Relevant characteristics of condenser tube materials are reviewed. To combat corrosion and erosion, PPFs have been used. The reported PPF performance measures are quoted as fixed values and no consideration is given to their varia- tion with time. In terms of mitigating fouling, anti-fouling coatings in literature
CHAPTER 2. LITERATURE SURVEY
have been shown to improve heat exchanger performance, by lessening the ef- fects of fouling. However, the literature does not provide sufficient information regarding the water-side fouling of PPFs, thus warranting further research. The fouling resistance can be measured either online with the condenser, or offline where separate tubes are tested outside the condenser. Various methods in lit- erature are considered for their suitability to test the fouling characteristics of PPFs. The chapter concludes by discussing fouling models as well as condenser modeling.
Chapter 3
Experimentation
3.1 Introduction
The waterside fouling of archetypal steam surface condenser tubes is experi- mentally investigated so that the temporal performance effects of incorporat- ing paint-based protective films (PPFs) can be quantified. Several different alloy tubes, with and without PPFs, are thus subjected to the identical water that also passes through an actual steam surface condenser. A conventional coal-fired thermal power plant is selected for this investigation. The plant has an open- type recirculating cooling water system with four natural draft wet-cooled cool- ing towers which supply cooling water to six surface condensers. This means the condensers suffer from fresh-water fouling2. Owing to the size, variability in condensation processes, inaccessibility, and poorly instrumented properties of the actual plant condenser, an off-line apparatus is used instead. Actual plant cooling water is tapped from the plant’s cooling water system at a point down- stream of the condenser and just before the cooling water enters the wet-cooling tower (figure 3.1).
The significance of using actual cooling water in real time will become appar- ent later. The distance between the tap-off point and the condenser is approx- imately 50 m, but the temperature measured at the condenser outlet water box compares within 1◦C of that measured at the tap-off point. Water exiting the con- denser is specifically chosen because it is the hottest point in the cooling system and therefore has the highest propensity for scaling. Even though scaling takes place within the condenser before this point, causing inversely soluble ions to precipitate out of solution, the cooling water exiting the condenser is still super- saturated with scalants. Furthermore this water is above 30◦C and is therefore in
the range of highest biofilm development (chapter 1). The cooling tower pond was not considered as a good candidate location because of the striation that oc- curs vertically in the pond. Also the water entering the condenser is around 20◦C
2Coversely some power plant condensers use sea water in open-type cooling systems and in
CHAPTER 3. EXPERIMENTATION
1
Figure 3.1: Apparatus installation in relation to the plant cooling water network – (1) tap-off point
and therefore not in the region of maximum biofouling potential as it is at the condenser outlet. Therefore this tap-off point was not considered suitable. The plant cooling water is referred to as the fouling fluid hereafter.
Knudsen (1981) explains how steam is the least desirable medium to heat the fouling surface in experimental apparatuses due to its inherent variability caused by surface weathering and non-condensible gases. Therefore sensible heating is used in this apparatus to simulate the condensing steam in the actual condenser – maintaining repeatable and known conditions such as heat flux and flow rate. Six identical double-pipe heat exchangers are formed by locating PVC tubes con- centrically over each of the test tubes creating a water jacket around each tube. The double-pipe heat exchangers are then arranged in a parallel configuration. Heated potable water passes through the annular region of each heat exchanger which heats the outer surface of the test tube, whilst the actual cooling water passes through the inside of the test tube at the design flow rate for the con- denser. The heat exchangers are operated in a co-current fashion such that the fouling fluid and heated potable water flow in the same direction. This configura- tion achieves a more uniform tube wall temperature compared to a counter-flow configuration.
Figure 3.2 depicts how the equivalent heat transfer conditions experienced by a tube inside a condenser are simulated by the heated water inside the test heat exchangers. Both the tube inside (a) the condenser and (b) the double-pipe heat exchanger have the fouling fluid passing through the inside of the tube, as
CHAPTER 3. EXPERIMENTATION
the foulant deposits at the fluid-solid interface. The velocity of the heated water is then controlled such that the sensible convective heat transfer coefficients are comparable to those for condensing steam.
1 2 2 3 4 5 6 7 Q Q |Q| |Q| 25◦C 45◦C 35◦C 40◦C 34◦C 39◦C 36◦C wall temperature
(a) condenser (b) double-pipe heat exchanger
Figure 3.2:Illustration comparing heat transfer conditions within the condenser and the double-pipe heat exchanger – (1) heated water flowing through annulus, (2) fouling fluid, (3) foulant, (4) tube wall, (5) insulation, (6) steam, (7) falling condensate