Compact Heat Exchangers. Selection, Design and Operation

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This results in the need of excess energy disposal by forms of waste heat which is performed by a pre-cooler. This device guarantees that the bleed air can be safely piped from the engine, through the fuselage and into the aircraft. Once the bleed air from the engines has entered the pneumatic system and been processed through the pre- cooler stage 1 , it flows through ducting in the wings before arriving at the air conditioning system.

These systems are used to cool and, if necessary, dehumidify the bleed air from the engines or APU before it is supplied to the aircraft cabin. The output of each individual pack links to a mix chamber where cold and hot air is mixed to allow for various temperature variations as desired by the cockpit settings. The diagram follows on dir- ectly from figure 3. This is achieved by decreas- ing the temperature and water content of the bleed air used in the conditioning process.

To analyse the air-conditioning sub-system in more detail, it is important to consider the thermodynamic processes that are involved within the condition- ing packs. These processes are typically based on vapour cycles or air cycles; the former being the one used in industry today [18]. Although vapour cycles have higher efficiencies than air cycles, they are generally heavier so are seldom used on commercial aircrafts. Air cycles, despite their slightly low efficiency state, have advantages such as the refrigerant air is free, overall mass is low, the compressor is already part of the engine and efficient heat transfer occurs.

The basic air cycle machine ACM system consists of a cold air unit com- pressor - turbine and a heat exchanger. In aircraft applications, an extra heat exchanger is employed primary heat exchanger before the compressor as cool- ing through this exchanger increases the efficiency of the ACM because it lowers the temperature of the air entering the compressor - thus less work is required to compress a given mass of air.

The mix manifold performs the task of taking up this air into a mixing chamber where it is then combined with an equal quantity of filtered recirculated air from the cabin. This reduces bleed demand and therefore lowers fuel burn requirements. The recirculated air entering the mix manifold is essentially sterile - These filters are similar to those used in critical wards of hospitals wards and industrial clean rooms [17].

A schematic of this system is outlined in figure 3. The risers take the air from below the floor to the overhead air distribution network runs the length of the cabin[18]. Air leaves the outlets at a velocity of more than 2. The return air grilles are located in the sidewalls near the floor and run the length of the cabin along both sides [18].

The remaining is thus exhausted back to the ambient via a nozzle and an outlet louvered door [13]. This expelling of ram air is important to allow for the refreshed supply of cool air to effectively enter the air conditioning pack. The typical passenger is covered in a volume of air amounting to about 1m3 [23]. To achieve the same level of air quality in an aircraft where the available space per passenger is much smaller and accounting for the provision of adequate smoke clearance in the event of smoke accumulation due to a system failure, a factor of 2 may be introduced to maintain comparable air quality to an office space.

Therefore, there is a need for 2. This standard states the quality of air and proper blood oxygenation is manly governed by the concentra- tion of Carbon Dioxide CO2 in a particular space instead of oxygen availability [24].

To allow for such an assessment using CO2 as a surrogate indicator of the adequacy of ventilation in an environment, it is important to determine the levels of carbon dioxide in air upon human exhalation. Normal adult breathing involves the intake of about 0.

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Based on the analysis of exhalation carbon dioxide level minimization, rate of breath at rest and subsequent levels of carbon dioxide concentrations in ppm, equation 3. In equation 3. It can be seen that mean CO2 concentrations ranged from to 1, ppm and other published studies [17] show CO2 concentration levels during flight average between ppm to 1,ppm. Equation 3. This value is essentially the flow rate of the hot air that passes through a single heat exchanger within the air conditioning packs.

This is calculated using the ideal gas law equation 3. Each important stage of the ACM is numbered where some sort of thermodynamic process occurs either changes in temperatures or pressures or both. However, since this project makes use of the isolated entropy minimization approach as mentioned in section 1 , it is important to evaluate the role the primary heat exchanger plays within the ACM so that it may be isolated.

It is therefore important to evaluate the conditions at each point in the ACM. This is ducted into the pack at the primary heat exchanger which is part of the ACM and the main design parameter in this project. The remaining air that is not bypassed, enters the compressor where it is increased in pressure with a corresponding increase in temperature point 3.

This removes water that may have formed during the heat exchange process - thus preventing freezing of the turbine blades and condensa- tion in the cabin. It is decelerated in a diffuser point 1r and contin- ues through the secondary and primary heat exchangers, where it is heated to the state 2r and 3r respectively. Before being discharged into the ambient point 3r , the stream is accelerated through a nozzle and then exhausted through a louvered exit door [13].

