Thermal Design Margins for Heat Exchangers

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Process Engineering Guide: GBHE-PEG-HEA-504

Thermal Design Margins for Heat Exchangers Information contained in this publication or as otherwise supplied to Users is believed to be accurate and correct at time of going to press, and is given in good faith, but it is for the User to satisfy itself of the suitability of the information for its own particular purpose. GBHE gives no warranty as to the fitness of this information for any particular purpose and any implied warranty or condition (statutory or otherwise) is excluded except to the extent that exclusion is prevented by law. GBHE accepts no liability resulting from reliance on this information. Freedom under Patent, Copyright and Designs cannot be assumed.



Process Engineering Guide: Thermal Design Margins for Heat Exchangers

CONTENTS SECTION 0 INTRODUCTION/PURPOSE 3 1 SCOPE 3 2 FIELD OF APPLICATION 3 3 DEFINITIONS 3 4 TERMINOLOGY 3 5 REASONS FOR SPECIFYING A DESIGN MARGIN 3 5.1 Instantaneous Rates 4 5.2 Future Uprating 4 5.3 Plant Upsets 4 5.4 Process Control 4 5.5 Uncertainties in Properties 4 5.6 Uncertainties in Design Methods 4 5.7 Fouling 4 6 COMBINATION OF DESIGN MARGINS 5

7 CRITICAL AND NON-CRITICAL DUTIES 5 7.1 General 5 7.2 Penalties of Over-design 6



8 OPTIMIZATION OF EXCHANGER DUTY 6 9 WAYS OF PROVIDING DESIGN MARGINS 6 9.1 The Provision of Excess Surface 7 9.2 Decreasing the Design Temperature Difference 7 9.3 Increasing the Design Process Throughput 7 9.4 Increasing the Design Fouling Resistance 8 9.5 Reducing the Design Process Outlet Temperature

Approach 8 9.6 Adjusting the Physical Properties 8

10 ACCURACY OF THE DESIGN METHODS FOR SHELL AND TUBE EXCHANGERS 8 10.1 Pressure Drop 8 10.2 Heat Transfer 9 11 SUGGESTED DESIGN MARGINS 10 11.1 No Phase Change Duties 10 11.2 Condensers 10 11.3 Boilers 10 12 EFFECT OF UNDER- OR OVER-SURFACE ON PERFORMANCE 10



FIGURES (1) EFFECT OF LENGTH ON EXCHANGER DUTY

COUNTERCURRENT FLOW, C* = 1.0 12 2 EFFECT OF NUMBER OF TUBES ON EXCHANGER

PERFORMANCE COUNTERCURRENT FLOW, C* = 1.0, ALL RESISTANCE IN TUBES 13

3 EFFECT OF TUBE LENGTH ON NUMBER OF TUBES,

AREA AND PRESSURE DROP 14 DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE 15



0 INTRODUCTION/PURPOSE This document is one of a series on heat transfer prepared for GBH Enterprises. When designing a heat exchanger it is usual to include some form of over-design or "safety factor" to allow for uncertainties in the design process. This can be done in many different ways, which have advantages and drawbacks. Unless the specifying engineer is aware of the implications of the chosen method, the effective safety margin may be different from the intention. 1 SCOPE This Guide explains the reasons for including a design margin, discusses the various ways in which one can be provided and comments on the relative merits of the different ways. It also gives some information on the accuracy of heat exchanger designs with special reference to shell and tube heat exchangers. 2 FIELD OF APPLICATION This Guide applies to process engineers in GBH Enterprises worldwide, who may be involved in the specification, design or rating of heat transfer equipment. 3 DEFINITIONS For the purposes of this Process Engineering Guide, the following definitions apply: HTRI Heat Transfer Research Incorporated. See GBHE-PEG-HEA-502. HTFS Heat Transfer and Fluid Flow Service. One of the suppliers of

thermal design software. See GBHE-PEG-HEA-502. With the exception of terms used as proper nouns or titles, those terms with initial capital letters which appear in this document and are not defined above are defined in the Glossary of Engineering Terms.



