13.4.1 Separators - part 3
Wire mesh separators
The early designs of combined moisture separator/ reheater supplied by GEC, Asea Brown Boveri and Westinghouse incorporated a wire mesh separator.
The water separation characteristics had been subject to theoretical and experimental investigation and was established in a number of industrial applications. Brodie , who studied the extraction properties of wire mesh in an atmospheric air/water flow, had demonstrated that a residual steam wetness of 0.1-0.2% could be obtained but only over a narrow range of very low steam velocities of the order of 1-2 m/s. Further tests carried out by GEC confirmed that good separation performance could be obtained for crossover conditions in wet steam turbines and gave good correlation with the previously predicted upper limiting velocity. These, and other investigations, showed that water is extracted when the approach velocity is sufficient to cause inertia impaction of droplets on the wires of the mesh. After deposition, the droplets coalesce and run along the surface of the wire under the influence of gravity until they reach a point where two wires cross. This point soon becomes overloaded with water and large drops fall from the mesh against the direction of the steam flow.
The wire-mesh mats must be disposed as near horizontally as possible and, in view of the necessary low velocity, the mats have to be very large (80 m2 for a 300 MW machine). This, in turn, requires a steam distribution system capable of collecting and draining the separated water falling from the mesh without causing re-entrainment, or impairing the uniformity of the steam flow. Different manufacturers achieve this using different ingenious methods. Figure 2.102 shows a cross-section through an Asea Brown Boveri arrangement. In common with all mesh separator designs, the wire-mesh mats are conveniently incorporated in the same vessel as the reheater tube banks. Steam is channelled along the lower side of the vessel (the left-hand side in the figure) and directed diagonally through three mesh trays in parallel. Water is collected on the sloping base of each tray and drained to the base of the vessel through funnels and pipework.
A scheme devised by GEC is shown in Fig 2.103. Steam enters from the base of the vessel and is distributed along the length of the vessel by a series of perforated plates which also act as water collectors. Each hole in the distribution plates is provided with a raised lip on the upper surface of the plate to prevent the separated water draining into the steam flow and becoming re-entrained. The holes are of such a size and pitch that they ensure diffusion and downstream mixing of the steam. Water is collected in drainage troughs set into the perforated distribution plates and is led to the bottom of the vessel. From there, the separated water is piped to a heater in the feed train.