6.3.2   Area required for flow through the tube bundle

 

As all HP heaters employ U-tubes, the number of holes to give the flow area needed is equal to the number of Us. Taking a typical 660 MW feed flow and two-bank heater configuration:

  • Flow/heater = 273.5 kg/s
  • Heater feed inlet temperature = 191°C
  • Internal tube diameter = 15 mm
  • Average feed temperature = 222°C

From ERA 1967 Steam Table, - 1.17·10—3 m3/kg specific volume

The missing parameter to find the flow area is the velocity of the feedwater in the tubes.

For the purpose of this specimen calculation, a water velocity of 2 m/s is taken, but the actual value varies with individual manufacturer's practice; however, the 'Standards for Closed Feedwater Heaters (HEI) 1979' [17] recommend not more than 2.4 m/s (8 ft/s) at maximum flow conditions.

If, for instance, one bank of HP heaters is bypassed in a two-bank system and the feed split is 60/40, with 60% through the active bank, the feed-water velocity will increase to 1.2 times normal velocity, which would give (1.2 x 2) = 2.4 m/s (i.e., the HEI maximum recommended feedwater velocity).

The heater needs 934 U-tubes and the tubeplate has 1868 holes. If a tube pitch of 25.4 mm is taken with a 60° triangular pattern, then allowing for a typical minimum U-tube bend and gaps for internal fittings, etc., the tube bundle can be assumed to have an overall diameter of 1300 mm.

 

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