Characterization of a titanium coaxial condenser

Abstract

In South Africa a leading engineering company, focusing on energy engineering services and products to the mining industry, developed a mobile refrigeration unit known as the air-cooling unit (ACU) MKII. The ACU MKII uses a stainless-steel tube-in-tube condenser, which in some cases has a higher than anticipated corrosion rate due to impurities in the mine supply water used as heat sink. A solution to this problem is the use of titanium for the inner tube due to its corrosion resistance. Advancements in manufacturing techniques have resulted in titanium being used in coaxial coils. The coaxial configuration with its pure counterflow characteristics results in an enhanced heat transfer compared to smooth tube heat exchangers while the physical compactness of the coaxial coil is beneficial. However, limited performance data, such as heat transfer and pressure drop characteristics, exists for these titanium coaxial heat exchangers. To better understand the applicability of a titanium coaxial condenser in the ACU MKII, a need exists to develop a thermal-fluid simulation model to predict the coil’s convection heat transfer and pressure drop for R-407C refrigerant inside the annuli and water through the inner tube. The simulation is based on a modelling approach in literature that was developed for copper coaxial condensers employing enhancement factors for the complex geometry. Enhancement factors are incorporated into published correlations to improve the accuracy of the titanium coaxial condenser predictions of convection heat transfers and pressure drops. The allowance for enhancement factors in the simulation model are made to account for any differences between a standard helical coil and the coaxial tube annulus. Experimental data were gathered from a test-bench and incorporated to calculate the enhancement factors through comparison between simulated and measured values. The resulting heat transfer and friction (pressure drop) enhancement factors were 0.5288 and 5.1534 respectively. Simulated heat transfer and pressured drop values, using no enhancement factor in the simulation model, were compared to the measured values and produced an average difference of 5.23% and 90.42% respectively. The average differences for the log mean temperature difference (LMTD) was 11.57%. After implementing the above-mentioned average enhancement factors in the simulation model, the average differences between the simulated and measured heat transfer and pressure drop were 5.79% and 32.62% respectively. The average LMTD differences was 6.83%.