Ch 3 Types Of Heat Exchangers

Chapter 3 Types of Heat Exchangers Learning Objectives At the end of this chapter students will: •

Know the various ways in which heat exchangers are classified

•

Have a knowledge of the structure and characteristics of available heat exchanger types

•

Have an understanding of the relative merits of different heat exchanger types for particular applications.

3.1

Classification of Heat Exchangers

Heat exchangers are described in a number of ways, according to their geometry and their application. Fig. 3.1a-f illustrates the classifications that will be used in this module, however this is not exclusive. Other designations are used in particular industries and standards are relevant to some applications. One important distinction that is not included in Figure 3.1 is that between fired and unfired heat exchangers. In this module we shall focus mainly on unfired heat exchangers although much of the analysis is applicable to fired units, except in the region of the combustion chamber. The classifications included in Fig. 3.1 are: a)

Classification by Heat Transfer Process

b)

Classification by Surface Area Density

c)

Classification by Number of Fluid Streams

d)

Classification by Flow Arrangement

e)

Classification by Heat Transfer Mechanisms

f)

Classification by Application and Industry

g)

Classification by Construction

Classification by Heat Transfer Process Heat may be transferred between two fluids by direct contact, in which case the fluids are permitted to mix. Examples of this arrangement include open cooling towers, many driers and direct contact feed heaters. In this module we shall be dealing 3.1

principally with indirect contact heat exchangers in which the two fluid streams are separated by an impermeable wall through which heat is transferred. An alternative arrangement of indirect heat exchanger incorporates a solid storage element which is alternately heated and cooled by the hot and cold process streams, respectively.

Heat Transfer Process

Direct Contact type

Indirect Contact type

Direct Transfer

Storage

Fluidised Bed

Figure 3.1a Classification by Heat Transfer Process

Classification by Surface Area Density Heat exchangers with a high ratio of heat transfer area to volume (and, by implication, small flow passages) are known as compact heat exchangers (CHEs). An arbitrary, but generally accepted boundary between compact and non-compact classifications is of the order of 300m2/m3, although some authors suggest values of 200m2/m3 or 700m2/m3 are more appropriate figures. The spectrum of area densities found in heat exchangers is found in figure 3.2 An area density of 300m2/m3 corresponds to a flow passage hydraulic diameter of 10mm.

Surface Area Density

Compact > 300m2/m3

Non-Compact 2 <300m /m3

Figure 3.1b Classification by Surface Area Density

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Figure 3.2 Overview of compact heat transfer surfaces

Classification by Number of Fluid Streams The majority of heat exchangers have two fluid streams - the hot and the cold stream. Electric heaters and heat sinks involve heat transfer from a solid to one fluid. In the process industries it is common for heat transfer to take place between a number of fluid streams within a single unit.

Number of Fluid Streams

One

Two

n, n>2

Figure 3.1c Classification by Number of Fluid Streams

Classification by Flow Arrangement The three simplest flow arrangements are illustrated in Fig. 3.3. These are described as parallel or co-current flow (both fluids flowing in the same direction); counter or countercurrent flow (the fluids flowing in opposite directions); and cross-flow (the fluids flowing at right angles). Multipass arrangements involve a combination of parallel and counter flow in parts of the heat exchanger and may include an element of

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crossflow. The flow arrangement has a quantitative effect on the performance of a heat exchanger. This is discussed in more detail in Section 5. Flow A rrangement

M ultipass

Single Pass

Parallel Flow C o-current Flow

C ounter-flow C ountercurrent Flow

C ross-flow

Figure 3.1d Classification by Flow Arrangement

Figure 3.3 Flow configerations through heat exchangers

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Classification by Heat Transfer Mechanisms The three convective heat transfer mechanisms occurring within heat exchangers are single-phase convection, boiling and condensation. In some circumstances radiation plays a significant part in the heat transfer.

Heat Transfer Mechanisms

Single phase convection both fluids

Single phase convection to boiling fluid

Condensing fluid to boiling fluid

Electric Heating to fluid

Condensing fluid to single phase convection

Convective and radiative heat transfer

Figure 3.1e Classification by Heat Transfer Mechanisms

Classification by Application and Industry Many descriptive terms are applied to heat exchangers based upon their application or function. Heat exchangers performing similar duties may be referred to by different names depending upon the application and the custom of the relevant industry. These descriptions may be misleading - for example no evaporation occurs in the typical domestic hot water boiler.

Application/industry

Boiler, Evaporator

Condenser

Oil cooler, air-cooled heat exchanger, “radiator”, etc

Radiator

Reboiler Heat sink

Figure 3.f Classification by Application and Industry

Classification by Construction The classification of heat exchangers by construction type is in many ways the most important of the classifications described here. When a heat exchanger is specified

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for a particular duty the one of the first tasks of the designer is to identify suitable construction types. These are described in Section 3.2

Construction

Tubular

Extended Surface

Plate Spiral

Lamella Rotary

Gasketed Plate

Double Pipe

Brazed Plate

Shell-and-Tube

Proprietary

Regenerative

Fixed Matrix

Welded Plate

Spiral Tube

Plate-Fin

Tube-Fin

Embossed Plate PCHE SPFHE Heat Pipe etc. etc

Figure 3.1g Classification by Construction

3.2

Heat Exchanger Construction Types

3.2.1 Tubular Heat Exchangers Tubular heat exchanges comprise an outer tube - the shell - and one or more tubes the tubes- within the shell. One fluid flows within the shell and the other in the tubes. Heat exchangers with relatively few tubes (typically less than 12) inside the shell, and with no baffles to guide the shell side fluid are referred to as double-pipe heat exchangers. Those with considerably more tubes and baffles to guide the shell side fluid are known as shell-and-tube heat exchangers. The shell-and-tube heat exchanger is the mainstay of the process industry and is widely used elsewhere- for example in power plant condensers and refrigeration plant. Although new types of heat exchanger are making some inroads into the traditional markets of the shelland-tube type, it is still the standard against which others compete. The Shell-and-Tube Heat Exchanger As stated above, the shell-and-tube heat exchanger comprises a number of tubes housed within a shell with their axes parallel to that of the shell. This arrangement is extremely flexible and can be arranged to give a wide range of flow configurations. A simplified diagram of a shell-and tube unit with one shell side pass and two tube side

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Specialist Materials Graphite, Teflon, Glass etc.

passes is illustrated in Figure 3.4. Some variations and a system for the nomenclature used for shell-and-tube units is given in Figure 3.5.

