Performance and fouling prediction model for finned-tube heat exchangers

Abstract

Finned-tube heat exchangers (FTHXs) are found in abundance in heating, ventilation, and air-conditioning (HVAC) systems. This vital piece of equipment is used to either cool or heat the occupational environment. The thermal conditions in the occupational environment could affect labour productivity, health and safety directly. Thus, FTHXs are of paramount importance to ensure continuity of operation and profit. FTHXs foul while operating in real-world conditions due to impurities in the hot and cold fluid streams. Consequently, the heat exchanger performance deteriorates, and at some point, system downtime is required to restore performance through maintenance activities. The need therefore exists for a performance, fouling and ideal maintenance interval prediction model for FTHXs. However, previous methods required instrumentation to be installed and the FTHX to operate at design conditions. None of the methods and models reviewed were compatible with a fouled FTHX operating without installed instrumentation at off-design conditions in the HVAC industry. The study objectives include a model capable of predicting the following at off-design conditions: the optimum and actual air-cooling duty as well as outlet temperature; performance loss due to fouling; and the ideal maintenance interval. This study presents a simplified, yet effective method for predicting FTHX performance, fouling and the ideal maintenance interval at off-design conditions and with no installed instrumentation. The method consists of three integrated models, namely; performance, fouling, and maintenance prediction. These models were derived from a combination of first principles of heat transfer and psychometry, the approaches followed by Bayesian and Markov to develop maintenance policies, and formulated thermal and production relations. Nine case studies were used to test the newly proposed method. Located 3.2 km underground, these case studies were found to operate at off-design conditions in a bulk air cooler system. After applying the performance model to the nine heat exchangers, the results revealed that the actual performance deviated by 29% from the optimal performance due to external and internal foulants being present on the heat transfer surfaces. The 29% reduction in air-cooling performance caused an overall 2 °C increase in air outlet temperature. The fouling prediction model further revealed that 11% (of the 29%) could be attributed to external fouling and the remaining 18% to internal fouling. These two factors increase the BAC outlet temperature by 0.7 °C and 1.3 °C, respectively. The external foulants were removed by means of water under high pressure, which resulted in the air outlet temperature decreasing by 0.7 °C. The formulated thermal and production relations indicated that the mine revenue could be improved by R58 million as a result of the temperature decrease. An additional R30 million could be gained if the air temperature is reduced further by 1.3 °C by eliminating the internal fouling. These figures highlight the importance of quantifying the impact of fouling and forecasting the ideal maintenance interval. The implementation and results obtained revealed that the newly proposed methodology met the study objectives and that the need for the study was addressed successfully.