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  • focused on process simulation

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    Heat exchanger simulation

    Problem

    The simulation of a heat exchanger is demonstrated in an example in this tutorial.

    Water (1,5 bar, 20 m/h) is to be heated from 10C to at least 90C with hot process water (130C,

    2.7 bar, 50 m/h). The heat exchanger is to be operated countercurrent. In doing so, the process

    water that flows in the tubes should not be cooled down by more than 50 K maximum. A tubular

    heat exchanger is dismounted from on old system and will then be used for this task. The data of

    the heat exchanger is stated in Table 1.

    Table 1: Geometric data of the heat exchanger

    Design TEMA R/ BEM (see Figure 1)

    Material Carbon steel (also C-steel or structural steel)

    Inner cladding diameter [m] 0.8

    Number of tubes 670

    Length of tubes [m] 4

    Tube alignment Turned triangular form (60)

    Tube spacing 1,25

    Tube dimensions = 19 ; = 16

    Number of baffle plates 11

    Spacing of the baffle plates [m] 0.32

    free cross section in shell side [%] 30

    Tube nozzle diameter (tube side) [m] 0.1

    Tube nozzle diameter (shell side) [m] 0.15

    The following task is to examine by means of a rating whether this heat exchanger delivers the

    required performance and that the pressure loss (tube as well as shell side) does not exceed 0.5

    bar.

    Figure 1: Design of the BEM heat exchanger http://www.engineeringpage.com/heat_exchangers/tema.html

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    Solution principle and assumptions:

    CC-THERM is required here besides CHEMCAD Steady State to solve the problem.

    CC-THERM is an add-on program and comprises the rigorous simulation of heat exchangers.

    CHEMCAD Steady State already offers the possibility to simulate heat exchangers through a simple energy and mass balance. However, a heat transfer coefficient is not calculated, and the construction parameters are not taken into consideration. CC-THERM offers rigorous calculations of the following heat exchanger types: tubular, plate and twin tube heat exchangers and air coolers. It is possible to select between the design of a new and the rating of an existing heat exchanger. With the rating of a heat exchanger, the construction data (e.g. number of tubes, tube dimensions, number of baffle plates, etc.) of the heat exchanger are already known. This way, it is possible to check whether the required output can be achieved with the available heat exchanger for a specified substance mixture. The heat exchanger is designed and calculated in accordance with international standards: TEMA, ASME, DIN or British Standard.

    For the simulation at hand, the IAPWS-IF97 steam table is used as thermodynamic model for the enthalpy calculation. A thermodynamic of mixing is not required because the only available component is water. Therefore, the simplest model (ideal Raoult's law, VAP) is chosen. Table 2 summarizes the most important simulation data.

    Table 2: Summary of the simulation data

    Units Components Thermodynamics Feed streams k

    Unit operations

    Common SI

    Water

    K: VAP, H: IAPWS

    Hot:

    = 130

    = 2,7

    = 0

    = 50 3/

    Cold:

    = 10

    = 1,5

    = 20 3/

    1 heat exchanger;

    2 feeds;

    2 products

    In addition, the hot flow is to have an outlet temperature in excess of 80 C, and the cold flow an

    outlet temperature of 90C.

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    Implementation of the heat exchanger simulation in CHEMCAD

    First, the flow sheet is generated with CHEMCAD Steady State and the feed streams defined as

    stated in table Table 2 (see Figure 2).

    Figure 2: Flow sheet with heat exchanger

    The heat exchanger can be specified via different parameters in the settings window (Figure 3). Amongst other parameters, the heat exchanger surface A and the heat transfer coefficient k can be defined, from which the process data of the outlet flows is calculated afterwards. Further settings options (e.g. statement of temperature, steam content, minimum temperature difference) are also possible. In Utility Option, the required mass flow of one of the feed streams can be calculated in dependence on the other and the set parameters. The stream flow and the view of the calculated results are defined in the menu item Misc. Settings.

