ch5_5

5.5. Special Surfaces Commercially available special surfaces used to enhance the boiling side heat transfer are finned surfaces, special plated surfaces, and porous surfaces produced by electroplating, sintering, or machining. The Trufin tubes, used for boiling and condensation enhancement, usually have 16 to 40 fins/inch, approximately 1/16 inch high and resemble a screw thread. The basic purpose is to increase the surface per unit length that is exposed to the boiling liquid and an area ratio increase of 2.2 to 6.7 is obtained. The fins are formed by an extrusion process and are available in most metals. The finning process also changes the surface nucleation characteristics and an improved surface factor is reported by Palen et al. (49). Although the heat transfer coefficients are high the fin efficiency is still high because of the very short fin length. For the high conductivity metals, such as copper, the fin efficiency is almost 100% but for low conductivity metals such as stainless steel the efficiency may drop into the 70-80% range. Surface enhancement is not a universally acceptable solution to the improved performance of reboilers. Careful consideration must be given to the particular operating conditions. Some of the factors to be evaluated are as follows. (1) These are expensive surfaces and cost comparisons should be made. (2) These surfaces are available only for a limited number of metals and the corrosion requirements of the system need evaluation. (3) The boiling range and fouling or corrosive characteristics of the liquids could significantly affect the final performance. Implied here is the ability to clean these surfaces. (4) The performance of a tube bundle can be significantly different from the performance of a single tube as described by Palen et al. (49). Here the relative effect of nucleate boiling to two-phase convection heat transfer needs to be determined. Further, apparent comparisons of tube bundle performance vs. single tubes needs a careful consideration of the effect of bundle layout, tube pitch, etc. on the circulation rate, hence two phase heat transfer, about which we are only beginning to understand. (5) The improvement of the boiling coefficient may not improve overall performance if the heating medium, fouling, or tube wall coefficients are limiting. The major application of enhanced surfaces is in boiling clean liquids at low temperature differences. Trufin tubes depend more on surface increase and seem less subject to problems of fouling and wide boiling range liquids. The maximum heat flux appears to equal that of a plain tube based on the projected area. Enhanced surfaces find industrial applications for two reasons: (a) For a given temperature difference the heat duty will be two or three times higher than for plain tubes at low ΔT. This can result in smaller reboilers with savings in space and weight. At high ΔT the relative performance of enhanced vs. plain tubes is less. (b) For a given duty the required temperature difference will be smaller than for plain tubes. This is of great importance when the cost of other equipment, such as compressors, and operating costs are considered. Heat transfer performance for some commercially available enhanced surface tubes are described by Yilmaz (50). 5.5.1. Boiling on Fins One of the problems of boiling from fins is the determination of fin efficiency. As the nucleate boiling coefficient is strongly dependent upon the temperature difference, which in a fin is varying along its length, the calculation of a fin efficiency requires a stepwise computer program. Fin efficiency calculations with a linear variation along the length were derived by Han (51) and Chen (52). A closer approach to 268 boiling conditions was made by Cumo (53) who made a numerical solution for the case where the heat flux was proportional to the third power of the local wall to liquid temperature difference. Haley and Westwater (54) solved a one dimensional general conduction equation. The effect of fin clearances was studied by Westwater (55) who found that bubble size was a factor but a 1/16-in clearance at atmospheric pressure was sufficient to avoid interference. 