8.8 External drain coolers
Two types of heat exchanger can be used as external drain coolers to extract the maximum amount of heat from the combined drains of the LP heaters.
Figure 3.62 shows diagrammatically the two types of drain cooler. The flashing type is similar to an LP heater, as the steam fraction of the flashed drains is condensed on the U-tubes of a heat exchanger. As seen from Fig 3.62 (c), the pressure in the shell is equal to the saturation pressure equivalent to the drains temperature. It is the temperature difference between the incoming feedwater and the drains outlet that determines the size of the heater. At the design point, the heat transfer surface is sized to maintain the drain cooler outlet temperature at the specified value.
The general construction of a flashing drain cooler is the same as an LP heater. The shell is extended to provide a flash chamber where the drains can flash into steam and water at the pressure determined by the thermal performance of the heaters. The steam will be about 30 to 40% of the incoming quantity and, because of the low pressure, volume will be large.
U-tubes are roller-expanded into a mild steel tube-plate and the shell is of all-welded mild steel construction. Internal baffles are provided to prevent the flashing steam and water impinging on the shell or tubes and causing erosion damage. The path taken by the steam and water is indicated in Fig 3.62 (a).
The alternative type of drain cooler uses the heat in the drain water to heat the incoming condensate by making the drains flow across the tube bundle through which the feed is flowing (Fig 3.62 (d)). In principle this is identical to the drain coolers already described for HP heaters. The heat balance to determine the rise over the drain cooler is shown in Fig 3.62 (0 and the resultant temperature gradient is illustrated by inset (e) of the figure. Again, U-tubes are roller-expanded into a mild steel tubeplate. The shell is of all-welded steel construction.
The baffle arrangement, however, is different as the baffles are required to guide the water across the tubes and to prevent leakage between the shell and baffle. The baffles are semicircular and a close fit in the shell. The path needed to be taken by the drain water is shown in Fig 3.62 (d).