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5.9. Special Considerations As the boil up rate is very sensitive to the T it is necessary to determine the range of the T to provide for the anticipated boil up range under both clean and fouled conditions. This T range should be within the controllable limits of the heating media. The boil up is basically controlled by varying the heating medium temperatures. For a condensing media a pressure control is used and for a sensible heat source media a by-pass control is used but here both temperature difference and heat transfer coefficient of the heating medium changes. For a condensing media, a problem arises when boiling at low loads with a clean vaporizer that the required pressure may be so low (or a vacuum) that condensate removal becomes difficult. For a sensible heat source, the amount of by-pass may so reduce the flow in the vaporizer that problems in distribution or accelerated fouling may occur. The vaporizer should have liquid in it and be started up at the lowest T possible. It is possible in those cases of large fouling allowances, excessive conservatism in design, or underestimated coefficients that the design T can be large enough for a clean reboiler to get into the film boiling regime. The design flux may also be low enough so that it can be met in the transition or film boiling modes. Under these conditions the effect of T is reversed from that of nucleate boiling thus the controls will be ineffective and the rate of fouling may be increased. If a condensing medium pressure becomes too low at low loads or clean condition, then a partial flooding of the surface should allow increasing the pressure into a controllable range. The surface should be flooded a fixed amount. Trying to control the boil up by varying the amount of flooding is difficult due to the slow response. With partial flooding consider the effect of stresses especially in horizontal units where the stresses are different in the flooded and unflooded tubes. Wide boiling range mixtures, where the boiling point of the heavy component exceeds the heating medium temperature, require reboilers in which circulation occurs even in the convective heat transfer region otherwise the light component will be stripped out and stagnation will occur. Operation near the critical pressure have several problems. The density difference between vapor and liquid is low so the driving head for circulation is low. The maximum heat flux and the critical T are also low. Operating under film boiling conditions might be considered as a possible solution. Low pressure (vacuum) operation requires careful analysis of the effect of static head on the boiling point and the possible temperature pinch thus caused at the top of the liquid zone. This boiling point elevation also results in much greater liquid preheat lengths and also reduces the available head for circulation. The large density differences cause higher acceleration pressure losses which result in reduced circulation. Low pressures require larger cavities for nucleation thus suppressing nucleate boiling coefficients, but the use of enhanced surfaces can be effective. Low T operation (8F) may sometimes be necessary because of process economics. Here the problem is nucleation and the use of enhanced surfaces is required. However, prediction of heat transfer coefficients is very uncertain and experimental tests on the specific surface liquid combination are recommended. Boiling curves developed from a small single tube pool boiling apparatus will provide a guide to the basic heat transfer coefficients which can be modified as necessary from the above discussed methods.

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Very high T is often encountered when vaporizing a low boiling point liquid; e.g., ammonia, with low pressure steam and here another problem of potential freezing of the heating medium must be considered. An available high temperature medium is another cause for high T. At high T the three limitations of film boiling, mist flow, and instability must be evaluated by the methods given above. Instability can be controlled within limits by throttling of the recirculation line or can be avoided by a shellside reboiler. Mist flow only represents a low heat transfer coefficient. Film boiling is a possible mode of operation if the potential for transition boiling is avoided where the control criteria are reversed. A possible solution to transition and fihn boiling is the use of medium- to high-finned tubes where the temperature drop along the fins is such that nucleate boiling occurs near the fin tips. Boiling on fins has been tested in the laboratory but no publication of a commercial application has been found. 5.9.1. Examples of Design Problems To design a heat exchanger requires that one first specify essentially all the dimensions of an exchanger and then one can proceed with the heat transfer calculations to determine whether the assumed design will give the desired performance and, if not, then the initial design is modified and the calculations repeated until an acceptable match of design and performance is obtained. A design problem is, therefore, a series of rating problems. In the rating of an exchanger its dimensions are known and only the heat transfer calculations are required; however, for vaporizers these calculations still involve considerable iterative trials to converge on an answer. It is obvious that the better the initial guess matches the final design the less the amount of calculation; hence, here is where experience is beneficial to the designer. In an attempt to aid a novice designer each of the examples below show in the initial steps one way of getting an initial design. In the second example, 5 steps are also used, however, in step 4 the guess of fraction vaporized is based on experience or as in this case prior knowledge of the experimental value.

