Heat Exchanger Zones Analysis

Zones analysis is useful for locating temperature pinches and phase changes that occur within the heat exchanger. Calculation of the average temperature driving force using the conventional logarithmic mean temperature difference (LMTD), derived from terminal temperature differences, will not be accurate when phase changes occur. PRO/II has a special capability to divide the exchanger into zones and calculate the Duty-weighted LMTD to yield better results.

Zones analysis can be performed only when both sides of the exchanger are defined. Zone boundaries are defined at the inlet and exit of the exchanger and at all points where there is a phase change. The default number of zones is five. Furthermore, zone sizes will be adjusted as necessary such that no zone accounts for more than 20% of the total exchanger duty.

Only Minimum Internal Temperature Approach (MITA) specification requires a zone analysis at calculation time to reach a solution. For "Area * U-value (Lumped Product)" and "Area and U-value (Separate Values for Each)" specifications zone analysis is always done during calculation, if requested. For all other specifications the zone analysis is usually done during the output generation, if requested. However, zone analysis for the latter can be requested during calculation time, if desired. This is CPU intensive and results in a significant increase in calculation time, especially for heat exchangers nested within recycle loops.

Usage

From the Simple Heat Exchanger Main Window push the Zones Analysis button to display the Heat Exchanger Zones Analysis Window.

1. Select "Include Zones Analysis" option.

2. Increase the number of zones (default is 5) if required.

3. Select when the zone analysis should be performed. For MITA and zoned MTD specifications it is always performed during calculation time. For other specifications it is advisable to perform the analysis during output time, since this results in faster simulation calculations.

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