Extended surface material and selection
The heat transfer sections of the HRSG frequently use extended surface to improve the overall heat exchange between the hot gases and the steam or water in the tubes. These extended surfaces are usually either a thin plate fin wraped helically around the tube or round or eliptical shaped studs. Following is a description of the more popular extended surfaces.
Segmented Fins:These are usually one of the two types shown below.
| High Frequency Continuously Welded |
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| Standard Frequency Spot Welded |
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The standard frequency, spot welded, design is not used as often since this design is normally selected when using very thin, high density finning such as in a large heat recovery boiler. Most HRSG designs use fins less than 0.049 inch thick. The standard frequency, spot welded fin also has a foot which presents a place where corrosion can occur if flue gases are corrosive or moisture is present.
Solid Fins:These are the most popular fins for modern HRSG's.
| High Frequency Continuously Welded |
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These are used generally when the fuel is No. 6 or higher.
| Resistance Welded |
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Thermal rating procedures for all these extended surface types are presented in the following pages. Both segmented fin types are rated using the same formulas.
For our sample boiler, that we have been developing throughout this material, we will choose to use 0.049" thick fins with a density of 6 fins per inch. We will use an 11% chrome alloy material for the superheater, A-176 TP409. For the evaporator and the economizer, we will use carbon steel, A 366, which should be good for all the temperatures in these sections. We will recheck our selections after doing the thermal calculations to confirm they are okay. It is important to note, that since the source of our gas is a gas turbine, it is a very clean service and we could have used a thinner fin at a higher density.
Disclaimer:
The formulas and correlations presented herein are all in the public domain and are to be used only as a learning tool. Note that any product, process, or technology in this document may be the subject of other intellectual property rights reserved by sponsors or contributors to this site. This publication is provided as is, without any warranty of any kind, either expressed or implied, including, but not limited to, the implied warranties of fitness for a particular purpose, or non-infringement.
The formulas, correlations, and methods presented herein should not be considered as being recommended by or used by the sponsors of this site. The purpose of this site is educational and the methods may or may not be suitable for actual design of equipment. Only a fired heater design engineer is qualified to decide if a calculation or procedure is correct for an application.