Heat Exchanger Selection

1. Heat Exchanger Selection Choosing the best exchanger for a given process application

2. Lecture series Introduction to heat exchangers Selection of the best type for a given application Selection of right shell and tube Design of shell and tube

3. The steps “Coarse filter” Rejecting those exchangers which will not be suitable on the grounds of operating pressure and temperature, fluid-material compatibility, handling extreme thermal conditions “Fine filter” Estimating the cost of those which may be suitable

4. “Coarse filter” Use information next few slides to reject those exchangers which are clearly out of range or are otherwise unsuitable The information is summarised in the table At this stage, if in doubt, include the exchanger (poor choices are likely to turn out expensive at the “fine filter” stage) The table in the accompanying Lecturer Pack should be copied for students use in the examples.The table in the accompanying Lecturer Pack should be copied for students use in the examples.

5. General points Tubes and cylinders can withstand higher pressures than plates If exchangers can be built with a variety of materials, then it is more likely that you can find a metal which will cope with extreme temperatures or corrosive fluids More specialist exchangers have less suppliers, longer delivery times and must be repaired by experts The last point means that specialist exchangers are not favoured in less developed parts of the worldThe last point means that specialist exchangers are not favoured in less developed parts of the world

6. Thermal effectiveness Stream temperature rise divided by the theoretically maximum possible temperature rise

7. Double pipe Normal size 0.25 to 200m2 (2.5 to 2000 ft2) per unit Note multiple units are often used Built of carbon steel where possible

8. Advantages/disadvantages of double-pipe Advantages Easy to obtain counter-current flow Can handle high pressure Modular construction Easy to maintain and repair Many suppliers Disadvantage Become expensive for large duties (above 1MW)

9.  Maximum pressure 300 bar(abs) (4500 psia) on shell side 1400 bar(abs) (21000 psia) on tubeside Temperature range -100 to 600oC (-150 to 1100oF) possibly wider with special materials Fluid limitations Few since can be built of many metals Maximum e = 0.9 Minimum DT = 5 K

10. Shell and tube Size per unit 100 - 10000 ft2 (10 - 1000 m2) Easy to build multiple units Made of carbon steel where possible

11. Advantages/disadvantages of S&T Advantages Extremely flexible and robust design Easy to maintain and repair Can be designed to be dismantled for cleaning Very many suppliers world-wide Disadvantages Require large plot (footprint) area - often need extra space to remove the bundle Plate may be cheaper for pressure below 16 bar (240 psia) and temps. below 200oC (400oF)

12. Scope of shell and tubeEssentially the same as a double pipe Maximum pressure 300 bar(abs) (4500 psia) on shell side 1400 bar(abs) (21000 psia) on tubeside Temperature range -100 to 600oC (-150 to 1100oF) possibly wider with special materials Fluid limitations Few since can be built of many metals Maximum e = 0.9 (less with multipass) Minimum DT = 5 K

13. Plate and frame Plates pressed from stainless steel or higher grade material titanium incoloy hastalloy Gaskets are the weak point. Made of nitrile rubber hypalon viton neoprene

14. Advantages of plate and frame High heat transfer - turbulence on both sides High thermal effectiveness - 0.9 - 0.95 possible Low ?T - down to 1K Compact - compared with a S&T Cost - low because plates are thin Accessibility - can easily be opened up for inspection and cleaning Flexibility - Extra plates can be added Short retention time with low liquid inventory hence good for heat sensitive or expensive liquids Less fouling - low r values often possible

15. Disadvantages of plate & frame Pressure - maximum value limited by the sealing of the gaskets and the construction of the frame. Temperature - limited by the gasket material. Capacity - limited by the size of the ports Block easily when solids in suspension unless special wide gap plates are used Corrosion - Plates good but the gaskets may not be suitable for organic solvents Leakage - Gaskets always increase the risk Fire resistance - Cannot withstand prolonged fire (usually not considered for refinery duties)