As has been previously outlined in section 3. Under isentropic conditions, the pressure at the diffuser exit point 1r can be determined from equation 3.

Compact Heat Exchangers Selection Design And Operation

Assuming the worst case scenario of an effectiveness of 0. The pressure, however, is important to consider as the pressure drop across the heat exchanger is an important factor that determines overall pumping power requirements. This is important in order to initiate te rating procedure as was outlined in the process flow diagram on page 9. Heat transfer in these devices usually involves the transfer from the hot fluid to the wall by con- vection, through the wall by conduction, and from the wall to the cold fluid again by convection. The major heat-exchanging element of an exchanger is the core which contains the heat transfer surface and fluid distribution elements such as fins, parting plates, and sidebars.

Heat transfer applications require different types of hardware and subsequently different configurations of heat transfer equipment. Considering restrictions on the size and weight of heat exchangers for use in an aircraft, the need for space min- imization and compactness of the heat exchanger in the bootstrap system needs to be of most importance when accounting for design parameters.

Heat exchangers currently being used in aircrafts have a volume of 0. These fins thus allow an increase in heat transfer surface area by a factor of 5 to 12 times the primary surface area [7]. Of these two, PFHEs are widely used in the auto mobile, aerospace, cryogenic and chemical industry applications [8] - with aerospace use dating back to [7]. Their attractive use springs from their ability to better handle as compared to tube-fin exchangers strict limitations on weight and volume which is of paramount importance when designing systems requiring high performance to-weight and compactness ratios.

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A decision matrix shown overleaf demonstrates in greater detail as to why plate-fin exchangers are most ideal for use in the airline industry. The matrix in table 3. This factor is then combined with an intuitive weighting to yield a final weighted score which can be used to determine which option is the most suitable. It was found that the plate-fin choice is the most ideal. Thermodynamically, the counterflow ar- rangement provides the highest effectiveness while the parallel flow geometry the lowest. The cross-flow arrangement, while giving intermediate thermodynamic performance, offers superior heat transfer properties and much simpler mechanical design - although at the expense of a slightly lower effectiveness [7].

Kays and London [30] state that although flow arrangements within the cross-flow setup include 1 Both streams unmixed; 2 One stream unmixed; and 3 Both streams mixed figure 3. Since no cross mixing occurs, the effectiveness that results is just lower than what would be found in counterflow exchangers.

Option 2 with one stream mixed produces effectivenesses which are less than option 1 and also causes drops with increasing NTU - an undesirable facet. Option 3 with both fluids mixed is considered both least effective more prone to large pressure drops due to non uniform flow [29]. This is also the most common flow arrangement used for extended surface heat exchangers used in the aviation industry [7] [8]. This is illustrated in figure 3. To achieve high heat transfer coefficients, it becomes important to optimize these fin geometries to best suit the conditions that need to be met.

A general approach toward increasing performance in all realms of the heat exchange that occurs is with minimization of the hydraulic-diameter flow passages. Small passages are favoured as they 1 increase turbulence and advection of fluid from the centre of the channel to the near-wall region; 2 are responsible for the breakup of the boundary layer; and 3 decrease the probability of appearance of stagnation areas and fouling [32].

Examples of commonly used fins are shown in figure 3. Although triangular and rectangular passages are more common, any complex shape desired can be formed depending on manufac- turing constraints. Triangular fins can be manufactured at high speeds and are less expensive than rectangular fins; although structurally weaker for the same passage size and fin thickness. They also have lower heat transfer per- formance particularly in laminar flow. Plain fins are used in those applications where core pressure drop is critical.

A heat exchanger with plain fins requires a smaller flow frontal area than that with interrupted fins for specified pressure drop, heat transfer and mass flow rate; but resulting in a longer passage length leading to a larger overall volume [7]. The heat transfer enhancement achieved with plain fins results mainly from increased area density, rather than any substantial rise in the heat transfer coefficient and is considered the lowest of all geometries [32].

The resulting wave form provides effective interruptions and induces a complex flow field.

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Heat transfer is enhanced due to creation of counter-rotating vortices which form while the fluid passes over the concave wave surfaces. The heat transfer and pressure drop charac- teristics of a wavy fin surface lie between those of plain and offset strip fins discussed next. Since there are no cuts in the surface, wavy fins are used in applications where an interrupted fin might be subject to a potential foul- ing or clogging problem due to particulates, freezing moisture and bridging louvers [7].

These fins may be visualised as a set of plain rectangular fins cut normal to the flow direction, and alternately offset lat- erally by shifting each strip by half the fin spacing left and right. As a result of this, the flow is periodically interrupted, leading to the creation of fresh boundary layers and consequent heat transfer enhancement.