Figure 3.4 Shell-and Tube Heat Exchanger

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Figure 3.5 Types of shell and tube heat exchangers

The tubes provide the heat transfer surface in a shell-and-tube heat exchanger, since one fluid flows through the tubes while the shell side fluid flows over the tubes. The tubes may be plain or may have low fins on either or both the internal and external surface to increase the effective heat transfer area and, in some cases, to enhance the heat transfer coefficient which can be achieved. The tubes are held in tubesheets, generally one at each end, the exception being the U tube design. The tubesheet is a round metal plate, drilled to locate the tubes. The tubes are expanded in the holes in the tubesheet to form a mechanical seal, additionally, brazing or welding may be employed to increase the strength and integrity of the tube to tubesheet joint. The shell is either rolled from plate and welded along its seam, or, for sizes up to about 0.6m diameter, cut from standard pipe. Flanged nozzles, normally made from

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standard pipe are welded into holes in the shell to provide inlets and outlets for the shell side fluids. Flanges are welded to each end of the shell to attach channels or bonnets (particularly in the case of U tube exchangers the bonnet may be welded directly to the shell at one end). An array of transverse baffles in the shell provides support for the tubes and guides the flow back and forth across the tube bundle. This serves to improve the heat transfer at the expense of some additional pressure drop. In the case of multi-pass shell side flow longitudinal baffles are included in the shell.

Figure 3.6a Plate baffles types, modified from Mueller (1973) (Taken from Fundementals of Heat Exchangers Design by Shah and Sekulic, John Wiley & Sons, 2003)

Although the shell-and-tube heat exchanger has been the most popular design of process heat exchanger since the advent of the process industry, research and development relating to heat transfer, manufacturing methods and materials means that improved designs are evolving. Two-phase (boiling and condensing) units can benefit from the use of micro-finned and treated surfaces. Conventional baffle

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arrangements are illustrated in Figure 3.6(a) while relatively recent developments are rod baffles (Figure 3.6(b)) and Helical Baffles (Figure 3.6(c)). The use of twisted tubes (Figure 3.6(d)) eliminates the need for baffles entirely.

Figure 3.6b - (a) Four rod baffles held by skid bars (no tubes shown); (b) tube in a rod baffle exchanger supported by four rods; (c) square layout of tubes with rods; (d) triangular layout of tubes with rods (Shah, 1981) (Taken from Fundementals of Heat Exchangers Design by Shah and Sekulic, John Wiley & Sons, 2003)

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Figure 3.6c Helical baffles shell-and-tube exchanger: (a) single helix; (b) double helix (Courtesay of ABB Lamus Heat Transfer, Bloomfield, NJ.) Taken from Fundementals of Heat Exchangers Design by Shah and Sekulic, John Wiley & Sons, 2003)

Figure 3.6d Twisted tube bundle for a shell-and-tube exchanger (courtesay of Brown Fintube Company, Houston, TX (Taken from Fundementals of Heat Exchangers Design by Shah and Sekulic, John Wiley & Sons, 2003)

The tube side fluid enters and leaves through nozzles welded to the tube side channel. This channel may incorporate one or more pass dividers to give the requisite number of tube side passes. Other important construction components that are found in most shell-and-tube heat exchangers include tie-rods, sealing strips, spacers and gaskets. The design of shell-and-tube heat exchangers will be covered in more detail in section 6.

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The Double-Pipe Heat Exchanger The double-pipe heat exchanger in its simplest form comprises two concentric tubes, one fluid flows in the inner tube while the other fluid flows in the annulus between the inner and outer tubes. This is shown schematically in figure 3.7 Frequently, double-pipe heat exchangers are arranged as "U" tubes so that, irrespective of the overall length of the heat exchanger the fluid inlet and outlet connections are located relatively close to each other, thus minimising pipe runs. Units with multiple internal tubes, but having the same general structure and with no baffles are also referred to as double-pipe heat exchangers. Fig. 3.8 illustrates arrangements of double-pipe heat exchangers.

Shell side fluid in

Tube side fluid in

Tube side fluid out

Shell side fluid out

Figure 3.7 Double Pipe Heat Exchanger Schematic

Figure 3.8b Multi-tube double-pipe heat exchanger

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While the tubular heat exchanger may be regarded as the workhorse of the heat exchanger world, at least in the process industries, there are an increasing number of alternatives which the innovative engineer should consider. Most of these are classed as compact heat exchangers and some are described in the later sections of this chapter. Compact heat exchangers (CHEs) are not a new technology although innovative designs are continuously being produced to suit market requirements. Compact heat exchangers are characterised by high heat transfer surface-area to volume ratios (typically 200 to 300 m2 per m3, or more) and high heat transfer coefficients compared to other exchanger types. Such designs are more efficient in terms of heat transfer although fouling and pressure are important design considerations and compact heat exchangers are not suitable for all applications. The students needs only to look at an air conditioiner, a radiator or modern central heating 'boiler' to see that compact design of heat exchangers are literally in everyday use.

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3.2.2 Plate Heat Exchangers As the name suggests, the fluids in a plate heat exchanger (PHE) are separated by thin plates, rather than tubes. The plates are unfinned, but are usually corrugated, the corrugations being important in that they give the plates structural rigidity and they induce turbulence thus enhancing the heat transfer. There are several variants of PHE. and this type of heat exchanger presents the greatest challenge to the shelland-tube type. A number of arrangements are discussed below. Plate and Frame (Gasketed Plate) Heat Exchanger The plate and frame or gasketed plate heat exchanger was one of the first compact exchangers to be used in the UK process industries, being described in patents dating from the late 19th century. However the design was first exploited in 1923 for use for the food and drinks processing, particularly in the dairy industry. It is currently second to the shell and tube heat exchanger in terms of market share. The most common variant of the plate and frame heat exchanger consists of a number of pressed, corrugated metal plates compressed together into a frame. These plates are provided with gaskets, partly to seal the spaces between adjacent plates and partly to distribute the media between the flow channels. Early designs used gunmetal plates, but these were soon superceded by stainless steel, now the most common plate material. Plate and frame heat exchangers first found widespread used in the food and dairy industries, where the ability to access plate surfaces for cleaning is imperative. As the materials of construction, in particular the gasket materials, developed the operating range and applicability of the units increased.

