Proceedings of ISEC 2005 2005 International Solar Energy Conference August 6-12, 2005, Orlando, Florida, USA ISEC2005-76232 PASSIVE HEAT EXCHANGER ANTI-FOULING FOR SOLAR DHW SYSTEMS Stephen J. Harrison Department of Mechanical and Materials Engineering Queen s University, Kingston Ontario, K7L 3N6, CANADA Phone (613) 533 2588, e-mail: harrison@me.queensu.ca ABSTRACT The interior surfaces of heat exchangers used in domestic hot water systems are particularly prone to fouling or complete blockage due to the accumulation of sediments, scale and mineral deposits. In many locations, mineral salts and other impurities may be present in the potable water supply and fouling may occur if the heat exchanger is not routinely cleaned or flushed of accumulated matter. In small residential installations, however, this is not practical due to the associated costs. In response to this need, a passive back-flushing system was designed that allows heat exchangers to be routinely backflushed many times a day. The action is a normal operation of the system and does not require user intervention, external power or controls to function. During back-flushing mineral deposits are washed out of the heat exchanger and flushed from the system. This paper describes the operation of the device and documents the results of accelerated tests undertaken to verify its operation. Keywords: Heat exchanger, fouling, solar hot water INTRODUCTION Thermal energy systems often incorporate heat exchangers for the transfer of heat from a heat source to a thermal load for a process or end use, or to a reservoir for storage. Modern heat exchanger designs are compact and offer high performance. To achieve this level of performance, small flow passageways are used to increase surface area and heat transfer. The interior surfaces of these devices are particular prone to fouling or complete blockage due to the accumulation of sediments, scale and mineral deposits or other matter that may be present in the heat transfer fluid [1]. In well-maintained commercial installations specific measures are taken to minimize the potential of this occurring. These include the careful monitoring and control of the chemical composition of the heat transfer fluid and frequent disassembly for chemical or power cleaning of the flow passages [2]. In the case of thermal systems designed to heat potable water, mineral salts and other impurities can often cause heat exchanger fouling [3]. An application, in which fouling of the heat exchanger is particularly acute, is in the case of residential potable hot water heating systems. In particular, if the system uses a heat exchanger or boiler to supply heat to the potable water, the continual transport of water through the unit, will result in the eventual deposition of mineral deposits in the flow passages of the heat exchanger [4, 5]. In climates subject to freezing conditions, solar domestic hot water (SDHW) systems typically incorporate a closed, antifreeze circulation loop and a heat exchanger to transfer heat from the solar collectors to the thermal storage. Recent efforts to produce viable SDHW systems for the residential market have lead to the use of cost-effective, high performance, compact heat exchangers. These units are usually placed between the solar collectors and the thermal storage, Fig.1. Potable water is circulated through one "side" of the heat exchanger and, in many geographic locations, it may become fouled with mineral deposits. This can lead to a significant drop in performance or complete blockage of the unit. To address this potential failure mode, a passive backflushing system that can be installed on the water supply to the heat exchanger was designed [6]. This device allows the heat exchanger to be routinely back-flushed many times a day. The action becomes a normal operation of the system and does not require user intervention, external power or controls to function. During back-flushing, mineral deposits are washed out of the heat exchanger into the thermal storage where they are re-dissolved and flushed from the system. This paper describes the operation of the device and documents the results of accelerated tests undertaken to verify its operation. 1 Copyright 2005 by ASME
Solar Collector use natural convection circulation on the water side of the heat exchanger to simplify the system, and reduce cost [7]. Consequently, the relatively low heat-exchanger circulation flow rates make this system configuration susceptible to fouling on the potable-water side of the heat exchanger. This anti-fouling method utilizes the water drawn for household use as the cleaning mechanism. At each draw of hot water by the occupant, cold water from the mains-water supply enters the heat exchanger from the top and exits from bottom, i.e., in the opposite direction to the flow in normal heating mode. It then flows into the storage tank creating a back-flush flow. To prevent the cold water from entering the tank top, a control valve was designed and installed at the top of the natural convection loop. The features and operation of the system are shown in Fig. 2(a) and 2(b). Anti-freeze circulation