16. Scope of plate-frame Maximum pressure 25 bar (abs) normal (375 psia) 40 bar (abs) with special designs (600 psia) Temperature range -25 to +1750C normal (-13 to +3500F) -40 t0 +2000C special (-40 to +3900F) Fluid limitations Mainly limited by gasket Maximum e = 0.95 Minimum DT = 1 K

17. Welded plates Wide variety of proprietary types each with one or two manufactures Overcomes the gasket problem but then cannot be opened up Pairs of plates can be welded and stacked in conventional frame Conventional plate and frame types with all-welded (using lasers) construction have been developed Many other proprietary types have been developed Tend to be used in niche markets as replacement to shell-and-tube

18. Air-cooled exchangers Inset figure is of an induced drought ACHE whereas a forced draught type was shown in the last lectureInset figure is of an induced drought ACHE whereas a forced draught type was shown in the last lecture

19. Advantages of ACHEs Air is always available Maintenance costs normally less than for water cooled systems In the event of power failure they can still transfer some heat due to natural convection The mechanical design is normally simpler due to the pressure on the air side always being closer to atmospheric. The fouling of the air side of can normally be ignored

20. Disadvantages of ACHEs Noise - low noise fans are reducing this problem but at the cost of fan efficiency and hence higher energy costs May need special features for cold weather protection Cannot cool to the same low temperature as cooling tower The evaporative cooling in a cooling tower produces cooler waterThe evaporative cooling in a cooling tower produces cooler water

21. Scope of Air Cooled Exchangers Maximum pressure - tube(process) side: 500 bar (7500psia) Maximum temperature: 600oC (1100o F) Fluids: subject to tube materials Size per unit: 5 - 350m2 (50 - 3500ft2 ) per bundle (based on bare tube)

22. Plate Fin Exchangers Formed by vacuum brazing aluminium plates separated by sheets of finning Noted for small size and weight. Typically, 500 m2/m3 of volume but can be 1800 m2/m3 Main use in cryogenic applications (air liquifaction) Also in stainless steel As a rough guide, a plate fin would be a fifth the size of a shell and tube for the same duty. Of course, a shell and tube exchanger is often not suitable for many plate-fin applications involving many streams and small temperature differences.As a rough guide, a plate fin would be a fifth the size of a shell and tube for the same duty. Of course, a shell and tube exchanger is often not suitable for many plate-fin applications involving many streams and small temperature differences.

23. Scope of plate-fin exchanger Max. Pressure 90 bar (size dependent) Temperatures -200 to 150oC in Al Up to 600 with stainless Fluids Limited by material Duties Single and two phase Flow configuration Cross flow, Counter flow Multistream Up to 12 streams (7 normal) Low DT Down to 0.1oC Maximum DT 50oC typical High e Up to 0.98 Important to use only with clean fluids

24. Printed Circuit Exchanger Very compact Very strong construction from diffusion welding Small channels (typically 1 - 2 mm mean hydraulic diameter) Can be made in stainless steel, nickel (and alloys), copper (and alloys) and titanium

25. Scope of PCHE Maximum Pressure 1000bar (difference 200bar) Temperature -200 to +800oC for stainless steel but depends on metal Fluids Wide range but must be low fouling Normal Size 1 to 1000m2 Flow configuration Crossflow or counterflow Effectiveness ? up to 0.98 Low ?T Yes Thermal cycling Has caused problems

26. Example Which exchanger types can be used for condensing organic vapour at -60oC and 60 bar by boiling organic at -100oC and 70 bar? Would you modify your choice if the boiling stream were subject to fouling requiring mechanical cleaning? The exchangers which can handle the pressure and temperature are Double pipe Shell-and-tube (with special material) Plate-fin Some welded plate designs could be investigated Fouling would rule out plate-fin and some welded plate designs.The exchangers which can handle the pressure and temperature are Double pipe Shell-and-tube (with special material) Plate-fin Some welded plate designs could be investigated Fouling would rule out plate-fin and some welded plate designs.

