Relationship between Flow Rate and Pressure Drop

Article details

Relationship between Flow Rate (flow rate) and Pressure Drop

(Pressure drop) in pipe systems such as cold water systems is important to system design and control.

HVAC, pump systems and other industrial processes have a fundamental relationship based on the laws of physics:


✅ Summary of main relationships

1. Quadratic Relationship

In the case of turbulent flow (Turbulent flow), which is commonly found in pipes.
⟶ Pressure Drop ∝ (Flow Rate)²

For example:
If the flow rate is increased to 2 times → the pressure drop will increase to 4 times.

2. In the case of smooth flow (Laminar flow)

Often found in very viscous or low velocity liquids.
⟶ Pressure Drop ∝ Flow Rate

For example:
Increase Flow rate 2 times → Pressure Drop increase 2 times.

🔧 Basic equations (Darcy-Weisbach Equation)

\Delta P = f \cdot \frac{L}{D} \cdot \frac{\rho v^2}{2}
Or in terms of Flow rate (Q):
\Delta P ∝ Q^2

where:

 • \Delta P = pressure drop (Pressure drop)
 • Q = flow rate (Flow rate)
 • f = coefficient of friction
 • L = pipe length
 • D = pipe diameter
 • \rho = fluid density

📈 Picture explaining the relationship

Flow Rate increases, Pressure Drop increases by how much (Turbulent)
1 times 1 times
2 times 4 times
3 times 9 times

🧠 Examples of usage
 • Pump size selection: You must know how much Flow increases → How much Pressure Drop increases so that the pump can overcome the pressure.
 • Piping system design: If the pipe is too small → Flow rate is high → Pressure drop is very high → The system is not energy efficient.