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MITA Mode

Overview

In MITA mode, the user specifies the desired Minimum Internal Temperature Approach (the smallest temperature difference between the hot and cold composite curves). The solver iterates on total heat duty to find the operating point that achieves the specified MITA.

How It Works

2-Stream Configuration (Fully Automatic)

With 2 streams (1 hot, 1 cold), MITA mode is fully determined — no additional specifications are needed:

  1. Identify hot and cold streams by inlet temperature
  2. Q-bisection: iterate on total heat duty
  3. At each trial Q, compute outlet temperatures via PH flash
  4. Build composite curves and calculate MITA
  5. Converge when \(|\text{MITA}_{\text{calc}} - \text{MITA}_{\text{spec}}| < 0.01\) K

3+ Stream Configuration

With \(N > 2\) streams, the user must specify \(N - 2\) outlet temperatures. The solver then:

  1. Classify streams as hot/cold based on inlet temperature relative to the group average
  2. Distribute trial heat duty proportionally to each stream's capacity
  3. Bisect on total Q until the composite curves achieve the target MITA

Configuration

  1. Select MITA from the Calculation Mode dropdown
  2. Enter the desired MITA value (in K or the selected temperature difference unit)
  3. Select flow direction: Counterflow or Co-current
  4. For 3+ streams: specify outlet temperatures for \(N - 2\) streams
MITA mode configuration
Figure 1. MITA mode configuration showing the MITA specification and flow direction selection.

Physical Significance

The MITA determines how closely the hot and cold composite curves approach each other:

MITA Interpretation
Large (> 20 K) Conservative design, small exchanger, low cost but low heat recovery
Moderate (5–15 K) Typical industrial practice
Small (1–5 K) Aggressive heat recovery, large exchanger, high capital cost
0 K Thermodynamic limit (infinite area), not practically achievable
< 0 K Infeasible — temperature crossover

Pinch Analysis

In pinch analysis (Linnhoff & Hindmarsh, 1983), the MITA is called \(\Delta T_{\text{min}}\) and determines the minimum energy requirement for the overall heat exchanger network. A typical value is 10 K for chemical processes and 3–5 K for cryogenic systems.

Results

Result Description
Outlet temperatures Calculated for all streams
Total heat duty Heat transferred to achieve specified MITA
UA calculated Resulting UA for the converged design
MITA calculated Should match specification (within 0.01 K)
Composite curves Visual confirmation of the pinch point location