Pressure drop is a critical design parameter that affects pump/fan sizing, operating costs, and system performance. This guide covers calculation methods for various heat exchanger configurations.
Components of Pressure Drop
Total pressure drop consists of:
- Frictional losses - Due to wall friction
- Acceleration losses - Due to velocity changes
- Gravitational losses - Due to elevation changes
- Minor losses - Due to fittings, bends, etc.
Single-Phase Tube-Side
Darcy-Weisbach Equation
ΔP = f × (L/D) × (ρV²/2)
Friction Factor Correlations
Laminar flow (Re < 2300): f = 64/Re
Turbulent flow (Blasius): f = 0.316 × Re^(-0.25) for Re < 10^5
Turbulent flow (Colebrook-White): 1/√f = -2log(ε/3.7D + 2.51/Re√f)
Typical Design Values
- Liquids: 10-50 kPa per pass
- Gases: 1-5 kPa per pass
Two-Phase Pressure Drop
Homogeneous Model
Treats mixture as single fluid with average properties: ΔP_tp = f_tp × (L/D) × (G²/2ρ_m)
Separated Flow Model (Lockhart-Martinelli)
ΔP_tp = ΔP_l × φ_l²
Where φ_l is the two-phase multiplier based on Martinelli parameter X.
Friedel Correlation
More accurate for refrigerants: φ_lo² = E + 3.24FH / (Fr^0.045 × We^0.035)
Air-Side Pressure Drop
Plain Fins
ΔP = f × (A_o/A_c) × (ρV_max²/2)
Wavy/Louvered Fins
Use appropriate friction factor correlations from literature.
Typical Design Values
- Evaporator coils: 50-150 Pa
- Condenser coils: 30-100 Pa
- Heating coils: 50-200 Pa
Shell-Side Pressure Drop
Bell-Delaware Method
Accounts for:
- Cross-flow pressure drop
- Window pressure drop
- Baffle leakage effects
ΔP_s = ΔP_c × R_b × R_l + ΔP_w × N_b
Kern Method (Simplified)
ΔP_s = f × G_s² × D_s × (N_b + 1) / (2ρ × D_e × φ_s)
Minor Losses
K-Factor Method
ΔP = K × (ρV²/2)
Typical K values:
- 90° elbow: 0.3-0.9
- Tee (branch): 1.0-1.5
- Sudden expansion: (1 - A₁/A₂)²
- Sudden contraction: 0.5(1 - A₂/A₁)
Equivalent Length Method
Convert fittings to equivalent pipe length: L_eq = K × D / f
Design Considerations
Allowable Pressure Drops
| Application | Tube-side | Shell/Air-side |
|---|---|---|
| Water systems | 35-70 kPa | 20-50 kPa |
| Refrigerant | 20-50 kPa | 30-100 Pa |
| Process fluids | 10-100 kPa | Varies |
Trade-offs
- Lower ΔP = larger equipment, lower operating cost
- Higher ΔP = smaller equipment, higher operating cost
- Optimize for total cost of ownership
Conclusion
Accurate pressure drop calculations ensure proper system design and efficient operation. Consider all components and use appropriate correlations for the specific application.