4 TERMINOLOGY Several alternative terms are commonly used to describe the "design margin", for example "safety factor", "% excess surface", "% over-design", "actual to required area ratio". 5 REASONS FOR SPECIFYING A DESIGN MARGIN An item of equipment may be designed to be larger than that needed to meet the average design throughput of the plant at design conditions for several reasons. The following list has been produced with heat exchangers in mind, but much of it is equally applicable to other items of equipment. 5.1 Instantaneous Rates The section of plant may be required to run at instantaneous rates above the normal plant throughput as part of the normal plant operation to allow for different availabilities of different sections of the plant. Designing for this condition does not represent a true design margin, as the higher rate represents a normal condition. 5.2 Future Uprating The engineer may wish to make provision for future plant uprating. If it is probable that the plant will be uprated at some future date, there may be a case for increasing the design throughput, with a corresponding increase in heat load. However, the heat transfer coefficient under the initial operating conditions will be lower than the design figure because of the lower velocities; the performance under the initial operating conditions should be checked to determine the expected safety margin at the initial conditions. Again, this does not represent a true design margin, as after the uprating there will be no margin left. Rather than installing the larger size unit initially, it may be preferable to make provision for increasing the size of the exchanger at some later date, either by replacing it with a larger unit, by adding an additional exchanger in parallel with the original one or by adding heat transfer enhancement devices.



5.3 Plant Upsets Variations in the inlet flowrates, temperatures or compositions of the feeds to the exchanger, due to disturbances in other parts of the plant, may require a duty above the nominal design. Although ideally such disturbances should be identified at the time when the duty was specified, and the worst case taken for design, it may be desirable to include an additional margin to allow for unforeseen disturbances. 5.4 Process Control The duty required from an exchanger may need to be above the steady state value in order to provide some control function for another piece of equipment. 5.5 Uncertainties in Properties Many heat exchangers are required to handle a complex mix of compounds where the physical properties of the mixture may be uncertain. This can result in errors in the required heat duty, the estimated heat transfer coefficients or the temperature driving force (by affecting the dew point of a condensing stream, for example). 5.6 Uncertainties in Design Methods In spite of improvements made over many years, there are still uncertainties in the predictive methods for heat transfer, especially for processes involving a phase change. It is generally advisable for a critical duty to provide some form of safety margin to allow for uncertainties in the design methods. 5.7 Fouling It is normal when specifying a heat exchanger duty to include the expected fouling resistances. The prediction of such resistances is not a precise science, often being more of a guess. As such, the fouling resistance may be considered itself to be a safety margin over the predicted clean performance, to allow for the (unpredictable) variation in fouling. However, it is normal to include an additional margin above that represented by the assumed fouling resistance. The extra pressure drop due to the fouling layer thickness should not be forgotten. See sub clause10.1.



6 COMBINATION OF DESIGN MARGINS Beware of over specifying design margins. During the course of a plant design, several stages occur between the overall concept and detailed equipment design. These might include: (a) Preparation of overall flowsheet. (b) Preparation of section flowsheet. (c) Specification of equipment duty. (d) Detailed equipment design. Each of these stages may be the responsibility of a different engineer. If each engineer adds his/her own design margin at each stage, the final item design may be considerably oversized for the duty. If the instantaneous section throughput has been increased to compensate for periods when the flowsheet rate cannot be achieved, it is unreasonable to design an air cooled exchanger for an ambient temperature which is exceeded for only a few hours each year. Frequently, there may be more than one type of uncertainty associated with the design of a heat exchanger, each of which might justify the inclusion of a design margin. For example, there may be uncertainties in fouling resistance, physical properties and ambient conditions. If the standard deviations for each area on uncertainty are d1, d2, d3, .... then the overall uncertainty of design will have a standard deviation of d0 where:

This is equivalent to saying that the combined margin for design M0 should be given by:

where M1, M2, M3 are the margins which would be applied for the uncertainties considered in isolation, expressed as fractional excess areas. This resulting margin will be less than that obtained by a straight summation of the individual margins. In particular, if the margin due to one particular factor is large compared with the others, then the other margins will be largely irrelevant.



