Experimental and Theoretical Study of Water Side Fouling Thermal Performance of Refrigerant to Water Condensers

Abstract

Brazed plate heat exchanges (BPHEs) and tube-in-tube heat exchangers (TTHEs) are commonly used in the refrigeration, air conditioning, and food industry as refrigerant-to-water condensers, in which the refrigerant rejects heat to water circulating in cooling tower loops. These heat exchangers often suffer from severe fouling issues because as the water in the cooling tower evaporates, the mineral concentration in the remaining water increases. Once the solubility limits are reached, the minerals precipitate and a layer of fouling formed on the heat transfer surfaces. Due to the fouling deposit, the thermal resistance between refrigerant and water gradually increases. The fouling formation penalizes the overall effectiveness of the refrigerant condensers, and thus, must be properly accounted for during the equipment design. This thesis focuses on fouling effects on the thermal and hydraulic performance of condensers in cooling tower systems. Two braze plate heat exchangers and a smooth tube-in-tube heat exchanger was experimentally investigated under fouling operating conditions by using a new experimental facility at Oklahoma State University. The aim was to measure the fouling resistance in real time and correlate the data with the heat exchanger internal geometry, water quality, and refrigerant saturation temperature. The fouling resistance in the TTHE was observed to have asymptotic trend, and the asymptotic limit was lower than that for BPHEs with soft corrugation angles and higher than that of BPHEs with hard corrugation angles operating at similar conditions. The hydraulic performance was similar to BPHEs with hard corrugation angles. Both refrigerant saturation temperature and water fouling potential increase would lead to a measurable increase in the fouling resistance inside the refrigerant to water condenser. A model for the mineral species dissociation and mineral precipitation on the heat transfer surfaces was verified. By considering a semi-empirical correlation for the fouling deposition strength factor, the simulation results predicted the fouling thermal resistance with an error of 30%. Model limitations and research needs for potential improvements are discussed.