Tham khảo tài liệu 'blast cleaning equipment_5', kỹ thuật - công nghệ, điện - điện tử phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Abrasive Hose Lines 145 Fig. Relationships between air velocity abrasive hose diameter and nozzle diameter based on O 2 8 dN vf V Apt pi For ApT 0 reduces to . For an assumed pressure drop of 25 and an air flow velocity Ỉ Ị. 15 m s delivers an optimum ratio dH dN . For the example mentioned earlier dN 12 mm the optimum hose diameter is dH . The next commercially available hose diameter is dH 38 mm which is very close to the requested value. Because the pressure drop is a function of the hose length the diameter ratio dH dN depends on hose length. In an optimum blast cleaning system the parameters dN dH and lH are well balanced and if one of the parameters changes value the other parameters must be adjusted accordingly. Optimum Flow Velocities in Abrasive Hoses Pressure drop in abrasive hose lines is due to air flow and abrasive material conveying in these hoses. Experience with pneumatic conveying systems has shown that pressure drop depended on air conveying velocity respectively dynamic air pressure and air flow Froude number and mass flow ratio abrasive air. One example is shown in Fig. more information can be found among others in Coulson and 146 4 Blast Cleaning Equipment Richardson 1968 and Marcus et al. 1990 . It can be recognised from the graph in Fig. as well as from Figs. and that the pressure drop for a solid-air flow is much higher compared with that for a plain air flow. For an air conveying velocity Ỉ Ị. 15 m s the pressure drop is ApT mbar m for air only but it is about ApT mbar m for mP kg min. It can also be noted from Fig. that pressure loss has distinct minimum values at certain optimum air conveying velocities. This optimum air flow velocity depends on the Froude number of the particle settling flow. For vertical conveying processes the optimum air flow velocity can be approximated as follows Ahland 1966 vOpt g0-2 dH2 v0-6 This equation is .