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Figure 3.9 - Exploded View of a ‘Food Style’ Plate and Frame Heat Exchanger (Courtesy of APV)

Figure 3.9 shows an exploded view of a typical plate and frame heat exchanger design. There are numerous suppliers of plate and frame heat exchangers. While all manufacturers follow the same basic construction method, the differences in performance claimed tend to be associated with the patterns on the plates that form the flow channels, and the choice of gasket materials. Newer designs can accommodate features such as grossly unequal flow rates on each side of the plate.

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Figure 3.10 Plate Design Arrangement (Coutesy of SWEP International)

The heat transfer surface consists of a number of thin corrugated plates pressed out of a high grade metal. The pressed pattern on each plate surface induces turbulence and minimises stagnant areas and fouling. Unlike shell and tube heat exchangers, which can be custom-built to meet almost any capacity and operating conditions, the plates for plate and frame heat exchangers are mass-produced using expensive dies and presses. Therefore, all plate and frame heat exchangers are made with a limited range of plate designs and sizes. However, as can be seen from figure 3.10, even with this limited range a manufacturer can produce wider range of channel properties, L. Figure 3.10(a) and (b) illustrates the gasketing arrangement around the ports of a gasketed plate. It is worth noting that the gaskets are arranged to avoid interstream leakage. Any leakage from the main body of the gasket is directly to the outside of the heat exchanger, while leakage from the region of the transfer port is into a vented area. This feature, together with the ease of disassembly for cleaning, make the Gasketed Plate Heat Exchanger attractive when handling foodstuffs. The plates

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are ordered so that the required flow arrangements are achieved. Single pass arrangements are shown in figure 3.11 (a) and (b) and a double pass arrangement is shown in figure 3.11(c).

Figure 3.10(a) – Close-up View of a Heat Exchanger Plate (Courtesy of APV)

Figure 3.10(b) Details of typical gasket arrangement for PHE (taken from Saunders-Heat Exchangers)

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Figure 3.11 (a) Schematic Representation of Flow Paths (single pass) Figure 3.11 (b) Most common flow arrangements in Plate and Frame Heat Exchangers

Figure 3.11(c) Examples of flow arrangements in PHEs

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The plate pack is clamped together in a frame suspended from a carrying bar. Gaskets are fitted to seal the plate channels and interfaces. The frame consists of a fixed frame plate at one end and a moveable pressure plate at the other. The moveable plate facilitates access for cleaning or exchanging the heat transfer surfaces. A feature of this type of heat exchanger is the ability to add or remove surface area as necessary. If, as shown in figure 3.11(a), the inlet and outlet ports (or nozzles) are all in the fixed plate this allows dismantling for cleaning, maintenance or for the addition or removal of plates without disturbing the pipework connections. The operating limits of gasketed plate and frame heat exchangers vary from manufacturer to manufacturer and is largely governed by the gasket material used. Typically, the operating temperature range is from -35oC to +200oC.

Design

pressures up to 30 bar can be tolerated, with test pressures to 40 bar. Higher temperatures can be tolerated if asbestos gaskets are acceptable. Typical gasket materials are listed below, together with their approximate maximum operating temperature: • Neoprene

110oC

• Nitrile rubber 135oC • Butyl rubber 150oC • EPDM.

150oC

• Viton.

175oC

• Asbestos Fibre

260oC

Heat transfer areas range from 0.02 m2 to 4.45 m2 (per plate). Flow rates of up to 3,500 m3/hour can be accommodated in standard units, rising to 5,000 m3/hour with a double port entry. Approach temperatures as low as 1oC are feasible with plate and frame heat exchangers. The surface pattern on the plates tends to induce good mixing and turbulence, and in general this type of heat exchanger has a low propensity for fouling.

Fouling

resistances of typically 25% of those for shell and tube heat exchangers have been measured by the Heat Transfer Research Incorporated (HTRI) in the USA.

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Gasketed plate and frame heat exchangers have a large range of applications typically classified in terms of the nature of the streams to be heated/cooled as follows: • Liquid-liquid. • Condensing vapour - liquid. • Liquid - evaporating liquid Plate heat exchangers are rarely used in applications involving single-phase gases. Gasketed units may be used in refrigeration and heat pump plants and, as stated earlier are extensively used in the processing of food and drinks, where the ease of plate cleaning and re-gasketting are important. In the chemicals sector, a substantial list of heating and cooling applications includes cooling isoparaffin, sulphuric acid, salt solutions, hexane and kerosene. Heating glycerine and condensing ethanol are other routine uses. The offshore chemical industry is also a large user in the UK. There are potential applications for plate heat exchangers on most chemical plants. Brazed Plate Heat Exchanger The brazed plate heat exchanger, as shown in section in fig

(see Figure 3.12)

comprises a pack of pressed plates brazed together, thus completely eliminating the use of gaskets. The frame can also be omitted.

