Heat Exchanger Design

Log Mean Temperature Difference

LMTD = (ΔT1 - ΔT2) / ln(ΔT1/ΔT2)

Q = U × A × F × LMTD

Where:
F = Correction factor for non-counterflow
U = Overall heat transfer coefficient
A = Heat transfer area

Effectiveness-NTU Method

1. Effectiveness Definition

   ε = Q_actual / Q_max
   Q_max = Cmin × (Th,in - Tc,in)
   

2. NTU Calculation

   NTU = UA / Cmin
   Cr = Cmin / Cmax
   

3. Effectiveness Relations

   • Counterflow: ε = (1-exp(-NTU(1-Cr)))/(1-Cr×exp(-NTU(1-Cr)))
   • Parallel flow: ε = (1-exp(-NTU(1+Cr)))/(1+Cr)
   • Shell-and-tube: Complex correlations by TEMA type

Shell-and-Tube Design

1. TEMA Designations

   Front End	Shell	Rear End
   A - Channel	E - One-pass	L - Fixed tubesheet
   B - Bonnet	F - Two-pass	M - Fixed tubesheet
   N - Channel	J - Divided flow	N - Fixed tubesheet
   -	X - Crossflow	P - Outside packed
   -	-	S - Floating head
   -	-	U - U-tube

2. Tube Layout

   • Triangular pitch (30°): Maximum tubes, poor cleaning
   • Square pitch (90°): Mechanical cleaning possible
   • Rotated square (45°): Higher turbulence

3. Baffle Design

   • Segmental: 20-45% cut
   • Double segmental: Reduced pressure drop
   • No-tubes-in-window: Vibration mitigation

Plate Heat Exchanger

1. Plate Selection

   • Chevron angle (25-65°): Trade-off h vs ΔP
   • Plate spacing: 2-5 mm typical
   • Pass arrangement: U or Z configuration

2. Design Considerations

   • Maximum pressure: 25-30 bar typical
   • Maximum temperature: 150-200°C (gaskets)
   • Fouling service: Not ideal

Air-Cooled Heat Exchanger

1. Configuration

   • Forced draft: Fan below bundle
   • Induced draft: Fan above bundle
   • Natural draft: No fan (limited duty)

2. Design Parameters

   • Face velocity: 2.5-3.5 m/s
   • Tube rows: 3-6 typical
   • Fin density: 275-435 fins/m

Fouling Considerations

Process Integration

• ME-012: Heat Exchanger Design and Rating
• ME-011: Thermal Management Design

Input Schema

{
  "design_type": "sizing|rating",
  "exchanger_type": "shell_tube|plate|air_cooled",
  "hot_fluid": {
    "name": "string",
    "flow_rate": "number (kg/s)",
    "inlet_temp": "number (C)",
    "outlet_temp": "number (C, for sizing)"
  },
  "cold_fluid": {
    "name": "string",
    "flow_rate": "number (kg/s)",
    "inlet_temp": "number (C)",
    "outlet_temp": "number (C, for sizing)"
  },
  "pressure_constraints": {
    "hot_side_max_dp": "number (kPa)",
    "cold_side_max_dp": "number (kPa)"
  },
  "fouling_factors": {
    "hot_side": "number (m2K/kW)",
    "cold_side": "number (m2K/kW)"
  }
}

Output Schema

{
  "duty": "number (kW)",
  "geometry": {
    "type": "string (TEMA designation or plate type)",
    "area": "number (m2)",
    "shell_diameter": "number (mm)",
    "tube_count": "number",
    "tube_length": "number (m)"
  },
  "thermal": {
    "LMTD": "number (C)",
    "F_factor": "number",
    "U_clean": "number (W/m2K)",
    "U_dirty": "number (W/m2K)"
  },
  "hydraulic": {
    "shell_side_dp": "number (kPa)",
    "tube_side_dp": "number (kPa)"
  },
  "performance": {
    "effectiveness": "number",
    "NTU": "number"
  }
}

Best Practices

1. Always include fouling factors appropriate for the service
2. Verify pressure drop constraints are met on both sides
3. Check for vibration potential in shell-and-tube designs
4. Consider maintenance access in configuration selection
5. Apply TEMA tolerances for manufacturing variations
6. Use conservative correlations for preliminary sizing

Integration Points

• Connects with CFD Analysis for detailed flow distribution
• Feeds into HVAC System Design for system integration
• Supports Thermal Analysis for component-level design
• Integrates with Process Design for plant-level optimization