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HT4.ppt Heat Exchanger Selection

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-

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HT4.ppt Heat Exchanger Selection

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    1. HT4.ppt Heat Exchanger Selection

    2. 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.

    3. Outline Motivation Basic philosophy General procedure General considerations - what you need to think about Typical questions and answers Examples

    4. 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. Links to other courses: process economics, materials, design.

    5. Basic Philosophy Rule out inapplicable exchangers (pressure, temperature, fluid compatibility, size range) Rank on economic basis

    6. 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

    7. 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

    8. 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) – 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?

    9. 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.

    10. General Aspects Specialised labour: plate, shell-and-tube Non-specialised: double-pipe

    11. 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

    12. 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 shell & tube get blocked less easily gasket materials frequently difficult to find for plate

    13. 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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