Figure 3.12 – Section Through a Brazed Plate Heat Exchanger (Courtesy of Alfa Laval Thermal Division)

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Brazed plate heat exchangers tend to be offered by the principal suppliers of the plate and frame type and tend to be directed at niche markets such as refrigeration, where they have largely displaced shell-and-tube exchangers. Brazed PFEs are available with heat transfer capabilities up to 600 kW, depending on the supplier. The brazed and gasketed PFE share many features, but obviously the brazed unit cannot be disassembled for cleaning or addition of plates. However, the corrugated plates induce a highly turbulent flow such that the scouring action of the turbulence reduces surface deposits in the heat exchanger. Brazed plate heat exchangers consist of a number of pressed stainless steel plates joined together by brazing. Typically a very high content copper braze is used, and the brazing process is carried out under vacuum. Capillary forces collect the brazing material at the contact points between the plates. The braze seals the periphery of the plates, and the internal herringbone contact points are also brazed, thus permitting higher allowable working pressures than would be found in gasketed units. Stainless steel is usually used as the plate material, temperature limitations are therefore dependent upon the braze properties. Copper brazed units are available for temperatures up to 225oC and a maximum operating pressure of 30 bar, but copper braze may produce an incompatibility with some working media. Nickel brazed units are available for temperatures up to 400oC and maximum operating pressures of 16 bar. The brazed plate unit is aimed at the refrigeration/heat pump market for evaporators and condensers (water-cooled), but it is also suitable for process water heating, heat recovery and district heating systems. Brazed plate heat exchangers can also be used as desuperheaters, subcoolers, economisers and oil coolers. The introduction of nickel brazed units has allowed brazed units to be used within the process industries, for duties such as de-mineralised water cooling and solvent condensing.

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Typically, a brazed plate heat exchanger is about 20-30% of the weight of a shell and tube heat exchanger for the same duty. For example, a brazed plate heat exchanger, used as a water-cooled refrigerant condenser with a duty of 70 kW, had a weight of 20 kg. Its height and width were 522 mm and 115 mm respectively. A conventional shell and tube condenser of the same duty would be 2,250 mm long, have a diameter of 200 mm, and weigh 130 kg. Welded Plate Heat Exchangers Welded plate heat exchangers, or more correctly partially welded plate heat exchangers combine advantages of gasketed plate and brazed plate units. Externally, partially welded plate heat exchangers (also referred to as

twin plate heat

exchangers) resemble fully-gasketed plate and frame units. However, the difference is the plate pack has alternating welded channels and gasketed channels as in the arrangement illustrated in figure 3.13. The advantage of welding the plate pairs is that, except for a small gasket around the ports, other materials are eliminated and corrosion and the consequence of gasked degradation is reduced.

Figure 3.13 – Flow Diagram of the LR4 APV Baker Laser-Welded Plate Heat Exchanger (Courtesy of APV)

The overall construction is similar to that of the gasketed plate and frame heat exchanger (described above), with one important exception: each plate pair is welded together, normally using laser welding. Porthole gaskets fabricated from highly resistant elastomer or non-elastomer materials, are attached to the ports in the welded pair using a glueless method. Plate construction materials are as for the 3.22

gasketed plate and frame heat exchanger. The plate material is normally selected for its resistance to corrosion. The operating limits are similar to those for the gasketed plate and frame type, but with the added protection from leaks afforded by the partially welded construction. Potential applications are also as for the gasketed plate and frame heat exchanger, but extended to include more aggressive media. The units may be disassembled and plate pairs added or the external surface of the pairs mechanically cleaned. The internal surfaces must be chemically cleaned. Partially welded plate heat exchangers are used for the evaporation and condensation of refrigerants such as ammonia and hydrochlorofluorocarbons (HCFCs), and for chemical and general process duties involving aggressive liquids. 3.2.3 Plate-Fin Heat Exchangers Plate-fin heat exchangers (PFEs) were developed some 50 years ago for use in the cryogenics industry. They are now used widely in aircraft and defence applications and their use in process applications, especially offshore is steadily increasing. Brazed PFEs

Fin

Fluid 'A'

Plat

Figure 3.14 Brazed PFEs

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Fluid 'B'

A brazed PFE comprises a series of metal sheets pressed, and sometimes cut, to form fins. Each sheet of fins is then sandwiched between parting plates with bars down each side and then the assembly is vacuum or salt brazed together to form the core of the heat exchanger. The fins act as both secondary heat transfer surfaces and as structural members. Headers are brazed onto the cores. At entry and exit to the core the fins are designed to ensure even flow distribution, and, particularly in smaller units, a significant part of the core may be used for flow distribution rather than heat transfer. The general arrangement of a plate fin heat exchanger is shown schematically in figure 3.14 and 3.15. An assembled unit is shown in figure 3.16, the size of this unit serves to illustrate that compact does not necessarily mean small compact heat exchangers designed for high thermal duties may well be very large in absolute terms, but small compared with a shell-and tube unit for the same duty. For example, a plate-fin heat exchanger with 6 fins/cm provides approximately 1,300 m2 of surface per m3 of volume. This heat exchanger would be approximately 10% of the volume of an equivalent shell and tube heat exchanger with 19 mm tubes.

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Figure 3.15 Core Structure of a Brazed Aluminium Plate-Fin Heat Exchanger (Courtesy of Chart Marston Limited)

Figure 3.16 – Aluminium Plate-Fin Heat Exchanger (Courtesy of Chart Marston Limited)

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(a) Plain Fins

(b) Perforated Fins

(c) Offset-Strip Fins

(d) Serrated Fins

Figure 3.17 Typical Fins for Plate-Fin Heat Exchangers

A range of fin types and dimensions are available the most common types are: plain, serrated (or offset strip), louvred and perforated, these are illustrated in figure 3.17 Plain fins may be pressed in a herringbone or wavy pattern, resulting in a tortuous flow path for the fluid and increased heat transfer. The choice of fin type and dimension depends upon the nature of the fluid stream and the allowable pressure drop. Some guidelines are presented in Table 3.1. It should be remembered that the fins may be chosen to suit the characteristics of each fluid stream - there is no requirement to use the same type or size of fin for each side of the heat exchanger.