(Strictly speaking, the above approach is only correct if the uncertainties follow a normal distribution, but it will be reasonable even if they do not). 7 CRITICAL AND NON-CRITICAL DUTIES 7.1 General Critical exchangers can be defined as: (a) Exchangers which, if they failed to perform as required, would have a

significant effect on plant safety; for example, the inability to control a potential runaway reaction.

(b) Exchangers which directly affect the plant production rate. Typical

examples are distillation column reboilers and condensers, some feed heaters and run-down coolers and some fired heaters.

Other exchangers will be "non-critical", if they do not perform as required then plant efficiency may be reduced and running costs increased with very little effect on plant production. Examples of these are compressor suction and interstage coolers, chillers, vacuum and refrigeration condensers and most interchangers. The distinction between "critical" and "non-critical" duties in some cases may be somewhat arbitrary. Ultimately, a trade-off needs to be made in some way between the cost of the exchanger and the consequences of under-design. .Whereas the provision of a suitable design margin for a critical duty may be necessary, this is not true for non-critical duties. In general, no margin should be provided for non-critical duties. 7.2 Penalties of Over-design Design margins are provided to compensate for uncertainties which could reduce the calculated performance. However, these uncertainties could be unfounded, or even act to improve performance, resulting in an oversized exchanger for the duty. In most cases, the only penalty associated with an oversized exchanger is the extra capital cost. However, there may be cases where an oversized exchanger can have positively harmful effects.



If the oversized unit is part of an exchanger network, its over-performance could result in problems with other exchangers in the network. It may be possible to overcome this by the provision of suitable control schemes, such as bypassing of some of the fluid round the unit, but this could result in other problems, such as excessive fouling. In the extreme, it may require the installation of additional trim heaters or coolers, thus detracting from the benefits of the network. Thermosyphon boilers can present particular problems. The turndown ratio of such units is limited; typically a 3:1 turndown is the most that can be achieved without running into problems with stability or total failure to circulate. The performance and stability of such units is influenced not only by the installed area but by also by the design of the circulation pipework and the distribution of pressure drop around the circuit. For steam heated boilers, it may also be necessary to run with sub-atmospheric steam to achieve turn-down conditions with an oversized boiler. This can lead to problems of condensate removal. It is imperative that the designer carry out performance runs for the design for the complete operating range under both clean and fouled conditions. For more information on vertical thermosyphon boilers, see GBHE-PEG-HEA-515 8 OPTIMIZATION OF EXCHANGER DUTY So far, it has been assumed that the required duty of the exchanger is fixed, and the margin is required to ensure that this duty can be met. The exchanger designer will then try to produce the "best" design which meets the duty within the constraints, with some agreed margin. However, in many cases, the duties of specific exchangers within the process are not fixed a priori. The duty of these exchangers should be optimized by the correct trade-off between capital and running costs, possibly with the assistance of heat exchanger network design methods. Discussion of these methods is beyond the scope of this guide. Many such exchangers can be classified as "non-critical". For these, designs based on normal fouling resistances without additional margins should be considered.



9 WAYS OF PROVIDING DESIGN MARGINS A thermal design margin (safety factor) may be provided in several different ways, which have their own advantages and disadvantages. It is important that the engineer understands the implications of these. The engineer should be wary of disclosing design margins to a supplier who is to perform the design, as the latter may be tempted to design with negative margins in order to maintain a competitive position, knowing that in many cases, actual performance checks under design conditions may be difficult or impossible. Because of this, it may be advisable to produce a separate data sheet to send to the manufacturer, on which certain items have been removed or altered. This sheet should be included, suitably annotated, in the plant manual, along with the correct data sheets, so that the true situation is recorded. 9.1 The Provision of Excess Surface Excess surface may be provided in one of two ways: (a) Adding extra surface in parallel:

Providing the extra surface by increasing the number of tubes or passages per pass over that theoretically necessary is generally unsatisfactory for cases where convective transfer is the dominant mechanism. It will result in a more expensive unit but because of the reduced velocity, and hence coefficient, there may be little effective increase in performance. See Clause 12 for more information.

