Heat exchange area of U-type heat exchanger
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I would like to ask everyone: in the new version of GB151-2014, the statement that \"the area of the U-shaped bends is generally not included\" has been removed regarding the heat exchange area of U-type heat exchangers. Is the area of the bends considered part of the heat exchange area in the new version?
It’s meant as a design margin, right? Is it the industry standard?
You can follow the original approach; in this standard definition, there seems to be a statement stating that if it can be determined that this part is involved in heat exchange or something similar, then it can be included. It seems to be related to the extent of heat exchange involved; this needs to be determined during process design, and it’s better not to consider it just to be safe
This cannot be considered the default setting, as older versions required that this part not be included. The new version no longer requires it, in Definition 151. P15 said: Whether to include it should be determined based on the flow conditions of the fluid in the shell side of the elbow section.
Under normal circumstances, if there is phase change (condensation or evaporation) on the shell side, and no baffles are needed to force flow and enhance heat transfer, this part can be included. If there is longitudinal heat transfer on the shell side with no phase change, then the U-value can be included, but a coefficient may be applied to it. If the shell side is in the form of baffle plates, and flow must be forced using those baffles to enhance heat exchange, then the U-end cannot be taken into account.
The heat exchange area can be found in the definition given in GB/T151-2014, on page P16; in simple terms, it refers to the area that is involved in heat transfer. As for whether the bent sections of the U-tubes should be included or not, this depends on the structural design of the equipment and must be determined by the designer.
It’s been removed; it seems that the heat exchange area now includes this part as well
Isn’t it better not to make this something mandatory? It’s up to you to decide whether it should be counted as heat exchange area; if the last element is a full support plate, then the area of the elbow cannot be included, but if it’s a baffle or part of a kettle-type evaporator, then it can be counted. It can also be designed as a margin; it’s hard to let you decide for yourself anymore.
Now, heat exchanger manufacturers take the area of the elbow bends into account, which results in the original design conditions no longer being met
There are several scenarios in this regard. One scenario is when you purchase a heat exchanger; in that case, the manufacturer is responsible for its heat exchange efficiency and heat exchange area. As long as the manufacturer believes that the bends play a role in heat exchange and takes overall responsibility for the heat exchanger, then there’s no problem with that. Another option is for you to specify the process conditions; in this case, your specifications serve as inputs. You should be clear as to whether the bent pipes are included in the heat exchange area, and you need to review the manufacturer’s drawings. If you notice such a situation, you can simply request that it be corrected. For the last option, where you create the drawings based on the given conditions, it’s still up to you to determine the heat exchange area.