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Fin Type

Application

Relative ∆p

Relative Heat Transfer

Plain

General

Lowest

Lowest

Perforated

Boiling streams

Low

Low

Herringbone

Gas streams with low allowable P,

High

High

Highest

Highest

High pressure streams Gas streams for hydrocarbon and natural gas applications Serrated

Gas streams in air separation applications General

Table 3.1 – Brazed Plate-Fin Types The maximum operating temperature of a plate-fin heat exchanger is a function of its construction materials. Aluminium brazed plate-fin heat exchangers can be used from cryogenic temperatures (-270oC) up to 200oC, depending on the pipe and header alloys. Stainless steel plate-fin heat exchangers are able to operate at up to 650oC, while titanium units can tolerate temperatures approaching 550oC. Aluminium brazed units can operate at up to 120 bar, depending on the physical size and the maximum operating temperature. Stainless steel plate-fin heat exchangers are currently limited to 50 bar, with developments expected that will extend the capability to 90 bar. Higher pressures can be tolerated by using a diffusion-bonded structure. The size of a plate-fin heat exchanger is a function of the procedure used to assemble the core and is limited by the size of oven used for the brazing process. In the case of aluminium vacuum-brazed units, modules of 6.25 m x 2.4 m x 1.2 m are available.

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If considering use of a brazed aluminium plate-fin exchanger, the engineer should ensure that: • All fluids must be clean and dry. Filtration must be used to remove particulate matter over 0.3mm. • Fluids must be non-corrosive to aluminium. Water is suitable if it is a closed loop and contains corrosion inhibitors. • Fluids must be in the temperature range –270 to +200 o C. • The maximum design pressure is less than 120 bar. The temperature and material compatibility criteria are obviously eased if stainless steel or titanium is used, but unless chemical cleaning is possible, the requirement for a clean fluid must be met. Acceptance by the process industries has been slower, largely because the most common material of construction, aluminium, is a poor structural material under cyclic loads and at moderate temperatures. The structural integrity of conventional plate-fin heat exchangers is further compromised in typical process applications by the presence of braze material and the heightened susceptibility to corrosion at the brazed joints. Stainless steel and titanium are both difficult to work, and combined with their inherent cost, make PHEs made with these materials expensive and the problem of the brazed joints remains. Proprietary designs overcome this, as described below. Diffusion Bonded Plate Heat Exchangers A recent development is the Rolls Laval super-plastic formed diffusion bonded heat exchanger (SPF/DB) made entirely of titanium. Rolls Laval near Wolverhampton, UK, is a joint venture involving Rolls Royce and Alfa-Laval, one of the world's leading heat exchanger manufacturers. As with the Printed Circuit Heat Exchanger, described in section 3.2.5, development of the SPF/DB heat exchanger required adoption of technologies which were first used in other fields: in this case the expertise of Rolls Royce in the manufacture of high integrity aero-engine components. Rolls Royce recognised that the techniques used to make hollow gas turbine blades, were ideally suited to the construction of quality heat exchangers.

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Certain metals, including titanium, exhibit superplasticity. Superplasticity is the ability of a material to sustain large strains without the onset of tensile instability or necking. Exploitation of this phenomenon permits the generation of complex shapes to close tolerance without the use of formers. Furthermore, the techniques of diffusion bonding, as described earlier in this article, and super-plastic forming had been used in the company for volume production of components to the highest quality standards. The steps involved in the manufacture of the Rolls Laval heat exchanger are as follows. Each element of the heat exchanger comprises three sheets of titanium: two parting sheets separated by the secondary heat transfer surface. Titanium sheets of the required thickness and surface finish are cleaned to a high standard to ensure that no surface contamination is present which might inhibit grain growth during the diffusion bonding process. A bond inhibitor is then deposited on the internal surfaces of the parting sheet in a pattern corresponding to the ultimate layout of the passages of the heat exchanger, effectively masking these areas and thus preventing bonding. The resulting sandwich is heated under external pressure so that diffusion bonding occurs. The next stage of the manufacture involves opening the flow channels in the plate assembly using superplastic forming. During the forming the assembly is placed in a closed die and heated to around 900°C. Gas is injected at a controlled high pressure into the sandwich forcing the parting plates to separate and the central sheet to stretch between them, thus forming the tins. The die is then closed under external pressure to flatten or "iron" the element, removing any irregularities in its external shape. A number of elements are diffusion bonded together to form the heat exchanger core. Addition of appropriate inlet and outlet ports and welding of the nozzles completes the unit. The steps in the manufacturing process are illustrated in figure 3.18 and figure 3.19 shows completed elements after super plastic forming.

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Figure 3.18 Manufacturing the Core of a Diffusion-Bonded Plate-Fin Heat Exchanger (Courtesy of Rolls Laval Heat Exchangers Ltd)

Figure 3.19 Example Elements of Diffusion Bonded Plate-Fin Heat Exchangers (Courtesy of Rolls Laval Heat Exchangers Ltd)

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The first three of these units to go into service are employed on an unmanned gas production platform cooling wet natural gas after compression. Costing £300,000 each and weighing over 4.5 tonnes, these heat exchangers are neither cheap nor small, but they are a tenth of the volume and less than a quarter of the weight of an equivalent shell-and-tube exchanger, as represented in figure 3.20 The savings in expensive titanium and valuable offshore space are substantial. A further five units have now been installed offshore and sixteen exchangers are being manufactured for new platforms. Although the titanium units currently available are particularly suited for sea water cooling offshore it is intended that the range of materials offered will be extended making the technology attractive in many other processing environments. Since this module was prepared manufacture of the Rolls-Laval SPF/DB heat exchanger has ceased. Its description remains in the notes as an example of application of different manufacturing techniques to heat exchanger manufacture. Despite the technical promise of this particular unit, it was not a commercial success.

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Figure 3.20 Size difference for Gas Cooling Heat Exchanger on a North Sea Platform (Courtesy of Rolls Laval Heat Exchangers Ltd)

3.2.4 Spiral Heat Exchangers The classic design of a spiral heat exchanger is simple; the basic spiral element is constructed of two metal strips rolled around a central core forming two concentric spiral channels. Normally these channels are alternately welded, ensuring that the hot and cold fluids cannot intermix. The heat exchanger can be optimised for the process concerned by using different channel widths. Channel width is normally in the range 5 to 30 millimetres. Plate width along the exchanger axis may be up to 2 m, as can the exchanger diameter, giving heat transfer areas up to 600m2 per unit. Figure 3.21(a) (b) and (c) show spiral heat exchangers with different flow arrangements.