(b) Adding extra surface in series:

Increasing the flowpath, by increasing the exchanger length or the number of passes, is generally more satisfactory. This will, however, increase the pressure drop, and it may be necessary to increase the number of passages as well, to restore the pressure drop to the desired value. A check on the predicted performance of the oversized exchanger will confirm the actual pressure drop to be expected.

Note that the "area ratio" obtained when rating an exchanger, or the "percentage over-design" obtained when rating an exchanger, is an indication of the extra length of exchanger above that required. It is not possible to use this approach without declaring it to the manufacturer.



9.2 Decreasing The Design Temperature Difference Sometimes a higher air or cooling water inlet temperature is specified for critical services than for non-critical duties. This suffers from the disadvantage that the actual margin on performance at normal air or water temperatures will depend on the required product temperature. A refrigerant condenser designed using this approach might have a 25% margin; for a reactor cooler/condenser, with a higher outlet temperature, it might be only 5%. The specification of design ambient temperature for air cooled heat exchangers is discussed in sub clause 3.5 of GBHE-PEG-HEA-513. It should be used to ensure that a critical unit is designed to meet its duty on warm days, but it is not recommended to use this parameter to control design margins at other ambient conditions. This approach can be useful when designing vertical thermosyphon reboilers. Because of the coupling between heat transfer performance and circulation, the extra length concept cannot safely be used. It is better to design the unit for operation with a lower steam pressure, and hence condensing temperature, than is available. A check on the predicted performance with the higher steam pressure will give the maximum heat duty possible; the ratio of this to the desired duty is a measure of the safety margin. However, beware that the higher steam pressure does not result in problems such as film boiling, particularly under clean conditions. See GBHE-PEG-HEA-515 for more details. 9.3 Increasing the Design Process Throughput As a means of providing a design margin, this suffers from the same disadvantage as increasing the number of tubes, namely that under normal conditions the tubeside performance will be poorer than design, so the margin may be less than expected. If this approach is used, and the higher throughput is not actually likely to occur, the allowable pressure drop supplied to the manufacturer should be increased above the actual value by the square law, in order that he be not unduly constrained. As the unit will end up being designed for a flowrate above that at which the plant will run, it will not be possible to do performance checks at design conditions.



9.4 Increasing the Design Fouling Resistance This reduces the overall heat transfer coefficient, hence resulting in a larger surface area being selected for the exchanger. The designer will seek to minimize the area, within the constraints of allowable pressure drop; the film coefficients used will not be affected by the "safety margin" as is the case for using an increased throughput. The approach is useful when dealing with a manufacturer, as it enables the safety margin to be hidden from him. However, it is good practice to disclose the actual safety margins in the final documentation, so the expected fouling resistance should be recorded in the final revisions of the data sheets. 9.5 Reducing the Design Process Outlet Temperature Approach In many ways this is the most satisfactory form of safety margin, and it does allow the final unit to be checked against design conditions. However, it suffers from the same drawback as does raising the design air or water temperature, in that the margin will appear greater for units with a low outlet temperature. 9.6 Adjusting the Physical Properties If there is uncertainty in the physical properties, it may be worth considering adjusting the values used. However, some care has to be taken over how this is done. Ideally, a sensitivity analysis ought to be performed on the effects of all properties on the predicted performance. In practice, this is unrealistic; even using only two values for each of the main properties for a two phase system (latent heat, quality, specific heat, viscosity and thermal conductivity for each phase) As a general rule, a "safe" design will be produced if the heat load and viscosity are overestimated and the thermal conductivity, specific heat and density are underestimated. The percentage error in the prediction due to an error in any one of these properties will be less than the percentage error in the property, typically around one half. Note that for single phase cases, the specific heat and heat load cannot be specified independently. For these cases, the "safe" design results from overestimating the specific heat.