27. Heat exchanger costing - “fine filter” Full cost made up of Capital cost Installation cost Operating cost The cost estimation method given here is based only on capital cost - which is the way it is often done Note: installation costs can be as high as capital cost except for compact exchangers Installation cost considerations can predominate on offshore plant

28. Scoping The cost estimate method given here is for the preliminary plant design stage - scoping Note that we are trying to estimate the cost of an exchanger before we have designed it Full design and cost would be done later

29. Quick sizing of heat exchangers We estimate the area from

30. FT correction factor This correction accounts for the two streams not following pure counter-current flow At the estimation stage, we do not know the detailed flow/pass arrangement so we use FT = 1.0 for counter flow which includes most compact ant double-pipe FT = 0.7 for pure cross flow which includes air-cooled and other types when operated in pure cross flow (e.g. shell-and-tube) FT = 0.9 for multi-pass FT = 1.0 if one stream is isothermal (typically boiling and condensation) Using an FT of 0.9 for multipass exchangers assumes that the designer is going to avoid having a value less than 0.8. It cannot be higher than 1.0 so 0.9 seems a reasonable average within the accuracy of these estimates.Using an FT of 0.9 for multipass exchangers assumes that the designer is going to avoid having a value less than 0.8. It cannot be higher than 1.0 so 0.9 seems a reasonable average within the accuracy of these estimates.

31. Estimating U This may be estimated for a given exchanger type using the tables from ESDU (given below) These tables give U values as a function of Q/?T (the significance of this group will become clear later) Example: high pressure gas cooled by treated cooling water in a shell-and-tube, where Q/?T = 30 000 W/K gives U = 600 W/m2K This includes typical fouling resistances The tables are included in the Lecturer Pack with the required table entry circled. It is worth also noting the the C value of 0.4 at this stage - the significance will become clear later.The tables are included in the Lecturer Pack with the required table entry circled. It is worth also noting the the C value of 0.4 at this stage - the significance will become clear later.

32. Estimating cost This has often been done by multiplying the calculated area, A, by a “cost per unit area” But, when comparing exchangers, U and A vary widely from type to type. It is also difficult to define A if there is a complicated extended surface. Hence, ESDU give tables of C values where C is the “cost per UA” - using 1992 prices Note, from our basic heat transfer equation UA = Q / DT The costs were obtained from manufacturers who looked a the typical costs of exchangers built for the different applicationsThe costs were obtained from manufacturers who looked a the typical costs of exchangers built for the different applications

33. ESDU ESDU gives tables for a range of heat exchanger types but we can only include here those for shell-and-tube and plate-and-frame Full data Item 92013 is available from ESDU International plc 27 Corsham Street London N1 6UA Tel 0171 490 5151 Fax 0171 490 2701 esdu@esdu.com

34. Steps in calculation Calculate ?Tln and hence estimate ?T Determine Q/?T Look up C value from table To determine C at intermediate Q/?T, use logarithmic interpolation - see next slide Calculate exchanger cost from - Cost = C(Q/?T) Taking the last shell-and-tube example, C = 0.4. Hence, Cost = £ 0.4 X 30 000 = £12 000 Make sure that you take account of footnotes in tables

35. Logarithmic interpolation In the example given previously, the Q / DT value happens to be in the table. Usually, however, you must interpolate between entries in the table. This is done effectively by plotting on log-log paper and doing a linear interpolation. The slide gives the formula for this.In the example given previously, the Q / DT value happens to be in the table. Usually, however, you must interpolate between entries in the table. This is done effectively by plotting on log-log paper and doing a linear interpolation. The slide gives the formula for this.

36. HEAd Heat Exchanger Advisor Helps guide you through the selection process Does the coarse and fine filter steps in one and provides extensive help text Although, HEAd is based on the ESDU item, some changes have been made in consultation with HTFS Members.Although, HEAd is based on the ESDU item, some changes have been made in consultation with HTFS Members.