Thermal and Chemical Analysis of Fouling Phenomenon in Condensers for Cooling Tower Applications
refrigerant condenser, tube-in-tube, BPHE, corrugation angle, mineral precipiation mass flux
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 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 deposit on the heat transfer surfaces: an undesirable yet unavoidable phenomenon. Due to the fouling deposit on the heat transfer surfaces, the thermal resistance between refrigerant and water gradually increases. The fouling resistance depends on several factors such as heat exchanger geometry, heat flux, water quality and water flow rates. The fouling factors penalize the overall effectiveness of the refrigerant condensers and thus must be properly accounted for during the equipment design. Predictions of the fouling allowances during the life service of the condenser characterize the degradation in thermal performance and provide guidelines about the maintenance and service of this type of equipment. In this work, a smooth TTHE was investigated 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 water quality and heat flux inside the refrigeration condenser. 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 BHPEs with hard corrugation angles operating at similar conditions. The hydraulic performance was similar as BPHEs with hard corrugation angles. The fouling deposit inside the tube-in-tube heat exchanger was further analyzed by using a CCD camera with a borescope probe and by adopting a chemical digestion process for the fouling particles inside the tubes. Fouling did not deposit uniformly inside the TTHE and it was evident that more fouling was present at the outlet section of the water side. The chemical analysis showed that more than 85% of the fouling material was CaCO3, which was expected from the chemical analysis of the water in critical saturation conditions.