Oscillations in the form of vortices shed from the trailing edges of the interrupted fins en- hance local heat transfer by continuously bringing in fresh fluid towards the heat transfer surfaces - an enhancement at the expense of higher pressure drops [7] [8]. For specified heat transfer and pressure drop requirements, the offset strip fin surface demands a somewhat higher frontal area compared to those with plain fins, but results in a shorter flow length and lower overall volume.

Compact Heat Exchangers

It is considered that with the shorter strip lengths, a better heat transfer performance is achieved [33]. It has been contended that the performance of the louvered fins is similar to or better than offset strip fins. Another limitation is the fact that the louver fins have a slightly higher potential for fouling than OSF. The flow structure in the louvered fin flow passage is dependent on the flow rate.

At very low rates, the main flow stream does not pass through the louvers, whereas at higher ones, the flow becomes nearly parallel to the louvers. It is thus concluded that the multi -louvered fin has the highest heat transfer enhancement relative to pressure drop in comparison with most other fin types.

While this geometry, with boundary layer interruptions, is a definite improvement over plain fins, its performance is generally poorer than that of a good offset strip fin. Furthermore, the perforated fin rep- resents a wasteful way of making an enhanced surface, since the material removed in creating the perforations is thrown out as scrap.

Perforated fins are now used only in limited number of applications such as turbulators in oil coolers. Pins may have a round, an elliptical, or a rectangular cross section. These types of finned surfaces are not widely used due to low compactness and high cost per unit surface area compared to multilouvered or offset strip fins. Due to vortex shedding behind the pins, noise and flow-induced vibration are produced, which are generally not acceptable in most heat exchanger applications [7]. In some cases, neglecting complications such as fins has resulted in the fin-less design technique so as to simplify heat exchanger model- ling, analysis, and numerical solution.

Based on the aforementioned discussion of section 3. It is a ratio of the Colburn factor j and Fanny friction factor f as defined by Kays and London [30]. The j and f factors for the fin geometries discussed in section 3. The greater the j factor and smaller the f factor, the greater the heat transfer and smaller the effects of friction - thus leading to lower pressure drops and power requirements.

Based on the discussion in section 3. Section 3. Due to these factors and since a low f value is preferred, these geometries can be eliminated as potential choices. With the offset strip fin sitting at a central position for both the Colburn and Fanny friction factors, it can be considered the most ideal geometry to be em- ployed in this project as it presents the best flow area goodness factor ratio and thus heat transfer opportunities. Along with its ability to minimize its weight to volume ratio much more effectively than others due to the employment of shorter flow lengths - something that has been of paramount importance in this project - it is also stated by Shah [7] that OSF are used extensively by aerospace, cryogenic, and many other industries.

A single layer schematic of a plate-fin exchanger employing offset strip fins is shown in figure 3. Furthermore, the material should exhibit strength to withstand the operating temperatures and pressures it will be exposed to during operation. Since the plate-fin exchanger type has been considered for this project - in specific the offset strip fin - its extended surfaces should ideally be manufactured from a metal that has good formability. Aluminium, in its vari- ous forms, offers clear possibilities to achieve these goals with its relatively high thermal conductivity, low density, strength at low temperatures, increased fin ef- ficiency, and low cost as it sets itself apart from other options such as copper, steel, nickel alloys and titanium.

Typical alloying elements include magnesium, manganese, zinc, cop- per, and silicon. All non-heat-treatable alloys 1xxx, 3xxx, 4xxx, and 5xxx series have a high resistance to general corrosion [8]. Consequently, brazing will be con- sidered as the more ideal option as aircraft use would ideally require premium op- eration characteristics despite higher costs and more precise control over tolerances thus resulting in a clean joint with no need for additional finishing.

Standard commercial metals are based on the Al 4xxx series - an aluminium-silicon-system [39]. The 6xxx series, however, has a slightly lower melting temperature and thus a higher susceptibility to silicon pen- etration. Referring to the Aluminium alloy table on page 51 published by Capalex [40] an Aluminium extrusion manufacturer in the UK , the Al alloy with major alloying constituents of 1. It can thus be considered the most suitable alloy for use in the heat exchanger for this project as the offset strip fins will re- quire substantial surface manipulation along with the headers and nozzles.