TABLE OF CONTENTSChapter 1Basic Heat Transfer1.1. Basic Mechanisms of Heat Transfer1.1.1. Conduction1.1.2. Single Phase Convection1.1.3. Two Phase (LiquidGas/Vapor) Flow1.1.4. Condensation1.1.5. Vaporization1.1.6. Radiation

1.2. Basic Heat Exchanger Equations1.2.1. The Overall Heat Transfer Coefficient1.2.2. The Design Integral

1.3. The Mean Temperature Difference1.3.1.The Logarithmic Mean Temperature Difference1.3.2. Configuration Correction Factors on the LMTD

1.4. Construction of Shell and Tube Heat Exchangers1.4.1. Why a Shell and Tube Heat Exchanger?1.4.2. Basic Components of Shell and Tube Heat Exchangers1.4.3. Provisions for Thermal Stress1.4.4. Mechanical Stresses1.4.5. The Vibration Problem1.4.6. Erosion1.4.7. Cost of Shell and Tube Heat Exchangers1.4.8. Allocation of Streams in a Shell and Tube Exchanger

1.5. Application of Extended Surfaces to Heat Exchangers1.5.1. The Concept of the Controlling Resistance1.5.2. Types of Extended Surface1.5.3. Fin Efficiencies and Related Concepts1.5.4. The Fin Resistance Method1.5.5. Some Applications of Finned Tubes.

1.6 Fouling in Heat Exchangers1.6.1. Typical Fouling Resistances.1.6.2. Types of Fouling1.6.3. Effect of Fouling on Heat Transfer1.6.4. Materials Selection for Fouling Services1.6.5. Removal of Fouling

NomenclatureBibliography

Chapter 2Sensible Heat Transfer2.1. Heat Exchangers with Low and MediumFinned Trufin2.1.1 Areas of Application2.1.2. Description of Low and MediumFinned Trufin

2.2. Basic Equations for Heat Exchanger Design2.2.1. The Basic Design Equation and Overall Heat Transfer Coefficient2.2.2. Fin Efficiency and Fin Resistance2.2.3. Mean Temperature Difference, F Factors

2.3. Heat Transfer and Pressure Drop During Flow Across Banks of Trufin Tubes2.3.1. Heat Transfer In Trufin Tube Banks2.3.2. Pressure Drop During Flow Across Banks of LowFinned Trufin Tubes2.3.3. Effect of Fouling on Trufin

2.4. Heat Transfer and Pressure Drop Inside Tubes2.4.1 Heat Transfer And Pressure Drop In Single Phase Flow Inside Round Tubes2.4.2. Heat Transfer in TwoPhase Flow Inside Tubes

2.5. Preliminary Design of Shell and Tube Heat Exchangers2.5.1 Basic Principles of Design2.5.2. Preliminary Design Decisions2.5.3. Procedure for Approximate Size Estimation

2.6. Delaware Method for ShellSide Rating of Shell and Tube Heat Exchangers2.6.1. Introduction2.6.2. Calculation of ShellSide Geometrical Parameters2.6.3. ShellSide Heat Transfer Coefficient Calculation2.6.4. ShellSide Pressure Drop Calculation

2.7. Examples of Design Problems for Low and MediumFinned Trufin in Shell and Tube Heat Exchangers2.7.1. Design Of A Compressor Aftercooler2.7.2. Design of A Gas Oil to Crude Heat Recovery Exchanger

NomenclatureBibliography

Chapter 3Trufin Tubes in Condensing Heat Transfer3.1. Trufin Tubes in Condensing Heat Transfer3.1.1. Modes of Condensation3.1.2. Areas of Application3.1.3. Types of Tubes Available

