I. Bernoulli: Energy Conservation of Fluid
For ideal, incompressible, steady flow, along a streamline:
p/ρ + v²/2 + g·z = constant
The three terms are pressure energy, kinetic energy, and potential energy. It explains why a narrower pipe gives faster flow and lower static pressure (Venturi effect), and why an elevated water tank can discharge by gravity.
II. Reality Adds a "Friction Account"
Real fluids have viscosity and pipe walls have roughness, so in practice:
p₁/ρ + v₁²/2 + g·z₁ = p₂/ρ + v₂²/2 + g·z₂ + hf
hf is the energy loss per unit mass of fluid, ultimately manifesting as pressure drop Δp. The pressure drop in a heat exchanger system comes from two parts:
- Friction loss (along path): Friction between fluid and pipe wall, growing with pipe length, roughness, and the square of flow velocity;
- Local resistance: Elbows, reducers, inlets/outlets, plate corrugation disturbances—each is a "hit the wall."
III. The Entanglement of Pressure Drop and Heat Transfer
As mentioned, h ∝ u^0.8, while pressure drop Δp ∝ u² (or even higher). Therefore:
- Doubling the flow velocity raises heat transfer by about 0.8 times, but fan power by about 2~3 times;
- There exists an economic flow velocity; beyond it, electricity cost exceeds heat transfer gains.
This is the fundamental reason a heat exchanger is not "the faster the better"—the Bernoulli and friction laws together draw a red line for flow velocity.
IV. How to Select Fans/Pumps
In selection, first calculate the total system pressure drop (heat exchanger body + piping + air outlets), then select equipment whose air volume/flow matches that pressure-drop point, leaving a 10%~20% margin is enough—do not blindly oversize: an oversized fan operating year-round away from its high-efficiency zone is a black hole of electricity. If during operation you find insufficient air volume, first check whether the plates are clogged or the filter is dirty—abnormally high pressure drop is often the first signal of ash accumulation.