loop Solar Storage (supplies Load) Heat Exchanger Pump Fig. 1. A typical closed-loop solar domestic hot water system. FOULING IN HEAT EXCHANGERS Fouling of heat exchangers in hot water systems is a common problem [3] and occurs when there is the deposition of unwanted materials onto the heat transfer surfaces. In some geographic locations, due to the hardness of the water, fouling can lead to poor system performance and even system failure. Most fouling problems are the result of a combination of causes [5]. Corrosion in the system depends on the physical, chemical, and biological characteristics of the potable water. Scaling, common in water systems, is due to the presence of inverse soluble salts that precipitate onto hot surfaces. Gravity acting upon suspended particles in water also causes precipitation fouling. These suspended particles are usually sand, salt or particles of corrosion. Chemical compounds found in water that can cause fouling problems in SDHW systems include: CaCO 3, Mg(OH) 2, CaSiO 3 and CaSO 4. ANTI-FOULING DEVICE DESIGN Mechanical methods of anti-fouling fall into two classifications: on-line and off-line cleaning. On-line cleaning can be continuous or intermittent. The components are not removed from the system. The advantages of this method over the off-line method include ease of operation, cost effectiveness and uninterrupted operation. For this project, an on-line back-flushing system [6] was designed to prevent fouling in the heat exchanger of a low flow SDHW system. This type of solar system is designed to Fig. 2(a). Conceptual operation of the passive back-flushing system during normal solar charging mode. Fig. 2(b). Conceptual operation of the passive back-flushing system during normal back-flush mode. 2 Copyright 2005 by ASME
M OPERATIONAL EVALUATION A test apparatus, Fig. 3, was constructed to evaluate the effectiveness of this anti-fouling design. Two identical natural convection heating loops, similar to the tank loop in an SDHW system, were built on each side of a storage tank. The backflush system was installed on one of the natural convection circulation loops. The hot side of the heat exchanger in each loop was connected in such a way that the same flow rate and inlet temperature was assured. For the back-flush loop, water was drawn from the bottom of the tank and used to back-flush the heat exchanger at one-hour intervals for 3-minute durations. In the back-flush system design, the check valve (item 12 in Fig. 3) in the natural convection loop is of great importance. This valve has to be designed in such a way that it will cause minimal pressure loss (or flow restriction) to the flow of heated water during normal charging, but must restrict the flow of cold water to the top of the tank. Figure 4 shows the measured temperatures in the flow loop before, during and after a backflush cycle (caused by a hot water draw) and illustrate the operation of the system. It may be seen that the outlet temperature from the loop to the tank (T4 in Fig. 4) remained constant during the back-flush, verifying the functioning of the flow valve. A sharp dip in temperature occurs shortly after the back-flush due to the restart of the natural convection flow as some of the cold water in the loop was forced upwards. Controller Fig. 3 M 12 P1 T1 T4 T3 T2 Cold Water Supply Storage Tank Cold Water Return Test apparatus for accelerated testing of back-flow device. P2 Hot Fluid Supply Hot Fluid Return 60 T4 T3 T2 T1 50 Temperature, C 40 30 20 Natural convection heating of tank water in heat exchanger Recovery of natural convection flow through heat exchanger 10 0 Start of water draw Backflush region with flow reversal through heat exchanger 0 1 2 3 4 5 Time, min End of water draw Fig. 4. Measured temperatures in the natural convection loop with back-flow design, (T1 to T4, P1 and P2 are measured temperatures and pressures, respectively) 3 Copyright 2005 by ASME
ACCELERATED FOULING TESTING To investigate the operation of the passive anti-fouling back-flow device, a controlled experiment was undertaken at Queen's University, Dept. of Mechanical Engineering. The test apparatus, as shown in Fig. 3, was used to evaluate the antifouling effect of the proposed design. For comparison, two identical natural convection loops, similar to the tank loop in an SDHW system, were built on each side of the 450 L tank. The passive back-flush, anti-fouling device was installed in one of the loops and the other loop was operated without the benefit of the device to act as a baseline. The primary side of each heat exchanger was supplied with hot water (at approximately 60 o C) from the same source in such a way that the same flow rates and inlet temperatures were maintained. For the loop with the back-flush device, water was drawn from the bottom of the common storage tank and used to back-flush the heat exchanger at one-hour intervals for 3 minutes. Flow circulation through both heat exchangers was driven by natural convection, caused by the density difference that existed between the heated water in the heat exchange loops and the cooler water in the common storage tank. To ensure that this