physical properties for heat exchanger design

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Process Engineering Guide: GBHE-PEG-HEA-500

Physical Properties for Heat Exchanger Design 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: Physical Properties for Heat Exchanger Design

CONTENTS SECTION 0 INTRODUCTION/PURPOSE 2 1 SCOPE 2 2 FIELD OF APPLICATION 2 3 DEFINITIONS 2 4 COMPONENT PROPERTIES 3

4.1 General 3 4.2 Use of Component Properties for Mixtures 3

5 INPUT OF MIXTURE CURVES 4 5.1 General 4 5.2 Generation of the Mixture Curves 4 5.3 Selection of Temperature Points 5 5.4 Extrapolation 6 6 IMMISCIBLE CONDENSATES 7



FIGURES 1 TEMPERATURE POINTS SELECTED FOR

EQUAL ENTHALPY CHANGE 6 2 TEMPERATURE POINTS SELECTED FOR GOOD

FIT TO CURVE 7



0 INTRODUCTION/PURPOSE This Guide is one of a series on heat transfer produced for GBH Enterprises. 1 SCOPE This Guide discusses how to provide physical property data for the computer aided thermal design of heat exchangers. It is NOT a guide to sources of property data, nor to methods of estimating such data. If information is required on such items, a physical properties expert should be consulted. 2 FIELD OF APPLICATION This Guide applies to all process engineers in GBH Enterprises worldwide. 3 DEFINITIONS For the purposes of this Guide, the following definitions apply: HTFS Heat Transfer and Fluid Flow Service. A cooperative research

organization, based in the UK, involved in research into the fundamentals of heat transfer and two phase flow and the production of design guides and computer programs for the design of industrial heat exchange equipment.

. HTRI Heat Transfer Research Incorporated. A cooperative research

organization, based in the USA, involved in research into heat transfer in industrial sized equipment, and the production of design guides and computer programs for the design of such equipment.



4 COMPONENT PROPERTIES 4.1 General As an alternative to using in-built data banks of commercially available programs , property data for individual components can be input. Data are normally given at two temperatures, however programs are now available to allow for up to 12 temperatures. 4.2 Use of Component Properties for Mixtures Component or automatic properties for mixtures should not be used unless it is ensured that they are reasonably ideal. In general, if data are given for several components, either by use of automatic property codes in commercially available programs or by direct input, or a combination of these, the programs will use ideal mixing rules to determine the mixture properties (non-ideal VLE behavior can be allowed for in some commercially available programs, using equilibrium constants, but all other properties are based on ideal mixing). 5 INPUT OF MIXTURE CURVES 5.1 General The use of mixture profiles is the preferred way to input physical property data for mixtures, as it allows for the user to model non-ideal behavior with more precision. However, it does require the generation of a large quantity of data. Some commercially available programs, (i.e., thermal rating programs) will accept physical properties in the form of a matrix of values for all properties, calculated at different temperatures, and in some cases also at different pressures. Versions exist that will allow up to three sets of data at different pressures. . Versions exist that will allow data at two pressures;



5.2 Generation of the Mixture Curves If all the components of a mixture are available, the recommended method of generating the MIXTURE curves is by means of commercially available programs. If not all data are available, it may be possible to insert the coefficients for the missing components when running the program. For further help, consult a physical properties expert. If the additional data are not available in a suitable form, it may be necessary to generate the data by hand. 5.3 Selection of Temperature Points The properties at the chosen temperature points, when linked linearly, should be a reasonable approximation to the actual curves. This is particularly important for the weight fraction vapor and the specific enthalpy or heat load from entry, as these properties often vary in a significantly non-linear manner with temperature. Obviously, the more data points provided by the user, the better the representation of the data will be. Commercially available programs will interpolate linearly between values supplied. The programs in general use a spline method of interpolation for specific enthalpy and weight fraction vapor which will give better results than the simple linear interpolation. For two phase systems, condensing or boiling, the dew point and the bubble point should always be included as points on the profile if they occur within the temperature range selected. Usually, only one extra point will be required for the superheated region, and one for the subcooled region unless there is a large superheated or subcooled zone and the properties vary in a non-linear manner. The majority of the points should be selected in the two-phase region, where, because of composition changes, properties are changing rapidly. One commercially available program, - has an option that first locates the dew and bubble points, if they occur within the input temperature range, and then divides the two-phase region into zones of equal enthalpy change.



For many mixtures, the equal enthalpy change approach is adequate. However, it can lead to significant errors for mixtures with temperature/enthalpy curves with regions of high curvature, for example, mixtures of mainly condensables, but with a small quantity of inerts. In these cases, it is desirable to have more points in the region of high curvature, if necessary at the expense of regions where properties vary in a linear manner. See Figures 1 and 2. The difference between these two approaches can lead to differences in performance prediction of more than 10%. Following a preliminary run, the estimated weight fraction vapor and stream enthalpy values should be plotted against temperature. This may be done using the graph plotting option of commercially available programs. Further runs may be necessary at different temperatures to obtain the full shape of the curves. Having obtained the full curve, a final set of temperatures can be selected, including dew and bubble point if they occur within the temperature range, and processed through the 'Manual' option to obtain the data files for running the exchanger program. 5.4 Extrapolation Mixture curves given should span the range of temperatures expected. If they do not, the program will extrapolate from the points given, often in a linear manner, which can result in significant errors or program crashes in some cases.



FIGURE 1 TEMPERATURE POINTS SELECTED FOR EQUAL ENTHALPY CHANGE



FIGURE 2 TEMPERATURE POINTS SELECTED FOR GOOD FIT TO CURVE



6 MMISCIBLE CONDENSATES Some vapor mixtures condense forming two wholly or partially immiscible liquid phases. A typical example is a mixture of steam and hydrocarbons. In general, such a system will have two 'dew points'. As the vapor is cooled, the first dew point will be reached where one liquid phase will condense. Further cooling will produce a second dew point, below which two liquid phases will be present. At present none of the heat exchanger programs available will handle this situation rigorously. Indeed, there is some doubt as to how such a system does perform in condensation. The recommended method for rating such exchangers, which is believed to underestimate the heat transfer coefficient, and hence will generally be safe, is as follows: (a) The two dew points should be selected as two of the temperature points

on the condensation curve. (b) Other temperature points should be selected to give a good representation

of the curve shape. (c) For temperatures between the two dew points there is only one liquid

phase. Use the liquid physical properties of this phase. (d) Below the lower dew point, where there are two liquid phases, the liquid

phase used by the heat transfer program should be assumed to have the transport properties of the phase with the worse properties, i.e. higher viscosity and lower thermal conductivity. For a water/organics system, this will almost certainly be the organics rich phase. For other systems, it may not be obvious which will give the worse results, and it may be necessary to try both options.

(e) Below the lower dew point the specific heat and enthalpy of the liquid

phase has to be taken as the weighted mean of the two phases, to conserve the heat balance.

Commercially available programs can be used to generate the values of the properties, assuming that the non-ideality can be modeled. The 'Automatic' temperature point method cannot be used in these circumstances, and the location of the two dew points and the bubble point will need to be determined by trial and error. Data for both liquid phases can be generated in tabular form

physical properties for heat exchanger design

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