Direct contact heat transfer with change of phase: Spray-column studies of a three-phase heat exchanger (original) (raw)
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Desalination, 2011
Vapor-compression desalination is a reliable and robust desalination technology that benefits from high heat transfer coefficients in evaporators/condensers. In this study, heat transfer coefficients were measured in vertical dimpled-sheet heat exchangers (0.762-mm-thick naval brass 464 or 0.203-mm-thick copper), some of which were bare (filmwise condensation) and others were coated with a thin layer of passive electroless Ni-P-PTFE (dropwise condensation). The heat exchanger sheets were mounted in a sealed two-chamber apparatus with condensing saturated steam on one side and boiling saturated water on the other. Shearing steam on the hydrophobic condensing surface enhanced the overall heat transfer coefficient by 1.6 times and forced convection on the boiling side increased it by an additional 1.4 times. Adding a 0.635-μm-thick layer of hydrophobic Ni-P-PTFE coating to naval brass 464 that was roughened on the boiling-side surface with sand-blasting increased the heat transfer coefficient by 4.3 times. Adding PTFE boiling stones to the boiling side of the hydrophobic copper plates increased the heat transfer coefficient by 1.15 times. The maximum overall heat transfer coefficient measured was U = 184 kW/(m 2 ·°C), which occurred with 0.203-mm-thick copper (k = 400 W/(m·°C)) coated with 0.635-μm-thick Ni-P-PTFE and PTFE boiling stones as a dynamic nucleation agent in the bulk liquid (P = 722 kPa, T =166°C, v steam =0.49 m/s, R ≈ 1 kg shearing steam/kg condensate, v liq =1.57 m/s, ΔT =0.20°C).
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An experimental investigation of the volumetric heat transfer coefficient in a three-phase direct contact condenser was carried out. A 75-cm long cylindrical Perspex column with a 4 cm diameter was used. Only 48 cm of the column was utilised as the active direct contact condensation height. Pentane vapour at three different initial temperatures (40ºC, 43.5ºC and 47.5ºC), with differing mass flow rates, and tap water at a constant initial temperature (19ºC) with five different mass flow rates were employed as the dispersed phase and the continuous phases, respectively. The results showed that the volumetric heat transfer coefficient increased with increasing mass flow rate ratio (variable dispersed phase mass flow rate per constant continuous phase mass flow rate), the continuous phase mass flow rate and holdup ratio. An optimal value of the continuous phase mass flow rate is shown for an individual dispersed phase mass flow rates. This value increases with increasing vapour (dispersed) phase mass flow rate.
Analytical Modelling of a Spray Column Three-Phase Direct Contact Heat Exchanger
ISRN Chemical Engineering, 2013
An analytical model for the temperature distribution of a spray column, three-phase direct contact heat exchanger is developed. So far there were only numerical models available for this process; however to understand the dynamic behaviour of these systems, characteristic models are required. In this work, using cell model configuration and irrotational potential flow approximation characteristic models has been developed for the relative velocity and the drag coefficient of the evaporation swarm of drops in an immiscible liquid, using a convective heat transfer coefficient of those drops included the drop interaction effect, which derived by authors already. Moreover, one-dimensional energy equation was formulated involving the direct contact heat transfer coefficient, the holdup ratio, the drop radius, the relative velocity, and the physical phases properties. In addition, time-dependent drops sizes were taken into account as a function of vaporization ratio inside the drops, whil...
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Chemical Engineering & Technology, 2005
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