Microsoft PowerPoint - How Can We Select Correct Heat Exchanger

Microsoft PowerPoint - How Can We Select Correct Heat Exchanger

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Unformatted text preview: How Can We Select Correct Heat How Exchanger Course By : Prof. Mohajer prepared by: S.M. rafigh 2011 Components of Course: What Stage are We Up To? Types of exchangers, revision of OHTCs, fouling factors. Heat exchanger selection. Thermal performance analysis (NTUs) for co& counter-current exchangers. Multi-pass exchangers (S&T). Condensation & boiling. Radiation. • • • • • • Outline Motivation Basic philosophy General procedure General considerations - what you need to think about Typical questions and answers Examples • • • • • • Motivation: Specification & Design Most common design situation is where • you need to specify type of new exchanger (always first step), estimate cost and assess suitability. suitability. Links to other courses: process economics, • materials, design. Basic Philosophy Rule out inapplicable exchangers (pressure, • temperature, fluid compatibility, size range) Rank on economic basis • General Procedure Must calculate heat duty • Minimise cost subject to constraints • fluid inlet and outlet temperatures – allowable pressure drops – compatibility of materials (corrosion) and fluids – (direct/indirect contact) maintenance (repairs) – availability (can we get it easily?) – sensitivity to other conditions – General Considerations Design pressures Design temperatures Heat duty / size range Fluid type / compatibility Boiling/condensation (“quality”) Temperature driving forces Allowable pressure drops Fouling tendency Space limitations Typical Questions to be Answered Although their construction is quite • complicated, large shell-and-tube heat exchangers are often used in industry (eg oil refineries) refineries) – why ? For a “small” heat duty (100 kW) and two non- • corrosive liquids, what types of exchangers should be considered, and what are some of the pro’s and con’s of each? Parts of the Answers Shell-and-tube exchangers are common, • because construction rules and operating practice are well established. Double-pipe (concentric pipe) exchangers are • cheap & simple in small sizes (<20 kW), can be constructed by non-specialised labour. It would be worthwhile to inquire about small plate exchangers. General Aspects Specialised labour: plate, shell-and-tube • Non-specialised: double-pipe • Plate & Spiral Designs Plate not for > 250oC or > 20 bar (sealing, • gasket materials) Spiral • like plate, rolled up – compact, high htcs like plate – not so flexible: cannot add area easily – have better sealing than plate – Not so common for two-phase flow • narrower gaps in plate, spiral (5 mm typical) than – the tube diameters (>19 mm typical) in shell & tube tube shell & tube get blocked less easily – gasket materials frequently difficult to find for – plate Conclusions General heat exchanger selection situation • involves minimising cost subject to a long list of possible constraints In general, robustness is a very important • factor - shell-and-tube exchangers may not be the most efficient, but they score highly in this category ...
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