5.5.2. Mean Temperature Difference In all boiling processes the liquid is superheated with reference to the vapor saturation temperature but the effect of superheat on the temperature difference calculation is neglected. The amount of superheat cannot be predicted and is usually small compared to the ΔTsat; thus this effect is ignored. For in-tube boiling the heat transfer coefficient calculational procedure requires the division of the tube into zones for each flow regime which in turn requires calculating the heat transferred zone by zone and thus the temperature difference is also included in the calculations. The net result is no separate calculation of a mean temperature difference is made for in-tube vaporization. For boiling outside of tubes the determination of a mean temperature difference is an arbitrary choice based on the amount of subcooling, the boiling range, and to some extent the geometry of the reboiler. The problem is the degree of mixing that occurs in the shell which depends on the circulation rate. Very little is presently known about shell-side circulation and methods for estimating the circulation rates are only in early stages of development. As shown by Palen et al. (32) in Figure 5.31 the bulk temperature within the tube bundle varies with length and the effective ΔT differs substantially from the assumed ΔT or the ΔT based on exit temperatures; thus depending upon the assumptions made, several methods of calculating the mean temperature difference can be used. Some of these choices are: (1) For a pure component or a narrow boiling range mixture, the vapor saturation temperature is used, and if a single component condensing vapor is the heating medium, the temperature difference is this difference. If the heating medium is transferring sensible heat, then a log mean of the vapor saturation temperature and the heating medium terminal temperatures is used. (2) For a wide boiling range mixture or where the sensible heat load is a substantial fraction of the total heat load, a counterflow log mean ΔT gives optimistic results (32, 34) and an LMTD based on exit vapor temperature is recommended. (3) When the effect of static head on the boiling point is significant; e.g., in large bundles and/or in vacuum service, then the boiling temperature should be based on the mean pressure in the bundle including the imposed static head. (4) When a horizontal thermosyphon reboiler is used, a counterflow LMTD is very optimistic and use of a cocurrent flow LMTD is suggested. Since experimentally measured boiling coefficients, and any resulting correlations, are dependent on the temperature differences used to calculate these coefficients from the basic data, then the same LMTD method should be used in the design of reboilers when using these data or correlations. However, the 269 generalized correlations given above have such a spread of data that the temperature difference determination is a minor factor in the data spread and the MTD suggestions in the above paragraph should be followed. 270 TABLE OF CONTENTS Chapter 1 Basic Heat Transfer 1.1. Basic Mechanisms of Heat Transfer 1.1.1. Conduction 1.1.2. Single Phase Convection 1.1.3. Two Phase (Liquid˚Gas/Vapor) Flow 1.1.4. Condensation 1.1.5. Vaporization 1.1.6. Radiation 1.2. Basic Heat Exchanger Equations 1.2.1. The Overall Heat Transfer Coefficient 1.2.2. The Design Integral 1.3. The Mean Temperature Difference 1.3.1. The Logarithmic Mean Temperature Difference 1.3.2. Configuration Correction Factors on the LMTD 1.4. Construction of Shell and Tube Heat Exchangers 1.4.1. Why a Shell and Tube Heat Exchanger? 1.4.2. Basic Components of Shell and Tube Heat Exchangers 1.4.3. Provisions for Thermal Stress 1.4.4. Mechanical Stresses 1.4.5. The Vibration Problem 1.4.6. Erosion 1.4.7. Cost of Shell and Tube Heat Exchangers 1.4.8. Allocation of Streams in a Shell and Tube Exchanger 1.5. Application of Extended Surfaces to Heat Exchangers 1.5.1. The Concept of the Controlling Resistance 1.5.2. Types of Extended Surface 1.5.3. Fin Efficiencies and Related Concepts 1.5.4. The Fin Resistance Method 1.5.5. Some Applications of Finned Tubes. 1.6 Fouling in Heat Exchangers 1.6.1. Typical Fouling Resistances. 