2.4.1 Online testing methods of measuring condenser fouling
Online condenser fouling may either be measured indirectly or directly. Indirect methods calculate the cleanliness factor by using actual plant data and compare this value to the design cleanliness factor. However, caution must be exercised when using this method as the calculated cleanliness factor represents fouling as well as any other steam-side changes in the convection coefficient. Addition- ally, fluctuations in plant variables must be handled by suitable selection of data points as shown by Prieto et al. (2001).
The direct measurement of fouling on steam surface condensers is compli- cated by the presence of excess non-condensible gases which accumulate from air ingress (Putman & Harpster, 2002). Air ingress occurs from equipment leaks in the steam circuit and these non-condensible gases accumulate in the condenser, causing an increase in the thermal resistance to heat transfer. To separate these two effects Putman & Harpster (2002) proposed measuring the total increase in thermal resistance of the condenser from its design condition, and then subtract- ing either the resistance offered by the cooling water fouling or that caused by the presence of the non-condensible gases.
According to Putman & Harpster (2002) the cooling water fouling may be measured using two methods: either the method given by the ASME standard,
Steam surface condensers (ASME, 2010) or Bridger Scientific Inc. (Garey, 1997).
The first method measures the outlet temperatures of two adjacent tubes in the condenser, one of which is to be cleaned prior to the test and the other is left in the fouled condition. Due to the spacial proximity of the tubes the outer con- vection coefficients, as well as the mass flow rates and inlet temperatures, are assumed to be identical. The second method also uses two adjacent tubes in the condenser, although one tube is blocked on either end allowing a temperature probe to be inserted into the tube such that the steam saturation temperature
CHAPTER 2. LITERATURE SURVEY
may be measured. A flow meter is installed upstream of the fouled tube, and the inlet and outlet water temperatures are measured.
2.4.2 Offline testing methods of measuring condenser fouling
Offline condenser fouling testing is either performed on test tubes removed from the condenser, or individual new tubes that are of the same material. Knudsen (1981) reviewed several apparatus types and techniques for measuring the foul- ing of heat transfer surfaces. Of these methods, the most apt to measuring the fouling of steam surface condenser tubes are: indirect electric heating, sensible heating from a fluid, and latent heating by a condensing vapor.
Indirect electrical heating can be achieved by winding resistance wire around the tube, or placing a thick-walled jacket over the tube with a resistance heater embedded in the jacket. The latter technique introduces an additional uncer- tainty of the contact resistance (Knudsen, 1981). Awad et al. (2007) used electric heaters in this way to study the effect of surface temperature on fouling rates. Specifically, the authors considered water flowing at 0.8 m/s having a suspended colloidal solution of solid particles (alluvium) with a concentration of 1 gm/L and a surface temperature in the range from 55◦C to 95◦C.
Izadi et al. (2011) also used the indirect electrical heating technique to mea- sure the composite fouling of sea water on 90/10 Cu/Ni tubing. The sea water samples were filtered through 10 µm filter paper to remove suspended solids. Most of the biological spores were removed in this way according to the author. Moreover, the tests were performed at 76◦C which suggests that the composite
fouling was a result of precipitation and corrosion fouling rather than biological fouling.
Sensible heating from a fluid uses an annular test section to test fouling on the inner or outer surface of test tubes (Knudsen, 1981). The test fluid can pass either through the annular region (fouling the outer surface of the tube) or inside the test tube (fouling the inside surface). The heating fluid then flows in a counter- current or co-current fashion, although Knudsen (1981) suggests that co-current flow can be advantageous to reduce variability in the surface temperature.
Using a tube-in-tube heat exchanger (double-pipe heat exchangers connected in a serpentine) as well as brazed-plate heat exchangers, Wu & Cremaschi (2013) investigated the effect of fouling in condensers for cooling tower applications. The potential for fouling of the water sample was measured in terms of the Lan- gelier Saturation Index (LSI) (Langelier, 1936). According to Wu & Cremaschi (2013) this number is calculated as the difference between the actual pH of the water sample and the pH at which concentration of the calcium carbonate in that water sample would be in equilibrium with total alkalinity. Wu & Cremaschi (2013) found that during these fouling tests the entire water loop experienced fouling. Only about 4-5% of the total amount of fouling occurred within the test heat exchanger. This raises an important consideration in the design of fouling apparatuses.
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The ASTM standard D4778-10 (2015) describes a test method to measure the fouling deposited (in terms of a mass measurement) on a test tube under heat transfer conditions. Cooling water passes over a heated section of the test tube (10 cm-20 cm) whilst keeping the flow rate and heat flux constant. A cartridge heater is inserted into the test tube (of 9.5 mm or 12.5 mm outside diameter) en- suring a snug fit. The cooling water passes through the annulus formed by fitting an acrylic tube (of 25 mm outside diameter) over the test tube. Thus, fouling oc- curs on the outside of the test tube. Since PPFs are applied to the internal surface of the tube, this apparatus is not directly applicable. Also the mass measurement of deposit is not directly relatable to calculating the fouling thermal resistance since the deposit porosity often varies.