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Figure 3.21 (a) Type 1 - Spiral Flow-Spiral Flow Heat Exchanger (Courtesy of Alfa Laval Thermal Division)

Figure Type 3.21(b) - Cross Flow-Spiral Flow Heat Exchanger (Courtesy of Alfa Laval Thermal Division)

Figure 3.21(c) Type 3 - Combination CrossFlow and Spiral Flow-Spiral Flow (Courtesy of Alfa Laval Thermal Division)

The spiral heat exchanger can be tailor-made to perform in a wide variety of duties in all metals that can be cold-formed and welded, such as carbon steel, stainless steel and titanium. High-grade alloys are routinely used for excellent resistance to corrosion and erosion. In some cases double spacing may be used, produced by

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simultaneously winding four strips to form two channels for each fluid. These double channel systems are used when there is a large flowrate or small pressure drop, but should not be used for fouling media or media containing solids. Spiral heat exchangers tend to be self-cleaning. The smooth and curved channels result in a lower fouling tendency with difficult fluids. Each fluid has only one channel and any localised fouling will result in a reduction in the channel cross sectional area causing a velocity increase to scour the fouling layer. The use of spiral heat exchangers is not limited to liquid-liquid services. Variations to the basic design give exchangers that are suitable for liquid-vapour or liquid-gas services. The flow configurations illustrated in figure 3.21 may be summarised: • Type 1 – Media in full counter-current flow (Fig 3.21(a)). The hot fluid enters at the centre of the unit and flows from the inside outward. The cold fluid enters at the periphery and flows towards the centre. • Type 2 – One medium in cross flow whilst the other is in spiral flow. (Fig 3.21(b)) The medium in crossflow passes through the open channels of the spiral usually in a vertical direction. The service fluid spiral flows through the other channel, welded shut, with side wall inlet and central outlet fed through the side wall. This design can be used as either a condenser or vaporiser. • Type 3 – Combination design. (Fig 3.21(c)) A gas or vapour mixture to liquid exchanger combines the above two designs; the hot stream enters at the top and flows tangentially through the exchanger exiting at the side. Typically, the maximum design temperature is 400oC set by the limits of the gasket material. Special designs without gaskets can operate with temperatures up to 850oC. Maximum design pressure is usually 15 bar, with pressures up to 30 bar attainable with special designs.

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The design is ideal for fluids prone to fouling, or polluted with particles as a result of the relatively large channel width. Hence, it is ideal for use in the food industry (sauces, slush and slurry) as well as in brewing and wine making. Spiral heat exchangers have many applications in the chemical industry including TiCl4 cooling, PVC slurry duties, oleum processing and heat recovery from many industrial effluents. Spiral heat exchangers also provide temperature control of sewage sludge digesters and have uses in other public and industrial waste plants. Counter flow spiral heat exchangers have perfect counter-current flow paths that permit the best possible overlap of exit temperatures. As such, they can maximise the heat recovery on large-scale cogeneration projects although they may be more expensive than plate designs. Spiral exchangers can be mounted directly onto the head of distillation columns acting in a condensing or reflux role. Specific advantages are ease of installation, low pressure drop and large flow cross-section. Consequently, there are many condensing applications in all process industries particularly for condensing under vacuum. Spiral designs have a number of advantages compared to shell and tube heat exchangers: • Optimum flow conditions on both sides of the exchanger. • An even velocity distribution, with no dead-spots. • An even temperature distribution, with no hot or cold-spots. • More thermally efficient with higher heat transfer coefficients. • Copes with exit temperature overlap, or crossover, whereas shell and tube units require multi-shells in series to handle temperature crossover. • Small hold up times and volumes. • Removal of one cover exposes the total surface area of one channel providing easy inspection cleaning and maintenance. For the same duty, a spiral heat exchanger heat transfer area would be 90m2 compared to 60m2 for a plate and frame design or 125m2 for a shell and tube design. The physical size comparison is shown in figure 3.22.

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Figure 3.22 Heat Exchanger Size Comparison for Plate, Spiral, and Shell and Tube Heat Exchangers (Courtesy of GEA Process Technology)

3.2.5 Printed Circuit Heat Exchangers (PCHEs) Printed circuit heat exchangers (PCHEs) are highly compact, corrosion resistant heat exchangers capable of operating at pressures of several hundred atmospheres and temperatures ranging from cryogenic to several hundred degrees Celsius. Developed and produced by Heatric in the late 1980s, the PCHE provided a compact alternative to the shell-and-tube heat exchanger for many applications where the latters dominance was unquestioned. The printed circuit heat exchanger design offers a unique combination of innovative manufacturing technology and potential breadth of application. In common with some other compact heat exchangers, it is potentially more than just a compact plate heat exchanger; the structure has applications in a variety of other unit operations, including reactors, mass transfer and mixers.

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Figure 2.23 Fluid Flow Paths on a Typical Printed Circuit Heat Exchanger Etched Plate (Courtesy of Heatric Ltd)

Printed circuit heat exchangers are constructed from flat alloy plates with fluid flow passages photo-chemically machined (etched) into them. This process is similar to manufacturing electronic printed circuit boards, and gives rise to the name of the exchangers. An example of a plate showing a 'herringbone' pattern of flow paths is shown in figure 2.23. Heatric originally developed printed circuit heat exchangers in Australia, where this type of heat exchanger first became commercially available for refrigeration and process applications in 1985. In 1990, Heatric moved to the UK and has supplied printed circuit exchangers into the offshore and process sectors, both in the UK and overseas. The standard manufacturing process involves chemically milling (etching) the fluid flow passages into the plates. This allows enormous flexibility in thermal/hydraulic design, as complex new plate patterns require only minimal re-tooling costs. This plate/channel forming technique can produce a wide range of flow path sizes, the channels varying typically from 0.5 to 2.0 mm in depth. Stacks of etched plates, carrying flow passage designs tailored for each fluid, are diffusion bonded together to form a compact, strong, all-metal heat exchanger core. A cross-section through a typical core sample is shown in Figure 3.24. No gaskets or brazing materials are

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required for the assembly. Diffusion bonding allows the plates to be joined so that the bond acquires the same strength as the parent metal. The thermal capacity of large heat exchangers is achieved by welding together diffusion bonded blocks to form the complete heat exchanger core. Finally, fluid headers and nozzles are welded to the cores, in order to direct the fluids to the appropriate sets of passages. Figure 3.25 shows a completed heat exchanger unit. Materials of construction include stainless steel (SS 300 series) and titanium as standard, with nickel and nickel alloys also being commonly used. A copper variant is being developed.

