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New Heat Exchanger Features in VMGSim 10.0 

New features have been added and upgrades have been made to heat exchangers in VMGSim. This article will provide a description of the changes made, which include:

  • Changes to the interface
  • New “Rated Overdesign” mode has been added
  • Added new options for calculating the boiling heat transfer coefficient
  • “Rating mode” is now called “Simulated Performance”
  • Calculations for possible vibration concerns can be selected in TEMA type exchangers
  • Hairpin heat exchangers can be selected
  • Longitudinal fins are now available for TEMA E, TEMA F, double pipe and hairpin exchangers
Updated Interface

Updates have been made to the heat exchanger interface for a couple of reasons. The “Rating Engines” that were in the “Detailed Rating” frame now have functions other than just rating so the naming of these engines and the frame has been changed. Starting with VMGSim 10.0.87 the name of the frame and the engines have been changed to “Detailed Geometry” and “Calculation Engine” respectively, and the “Rating” tab that would appear when VMGSim was selected has been renamed to “VMG Detail”. 

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On the “VMG Detail” tab the selection of certain options requires the need to display numerous variables which could sometimes be hidden if the heat exchanger form was not “tall” enough. In order to make navigation to these variables simpler, the input frames have been changed into a single frame with nodes and a scroll bar was added. Radio buttons for switching between the different modes were also added. The different available modes are described below.

The changes to the interface described above and the different modes that are described below will be added to air coolers in the next official update.

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“Simulated Performance” and “Rated Overdesign” Modes
Simulated Performance

Simulated performance is still the default mode when “VMG” is first selected as the calculation engine. The function of this mode is the same as “Rating” mode from previous versions of VMGSim. “Simulated Performance” mode requires both inlet material ports and the geometry of the exchanger to be specified in order to calculate the conditions of the outlet streams. This mode gives the user the expected performance of the entered geometry for the current inlet conditions.

Rated Overdesign

“Rated Overdesign” calculates the area overdesign required to match the geometry to the specified energy exchanged. Information for both the inlet and outlet streams needs to be provided in order for this mode to be able to solve.

Area Overdesign refers to a linear extrapolation of “Percent Excess Heat Transfer Area” for the specified process conditions to some other (unknown) process conditions. Without knowing these other conditions, it can only be assumed that the same Overall Heat Transfer Coefficient (U) and the same Effective Mean Temperature Difference (EMTD) could also be maintained for that unknown situation. However, U varies with flowrate and EMTD varies with temperature approach (or heat duty, or flowrate), even for the same heat exchanger geometry.

For example, a result of 10% Overdesign does not necessarily guarantee that 10% more flow could be brought to the same outlet temperatures with the same exchanger geometry. Similarly, it does not mean that 10% more cooling of the hot side fluid and/or 10% more heating of the cold side fluid could be achieved with this same exchanger geometry. This can easily be explained by considering the following:

  • The outlet temperature approaches in that case will become narrower
  • Any geometry, other than a pure countercurrent tube pass arrangement, will include an F-factor correction from LMTD to EMTD, and that F-factor may decrease drastically due to the closer approach temperatures
Boiling Heat Transfer Coefficients 

Several correlations for calculating boiling heat transfer coefficients are available and, for the same system, these different correlations can give heat transfer coefficients that are orders of magnitude different [1]. This large variation is also seen when heat transfer coefficients are determined from experimental data, so to be able to better match this variation new correlations have been added. The new available options are Rohsenow, Mostinskii, Bier, and Cooper. The boiling heat transfer coefficient correlation can be changed in the “VMG Detail – Numerical Settings” frame on the “Settings” tab of the heat exchangers. Since the roughness of the surface strongly affects boiling heat transfer a boiling roughness parameter has been added to allow for the direct tuning of the boiling heat transfer coefficient.

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Vibration Calculations

When a fluid flows across the tubes in a shell and tube heat exchanger damage caused by vibrating tubes can occur. Methods for calculating the critical velocity and the amplitudes of vibration outlined by the Tubular Exchanger Manufacturers Association (TEMA) [2] have been added to the VMG Detail heat exchanger calculations. These calculations allow for the prediction of possible vibration problems inside a specified shell and tube heat exchanger. To activate the vibration calculations, select the “Include Vibrational Calculations” check box under the “Automatic Calculations” frame and then provide values for the required inputs. Once solved, the “Vibration Concerns” box in the output frame will show if vibration damages are possible, and under the Detailed Output frame the different areas of concern are displayed under the “Vibrational Considerations” node. Vibrations are expected to be a concern if the shell velocity is larger than the critical velocity or if either of the amplitudes are larger than 2% of the tube diameter. The location of all of these variables are shown below.

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Due to the complexity of fluid flow in heat exchangers the calculations provided are only meant to give an estimate of vibration concerns. TEMA’s guarantee does not apply to the vibration equations provided in their standards, and currently VMGSim does not calculate the local velocities in all of the areas of heat exchangers so not all areas can be checked for issues.

Hairpin Heat Exchangers

Hairpin heat exchangers can now be selected from the shell drop down on the “VMG Detail” tab. It is possible for these exchangers to be single or multi-tubed, they can have transverse and longitudinal fins, and they can be with or without baffles. TEMA front ends are not available with hairpin heat exchangers and there are different options for the rear ends because hairpin heat exchangers are not a TEMA type heat exchanger. The options for rear ends are “U-Bypass”, where the shell and tube side fluids are not in contact between the passes of the exchanger, and “U-Through” where the fluids are in contact between passes.

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Longitudinal Fins

Longitudinal fins can now be added to tubes if the selected shell type is TEMA E, TEMA F, double pipe or hairpin. Longitudinal fins can’t be added if any other shell type is selected. Longitudinal fins and baffles cannot be selected at the same time. The methods outlined in [1] are used to calculate the heat transfer and pressure drop in exchangers with longitudinal fins. Single and double longitudinal fins can be selected.

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Jordan Mangold, EIT, Technical Support, VMG Calgary

Please contact your local VMG office for more information.

References

[1]  G. F. Hewitt, G. L. Shires and T. R. Bott, Process Heat Transfer, CRC Press, 1994. 

[2]  Tubular Exchanger Manufacturers Association, Standards of the Tubular Exchanger Manufacturers Association, New York, 2007.    

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