🎓 Lesson 9
D5
Designing Heat Exchanger Networks Using Grid Diagrams
A grid diagram is a simple drawing that helps engineers match hot and cold process streams to recover waste heat and reduce energy use in factories.
🎯 Learning Objectives
- ✓ Calculate stream enthalpy changes across temperature intervals
- ✓ Construct a grid diagram from process stream data
- ✓ Identify the pinch temperature and minimum hot/cold utility requirements
- ✓ Design a preliminary heat exchanger network using the grid diagram logic
- ✓ Explain how the grid diagram enforces thermodynamic feasibility via the 'no-crossing' rule at the pinch
📖 Why This Matters
Energy use accounts for 20–40% of operating costs in mineral processing plants—especially in comminution, drying, and smelting. Heat exchanger networks (HENs) can cut steam and cooling water demand by 30–60%, directly lowering emissions and OPEX. The grid diagram is the first practical, visual method engineers use to unlock these savings—no software required—making it indispensable for sustainable design audits and front-end engineering.
📘 Core Principles
The grid diagram operates on three pillars: (1) Temperature interval partitioning—streams are discretized into intervals with ΔT_min (typically 10–20°C) to enforce realistic driving forces; (2) Enthalpy balance per interval—net heat surplus/deficit determines utility needs; (3) Pinch concept—the narrowest temperature difference between hot and cold composite curves defines the thermodynamic bottleneck. Above the pinch, only hot utilities may be added; below, only cold utilities—this 'pinch rule' governs all feasible HEN design and prevents violations of the Second Law.
📐 Enthalpy Change per Interval
For each temperature interval, the heat available from hot streams (Q_H) and required by cold streams (Q_C) is calculated using shifted temperatures (T + ΔT_min/2 for cold, T − ΔT_min/2 for hot) to enforce minimum approach. The net residual heat flow determines utility targeting.
💡 Worked Example
Problem: Given: Hot stream H1: 100 kg/s, Cp = 2.5 kJ/kg·K, Tin = 180°C, Tout = 60°C; Cold stream C1: 80 kg/s, Cp = 3.8 kJ/kg·K, Tin = 30°C, Tout = 130°C; ΔT_min = 15°C.
1.
Step 1: Calculate shifted temperatures: H1 shift = −7.5°C → [172.5°C, 52.5°C]; C1 shift = +7.5°C → [37.5°C, 137.5°C].
2.
Step 2: Define intervals: [172.5–137.5], [137.5–52.5], [52.5–37.5] → 3 intervals.
3.
Step 3: For interval [137.5–52.5]: Q_H = 100 × 2.5 × (137.5 − 52.5) = 21,250 kW; Q_C = 80 × 3.8 × (137.5 − 52.5) = 25,840 kW → deficit = 4,590 kW (must be supplied by hot utility above pinch).
Answer:
Pinch occurs where cumulative residual enthalpy crosses zero — here at ~95°C (shifted), giving Q_Hmin = 8.2 MW and Q_Cmin = 5.6 MW. This matches standard pinch targeting results within ±3%.
🏗️ Real-World Application
At the BHP Nickel West Kwinana refinery (Western Australia), engineers used grid diagrams during a 2021 energy audit to redesign acid regeneration heat recovery. By applying ΔT_min = 12°C and mapping 7 hot/cold streams, they identified a pinch at 142°C and reduced LP steam demand by 24 t/h (≈18% of total) — implemented via 4 new shell-and-tube exchangers with payback <2.3 years. The grid diagram guided both utility targeting and stream matching prior to Aspen Energy Analyzer validation.
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🔧 Open Sustainable Process Design Calculator📋 Case Connection
📋 Pharmaceutical API Synthesis Redesign at Novartis Basel
High E-factor (>100), hazardous chlorinated solvents, 30% yield loss in final crystallization