8.6   Bled-steam inlets and drain outlets

 

As with HP heaters, high local velocities adjacent to the bled-steam inlets must be avoided to prevent damage to tubes.

Impingement plates are provided under the bled-steam inlet branches on the heater illustrated in Fig 3.60. The manufacturer of the other heater, however, has chosen to provide steam inlet belts with ports placed around the circumference of the shell to provide a large flow area and consequent low inlet velocities. A drain is provided on the bottom of the steam inlet belt to prevent water accumulation.

As condensate is formed in the heater, it is drained from the bottom of the heater shell.

The drainage of horizontal heaters depends upon the water flowing from each end of the heater to a drainage point on the bottom of the shell.

A natural water level is established which is dependent upon outlet diameter, rate of flow of condensate out of the vessel and the radius of the horizontal vessel. The relationship is complex and reference should be made to Gardner and Crow [20], Kubie, Rowe and Jones [2] and Cranfield [21] for a full explanation of the influence that various parameters have on the depth of water in the heater. The determination of the natural water depth is important. The water level must be below the bottom tubes or the heat transfer surface will be lost. The level at which the high water level switches should be set is also dependent on the maximum working level under the permitted operating conditions.

 

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