This alloy also has a high resistance to corrosion through the atmosphere and moderate strength overall - making it an already well established material choice for many plate-fin heat exchangers in industry [7] [8] [41]. Whilst every effort is made to ensure the accuracy of the data provided, Capalex does not guarantee or accept liability for its accuracy. Actual values will depend on selection specification. Useful information Standard gauge swg to metric conversion table: Standard gauge swg 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 Thickness mm 4.

In addition, extra alloy- ing of copper acts to lower both solidus and liquidus temperatures and increase the corrosion susceptibility of the brazed joints. In certain cases, elements like magnesium, zinc or bismuth are added to improve the brazeability of the overall process [39]. Following this, typical Al 4xxx series filler materials used in industry are tabulated in table 3. Most common filler metals are Al , and In addition, the bonding strength is higher than the ultimate strength of the Al alloy - thus under this condition, the brazing process is certain to fail using the traditional AlSi filler metal.

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For this project, it is thus recommended that the Al-7SiCu-2Sn-1Mg filler material choice be considered despite it not being an established industry standard currently. In this process, the specification of heat exchanger dimensions will be defined iteratively until they meet the required performance characteristics. To aid in this highly iterative approach to meet the desired optimum conditions, computational software will be used in order to con- verge to a desired solution for a given set of constraints in this case the required performance characteristics.

These characteristics can be outlined as: 1. Required outlet temperatures both hot and cold side 2. Required outlet pressures both hot and cold side 3. Minimized entropy number 4. Minimized overall volume; and therefore mass In order to evaluate these characteristics, each item listed above 1 to 4 can be assigned a base equation needed for the numerical evaluation.

Although, many of these equations are dependant on surface geometrical properties and basic char- acteristics of the selected fin geometry for the PFHE. Since the offset strip fin was chosen as the ideal option in section 3. Following this, sections 4. Note: All equations outlined throughout this chapter are those that will be input into MathCAD for the convergence to the optimum solution please refer to figure 2. The lateral fin offset is generally uniform and equal to half the fin spacing including fin thickness. Figure 4. These are outlined in figure 4.

Referring to figure 4. The primary area consists of the plate area figure 4. The secondary fin area consists of the sum of the fin height area figure 4. Due to the overlap of edge widths between two offset fins, only half of the edge width area is available for heat transfer at the front and half at the back edge of each fin. Common fin thickness t typically range between 0. The parting sheet thickness tw usually involves a finite element analysis to determine the adequate thickness for sufficient resistance of tensile stresses due to pressure forces.

Due to the complexity associated with this, it falls beyond the scope of this project and thus a typical sheet thickness value between 0. With the initial correlations provided by Manson et al. However, one of the limitations of the Kays and London Colburn and friction factors is their accuracy. Following these initial developed correlations, Weiting obtained correla- tions for the laminar and turbulent flow regions but not for the transition region. The most comprehensive correlations for j and f factors have been de- veloped by Manglik and Bergles [33] as they describe the right trend in the heat transfer and friction loss behaviour of offset strip fins in laminar, transition, and turbulent flow regimes.

This is particularly useful as one equation may be used for flow in any of the above cases making the analysis much simpler. These correl- ations are shown in equations 4. Manglik and Bergles [45] provide the most accurate definition of the hydraulic diameter Dh specifically for OSF - one that differs from the traditional definition. This new definition shown in equation 4.

Vertical and lateral fin edges have also been accounted for thus extending the understanding of London and Shah and Joshi and Webb who considered only vertical edges [34]. This is typically due to the relationship of being inversely proportional to the core free-flow area Ao for fixed operating conditions.

The correlations for the j and f shown in equations 4. The area goodness factors of four geometries given in figure 4. An attempt will therefore be made to ensure the fin geometries for the OSF em- ployed in this project result in such values for the ratios. Mass Velocity The mass velocity G is given by equation 4. Following from the objective function outlined in equation 2. In order to evaluate equation 4. As a result, values pertaining to those specified in table 3. The following are the major assumptions made for the pressure drop analysis employed in the design; as adopted from Shah [7].

Flow is steady and isothermal, and fluid properties are independent of time. Fluid density is dependent on the local temperature and treated as a con- stant. The pressure at a point in the fluid is independent of direction. Body forces are caused only by gravity i. There are no energy sinks or sources along a streamline; thus no energy dissipation.

Equation 4. This expression accounts for three major parameters occurring in the heat exchanger during a typical heat transfer process namely; 1 a pressure drop at the core entrance due to sudden contraction; 2 a pressure drop within the core; and 3 a pressure rise at the core exit. The property ratio method is used for this wherein all properties are evaluated at the latest bulk temperature Tb , and then all variable-property effects are lumped into a correction factor.