10 ACCURACY OF THE DESIGN METHODS FOR SHELL AND TUBE EXCHANGERS

It was stated earlier that one of the reasons for providing a design margin was due to uncertainty in the design methods used. This Clause gives some indication as to the likely magnitude of such errors. 10.1 Pressure Drop 10.1.1 Tubeside Flow For single phase flow in clean heat exchanger tubes, the estimated pressure drop is likely to be accurate to within 2%, assuming the physical properties are known. The pressure drop in a fouled tube may be significantly higher, and may not be estimated correctly by the computer programs used for exchanger design. There are two factors resulting from fouling which are important here.

(a) Firstly, the fouling layer reduces the effective bore of the tube. For a smooth tube, in turbulent single phase flow, the pressure drop is inversely proportional to the diameter raised to the power 4.75. Thus, a dirt layer which reduces the bore by 10% will increase the pressure drop by 65%, all other things being equal.

(b) Secondly, the dirt layer is likely to increase the relative roughness

of the tube. The roughness of moderately rusty carbon steel is typically 10 times that of clean steel. The effect this has on pressure drop increases with Reynolds number. At a Reynolds number of 10,000 it will give an increase in pressure drop of about 30-40%, at Re=100,000 about 90-100% and at Re=1,000,000 about 250%. These increases will be compounded with those due to the reduction in bore.

Commercially available programs, allow the user to input the thickness of the fouling layer, and use this to determine the effective tube diameter for pressure drop calculations. However, these calculations take no account of the effect of fouling on roughness. Some programs make no allowance for the effect of fouling on pressure drop.



The basic correlations for two phase pressure drop have considerably greater error than is the case for single phase flow, even without considering the effects of fouling. Errors of up to a factor of 2 in the estimated frictional pressure drop in smooth tubes may occur. However, the effects of surface roughness are less pronounced for two phase systems. 10.1.2 Shell-side Flow Shell-side flow is considerably more complex than tubeside flow. The models used in some commercially available programs are based on a method usually known as "stream analysis". The shell-side flow is divided between five parallel routes: tube-to-baffle leakage, cross-flow over the bundle, bypassing round the outside of the bundle, baffle-to-shell leakage and pass-partition lane leakage. (referred to in some programs as the "A", "B", "C", "E" and "F" streams respectively.) The models adjust the flow split until the calculated pressure drops for each stream are equal. The relative magnitudes of these streams affect not only the heat transfer, but also the pressure drop. It is not possible to give simple guidance here on the magnitude of such effects. However, some feel for the problem can be obtained by performing computer runs with different clearances. This simulates the blockage of the leakage paths by fouling deposits. It is not unusual for the predicted pressure drop to double if the baffle-to-shell and tube-to-baffle clearances are reduced from the normal design figures to zero. Some programs will adjust the clearances between tube and baffle to allow for the effect of the fouling layer thickness input by the user. Some programs do not; in order to simulate the fouled condition, the clearance has to be input. Note that in some programs a value of zero clearance may be interpreted as a request for the default value. If zero clearance is required, it may be necessary to input a small number. See program manuals of the software for more detail. 10.2 Heat Transfer

10.2.1 Tube Side

For single phase turbulent flow in tubes, the ESDU correlation, has a claimed (root mean square) error of 10.2%. Similar accuracy can be expected from other correlations. For transitional and laminar flow, higher errors can be expected.



10.2.2 Shell Side

As for pressure drop, heat transfer predictions for the shell side of shell and tube exchangers are complicated by the flow distribution. Some programs suggest an accuracy of 25% for single phase turbulent flow.

