Tube Heat Exchangers Managing Pressure Drop in the Design of Shell and Tube Heat Exchangers
Shell and tube heat exchangers find their applications in a variety of sectors. However, one of the major constraints that stands in the way of optimizing its thermal design is the pressure drop. Here’s an overview of why the managing the pressure drop is an essential aspect of its design and what are the considerations that need to be met.
Pressure drop is a major constraint in thermal design of shell and tube heat exchangers. A thermal design of a shell and tube heat exchanger is meaningful only when it is optimum and the extent of the optimality is constrained by the pressure drop. Optimization of thermal design requires maximization of overall heat transfer coefficient and / or effective mean temperature difference (EMTD) so as to minimize the heat transfer area subject to the constraints, pressure drop being the major one. Other constraints may be flow induced vibration, space limitation, etc.
Overall heat transfer coefficient can be maximized by maximizing shell side and tube side flow velocities, which, in turn, is governed by the allowable pressure drop as higher velocity means higher pressure drop.
Maximization of EMTD is achieved by a pure counter current flow. If there are two passes in tube side or shell side, a pure counter current flow requires two passes in other side, but allowable pressure drop in that side may not allow the same.
Why Measuring the Pressure Drop is Crucial for Heat Exchangers
Therefore, it is significant to manage the pressure drop during thermal design of the exchanger. If during the course of thermal design, calculated pressure drop is far more than the allowable pressure drop, the pressure drop becomes a limiting factor. However, there may be situations when the calculated pressure drop is far less than allowable pressure drop, i.e., the pressure drop becomes surplus.
The pressure drop should be managed in such a way that the calculated pressure drop is within and as close as possible to the allowable pressure drop. In other words, if the pressure drop is a limiting factor during thermal design, the calculated pressure drop should be reduced such that it is closest possible to the allowable pressure drop without exceeding the same. On the other hand, if pressure drop is surplus during thermal design, the calculated pressure drop should be increased as close as possible to the allowable pressure drop.
Thermal design of a shell and tube heat exchanger generally starts with the following configuration:
- Tubular Exchanger Manufacturers Association (TEMA) E shell,
- Segmental baffle with 25 per cent cut,
- ¾ inch tube outer diameter,
- One tube pass, and
- Triangular tube pitch with a pitch ratio of 1.25 unless shell side mechanical cleaning requirement necessitates square pitch with a minimum cleaning lane of 0.25 inch as recommended by TEMA, which requires a tube pitch ratio of 1.33 for ¾ inch tube outer diameter.
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