What you can expect from method changes in Version 8.2

Xist^® window pressure drop models

Xist 8.1 has two different models of window pressure drop for single-segmental baffles, depending on baffle cut and baffle spacing. The original model in S-SS-3-1 was developed for single-segmental baffles. A subsequent model, reported in S-SS-3-4, was developed for geometries such as double-segmental baffles, in which there is significant longitudinal flow. The S-SS-3-4 model was also applied to single-segmental baffles with large baffle spacing and/or large baffle cuts. In a few single-segmental and NTIW cases, the two window models can lead to abrupt transitions between the window pressure drop predictions when adjusting baffle cut and spacing. 

Version 8.2 incorporates changes to the window flow model that mitigate discontinuities in the results. There is proration between the two frictional pressure drop models, and the flow areas and fractions used for the turning and accelerational losses are made consistent. The revised model was validated with a CFD study and compared with the extensive HTRI research data set covering laminar and turbulent flow. Overall, Xist 8.2 gave slightly better agreement with the data than Xist 8.1.

Users will notice little change in the predictions for most cases operating within HTRI design guidelines. However, for unusual geometries such as those with high shell/bundle or inline pass partition flow fractions, more noticeable differences in heat transfer and pressure drop can be observed.

Additional reading: S-SS-3-1 and S-SS-3-4. Design Manual updates (forthcoming)

Revised method for finding the delta correction factor in X shells

The delta correction factor imposes a reduction on the mean temperature difference (MTD) when there is a close temperature approach together with a significant ineffective flow fraction for heat transfer. The calculation depends on determining the extent of mixing between the effective and ineffective flow streams. The delta method in Xist 8.1 and previous versions employs a mixing factor derived using HTRI research data for baffled E-type exchangers. This method has been unreliable when applied to TEMA X shells. 

CFD calculations have developed a mixing factor expression for X shells which has been incorporated into Xist 8.2. The new method usually predicts lower values of the delta correction factor than that in Version 8.1 (although in many well-designed X shells, there will be very little difference), hence providing improved detection of design characteristics that could lead to underperformance.

Shellside cooling monitor

The shellside subcooling monitor included in Xist until Version 6.3 has been re-enabled in Version 8.2. The original monitor was disabled due to problems with condensate level predictions in gravity-controlled condensation based on methods in TPG-3. Subsequent research developed methods, documented in CS-16, which were then validated by experimental data discussed in CS-21.

The Version 8.2 subcooling monitor performs post-processing calculation assuming no tube removal from the bottom of the shell to quantify the height of subcooled liquid, the non-equilibrium condensate temperature, and other related parameters down the length of a horizontal exchanger with vertical baffle cuts. The subcooling calculations are performed after the heat transfer and pressure drop calculations have converged, which means that the subcooling calculations have no impact on the standard case results. The results can be used to help users understand the condensate liquid levels and temperature distribution with flooded and unflooded condensers with subcooling for full bundles. If the rigorous tube layout has tubes removed from the bottom of the bundle, then post-processing calculations result in lower condensate temperatures. 

Additional reading: TPG-3, CS-16, and CS-21

Turndown limits for vertical tubeside thermosiphons

Experiments were performed to determine conditions that may cause loss-of-liquid recirculation in a vertical tubeside thermosiphon reboiler (VTTR). Warning messages have been included in Xist 8.2, based upon the boiling range of the fluid.

• Boiling range less than 5 °C 
  Checks are made to determine if the wall superheat exceeds that for the onset of nucleate boiling and if a heat flux, scaled with the latent heat exceeds a certain value. Here a “pool boiling” type of event may occur where the vapor flow generated may approach or exceed the circulation rate giving rise to an unsteady flow and low duty.
• Boiling range greater than 30 °C 
  An abrupt loss-of-liquid circulation may occur when the non-dimensional average heat flux is less than the minimum non-dimensional heat flux. The wide boiling range mixture promotes loss of circulation due to the light components being preferentially stripped, reducing the effective mean temperature difference. In addition, for a low intube mass flux, a warning indicates that the accumulation of the heavier components may cause the heat transfer to be overestimated. 
• Boiling range between 5 to 30 °C 
  A warning indicates that no turndown analysis is performed.

The warnings do not affect thermal calculations, but will raise awareness of potential operating issues or increased uncertainty in the results.

