The risers which we will be dicussing on this page are those that are generally external to the heat transfer sections. Since when the riser and the heat transfer are the same, the selection and sizing are based on the heat transfer requirement, i.e., the steam rate, and the circulation design. The circulation for the HRSG is discussed in previous pages.
The risers which we are now concerned with would be those found on I-Frame evaporators, horizontal tube evaporators, etc. This type riser is normally constructed of standard pipe and is connected to a header at the evaporator outlet with the other end connected to the steam drum.
Minimum riser flow area should be based on the following:
| Amin | = | Riser area, in |
| Wg | = | Gross water/steam flow, lb/hr |
| Volv | = | Vapor specific volume, ft3 |
| Voll | = | Liquid specific volume, ft3 |
| A | = | Circulation ratio of evaporator coil |
The target velocity in an individual riser pipe should be less than 12 ft/sec. The minimum number of riser pipes per header should be two. If the header is more than six feet and less than twelve feet in length, the preferred number of risers is three. For longer lengths, a similar number should be provided to prevent distribution problems in the header or manifold.
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