Experimental study on water separation process in a novel spray flash vacuum evaporator with heat-pipe

Based on the high heat transfer performance of heat pipe (HP) and large specific surface area of small diameter droplets, HP and spray technologies are combined in a novel single stage vacuum evaporator and artificial seawater with 3% saltness is studied experimentally. Parametric studies are carried out, such as cold source and heat source temperature, spray temperature, and spray flow. The results show that: the maximum heat flux density reaches 32 W cm−2 on evaporator bland plate; HPs absorb energy effectively from low grade heat source (40– 80 °C) then transfer the energy to the droplets already flashed, so as to maintain or even increase the superheat degree of droplets during evaporating process; This method can significantly improve the water separation rate (quality of condensed water / quality of dilute solution ∗ 100%); Changing the parameters can regulate water separation from 0% to 65%. Thus it has important significance for utilizing lower grade heat source effectively, gaining brine, and promoting the fresh water yield per unit volume and adaptability to variable load in desalination project. © 2016 Elsevier B.V. All rights reserved.

To promote single-stage water separation rate and energy efficiency of low grade heat source in traditional desalination device, a novel solution is put forward. Centrifugal nozzle generates micro droplets with diameter of 5–10 μm and sprays them into the evaporator to flash. And HPs absorb heat from low grade heat source then transfer the heat to the droplets already flashed with high heat flux density. The coupling between energy flow and mass transfer in spray flash and droplet evaporation reheating is achieved. Compared with traditional single-stage flash evaporator, the new scheme lifts the water separation rate to 48% from 5%, helps to compact the unit and uses low grade heat source as low as 40 °C. The scheme has great significance in using low grade heat source effectively, improving freshwater yield per unit volume and gaining concentrated brine, the main conclusions are made as follows. (1) Solution spray has already fully spread before reaching the top section of HPs. When nozzle height is about a certain range, the impact of nozzle height on separation rate can be ignored. (2) The actual average separation rate is slightly less than the theoretical value with and without heat source. Because it is affected by fluctuation of evaporator pressure, heat preservation material performance limitation and the existence of imbalance temperature difference. But their impacts are not significant. (3) The heat source temperature is determinant of fresh water yield. The higher heat source temperature is, namely the higher HP's cold end temperature is, the more heat that droplets can obtain and the faster evaporation process will be. Water film on the HP wall becomes thinner when solution is atomized and heat transfer performance is enhanced, but permanent dry wall phenomenon should be avoided because it can cause the deterioration of heat transfer. (4) The cooling water temperature is an important factor of fresh water yield. Flash equilibrium pressure and the steam temperature decrease with the decreasing cooling water temperature, so the initial degree of superheat and steam flow rate rise. Thus heat transfer performance is enhanced and the separation rate is improved. (5) For the device taking the new scheme, changing the parameters can regulate water separation from 0% to 65%. Theoretically, this scheme can realize and regulate the separation rate above 65% if the heat source temperature is higher or recirculating spraying is achieved.