Figure 3.24 Section of a Typical Printed Circuit Heat Exchanger Core (Courtesy of Heatric Ltd)

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Figure 3.25 Gas Dew Point Control Printed Circuit Heat Exchanger (Courtesy of Heatric Ltd)

Mechanical design is flexible; etching patterns can be adjusted to provide high pressure containment where required. Due to its construction, the printed circuit heat exchanger is able to withstand substantial pressures. Pressures as high as 200 bar are routine, with values in the range 300 - 500 bar being possible. The limitation usually being imposed by the headers, rather than by the core itself. The all welded construction is compatible with very high temperature operation, and the use of austenitic steel allows cryogenic operation. Operating temperature ranges from -200oC to +900oC, the upper limits depending on the metal selected and the pressure duty. Passages are typically of the order of 2 mm semi-circular cross-section (i.e. 2 mm across and 1 mm deep) for reasonably clean applications, although there is no absolute limit on passage size. Primary heat transfer surface densities, expressed in terms of effective heat transfer area per unit volume, can be up to 2500m2/m3 This is higher than primary surface density which can be achieved in gasketed plate exchangers, and an order of magnitude higher than normal primary surface densities in shell and tube exchangers.

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Printed circuit heat exchangers are all welded so there is no braze material employed in construction, and no gaskets are required. Hence the potential for leakage and fluid compatibility difficulties are reduced and the high level of constructional integrity renders the designs well suited to critical high pressure applications, such as gas compression cooling exchangers on offshore platforms. The thermal design of printed circuit heat exchangers is subject to very few constraints. Fluids may be liquid, gas or two-phase, multi-stream and multi-pass configurations can be assembled and flow arrangements can be truly countercurrent, co-current or cross-flow, or a combination of these, at any required pressure drop. Where required high heat exchange effectiveness (over 98%) can be achieved through very close temperature approaches in counter-flow. To simplify control, or to further maximise energy efficiency, more than two fluids can exchange heat in a single core. PCHEs may be designed for heat loads ranging from a few watts to many megawatts, in exchangers weighing from a few kilograms to several tonnes. Flow induced vibration, an important source of failure in shell and tube exchangers, is absent (as are the tubes!) from printed circuit heat exchangers. Concern may be expressed regarding fouling and blockage of the small channels, however often a simple strainer upstream of the unit will remove outsize particles, while the corrosion resistant materials of construction for printed circuit heat exchangers, the high wall shear stresses, and the absence of dead spots assist in resisting fouling deposition. Detailed thermal design of printed circuit heat exchangers is supported by proprietary design software developed by the manufacturer that allows infinite geometric variation to passage arrangements during design optimisation. Variations to passage geometry have negligible production cost impact since the only tooling required for each variation is a photographic transparency for the photo-chemical machining process. Counterflow and crossflow arrangements (or a combination) can be accommodated. Pressure drops can be specified, however as with all heat

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exchangers, lower allowable pressure drops will result in lower heat transfer coefficients and hence larger exchangers. Printed circuit heat exchangers extend the benefits of compact heat exchangers into applications where pressure, temperature or corrosion prevents the use of conventional plate or plat-fin heat exchangers. The printed circuit heat exchanger can handle gases, liquids and two-phase flows. Heatric cites four main application areas, as listed below: • Fuels processing: - Gas processing e.g. compressor cooling, liquids recovery. - Dehydration. - Synthetic fuels production e.g. methanol. - Reactor feed/effluent exchange. • Chemical processing: - Acids e.g. nitric, phosphoric. - Alkalis e.g. caustic soda, caustic potash. - Fertilisers e.g. ammonia, urea. - Petrochemicals e.g. ethylene, ethylene oxide, propylene. - Pharmaceuticals. - Plastics e.g. formaldehyde, phenol. • Power and energy: - Feedwater heating. - Geothermal generation. - Chemical heat pumps. • Refrigeration: - Chillers and condensers. - Cascade condensers. - Absorption cycles. Figure 3.26 illustrates the size difference between a comparable printed circuit heat exchanger and stack of three series shell and tube units used for gas dew point control. The duty is 2,350 kW across a 4oC LMTD. The printed circuit heat

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exchanger illustrated in Figure 3.21 has 600m2 of surface and a design pressure of 124 bar. Its weight is 15 tonnes, compared to 105 tonnes for equivalent shell and tube heat exchangers. Printed circuit heat exchanger cores are typically 5 to 10 times smaller than shell and tube exchangers tube bundles of equivalent performance.

Figure 3.26 Comparison of Printed Circuit Heat Exchanger and Shell and Tube Heat Exchangers of Equivalent Capacity (Courtesy of Heatric Ltd)

3.2.6 Plate and Shell Heat Exchanger The plate and shell heat exchanger combines many of the merits of shell and tube with plate heat exchangers, while externally resembling the former in some respects. Plate and shell heat exchangers feature an outer shell enclosing circular plates welded into pairs. The cooling medium flows on the shell side between the pairs of plates. As a plate is more thermally efficient than a tube, this can achieve a significantly higher level of heat transfer. The construction of a plate and shell heat exchanger involves welding together, in pairs, circular plates of a similar surface form and material to those of plate and frame heat exchangers. The plates are then located inside a shell, as shown in Figure

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3.27. A ‘closed’ model has a welded shell or an ‘open’ model has a removable end flange to facilitate shell-side cleaning.

