Forward speed loss

The backward curved fan has blades that are curved backward against the direction of rotation. They are useful with light dusts, fumes, and condensates. They operate at higher speed than the forward curved fans and hence tend to be noisier and somewhat more costly, but, as a compensatory advantage, they are responsive to situations of varying resistance without loss of efficiency. 

Suitable screw configuration and screw speed ensure compensation of heat loss, particularly after flash and also after the initial forward venting. The vacuums in the individual degassing zones are designed to avoid excessive foaming of the polymer solution or even crumbs in the degassing opening. 

The COAST computer program is used to calculate the reactor coolant flow coast down transient for any combination of active and inactive pumps and forward or reverse flow in the hot or cold legs. The equations of conservation of momentum are written for each of the flow paths of the COAST model assuming unsteady one-dimensional flow of an incompressible fluid. The equation of conservation of mass is written for the appropriate nodal points. Pressure losses due to friction, and geometric losses are assumed proportional to the flow velocity squared. Pump dynamics are modelled using a head-flow curve for a pump at fiill speed and using four-quadrant curves, which are parametric diagrams of pump head and torque on coordinates of speed versus flow, for a pump at other than full speed. 

Drive pressure (e.g., in the hydraulic stroke drive of the screw) and injection pressiue (e.g., in the screw forward cavity) are practically proportional to one another (independent of the injection conditions) and display the same cycle (if we disregard the friction losses as the screw moves forward) - see Figure 8.2. The internal mould pressiue, on the other hand, is lower than the injection pressure, conditioned by the flow losses during injection, which depend on the viscosity of the melt, the injection speed, and the geometry of the flow path. 

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