The downcomers which we will be dicussing on this page are those that are generally external to the heat transfer sections. The circulation for the HRSG is discussed in previous pages.
The downcomers which we are now concerned with would be those found on I-Frame evaporators, horizontal tube evaporators, O-Frame, A-Frame and D-Frame evaporators with external downcomers. This type downcomer is normally constructed of standard pipe and is connected to upper drum with the other end connected to the lower drum or header.
Minimum downcomer flow area should be based on the following:
| Amin | = | Downcomer area, in |
| Wg | = | Gross water flow, lb/hr |
| Voll | = | Liquid specific volume, ft3 |
| A | = | Circulation ratio of evaporator coil |
The target velocity in an individual downcomer pipe should be less than 6 ft/sec. The minimum number of downcomer pipes per header is one. If the header is more than six feet and less than twelve feet in length, the preferred number of downcomers is two. For longer lengths, a similar number should be provided to prevent distribution problems in the header or manifold.
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