Research On Fouling Prevention Technology With Steel Wire Helical Coil In Heating Tubes Of Tube Type Falling Film Evaporator

Falling film evaporator is widely used as an efficient heat transfer equipment.However, fouling problem of the inner wall in heating pipes is widespread and hasnot been resolved effectively. A new automatic fouling prevention technology calledfalling film helical coil is developed based on the flow characteristics of falling film andfouling prevention methods on falling film evaporator. The effect of fouling preventiondepends on the movement of the steel wire helical coil and the flow field of liquid film.Fouling prevention mechanism is obtained:(1)fouling is delayed because of thechange of velocity boundary layer and thermal boundary layer.(2)Tiny crystals arecleared away by the mechanical disturbance in boundary.The flow field in falling film tubes that one end of steel wire helical coil is fixedis analysised. The results show that the thickness of falling film is changed periodically.The velocity field is also changed that although the change of axial velocity decreases by15%because of the resistance of wire helical coil, the ability of fouling prevention andcleaning is improved by the generation of radial and tangential. meanwhile, foulingdeposition decreases with the enhancement of the turbulence intensity of falling film. Theconvection heat transfer of falling film is strengthened efficiently. Compared with emptytubes, the total heat transfer coefficient increased by32%.A cold simulation test is set up with clear water as working medium. From the pointof view of best fouling prevention and cleaning, spiral liquid film distributor and steelwire helical coil are optimized through the analysis of liquid film distribution and flowcondition after going through spiral liquid film distributor, the feature of movement ofwire helical coil and the influence of liquid film: spiral liquid film distributor is preferablewhich use single spiral flow channel, helix angle15°. The height above the tube rises to50mm because of the generated resistance. Fouling prevention is fit to realized throughmotion state and influence to falling film by steel wire helical coil with the thread pitch of18～22mm, the gap of0～2.25mm, and the wire diameter of1.5mm. The findings alsoshow that the nominal flow rate of falling film and the coefficient of elasticity of steelwire helical coil as the key factor of axial vibration of steel wire helical coil should beconsidered significantly while designing.Heat transfer test on the automatic fouling removal technology of falling filmhelical coil is developed to verify the effect of fouling removal and cleaning, ascertain themaximal ability of cleaning and find out the major factor which influence the ability of fouling prevention. The results show that falling film tube with steel wire helical coilinstalled in is provided with the effect of fouling prevention. The critical csurthat theinitial fouling rate is balance with cleaning rate increases with the enhancement ofnominal flow rate. Axial vibration which is the key factor of fouling prevention willrepresent a greater proportion in fouling prevention ability. The difference betweenfouling prevention ability of steel wire helical coil with one end fixed completely and theone with rotation is only within9%when the nominal flow rate is at0.32～0.4m/s. Thetwo kinds of steel wire helical coil can be exchanged completely. Steel wire helical coilwith one end fixed completely is provided with broad application because of reducingattrition, although the fouling prevention ability is not as good as the one with rotationwhen the nominal flow rate is under0.32m/s.