12th International Symposium on District Heating and Cooling

The <strong>12th</strong> <strong>International</strong> <strong>Symposium</strong> on District Heating and Cooling,September 5 th to September 7 th , 2010, Tallinn, Estoniamethod seems also to be very stable in detecting thefouling.DISCUSSION AND CONCLUSIONThe results indicate that the method proposed can beused to detect fouling in cross flow heat exchangersoperating under dynamic condition by usingmeasurements that can be obtained under normaloperation. The detection method is based on the wellknown method of Number of Transfer Units, withaddition of empirical relations to make the method validover wide range of mass flow rates.Figure 4. The CuSum chart quickly detects the shift in theoverall heat transfer coefficient.Comparison of Figures 3 and 4 shows that it is possibleto detect fouling in heat exchangers operating indynamic condition with quite good accuracy by usingthe NTU method and empirical relations.In Table 1 a comparison between the method in [4] andthe method presented in this paper is shown. From thetable it is apparent that the method presented in thispaper gives better results. The fouling detection intervalfor the drift corresponds to fouling factors on theintervals [0.00002, 0.00004] and [0.00001, 0.00003]respectively for the fast and slow fouling. The methodis therefore giving considerable better results than themethod described in [4].Table 1: Comparison of detection time between the twomethods, where method 1 is from [4]PercentilesMethod 1 Method 2Fast2.5% 0.59 0.2650% 0.83 0.3597.5% 0.98 0.40PercentilesSlow2.5% 0.63 0.2350% 0.81 0.3097.5% 0.93 0.35Typical fouling factors are, as stated above, on theinterval [0.0001, 0.0007]. The results therefore indicatethat the method can be used to detect fouling in crossflow heat exchangers that are operating in non-steadystate condition prior to the time a typical fouling factorheat exchangers are designed for is reached. TheBy monitoring the calculated overall heat transfercoefficient, it is possible to detect changes that are dueto fouling or changes in the working fluid. Unlikeconventional methods, this method can detect foulingin heat exchangers that are not operated in steadystate conditions. The fouling detection is performedwithin the designed fouling factor interval.Further work will include application of the method ondata from a real heat exchanger.ACKNOWLEDGEMENTThis work has been supported by the Environmentaland Energy Research Fund of Orkuveita Reykjavíkur,National Energy Fund and Energy Research Fund ofLandsvirkjun.REFERENCES[1] J. P. Holman Heat Transfer. Ninth edition, McGrawHill, 2002.[2] M. Mishra, P. K. Das and S. Sarangi. "Effect oftemperature and flow non-uniformity on transientbehaviour of crossflow heat exchanger".<strong>International</strong> Journal of Heat and Mass Transfer,2008, p. 2583-2592.[3] H. Kou and P. Yuan. "Thermal performance ofcrossflow heat exchanger with nonuniform inlettemperatures". <strong>International</strong> Communications inHeat and Mass Transfer, 1997; 51(9-10):357-370.[4] O. Gudmundsson, H. Palsson and O. P. Palsson."Simulation of fouling in cross-flow heat exchangerand a fouling detection based on physicalmodeling". In: Proceeding of The 50th Conferenceon Simulation and Modelling, Fredericia, Denmark,7-8th of October, 2009.[5] W. L. Pope, H. S. Pines, R. L. Fulton and P. A.Doyle. "Heat exchanger design "why guess afouling factor when it can be optimized?". EnergyTechnology Conference and Exhibition. Huston,Texas, 1978.308