    For this simulation, the outlet temperature of the cold flow is defined. According to the task, this should be at least 90C. A temperature of 95C is specified. One statement is sufficient for the simulation of a simple heat exchanger.

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    Figure 3: Settings window of the HTXR heat exchanger

    The simulation is started. This leads to the results shown in Figure 4. Both flows have an outlet temperature of approximately 95C, which means that the condition stating that the hot stream should not be cooled down below 80C is fulfilled.

    Figure 4: Results table of the simple heat exchanger

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    For the rigorous simulation, CC-THERM is now called up at "Sizing: Heat Exchangers" (Figure 5).

    Figure 5: Path for calling up CC-THERM

    After selecting the tubular heat exchanger, the module first asks which feed stream is supposed to flow through the heat exchanger in the tubes first (Figure 6), and - if not already selected - which heat exchanger in the flow sheet is concerned. In this example, the hot process water flows in the tubes.

    Figure 6: Definition of the feed stream flowing in the tubes

    Afterwards, CHEMCAD generates the Q-T diagram (Heat Curve, Figure 7). This is generated with 11 data points (pre-setting). It is also possible to choose between concurrent and countercurrent in the settings window "Heat Curve Parameters".

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    Figure 7: Parameter window of the Q-T diagram

    The Q-T diagram already shows whether a phase change occurs inside the heat exchanger. With a pure component system, it can be expected that the temperature will no longer increase in the case of a phase change, so that the curves will become horizontal. In the examined diagram, this results in the Q-T diagram displayed in Figure 8. There is no phase change. It is also possible to read the output temperatures to be expected. The data used in the diagram originates from the previously generated heat exchanger simulation using the CHEMCAD Steady State module.

    Figure 8: Q-T diagram

    After confirming with "OK", the next window for the general settings of the heat exchanger opens automatically (Figure 9). In Calculation mode, it is possible to choose between the design or the rating case. The calculation basis for the geometric details is selected in TEMA class/standard . Different international standards are available for this purpose. The TEMA standard R (Tubular Exchanger Manufacturers Association Type R) is preset.

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    The geometric layout of the heat exchanger is defined in the next step. The layout is stated using the TEMA standard.

    In this example, a pre-defined heat exchanger is to be rated for the given streams. This heat exchanger is based on the TEMA design. It concerns the TEMA Class R with the BEM form (Figure 1). From the pre-set process parameters, CHEMCAD automatically detects whether a phase change occurs and selects the respective calculation model in Process type. In CHEMCAD, the fouling factor is pre-set at 0.000176109 mK/W but can be adjusted manually.

    Figure 9: General settings window of the CC-THERM module for tubular HE

    In Modeling Methods (Figure 10) the calculation methods, e.g. for laminar or turbulent flows, can be adapted if required.

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    Figure 10: Settings window of the CC-THERM module for tubular HE, tab: Modeling methods

    The next settings window opens automatically after confirmation. The geometric data of the heat exchanger is defined in more detail in the following. Details of the tube bundle are entered first (Figure 11).

    Figure 11: Setting the tube bundle

    In the case of rating, all entry fields can be edited. In the case of design, the number and length of the tubes are calculated and therefore cannot be edited. CHEMCAD already contains pre-set

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    values for a tubular heat exchanger. The dimension of the tubes is 3/4 inch. However, all values can be overwritten manually.

    The geometric data of the heat exchanger is entered as stated in Table 1. The pre-set values, such as the tube sheet thickness, are applied accordingly.

    The shell settings screen opens next (Figure 12). Besides stating the shell diameter, it is also possible to enter whether several heat exchangers are to be connected in parallel or in series.

    Figure 12: Shell settings

    The nozzles can be specified in the subsequent window (Figure 13). Pre-set values are also available here, that can be adapted manually if required. The nozzle diameter is 0.1 m for the tube side and 0.15 m for the shel

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