| In the multiple effect evaporation desalination process of seawater and brackish water, the operation pressure and NaCl concentration are the two most important factors for the system efficiency and product water cost.In the present work,boiling heat transfer performance of NaCl solution with concentration of 0-25%is tested on smooth stainless tube at atmospheric pressure and lower pressure.The boiling hear transfer coefficient impacted by the system pressure and concentration of NaCl was mainly investigated.The results show that the efficiency of boiling heat transfer increases with the NaCl concentration increasing below atmospheric pressure.The heat transfer performances of pure water and 3.5%brine increase with the system pressure increasing.However,for the brines with 7%and 15%concentrations,the heat transfer performance first increases and then decreases.When the concentration of NaCl was raised to 25%,the heat transfer coefficient decreased with the system pressure increasing.The enhancement of modified PFA coatings on boiling heat transfer was investigated. The performance of coatings formed by different coating arrangement was also tested.There were three kinds of PFA coating surfaces named E-1,E-2 and E-3 which the coating covered approximately 100%,50%and 25%of the heating surface respectively.The heat transfer coefficient increased for E-2 and E-3 surfaces.The performance of E-3 was better than that of E-2 ant high flux.But the heat transfer coefficient was considerably reduced when we used E-1 surface.For this condition the bubbles spread over the surface and coalesced with bubbles formed at other sites,causing the entire surface to become vapor-blanketed.The boiling heat transfer was studied for brine with the concentration of 7%and 25%on E-3 heating surface.The E-3 surface showed considerably enhancement at atmospheric pressure.But the enhancement was not so obvious when the system pressure is very low (13kPa).
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