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, EstoniaANALYSIS OF HEAT TRANSFER IN HEAT EXCHANGERS BY USINGTHE NTU METHOD AND EMPIRICAL RELATIONSO. Gudmundsson, O. P. Palsson and H. Palsson 1Faculty of Industrial Engineering, Mechanical Engineering and ComputerScience Hjardarhagi 2-6, IS-107 Reykjavik, IcelandABSTRACTHeat exchangers are widely used in domestic andindustrial applications involving transfer of energy fromone fluid to another, for example in district heatingsystems. The wide usage underlines the importance tohave a good technique to detect if the effectiveness ofan heat exchanger is diminishing. There are number ofthings that can cause diminishing effectiveness of anheat exchanger, for example fouling, changes in fluidproperties as well as corrosion. In many cases thefouling is a particular problem, for example whengeothermal water is used. Geothermal water is verymineral rich which can cause serious fouling problems.The method presented in this paper is simple and easyto use and can be used to detect a diminishing heattransfer coefficient in many types of heat exchangers,in this paper the method is used on cross flow heatexchanger. The method uses measurements of theinlet and outlet temperatures as well as the mass flows,these measurements are usually easy to gather undernormal operation. The method uses the well knownNumber of Transfer Units (NTU) method as well asempirical relations to estimate the overall heat transfercoefficient, which is then statistically analyzed. Thedata used in this study was gathered from a simulatedcross-flow heat exchanger where the overall heattransfer coefficient was gradually decreased tosimulate diminishing effectiveness of the heatexchanger. The conclusion of this study shows that thederived detection method can detect fouling based onthe data from a simulated cross-flow heat exchanger,with a good accuracy and consistency. Further analysison real data is scheduled.INTRODUCTIONHeat exchangers are widely used in domestic andindustrial applications involving transfer of energy fromone fluid to another. General classification of heatexchangers are parallel flow, counter flow and crossflow. Their size and complexity can also vary greatly.Their operating conditions can be classified into twomain classes, steady operation where mass flow andtemperatures are relatively constant and dynamicoperation where mass flow and temperatures can varygreatly with time.During operation it is important to have someknowledge of the condition of the heat exchanger. Fora steady state condition it has proven to be relativelysimple, since analytical and empirical relations can bederived for different heat exchanger types and used forall necessary calculation regarding time invariantconditions, see e.g. [1]. If a dynamic operation exists itbecomes more complex to monitor the condition of theheat exchanger and more complex models are used,see e.g. [2] and [3].In this study, a mathematical model is used that hasbeen developed to simulate accurately the temperatureand flow transients in a cross flow heat exchanger. Themodel is based on the finite volume method (FVM)where a mathematical representation of a generalcross flow heat exchanger is solved numerically. Onepossible application of such a model is to generatedata that can be used to compare and tune moresimple dynamic models based on either black boxmethods or state space modelling. An importantapplication in this context involves methods to detectfouling in heat exchangers under dynamic operation.Description of the model can be seen in [4].Fouling in heat exchanger can be categorized in thefollowing categories, precipitation fouling, chemicalreaction fouling, corrosion fouling, particulate fouling,biological fouling and freezing fouling. Usually fouling isa combination of the categories. The fouling process inheat exchanger can be described as a process wherethe separating metal inside the heat exchangeraccumulates deposits from the fluids. This is verycommon and poses problems and results in reducedefficiency of the heat exchangers. There are numerousmethods available to address the effect of fouling, see[5–8]. Finally, decrease in the thermal efficiency of aheat exchanger due to property changes in a workingfluid will have similar effect on the heat exchanger asfouling.There are number of ways to detect fouling butaccording to [9], classical methods involvea) examination of the heat transfer coefficient,b) simultaneous observations of pressure drops andmass flow rates, c) temperature measurements,d) ultrasonic or electrical measurements ande) weighing of the heat exchanger plates. Methodsa–c) require the heat exchanger to be operating insteady state condition, d) can only monitor local foulingand e) requires the process to be stopped. Theserestrictions can be too strict or costly. Another305