Lect 2

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

© Hyprotech 2002

Lecturer’s Guide Copyright Hyprotech UK Ltd holds the copyright to these lectures. Lecturers have permission to use the slides and other documents in their lectures and in handouts to students provided that they give full acknowledgement to Hyprotech. The information must not be incorporated into any publication without the written permission of Hyprotech.

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 © Hyprotech 2002

Lecturer’s Guide

Q = U A ∆T

The steps • “Coarse filter” – Rejecting those exchangers which will not be suitable on the grounds of operating pressure and temperature, fluidmaterial compatibility, handling extreme thermal conditions

• “Fine filter” – Estimating the cost of those which may be suitable © Hyprotech 2002

Lecturer’s Guide

“Coarse filter” • Use information on 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)

© Hyprotech 2002

Lecturer’s Guide The table in the accompanying Lecturer Pack should be copied for students use in the examples.

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 © Hyprotech 2002

Lecturer’s Guide The last point means that specialist exchangers are not favoured in less developed parts of the world

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

ε=

T1,in − T1,out T1,in − T2 ,in

T1,in

T1,out

T2,in

T2,out © Hyprotech 2002

Lecturer’s Guide The effectiveness can be calculated for each stream. The higher of the two is the one that is important. Typically, exchangers are designed with an effectiveness of 60 - 80 per cent. All exchanger types can handle this. However, more specialist exchangers are required for an effectiveness above about 90 per cent, as will be seen.

Double Pipe Simplest type has one tube inside another - inner tube may have longitudinal fins on the outside

However, most have a number of tubes in the outer tube - can have very many tubes thus becoming a shell-and-tube © Hyprotech 2002

Lecturer’s Guide

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 © Hyprotech 2002

Lecturer’s Guide

Advantages/disadvantages of doublepipe • 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) © Hyprotech 2002

Lecturer’s Guide

Scope of 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 ε = 0.9 • Minimum ∆T = 5 K © Hyprotech 2002

Lecturer’s Guide It should be noted that the ranges and limits quoted above are a guide as to what is normal today. This limits are being extended. Also, with care in design and with specialist manufacture, it is possible to extend the limits, although this may be at additional cost.

Shell and tube

• Size per unit 100 - 10000 ft2 (10 - 1000 m2) • Easy to build multiple units • Made of carbon steel where possible © Hyprotech 2002

Lecturer’s Guide

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) © Hyprotech 2002

Lecturer’s Guide

Scope of shell and tube Essentially 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 ε = 0.9 (less with multipass) • Minimum ∆T = 5 K © Hyprotech 2002

Lecturer’s Guide

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

© Hyprotech 2002

Lecturer’s Guide

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 © Hyprotech 2002

Lecturer’s Guide

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) © Hyprotech 2002

Lecturer’s Guide

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 ε = 0.95 • Minimum ∆T = 1 K © Hyprotech 2002

Lecturer’s Guide

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 allwelded (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 © Hyprotech 2002

Lecturer’s Guide

Air-cooled exchangers

© Hyprotech 2002

Lecturer’s Guide Inset figure is of an induced draught ACHE whereas a forced draught type was shown in the last lecture. Induced draught tends to give better air-flow distribution. However, the fan is working in hotter air and is less efficient. Furthermore, access and maintenance are more difficult with induced draught.

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

© Hyprotech 2002

Lecturer’s Guide

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

© Hyprotech 2002

Lecturer’s Guide The evaporative cooling in a cooling tower produces cooler water

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)

© Hyprotech 2002

Lecturer’s Guide

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 © Hyprotech 2002

Lecturer’s Guide 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.

Scope of plate-fin exchanger

• Max. Pressure • Temperatures • • • • • • •

90 bar (size dependent) -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 ∆T Down to 0.1oC Maximum ∆T 50oC typical High ε Up to 0.98 Important to use only with clean fluids

© Hyprotech 2002

Lecturer’s Guide The standards of ALPEMA (Brazed Aluminium Plate-fin Exchanger Manufacturers Association) may be downloaded free of charge from the ALPEMA web site www.alpema.org

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

© Hyprotech 2002

Lecturer’s Guide

Scope of PCHE • Maximum Pressure • Temperature • Fluids • • • • •

Normal Size Flow configuration Effectiveness Low ∆T Thermal cycling

1000bar (difference 200bar) -200 to +800oC for stainless steel but depends on metal Wide range but must be low fouling 1 to 1000m2 Crossflow or counterflow ε up to 0.98 Yes Has caused problems

© Hyprotech 2002

Lecturer’s Guide

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?

© Hyprotech 2002

Lecturer’s Guide 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.

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 © Hyprotech 2002

Lecturer’s Guide

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

© Hyprotech 2002

Lecturer’s Guide

Quick sizing of heat exchangers ∆Tb

∆Ta

We estimate the area from Where

A =

Q U∆T

∆T = FT ∆Tlm ∆Tlm =

∆Ta − ∆Tb ln(∆Ta / ∆Tb )

© Hyprotech 2002

Lecturer’s Guide

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 and 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-andtube) – FT = 0.9 for multi-pass – FT = 1.0 if one stream is isothermal (typically boiling and condensation) © Hyprotech 2002

Lecturer’s Guide 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.

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 © Hyprotech 2002

Lecturer’s Guide 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.

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 / ∆T © Hyprotech 2002

Lecturer’s Guide The costs were obtained from manufacturers who looked a the typical costs of exchangers built for the different applications

ESDU • ESDU gives tables for a range of heat exchanger types but we can only include here those for shell-and-tube and plate-andframe • Full data Item 92013 is available from ESDU International plc 27 Corsham Street London N1 6UA Tel 0171 490 5151 Fax 0171 490 2701 [email protected] © Hyprotech 2002

Lecturer’s Guide

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 © Hyprotech 2002

Lecturer’s Guide

Logarithmic interpolation ln(C1) ln(C) ln(C2) ln(V1)

ln(V)

ln(V2)

Where the Vs are the values of Q/∆ T. V1 and V2 are the values either side of the required value V

 ln(C1 / C2 )ln(V / V1 ) C = expln(C1 ) +  ln(V1 / V2 )   © Hyprotech 2002

Lecturer’s Guide In the example given previously, the Q / ∆T 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.

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 HEAd

© Hyprotech 2002

Lecturer’s Guide Although, HEAd is based on the ESDU item, changes have been made in consultation with HTFS Members.