Figure 2.27 General Arrangement of a Plate and Shell Heat Exchanger (Courtesy of APV)

Generally the hot fluid is passed through the plate side, while the cooling fluid is directed on the shell side. The shell side fluid is routed through individual passes via a baffle plate similar to the shell in the tubular type heat exchanger. Multi-pass arrangements are possible, flow directors on both the shell and plate side adjust the flow paths. Current plate and shell heat exchanger models accommodate up to 600 plates in a shell 2.5 m long with a 1 m diameter. Plate and shell heat exchangers are available with a heat transfer surface area of up to 500m2. Standard plate materials are Titanium B265, Avesta 254 SMO and AISI 316. The shell can be made of St 35.8 or AISI 316 or other materials, such as Hastelloy or nickel, if necessary. The maximum operating temperature of a plate and shell heat exchanger is 900oC, and maximum working pressure is 100 bar. Single units, which can be operated in parallel for higher throughputs, can currently handle flow rates of 11 litres per second on the shell side. Plate and shell heat exchangers can work with aggressive media and acids, which cannot be handled by conventional gasketed plate heat exchangers. They can also withstand extreme temperature shocks and pressure shocks due to their rigid and compact construction. The principal applications for plate and shell heat exchangers are:

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• Heating including district heating. • Cooling including cryogenic applications. • Heat recovery. • Combined exchanger/reactors vessels. • Condensation/evaporation. Data that directly compares the shell and plate unit with a shell and tube heat exchanger are not available, but shell and plate heat exchangers have been compared with brazed plate heat exchangers. Like brazed plate heat exchangers, plate and shell heat exchangers reach very close approach temperatures. Furthermore due to the flexible layout of flow path configurations, overlapping or crossover of exit temperature is possible. For heat exchangers of equivalent area and capacity, plate and shell designs are smaller due to the higher ratio of heat transfer area and specific volume. It is claimed that the plate and shell heat exchanger will occupy only 20 to 30% of the footprint of equivalent capacity shell and tube types. The maximum operating pressure of the plate and shell unit will also be higher. 3.2.7 Polymer Heat Exchangers While most of the heat exchangers used in the process industries are metallic, other materials are available, and may be used when handling extremely aggressive fluids. Carbon, for example, is used for sulphuric acid, TEFLON and glass are occasionally used where extensive corrosion may occur. Ceramic units are available for use at high temperatures. The application of polymers in process heat exchangers is often stimulated by the need to protect against corrosion, in other applications the light weight and low cost of some polymers is advantageous. Polymer heat exchangers are available for heating, ventilating and air conditioning duties, where pressures are low and very thin plates may be used, thus mitigating the principle thermal disadvantage of polymers, i.e. their low thermal conductivity relative to metals. TEFLON Heat Exchangers Heat exchangers incorporating TEFLON were first introduced for corrosive or abrasive applications in chemical plants. As plastics have a relatively low thermal

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conductivity, small-bore tubes with thin wall sections were used. Typically 2.5 mm o/d tubes were used with a wall thickness of 10% of the outside diameter. TEFLON heat exchangers are available as shell and tube designs, or as immersion coils. TEFLON “Q” is a resin development that increases the temperature capability up to 200oC and has approximately twice the thermal conductivity of normal TEFLON. In addition, this resin is tougher and more abrasion resistant. Tube diameters have been introduced from 2.5 to 9.5 mm to increase flexibility. Polymer shell and tube units tend to be single pass, counter-current designs incorporating flexible tubes of TEFLON FEP or TEFLON “Q” fused at both ends to form a honeycomb structure. Shell-side baffles promote cross-flow and optimise thermal efficiency. All surfaces exposed to the process stream are made of TEFLON to resist fouling and corrosion. The small bore tubes produce a large surface area for a given volume; for example 1000 tubes of 4.45 mm o/d inside a 10 inch shell gives a heat transfer area of 275 m 2 /m 3 . Usually the shell is carbon steel although other shell materials are available. In the case of heat exchange between two corrosive streams, the shell can be TEFLON lined. Shell diameters range from 76 to 355 mm in lengths from 0.6 to 7.3 m. Immersion Coils Slimline coils are used in medium and large process tanks for heating or cooling purposes. Typically 300 tubes of 3 mm diameter give 166 m2/m3 Units are available in lengths from 1.22 to 4.9 m with surface areas from 3.2 to 23.7 m2. Process stream temperatures are restricted to less than 200oC These specialist exchangers are used for corrosive process streams, such as hydrochloric acid, or for abrasive process streams.

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3.3

Choice of Heat Exchanger Types

The choice of heat exchanger for a particular application depends upon any factors, some technical, others economic and in many cases, custom and practice. While enthusiastic heat transfer engineers may favour compact heat exchangers, in many applications there is considerable inertia resisting change from established (usually shell-and-tube) technology. This is understandable, the cost of a heat exchanger is likely to be small (in total perhaps 5-10% of the overall cost of a project), while failure of a heat exchanger may result in shut down of the plant with massive cost implications. Against this background, tried and trusted solutions are strong favourites. Novel heat exchangers have gained acceptance slowly, often beginning in niche applications. For example the plate-and-frame heat exchanger has considerable advantages in the food processing industry and its success in this area has given users confidence in wider applications. Other compact units have become established in applications where space and weight are critical, for example, the PCHE is a popular choice offshore, and its earliest use in the process industries included retrofit applications where shell-and-tube units had failed and the PCHE's small size made retrofit relatively easy. Guidelines given by Saunders are given here as in textbox 1 and a summary of compact heat exchangers is included as Table 3.2 It can be seen that in many situations a number of heat exchanger types may be suitable. In these circumstances the specifier should carry out some preliminary design calculations and assess the options more carefully.

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TextBox 1 - Heat Exchanger Type Selection

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TextBox 1 - Heat Exchanger Type Selection (cont.)

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Table 3.2 Comparative Summary of Heat Exchangers

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Table 3.2 Comparative Summary of Heat Exchangers Features (contnued)

Summary Points •

Heat exchangers may be classified in various ways.

•

The operating conditions and economic factors determine the type of heat exchanger which is best suited to a particular duty.

•

Several different types of heat exchangers maybe suitable for a particular duty, and some preliminary work may be necessary to determine the most appropriate.

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