Casanueva et al. (2003) designed and constructed a portable pilot plant to study the effects of different tube materials, diameters, and various chemical treatments on biological fouling caused by seawater. The schematic of their pi- lot plant is shown in figure 2.2. The shell and tube heat exchanger (3.1 m long) was constructed from PVC to prevent galvanic corrosion. The equipment was housed inside a standard 20 ft container to allow for in-situ testing using actual plant cooling water. The authors obtained a mean conductivity of the fouling film equal to 0.273 W/(mK) after 98 days of experimentation, but they conceded that since this was obtained under very specific conditions it was not possible to extrapolate to other fouling situations.
Figure 2.2:Schematic of pilot plant (source: Casanueva et al. (2003))
The apparatus described by Knudsen (1981) using a condensing vapor to heat a fouling fluid is formed by orientating a vertical test tube concentrically inside
CHAPTER 2. LITERATURE SURVEY
a larger tube. However, according to Knudsen (1981) a major problem with con- densing vapor is the variation in the condensing fluid convection heat transfer. Both surface weathering and the presence of non-condensible gases can change the condensation coefficient of heat transfer.
Another method, which is not included by Knudsen (1981), is radiant heating as proposed by Shinzato et al. (1990). The authors purport an accuracy of 1 × 10−5m2K/W of the measured fouling factor using this method of heating.
2.4.3 Fouling modeling
Fouling modeling is an important facet of designing and operating heat exchang- ers. Müller-Steinhagen (2011) reviews fouling modeling from 1960 to 2011. The Kern-and-Seaton model (equation 1.1) was integrated with respect to time and resulted in the following:
Rf = R∞f (1 − e−constant×time) (2.1)
where R∞
f is the asymptotic fouling resistance. This approach had several limi-
tations, the most notable of which is that it included empirical parameters that could only be obtained from operational data and also did not include chemical reactions (Müller-Steinhagen, 2011). Despite these shortcomings, from 1960 to about 1980 the Kern-and-Seaton model (equation 1.1) and the Tubular Exchang- ers Manufacturers Association (TEMA) fouling resistances were fundamental in most of the fouling models and heat exchanger designs.
Thereafter the work of Epstein (1983) altered the modeling approach such that fouling is considered to be made up of five processes: initiation, transport to the surface, attachment, removal (transport from the surface), and ageing. Nu- merous models to correlate available data followed, except many correlations can only be applied to idealized fouling cases. Müller-Steinhagen (2011) sug- gests that this may be a result of the non-linear nature and unsteady properties of fouling in addition to the numerous variables and different processes.
Recent developments to model fouling include: neural networks, computa- tional fluid dynamics (CFD) and molecular modeling. Fan & Zhong (2013) devel- oped a novel model based on Fuzzy stage identification and Chebyshev neural network, which showed better prediction of experimental work performed on a steam surface condenser than an asymptotic fouling model. Nebot et al. (2007) modified the kinetic fouling model defined by Konak (1973), in order to inves- tigate the effect of water velocity and tube material on fouling of steam surface condensers cooled by seawater. This evidence shows fouling models have been applied to steam surface condensers, but there are no reports of fouling models incorporating the effects of PPFs.
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2.4.4 Condenser modeling
The inherent complexity associated with the condensation process within con- densers makes the steam-side convection coefficient difficult to predict. Vapor shearing, condensate inundation and the presence of non-condensible gases are the most notorious processes affecting the condensing coefficient. Pioneering analysis in this field, performed by Silver (1963), put original ideas forward per- taining to the analysis of such effects, although the authors admit that (at that time) the ideas were yet unproven.
Individually these effects have been studied. For example, tube inundation effects were studied by Kern & Seaton (1959) in simple tube arrays. Rose (1980) developed approximate equations describing the convection coefficient in the presence of non-condensible gases. Recently Lakshmi et al. (2011) analyzed film condensation of pure vapors flowing normal to a horizontal condenser tube, whilst the tube experiences constant heat flux. All of these effects combined with spatial variations in the conditions within the condenser, such as non-uniform steam loading, means that the problem of condenser modeling often resorts to discretization of the condenser and ultimately computational fluid dynamic (CFD) techniques are used.
CFD models can either be two- or three-dimensional (Ramón & González, 2001; Prieto et al., 2003; Tarrad & Majeed, 2010). Using a CFD model, Rhodes & Hardy (2005) were able to study the impact on performance of the Pilgrim Nu- clear power station condenser, caused by changes in the following variables: a partially-filled waterbox, tube plugging, air-inleakage, and steam leakage from the moisture separator dump valve. Their CFD mesh consisted of 338 142 cells, although air extraction equipment was not explicitly modeled. Model verifica- tion indicated a maximum temperature difference of 4◦C between the measured
and calculated temperature rise across the west shell of the condenser. The au- thors suggested that this temperature difference can be a result of a difference between the actual heat rejected by the condenser and that assumed as a bound- ary condition in the model.
CFD modeling is gaining popularity and has been shown to be a successful tool in condenser performance modeling. There does however appear to be no such models which account directly for PPFs. Additionally there is a lack of evi- dence suggesting that fouling models that change with time have been incorpo- rated in previous CFD models.