0.5 I slightly corrosive but non-scale forming I I -2.0 I serious corrosion I Carrier 1965 ASHRAE Transactions: Symposia 551 Tab
30、le 3. Interpretation of the Ryznar Stability Index* RSI Index Value 4.0-5.0 Indication heavy scale 5.0-6.0 6.0-7.0 I 7.0-7.5 I corrosion significant I light scale little scale or corrosion 7.5-9.0 9.0 O I (2.6) 2 (5.2) 3 (7.8) Water Usage in gatlday (Ud) x I000 heavy corrosion corrosion intolerable
31、- 120F(49C) MOF(6OC) 1 SOF(65C) 160F(71C) 180F(82C) - -F -e- - * Carrier1965 (ASHRAE 1995) the calculations is that if the calcium carbonate content exceeds the level that can be maintained in solution, scale will occur. At lower pH, corrosion will occur. In terms of general corrosion in systems con
32、structed of primarily ferrous materi- als, this is a valid assumption for corrosion. In heat pump systems where the materials are more likely to be copper or cupro-nickel, there are other chemical species that can cause serious corrosion that are not accounted for in the RSVLSI calculations. These w
33、ould include hydrogen sulphide (H,S) and ammonia (“,) among others. As a result, for GHP systems, the RSI/LSI indices should be used as scaling rather than corrosion predictors. Calculation of the value for pHs can be accomplished using lab analysis results and the nomograph found in various referen
34、ces (ASHRAE 1995; Carrier 1965) or through the use of the following equation: pH, = (9.3 +A + B) - (C + D) (Edstrom 1998) where A = (log(TDS) - 1) / 10 TDS in ppm B = (-13.12 log(“C + 273) + 34.55 Temperature in “C C = (log (calcium hardness) - 0.4 Ca hardness in ppm (as Caco3) Figure 1 A plot of sc
35、ale deposit at various temperatures for a water containing I7ppm hardness. Same water at 150F (65.6OC) LSI = 1.2 (scale forming) RSI = 5.8 (light scale) It is apparent from this example that the results are heavily influenced by temperature. The water chemistry above is non- scaling to corrosive at
36、lower temperature and scaling at higher temperature. For heat pump applications-particularly for open-loop systems-this has important implications. Obvi- ously, the scalingproblem will be more significant in the cool- ing mode where the temperatures in the refrigerant-to-water heat exchanger are hig
37、hest. In the heating mode, the temper- ature may be low enough to eliminate scaling. For heat pump applications, it is important to calculate the scaling index at temperatures reflective of both heating and cooling modes so as to clearly indicate those conditions under which scaling may occur. As an
38、 alternative to the calculations above, a water testing laboratory can provide values for LSI and RSI directly. D = logalkalinity) Example: pH = 8.2, TDS = 500 ppm, calcium hardness = 165 ppm as Caco, alkalinity = 100 ppm as Ca CO, temperature = 55F ( 12.8“C) kin as Standard practice is to make thes
39、e calculations at the well temperature. For heat pump applications, it is important to spec for the lab the maximum temperature to which the water will exposed so the results are appropriate to the appli- cation. Figure 1 is a plot of scale deposit at various temperatures for a water containing 170
40、ppm hardness. The relationship (Caco31 A = (log(5OO)- 1)/10=0.17 B = (-13.12 10g(12.8+273) + 34.55 r2.33 between temperature and scaling is clearly demonstrated. Figure 2 presents a plot of LSI vs. pH for a collection of 260 water samples (Carrier 1965) from across the U.S. It is C = log 165 -0.4= 1
41、.82 D = log 100=2.0 pH values less than 7.5. apparent that serious scale problems (LSI 1) are unlikely at Figure 3 presents a plot of LSI vs. hardness for the same group of samples. It is equally clear from these data that seri- pHs=(9.3 +0.17 +2.33)-(1.82 +2.0)=7.98 LSI = 8.2 - 7.98 = 0.22 (balance
42、d) ous scale problems are unlikely at water hardness values below 80 ppm. In addition, hardness values above 200 pprn RSI = 2(7.98) - 8.2 = 7.76 (heavy corrosion) suggest the potential for serious scaling. 552 ASHRAE Transactions: Symposia 2 21 U C C - o0 e a 0.2 ppm, and temperatures of 46F (5C) to
43、 61F (16C) (Hackett and Lehr 1985). These conditions include most of the waters of the U.S. More restricted environmental criteria can be described for the organisms using an index known as “reductivity intensity” (rH) according to the following relationship: rH = Eh / (0.0992T + 2 pH) (Hasselbrath
44、and Ludeman 1972) where: Eh T pH The authors report that iron bacteria require an rH value of 14.5*1 .O for growth. Iron bacteria can be microscopically identified, but labo- ratory culture techniques are not well established. Culturing is the process of artificially growing a large enough populatio
45、n of organisms in a sample to permit effective laboratory iden- tification. As a result, if the organisms are present in sufficient quantity in a sample, they can be identified. Normally this only occurs after they have colonized a site in sufficient density to cause other problems (plugging). In te
46、rms of evaluating a = measured redox potential of the water in volts = absolute temperature in “K = measure pH of the water. particular site for the bacteria, lab analysis is not an effective tool. Background investigation of local experience through well drillers, groundwater professionals, and exi
47、sting well owners is a more effective approach. Iron bacteria infestations can be effectively treated; however, periodic treatments will be necessary over the life of the system in most cases. The frequency of well maintenance is very much a function of the effectiveness of the treatment methods use
48、d. Several treatment methods are available for iron bacteria (biocides, heat, ultraviolet, and disinfection) (Hackett and Lehr 1985), but the most common is disinfection with chlori- nation. For maximum effectiveness, several parameters must be carefully observed in the course of treatment. These in
49、clude dosage, residual chlorine concentration, exposure time, pH, agitation, and redevelopment. Lack of attention to any single issue can compromise treatment. Recommended residual concentration (that concentration remaining after some of the original dosage has been consumed by other non-iron bacteria biological material in the water) of chlorine is 500 ppm for effective treatment. To achieve this residual concentration, the chlorine dosage added should exceed the desired residual. Anecdotal evidence suggests that many contractors use an initial dosage of twice t