3.2. Condensation of Vapor Inside High-Finned Trufin Tubes3.2.1. VaporLiquid TwoPhase Flow3.2.2. Condensation Heat Transfer3.2.3. Mean Temperature Difference for InTube Condensation

3.3. Condensation of Vapor Outside Low and MediumFinned Trufin Tubes3.3.1. Shell and Tube Heat Exchangers for Condensing Applications3.3.2. The Basic Design Equations3.3.3. Mean Temperature Difference3.3.4. Condensation of a Superheated Vapor3.3.5. Condensation with Integral Subcooling On the Shellside3.3.6. Filmwise Condensation on Plain and Trufin Tubes3.3.7. Filmwise Condensation on Tube Banks3.3.8. Pressure Drop during Shell Side Condensation

3.4. Examples of Design Problems for Low- and Medium-Finned Trufin in Shell and Tube Condensers3.4.1. Condenser Design for a Pure Component: Example Problem3.4.2. Condenser Design for a Multi-component Mixture: Example Problem

NomenclatureBibliography

Chapter 4Trufin Tubes in Air-cool Heat Exchangers4.1. Heat Exchangers with HighFinned Trufin Tubes4.1.1. Areas of Application4.1.2. HighFinned Trufin4.1.3. Description of Equipment

4.2. Heat Transfer with HighFinned Trufin Tubes4.2.1. Fin Temperature Distribution and Fin Efficiency4.2.2. Effect of Fouling on HighFinned Trufin4.2.3. Contact Resistance in Bimetallic Tubes

4.3. Heat Transfer and Pressure Drop in HighFinned Trufin Tube Banks4.3.1. Heat Transfer Coefficients in Crossflow4.3.2. Mean Temperature Difference in Crossflow4.3.3. Pressure Drop in Crossflow4.3.4. Other AirSide Pressure Effects

4.4. Preliminary Design Procedures4.4.1. Principles of the Design Process4.4.2. Selection of Preliminary Design Parameters4.4.3. Fundamental Limitations Controlling AirCooled Heat Exchanger Design

4.5. Final DesignNOMENCLATUREBIBLIOGRAPHY

Chapter 5Trufin Tubes in Boiling Heat Transfer5.1. Trufin in Boiling Heat Transfer5.1.1. Pool Boiling Curve5.1.2. Nucleation5.1.3. Nucleate Boiling Curve5.1.4. Maximum or Critical Heat Flux5.1.5. Film Boiling5.1.6. Boiling Inside Tubes5.1.7. Subcooling and Agitation

5.2. Vaporizers Types and Usage5.2.1. General5.2.2. Boiling Outside Tubes5.2.3. Boiling Inside Tubes5.2.4. Other Types of Evaporators

5.3. Boiling Heat Transfer5.3.1. Pool Boiling Single Tube5.3.2. Single Tube in Cross Flow5.3.3. Boiling on Outside of Tubes in a Bundle5.3.4. Boiling Inside Tubes5.3.5. Boiling of Mixtures

5.4. Falling Film Heat Transfer5.4.1. Vertical InTube Vaporizer5.4.2. Horizontal ShellSide Vaporizer5.4.3. Dry Spots Film Breakdown

5.5. Special Surfaces5.5.1. Boiling on Fins5.5.2. Mean Temperature Difference

5.6. Pressure Drop5.6.1. TubeSide Pressure Drop5.6.2. ShellSide Pressure Drop

5.7. Fouling5.8. Design Procedures5.8.1. Selection of Reboiler Type5.8.2. Pool Type Reboilers5.8.3. Intube or Thermosyphon Reboilers

5.9. Special Considerations5.9.1. Examples of Design Problems

5.10. Example of Design Problems for Trufin in Boiling Heat Transfer5.10.1. Design Example Kettle Reboiler5.10.2. InTube Thermosyphon Example Problem5.10.3. Boiling Outside Trufin Tubes Example Problem

NOMENCLATUREBIBLIOGRAPHY