This new bulk temperature is important because the initial value was evaluated at an ap- proximate value for the outlet temperature refer to table 3. Therefore, methodologies outlined in studies that implement system entropy min- imization for the entire ACM [3] [2] [4] will not be followed. Since this project focuses on the primary heat exchanger, it is sufficient to conduct an entropy min- imization only on this component as an isolated device from the larger installation of the ACM.

Recalling the entropy number by Bejan [10] as discussed in section 2. The review by Li et al. The structures, heat transfer enhancement mechanisms, advantages, and limitations are summarized, and an example of an application as a solar receiver is given. It also referred available correlations for heat transfer and friction factor developed by various researchers. Microchannels represent the next step in heat exchanger development due to their high heat transfer performance and reduced weight as well as their space, energy and materials savings potential.

Khan and Fartaj [ 12 ] made a survey of the published literature on the status and potential of microchannels, identifying research needs. They also developed an experimental infrastructure to investigate the heat transfer and fluid flow for a variety of working fluids in different microchannel test specimens. The selected areas discussed in this review are the ones that attracted more attention recently, namely compactness and downsizing without the loss of performance, which is crucial for the industry; theoretical developments; reducing fouling and corrosion of plates in sever processes, with a direct impact on operational cost; and using nanofluids.

In the paper by Wang et al. Extensive results from experiments and numerical simulations indicated that these heat exchangers have better flow and heat transfer performance than the conventional baffled heat exchangers, therefore allowing to save energy, reduce cost, and prolong service life and operation time in industrial applications. The heat pipes are accepted as an excellent way of saving energy due to the high heat recovery effectiveness of these devices.

A brief literature review was performed by Srimuang and Amatachaya [ 15 ] on the applications of heat pipes heat exchangers for waste heat recovery in both commercial and industrial applications. This chapter briefly discusses the importance of heat exchangers and their advanced applications in terms of energy efficiency and process intensification, minimizing environmental and societal impacts.

It highlights main research and findings from each contributed chapter of this book. It also provides key topics related to advanced features and applications of heat exchangers and corresponding reference sources. We believe that this book will be a useful reference source of information on advanced features and applications of heat exchangers.

Compact Heat Exchangers: Selection, Design, and Operation - John E. Hesselgreaves - Google книги

Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution 3.

Heat Exchanger Plates Explained (Industrial Engineering)

Help us write another book on this subject and reach those readers. Login to your personal dashboard for more detailed statistics on your publications. Edited by S. Sohel Murshed. We are IntechOpen, the world's leading publisher of Open Access books. Built by scientists, for scientists. Our readership spans scientists, professors, researchers, librarians, and students, as well as business professionals. Downloaded: Introduction This chapter aims to provide an outline of the various applications of heat exchangers, taking in particular attention related to advanced features as well as to briefly highlight the main aspects from each chapter contribution of this book.

Contributions highlight Each chapter contribution of this book has been briefly highlighted in this section. Other selective complementary sources A very general review on advances in heat transfer enhancements was performed by Siddique et al.

follow Materials The conventional heat exchangers are manufactured in metal such as stainless steel, copper and aluminum and have disadvantages in terms of weight and cost. Special design The efficient design of heat exchangers is more critical in some applications, requiring devices having superior performance and reliable mechanical characteristics at high pressure and high temperature and complying with geometric constraints.

Conclusions This chapter briefly discusses the importance of heat exchangers and their advanced applications in terms of energy efficiency and process intensification, minimizing environmental and societal impacts. More Print chapter. How to cite and reference Link to this chapter Copy to clipboard. K25 Unknown. More options. Find it at other libraries via WorldCat Limited preview. Bibliography Includes bibliographical references and index. While retaining the basic objectives and popular features of the bestselling first edition, the second edition incorporates significant improvements and modifications.

This second edition offers: introductory material on heat transfer enhancement; an application of the Bell-Delaware method; new correlation for calculating heat transfer and friction coefficients for chevron-type plates; and, revision of many of the solved examples and the addition of several new ones. The authors take a systematic approach to the subject of heat exchanger design, focusing on the fundamentals, selection, thermohydraulic design, design processes, and the rating and operational challenges of heat exchangers.

It introduces thermal design by describing various types of single-phase and two-phase flow heat exchangers and their applications and demonstrates thermal design and rating processes through worked examples, exercises, and student design projects. Much of the text is devoted to describing and exemplifying double-pipe, shell-and-tube, compact, gasketed-plate heat exchanger types, condensers, and evaporators.

Subject Heat exchangers. Bibliographic information.