Additional reading: BG1-18 and BG1-19 (forthcoming)

Straight pipe elements displayed on flow regime maps for detailed piping

Flow regime predictions for detailed piping have been updated for horizontal and vertical straight elements. Instead of multiple line items displaying flow regime predictions, a single line item provides the outlet piping flow regime for straight pipe elements. Predictions for horizontal piping are based on the Taitel and Dukler map; those for vertical piping use the Dukler map. Furthermore, users can now visualize where a particular piping element is located on the appropriate flow regime map. Detailed piping flow regime maps are now displayed under Graphs, where the user can view the flow regime for each straight piping element and its proximity to flow regime transition lines.

Single- and two-phase pressure drop for bends

For single-phase and two-phase boiling pressure drop calculations in Xist and Xace^®, a correction is made to the current Blevins’ method for U-tubes and piping bends. The new method provides a smooth transition from moderate to sharp bends when the Reynolds number is higher than 5(10⁵). The new method reduces pressure drop for cases with a high Reynolds number.

Additional reading: TPF-7; Design Manual updates (forthcoming)

Updates in HPT Microfin Type 2

One option for tube internals in Xist, HPT Microfin Type 2, simulates the performance of the NEOTISS Type 2 microfinned ID enhancement. The calculations are based on NEOTISS proprietary methods validated with experimental data measured at the HTRI Research & Technology Center. Xist 8.2 includes methods based on re-correlation of the experimental data for condensing applications. Compared to Version 8.1, the heat transfer coefficients are lower for pure components, and pressure drop is up to 10% less for mixture condensation. 

Revised liquid holdup for vertical intube boiling

Version 8.2 revised HTRI void fraction calculations for intube upflow boiling. This change increases the static pressure drop under low mass velocities (less than 50 kg/s m²). Circulation flow rates are reduced for vertical tubeside thermosiphons under turndown conditions. For mass velocities greater than 50 kg/s m², the change in static pressure drop is negligible. 

Additional reading: BG1-18 and BG1-19 (forthcoming)

High effectiveness

Thermal effectiveness is a measure of how well the exchanger performs compared to ideal performance. It is defined as the ratio of actual duty to the thermodynamic maximum duty. With one single-phase fluid stream, the temperature effectiveness is readily calculated. For single-phase hot side, it is calculated as

For single-phase cold stream, it is calculated as

Exchangers with very high thermal effectiveness require close to pure counterflow and near perfect flow distribution to meet the required duty. They are sensitive to slight changes in operating conditions or imperfections in manufacturing. We have added a warning message to alert the user when the program predicts a temperature effectiveness greater than 0.9. 

New fan on/off logic

Xace 8.2 simplifies the procedure for assessing individual fan-off operation with the introduction of new logic in the Control > Flow Distribution panel. Users can readily identify off-fans by selecting checkboxes, thereby avoiding the need to manually specify the flow maldistribution profile. To facilitate the fan-off calculations, the new logic is able to modify the unit incrementation as a function of the number of fans, adjusting the number of length increments to an integer multiple of the number of fans. The adjusted incrementation is reflected in the 3D data and monitor reports. 

The new logic constitutes an automation of the previously recommended workaround procedure for modeling one fan-off operation in Xace. The results obtained via the new logic should be comparable to those obtained using the former workaround, although small differences may be observed in units with three or four fans, owing to the dynamic incrementation mentioned above.

Heat release curves for kettles with pure components

The natural circulation in a kettle reboiler causes recirculated liquid to remix with feed composition liquid at the bottom of the bundle. For mixtures, this recirculation ensures that the composition of the feed is different than the composition in the bundle. Xist accounts for this difference by generating a simplified “kettle heat release curve”. In Xist 8.1 and previous versions, this simplified kettle heat release curve is calculated for pure components and mixtures. For pure components, the kettle heat release curve introduces unnecessary simplifications into the calculations of bundle temperature distribution. Any errors in pure component cases are negligible except for low MTD (less than 3 °C).

Xist 8.2 has implemented an option to use the user-specified heat release data when Use modified heat release curve for kettles is specified as No (Control > Methods panel) for kettle reboilers. For most pure component cases, the average change in results is less than 10%. For certain pure component cases with low temperature approach (3 °C or less), the change in results can be significantly large.

Additional reading: BK1-13 for more information on the calculation of the kettle heat release curve; Lessons learned: Challenges in modeling kettle vaporizers: three point heat release curve (presentation from 2019 Global Conference)

Shellside condensation flow regime map

Version 8.2 adds a shellside condensing flow regime map. Previously, the shellside condensation data was printed on the HTRI flow regime map, which was developed specifically for tubeside condensation data. The new map shows shear-controlled, transition, gravity-controlled, and intermittent flow regimes. Boundaries between shear- and gravity-controlled regions are different than those on the tubeside flow regime map. The new flow regime map does not change results but provides clarity to the method we use in the thermal results.

Additional reading: Section B4.4 of Design Manual