flow would continue, heat was removed from the common storage tank by an immersed coil that circulated cool mains water. Through this arrangement, the common storage was maintained at a cool temperature representative of a typical mains water supply condition. To accelerate the test, the tank water was saturated with CaCO 3, CaSiO 3 and Mg(OH) 2. During the test interval from October 2001 to February 2002, 68.5gms of CaCO 3 was added at three intervals. This quantity ensured that the tank water was a saturated CaCO 3 water solution. The hardness of the test water during the test period is shown in Table 1 as determined from a sample taken from the bottom of the tank on February 15, 2002. Table 1. Hardness of the water as tested. Parameter mg/l CaSiO 3 0.99 Mg(OH) 2 21 CaSO 4 47 CaCO 3 49.8 Hardness 122 pressure drop across the heat exchanger and associated temperatures were consistent with those observed at the start of the test. When the baseline heat exchanger was removed, a large quantity of solid precipitate was found in the secondary side of the heat exchanger. The flow passages were effectively blocked. Consistent with this situation, the pressure head across the secondary side of the heat exchanger was observed to be much higher than at the beginning of the test and that of the back-flushed heat exchanger, (e.g., ~90 Pa versus 30 Pa). This relatively high pressure was indicative of a no-flow condition through the fouled heat exchanger [7]. A review of the temperatures in the baseline heat exchanger indicated that the rate of heat transfer across the heat exchanger was significantly reduced as indicated by a very low temperature drop in the primary fluid as it flowed through the fouled heat exchanger. Approximately 7 gm of solid substance, (largely comprised of precipitated and solidified CaCO 3 was recovered from the body of the baseline, non-back-flushed heat exchanger. Figures 5 and 6 show the baseline heat exchanger at the time of removal showing the precipitated CaCO 3 flowing from the body of the heat exchanger and a close-up photo of the inlet of the heat exchanger showing the "fouling" of the flow passages on the secondary side. No precipitate was found in the passively back-flushed heat exchanger, Fig. 6 Fig. 5. Photo of the "baseline" heat exchanger at time of removal from the test rig. Notes: Calculation of the above concentrations assumes that all the Si is in the form of CaSiO 3, all the Mg is in the form of Mg(OH) 2, all the SO 4 2- is in the form of CaSO 4 and all the Ca is present as CaSiO 3, CaSO 4 and CaCO 3. Note that these compounds will dissociate into ions in water. RESULTS OF ACCELERATED TESTING After 5 months of continuous testing the system was stopped and both heat exchange loops examined. Observation of the heat exchanger with the passive back flush system revealed that all the flow passages of the heat exchanger were clear and free of any residue or blockage. In addition, the Fig. 6. Comparison photo of the flow channels of the "baseline" heat exchanger (left) and the heat exchanger equipped with the passive back flow system (right) at the end of the testing period. 4 Copyright 2005 by ASME
CONCLUSIONS Fouling of heat exchangers in solar domestic hot water systems is a significant problem that may lead to the premature failure of components and the complete system. To address this problem, a passive back-flushing system that can be installed on the water supply to the heat exchanger was designed. To verify the operation of the proposed system a series of accelerated tests were undertaken. Results of the testing confirmed the successful operation of the passive antifouling device. Specifically, testing showed that a baseline heat exchanger operating under identical water and temperature conditions became significantly "fouled" with precipitated CaCO 3, while the heat exchanger fitted with the passive backflush device was unaffected and continued to operate as initially installed. ACKNOWLEDGEMENT The author would like to acknowledge the support of the Industrial Research Assistance Program (IRAP) of National Research Council of Canada and Enerworks Inc. REFERENCES 1. Mukherjee, R., 1996, Conquer Heat Exchanger Fouling, Hydrocarbon Processing, vol. 75: 121-127. 2. Bott, T.R., 1990, Fouling of Heat Exchangers, Institution of Chemical Engineers. 3. Plante, R.H., 1983, Solar Domestic Hot Water - A Practical Guide to Installation and Understanding, Wiley & Sons, New York, NY. 4. Baker, D.K., and G.C. Vliet, 2002, Identifying and Reducing Scaling Problems in Solar Hot Water Systems, Solar 2002, Annual Conference of ISES, Reno, NV. 5. Purdy, J.M., and S.J. Harrison, 1998, The Fouling of Heat Exchangers in Natural Convection Applications, Proceedings of the Solar Energy Society of Canada Inc., Annual Conference, Montreal, P.Q. 6. United States Patent No.: US 6,827,091 B2, Passive Backflushing Thermal Energy System, Dec. 7, 2004. 7. Lin, Q., Harrison, S.J. and Lagerquist, M., 2000, "Analysis and Modeling of Compact Heat Exchangers for Natural Convection Application", Proceedings of EuroSun 2000, 3rd ISES-Europe Solar Congress, Copenhagen. 5 Copyright 2005 by ASME