1.6.2. Types of Fouling 1.6.3. Effect of Fouling on Heat Transfer 1.6.4. Materials Selection for Fouling Services 1.6.5. Removal of Fouling Nomenclature Bibliography Chapter 2 Sensible Heat Transfer 2.1. Heat Exchangers with Low˚ and Medium˚Finned Trufin 2.1.1 Areas of Application 2.1.2. Description of Low˚ and Medium˚Finned Trufin 2.2. Basic Equations for Heat Exchanger Design 2.2.1. The Basic Design Equation and Overall Heat Transfer Coefficient 2.2.2. Fin Efficiency and Fin Resistance 2.2.3. Mean Temperature Difference, F Factors 2.3. Heat Transfer and Pressure Drop During Flow Across Banks of Trufin Tubes 2.3.1. Heat Transfer In Trufin Tube Banks 2.3.2. Pressure Drop During Flow Across Banks of Low˚Finned Trufin Tubes 2.3.3. Effect of Fouling on Trufin 2.4. Heat Transfer and Pressure Drop Inside Tubes 2.4.1 Heat Transfer And Pressure Drop In Single Phase Flow Inside Round Tubes 2.4.2. Heat Transfer in Two˚Phase Flow Inside Tubes 2.5. Preliminary Design of Shell and Tube Heat Exchangers 2.5.1 Basic Principles of Design 2.5.2. Preliminary Design Decisions 2.5.3. Procedure for Approximate Size Estimation 2.6. Delaware Method for Shell˚Side Rating of Shell and Tube Heat Exchangers 2.6.1. Introduction 2.6.2. Calculation of Shell˚Side Geometrical Parameters 2.6.3. Shell˚Side Heat Transfer Coefficient Calculation 2.6.4. Shell˚Side Pressure Drop Calculation 2.7. Examples of Design Problems for Low˚ and Medium˚Finned Trufin in Shell and Tube Heat Exchangers 2.7.1. Design Of A Compressor Aftercooler 2.7.2. Design of A Gas Oil to Crude Heat Recovery Exchanger Nomenclature Bibliography Chapter 3 Trufin Tubes in Condensing Heat Transfer 3.1. Trufin Tubes in Condensing Heat Transfer 3.1.1. Modes of Condensation 3.1.2. Areas of Application 3.1.3. Types of Tubes Available 3.2. Condensation of Vapor Inside High-Finned Trufin Tubes 3.2.1. Vapor˚Liquid Two˚Phase Flow 3.2.2. Condensation Heat Transfer 3.2.3. Mean Temperature Difference for In˚Tube Condensation 3.3. Condensation of Vapor Outside Low˚ and Medium˚Finned Trufin Tubes 3.3.1. Shell and Tube Heat Exchangers for Condensing Applications 3.3.2. The Basic Design Equations 3.3.3. Mean Temperature Difference 3.3.4. Condensation of a Superheated Vapor 3.3.5. Condensation with Integral Subcooling On the Shellside 3.3.6. Filmwise Condensation on Plain and Trufin Tubes 3.3.7. Filmwise Condensation on Tube Banks 3.3.8. Pressure Drop during Shell˜ Side Condensation 3.4. Examples of Design Problems for Low- and Medium-Finned Trufin in Shell and Tube Condensers 3.4.1. Condenser Design for a Pure Component: Example Problem 3.4.2. Condenser Design for a Multi-component Mixture: Example Problem Nomenclature Bibliography Chapter 4 Trufin Tubes in Air-cool Heat Exchangers 4.1. Heat Exchangers with High˚Finned Trufin Tubes 4.1.1. Areas of Application 4.1.2. High˚Finned Trufin 4.1.3. Description of Equipment 4.2. Heat Transfer with High˚Finned Trufin Tubes 4.2.1. Fin Temperature Distribution and Fin Efficiency 4.2.2. Effect of Fouling on High˚Finned Trufin 4.2.3. Contact Resistance in Bimetallic Tubes 4.3. Heat Transfer and Pressure Drop in High˚Finned Trufin Tube Banks 4.3.1. Heat Transfer Coefficients in Crossflow 4.3.2. Mean Temperature Difference in Crossflow 4.3.3. Pressure Drop in Crossflow 4.3.4. Other Air˚Side Pressure Effects 4.4. Preliminary Design Procedures 4.4.1. Principles of the Design Process 4.4.2. Selection of Preliminary Design Parameters 4.4.3. Fundamental Limitations Controlling Air˚Cooled Heat Exchanger Design 4.5. Final Design NOMENCLATURE BIBLIOGRAPHY Chapter 5 Trufin Tubes in Boiling Heat Transfer 5.1. Trufin in Boiling Heat Transfer 5.1.1. Pool Boiling Curve 5.1.2. Nucleation 5.1.3. Nucleate Boiling Curve 5.1.4. Maximum or Critical Heat Flux 5.1.5. Film Boiling 5.1.6. Boiling Inside Tubes 5.1.7. Subcooling and Agitation 5.2. Vaporizers ˚ Types and Usage 5.2.1. General 5.2.2. Boiling Outside Tubes 5.2.3. Boiling Inside Tubes 5.2.4. Other Types of Evaporators 5.3. Boiling Heat Transfer 5.3.1. Pool Boiling ˚ Single Tube 5.3.2. Single Tube in Cross Flow 5.3.3. Boiling on Outside of Tubes in a Bundle 5.3.4. Boiling Inside Tubes 5.3.5. Boiling of Mixtures 5.4. Falling Film Heat Transfer 5.4.1. Vertical In˚Tube Vaporizer 5.4.2. Horizontal Shell˚Side Vaporizer 5.4.3. Dry Spots ˚ Film Breakdown 5.5. Special Surfaces 5.5.1. Boiling on Fins 5.5.2. Mean Temperature Difference 5.6. Pressure Drop 5.6.1. Tube˚Side Pressure Drop 5.6.2. Shell˚Side Pressure Drop 5.7. Fouling 5.8. Design Procedures 5.8.1. Selection of Reboiler Type 5.8.2. Pool Type Reboilers 5.8.3. In˚tube or Thermosyphon Reboilers 5.9. Special Considerations 5.9.1. Examples of Design Problems 5.10. Example of Design Problems for Trufin in Boiling Heat Transfer 5.10.1. Design Example ˚ Kettle Reboiler 5.10.2. In˚Tube Thermosyphon ˚ Example Problem 5.10.3. Boiling Outside Trufin Tubes ˚ Example Problem NOMENCLATURE BIBLIOGRAPHY