Friday, March 6, 2009

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.

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