In shell-and-tube heat exchangers there is an unavoidable clearance between the tube bundle and the shell wall. This clearance has a relatively low flow resistance and hence a relatively large proportion of the shell-side flow will pass through it, bypassing the tube bundle and thus avoiding the heat transfer surface. The clearance can be blocked by sealingstrips, which eliminate the gap and divert the bypassing flow back into the tube bundle. This thesis details an investigation of bypass lanes in shell-and-tube heat exchangers and the use of sealing-strips. A model exchanger was designed and built to represent a rectangular tube bank in which the bypass lane width could be varied, sealing-strips could be inserted at various positions along the bypass to block completely the bypass wall-to-tube bundle clearance. For four different tube arrays, the pressure drop and exit bypass mass flow fraction were found for isothermal air flow over a range of total flow rates. Three different tube bank geometries were investigated, i) with no bypass lane (ideal bundle), ii) for a range of bypass lane widths, iii) for a range of bypass lane widths blocked by various numbers of sealing-strips. For one tube array type, flow distributions upstream of the tube bank were found using a hot-wire anemometer; from these results the inlet bypass flow fractions were estimated. From these studies it was found that ESDU generally overestimates the pressure drop coefficients when bypassing is present. Bell's (1960) correction factor generally underestimates the pressure drop coefficient when bypassing is present and overestimates the effect of sealing-strips in increasing the pressure drop over the bank. The rectangular tube bank model was modified to represent a section of a cylindrical exchanger. The effect on the pressure drop over a bank with a non-uniform bypass lane width, in the flow direction, was investigated. It was found that the inverse root mean square of the bypass clearances best characterised the "effective" bypass clearance of the whole bank. Flow visualisation studies were undertaken of shell-side flow in a cylindrical exchanger made of glass in which all leakage flows, except bypassing, were eliminated. From the dye traces produced, the shell-side flow was seen to be highly complex. For the geometries examined it appeared that there was little interchange of flow between the bypass and crossflow stream over the crossflow section of the exchanger, but that the bypass stream became crossflow at each window region, with crossflow transferring into the bypass lane. Sealing-strips were seen to produce a thorough mixing of the bypass and crossflow streams
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:292456 |
| Date | January 1989 |
| Creators | Martin, David John |
| Publisher | University of Bristol |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1983/e41ec89a-57fd-41ed-b2df-b94ea2e6fe28 |
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