8.9 Direct contact low pressure heaters
In both thermal and mechanical design, direct contact (DC) LP heaters are very similar to a de-aerator heater head. They are cylindrical vessels arranged in either the horizontal or vertical attitude, equipped internally with a means to maximise the heat transfer surface of the incoming condensate for presentation to the heating steam.
The maximum heat transfer coefficient is obtained by using sprays or trays to form film/ droplet flow.
The shells and dished ends are constructed of mild steel. As the last station to be constructed with DC heaters was ordered before 1976, all the DC heaters were constructed in accordance with the now superseded BS1500. The size of the heater shell is determined by the need to accommodate the sprays or trays to pass the feed flow, and so varies from heater to heater. Figure 3.63 shows a typical horizontal attitude DC heater of NEIP manufacture, employing a system of jets which impinge on plates to produce a fan-shaped film. The feedwater from the previous heater is introduced into the channels which run along the axis of the vessel. These distribution channels are bridged by pipes across the inside of the shell. Holes of the appropriate size and position direct jets of water onto the 'spray plates'. Sufficient depth below the spray plates is provided for the steam to heat the water to the saturation temperature corresponding to the heater body pressure.
A small baffle which runs the length of the shell at each side forms a ledge from under which the air is extracted. The heater is continuously vented to the condenser. The steam inlet has a baffle which directs the steam along the axis of the shell.
The drain outlet is at the centre of the heater and is fitted with an antivortex device. As discussed earlier in this section for horizontal LP heaters, a natural drainage level in the heater exists dependent on the flow rate into the heater, the heater shell diameter, etc.
The DC heater illustrated in Fig 3.64 was manufactured by GEC. The design philosophy is similar to the de-aerator head shown in Fig 3.57, but with minor differences to adapt the principles for DC heater service.
A water distribution header directs the incoming condensate through perforated windows in its underside onto perforated trays below. The resultant rainfall is heated by the condensation of the steam to the saturation temperature equivalent to the shell pressure.
The illustration shows the provision made to allow the steam free access to the water cascading over the trays. Steam flow is directed to the lower half of the heater to ensure that water cascading from the bottom trays is heated to the full saturation temperature. Air is vented to the condenser from each nest of trays from just below the cold water entry point.
Double drain outlets are provided to ensure adequate drainage, taking into account the principles discussed earlier in this section for horizontal LP heaters.
The trays and perforated distribution inlet window are made of stainless steel to prevent erosion/corrosion.