I. Where System Pressure Drop Comes From
The total pressure drop of a heat exchange system = heat exchanger body + piping + air outlet/filter + elbows and reducers. If any link is enlarged, the fan electricity bill rises with it. Still taking the 12000 m³/h air volume as an example, assume a system total pressure drop of 350 Pa.
II. Calculate Fan Shaft Power
Air volume Q = 12000 / 3600 ≈ 3.33 m³/s, fan efficiency η taken as 0.6:
P = Q · Δp / η = 3.33 × 350 / 0.6 ≈ 1942 W ≈ 1.94 kW
Annual operation 8000 hours, electricity cost ≈ 1.94 × 8000 × 0.7 ≈ 10,900 RMB.
III. The "Square Penalty" of Pressure Drop
Approximately Δp ∝ u²; raising the wind speed by 20% increases pressure drop by about 44%, and the electricity bill rises accordingly. Therefore:
- Do not design the flow channel too narrow, avoiding needless high velocity;
- Select filter and heat exchanger by resistance; do not leave excessive margin "for safety";
- Select the fan in its high-efficiency zone; a 10%~20% margin is enough, too large and it operates year-round away from the high-efficiency point.
IV. Selection Checklist
| Item | Practice |
|---|---|
| Air volume | By max condition + 10% margin |
| Total pressure | System total resistance + 10%~20% margin |
| Efficiency | Select the point on the fan characteristic curve's high-efficiency zone |
| Variable frequency | Use VFD when load fluctuates greatly, supply air on demand |
The heat exchanger's gain is in heat, the fan's cost is in electricity; the two must be put on the same ledger.
Related Reading
- The Bernoulli Equation and Pipeline Pressure Drop: The Energy Ledger of Fluid in a Heat Exchanger
- Boundary Layer and Flow Regime: How Laminar and Turbulent Flow Affect Heat Transfer
- Fluid Thermophysical Properties: The Impact of Density, Viscosity, Specific Heat, and Thermal Conductivity on Heat Transfer