11 SUGGESTED DESIGN MARGINS The choice of design margin ultimately lies with the process engineer and the designer and will be influenced by the nature of the process and the criticality of the exchanger in question. The degree of uncertainty in the fouling resistances should also be taken into consideration. The figures given in 11.1 to 11.3 should be regarded only as guides. 11.1 No Phase Change Duties For non-critical duties, a margin of 0 - 5% on the exchanger length should be adequate, and it may be worth even considering small negative margins if this leads to a design which fits in better with any standard tube length chosen for the process. For more critical duties, a value of 5-10% is appropriate. 11.2 Condensers For pure component systems, the condensing coefficient is unlikely to be limiting; the values given above for single phase cases will be appropriate. For multi-component cases, values around 10% for non-critical duties and 20% for critical duties are suggested, the margin being provided by extra tube length. 11.3 Boilers For boilers, it is generally worth considering the design margin in terms of the ratio of the maximum to desired evaporation capacity. For cases where the boiling resistance is dominant, particularly for multi-component systems, a margin of 10-20% is recommended.



12 EFFECT OF UNDER- OR OVER-SURFACE ON PERFORMANCE An exchanger which has less than the necessary surface will have a lower heat duty than required. However, the effect is generally not directly proportional to the shortfall in surface; an exchanger with only 90% of the required surface will generally perform more than 90% of the required duty. For single phase duties it is possible to estimate the effects of over- or under-surface on exchanger performance from a theoretical analysis. The change in heat load for a given change in surface area depends on the exchanger pass arrangement, the ratio of the product of heat capacity and flowrate for the hot and cold streams, C*, and the number of heat transfer units in the exchanger, NTU, (NTU = U.A/Cmin, where U is the overall heat transfer coefficient, A is the area and Cmin is the product of heat capacity and flowrate for the stream showing the greater temperature change.) The NTU value is a measure of the "thermal length" of the exchanger; duties with a large temperature overlap between the streams have a large value of NTU. Figure 1 shows the effect of changes in exchanger length on heat load for a pure countercurrent exchanger with C* = 1. It can be seen that as the number of transfer units for the base case is increased, the effect on performance of a given fractional change in length reduces. For a duty requiring an NTU value of 5, a 50% increase in length only results in a 6% increase in duty. Conversely, an exchanger of only the necessary length is still capable of 85% of the required duty. Figure 2 shows the effects on performance of changes in the number of tubes. Again, a pure countercurrent flow is assumed, with C* = 1. For this case, all the thermal resistance is assumed to occur on the tubeside. It can be seen that the performance is very insensitive to the number of tubes. Even for a very low value of NTU, a 50% increase in the number of tubes gives only 8.5% improvement in performance, whilst for NTU=5 the improvement is only 1.3%. This confirms what was said above, that the provision of a design margin by adding additional surface in parallel is not a good policy. The above analysis is based on several assumptions, including a constant heat transfer coefficient along the exchanger and linear temperature/enthalpy relationships. It is less easy to perform a theoretical analysis for cases involving multi-component phase change, but experience suggests that a similar behavior can be expected. For example, the heat transfer performance of vertical tubeside inerts condensers is generally insensitive to the number of tubes, but does depend significantly on tube length.



FIGURE 1 EFFECT OF LENGTH ON EXCHANGER DUTY COUNTERCURRENT FLOW, C* = 1.0



FIGURE 2 EFFECT OF NUMBER OF TUBES ON EXCHANGER PERFORMANCE COUNTERCURRENT FLOW, C* = 1.0, ALL RESISTANCE IN TUBES



FIGURE 3 EFFECT OF TUBE LENGTH ON NUMBER OF TUBES, AREA AND PRESSURE DROP



DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE This Process Engineering Guide makes reference to the following documents: GBH ENTERPRISES ENGINEERING GUIDES Glossary of Engineering Terms (referred to in Clause 3). GBHE-PEG-HEA-502 Computer programs for the thermal

design of heat Exchangers (referred to in Clause 3 and 9.1).

GBHE-PEG-HEA-515 The design and layout of vertical

thermosyphon reboilers (referred to in 9.2).

GBHE-PEG-HEA-513 Air cooled heat exchangers

(referred to in 9.2).

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Thermal Design Margins for Heat Exchangers

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