Develop a set of specialized calculations for plate-and-frame heat exchangers that could be used while running refrigeration loop simulations to help optimize and streamline a complex heat exchanger design process.
CRYOGENIC REFRIGERATION MODELING ASSISTANCE AND CHEMCAD/EXCEL INTERFACE DEVELOPMENT
The client constructs and sells cryogenic refrigeration loops for a variety of industries (semiconductors, flat panel displays, optical networking). Chemstations’ CHEMCAD computer process simulation software is used by the client to design and assess performance of brazed plate-and-frame heat exchangers in their refrigeration loops. These specialized exchangers are not represented sufficiently by any standard CHEMCAD heat exchanger unit operation model. The client requires more detailed input and output than is available in standard CHEMCAD plate-and-frame exchanger sizing models. The client contracted Process Engineering Associates, LLC (PROCESS) to use its experience with the CHEMCAD/Excel Unit Operation functionality to develop a set of specialized calculations for these exchangers that could be used while running the refrigeration loop simulations.
The CHEMCAD/Excel Unit Op is a unit operation icon that can be placed in a simulation flow sheet. Inlet and outlet streams can be connected to this icon as with any other unit operation used in a process simulation. The icon represents CHEMCAD’s capability to activate and use a personalized Microsoft Excel file. The Excel file is a standard worksheet that uses Visual Basic for Applications (VBA) programming to allow communication between CHEMCAD and Excel. When the icon is run from the flowsheet, the Excel Worksheet is activated and Visual Basic programs are executed. These programs take the input stream info from CHEMCAD, perform user-defined algorithms for specialized equipment, and create output streams that are returned to CHEMCAD. This capability allows CHEMCAD users to develop their own unit operations for special equipment. The CHEMCAD interface dialog for this unit operation is also custom-built specifically to send inputs and receive outputs from the Excel Worksheet file.
The client desired to input geometry for standard exchanger types and determine suitability for the process. Therefore, with a given exchanger area/geometry and a process duty specification, the program calculates a required area and compares the calculated area to the selected exchanger.
PROCESS created a customized input-output dialog box for the user to enter unit operation specifications for the heat exchanger. These specifications include exchanger geometry, outlet temperature specification and inlet/outlet piping configurations. The dialog interface also contains output areas to accept results from the calculations performed by the Excel VBA code. Outputs include exchanger area, percent excess (positive or negative), pressure drop, and heat transfer coefficients.
CHEMCAD user inputs for the outlet temperature specification and user-configured CHEMCAD inlet streams establish an overall heat duty. The heat exchanger is divided into a user-defined number of zones. The zone duty is set by the overall duty divided by the number of zones.
Wall and film heat transfer coefficient correlations specific to brazed plate-and-frame heat exchangers were used from literature for single-phase, condensing, and evaporating refrigerants and water. The Excel Worksheet uses algorithms coded into Visual Basic to calculate these local heat-transfer coefficients for each side of the brazed plate-and-frame heat exchanger.
Each exchanger zone was balanced by a nested iteration of two variables (zone-specific area and wall temperature) until the heat balance as represented by U x A x ΔTlm = m x Cp x ΔT was satisfied to within a user-set tolerance for each zone. The outer loop for zone area converged using direct substation while the inner loop of wall temperature utilized the bisection method. The zone areas were summed to calculate the total area required for the process specification. Special pressure drop equations from literature sources for condensing and evaporating refrigerants were also incorporated into the program to calculate the exchanger pressure drop.
The client desired, and PROCESS provided, full documentation of the heat exchanger geometry, heat transfer, and physical properties calculations, the Excel worksheet containing the calculations, and the visual basic for applications (VBA) code.
- Process Equipment Design and Manufacturing
- Advanced heat exchanger design
- Advanced process simulation interface development
- Equipment designer/manufacturer support
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