Flow Fraction – Part II

In the previous article on flow fraction, there are issues were discussed on the importance of the consideration while designing the heat exchanger. Although there are about five main streams when analyzing the flow fraction but the mainly emphasized are flow B and flow E. As highlighted in previous article, B must be greater than 40% and E must be less than 20% and the remaining can be mixture of the portion. So, some time or most of the time, heat exchanger will be over designed or over sized to cater the desired temperature but very rare consideration is given on the flow fraction analysis. In this article, flow fraction analysis will be covered on how it can be optimized.

This flow fraction takes place in shell side and directly affected by the shell side configuration and its fluid. So, configuration on the shell side can be modified in order to enhance the flow fraction.

Direct configuration that affecting the flow fraction are both baffle spacing and baffle cut. So, baffle spacing and cut shall be optimized as recommended by TEMA (Tubular Exchanger Manufacturer Association). For example, minimum baffle spacing recommend by TEMA is about 1/5 of shell inner diameter and 20-40% of baffle cut.

Nowadays all these can be done via sophisticated software such HTRI, HTFS, etc. So, as part of heat exchanger analysis, flow fraction analysis can be done since it could be presented by the software. However, as ultimate of heat exchanger warranty again dependent on vendor design and sometime the vendor would declare that they will guarantee the performance even sometime won’t behave as claimed by them. Usually the owner will let the vendor finalize it but as an engineer that designing and reviewing for engineering firm and owner respectively can use this approach to challenge the design done by the vendor. It’s again “Vendor Package” that cannot be argued so much.

Flow Fraction

Flow fraction usually deal with the vapor fraction and liquid fraction. However, here is something different will be discussed which is related to shell and tube heat exchanger. There are bunch of criteria that need to be considered but flow fraction on the shell side flow fraction will be highlighted since this criteria is not really taken into consideration when designing a new shell and tube heat exchanger, (STHX).

As illustrated above, there are about five main flow in the shell side where only one (B) out of five is cross flow and the remaining (A, C, E, and F) are leak-flow or bypass. There are proposed calling for these streams; main cross-flow stream (B), a tube-to-baffle-hole leakage stream (A), a bundle bypass stream, a pass-partition bypass stream (F), and a baffle-to-shell leakage stream (E).

As vapor and liquid fraction, there is a requirement to meet the specification of any particular product before sell it off. So, same goes to the shell side flow fraction of the STHX where the minimum design criteria is that B fraction must be greater than 40% and for E is less than 20%. So, it’s not necessary yet to increase the size of the heat exchanger if this specification not met. There are couple of optimization can be done to get the desired temperature for any particular stream.

The tricky part of the optimization of STHX will be elaborated in detail in next article. Those will explain the areas that can be used to challenge design engineer during design review. In the meantime, design engineer can use that info also to enhance the design