Pneumatic hose, pipe diameter, pressure and flow rate - the basis for scientific selection and calculation

Introduction
Choosing pneumatic hoses based solely on experience or the principle of "larger is better" may lead to slow system response, increased energy consumption, or wasted costs. Scientific selection is based on an understanding of the relationship between pipe diameter, pressure drop, and flow rate.

Core concept: Pressure drop is the key limiting factor When compressed air flows through a hose, pressure loss (pressure drop) occurs due to friction. Excessive pressure drop can lead to insufficient pressure at the end of the actuator (cylinder), decreased output, and slower movement. The goal of selecting a hose is to control the pressure drop in the pipeline within an acceptable range (it is generally recommended that the pressure drop of the entire pipeline system not exceed 10% of the working pressure) while meeting the flow demand.

Selection calculation steps

1. Determine system requirements:

   • Working pressure (P): For example, 0.6 MPa.

   • Required flow rate (Q): Calculate or estimate the maximum air consumption when the actuators operate simultaneously (usually measured in standard liters per minute, SLPM, or cubic meters per minute).

   • Hose length (L): The actual length of the hose from the air supply point to the farthest air consumption point.

2. Estimate using a pressure drop table or formula:

   • The easiest way is to refer to the "pressure drop-flow rate-diameter" comparison table provided by the hose manufacturer.

   • General estimation formula (simplified for laminar flow state): ΔP ≈ (Q * L * μ) / (π * d^4 * ρ) (where ΔP is pressure drop, d is inner diameter, μ is viscosity, and ρ is density). This formula indicates that the pressure drop is proportional to the first power of the pipe length and inversely proportional to the fourth power of the pipe inner diameter. Increasing the pipe diameter has a significant effect on reducing the pressure drop.

3. Select pipe diameter:

   • Based on the estimated flow rate and length, find the minimum pipe diameter corresponding to the target pressure drop (such as 0.05 MPa) in the comparison table.

   • Empirical rule: For small and medium-sized equipment, the main pipeline can be Φ10 or Φ12; the branch to the valve can be Φ8; and the valve to the cylinder can be Φ6 or Φ8. When driving large-diameter cylinders at high frequencies or over long distances, the pipe diameter needs to be increased.

Instance description
A device with a main air pipe length of 10 meters, a maximum instantaneous flow rate of 1000 SLPM, and a target pressure drop less than 0.07 MPa Check the pressure drop table of a certain brand of PU tube:

• The Φ10 pipe has an inner diameter of approximately 6.5mm: at a flow rate of 1000, the pressure drop is approximately 0.12 MPa/10m → exceeding the standard.

• The Φ12 tube has an inner diameter of approximately 8.5mm: at a flow rate of 1000, the pressure drop is approximately 0.04 MPa/10m → acceptable.
  Therefore, a hose with an inner diameter of not less than 8.5mm (corresponding to an outer diameter of Φ12) should be selected.

Conclusion
Scientific pipe selection is not a complex mathematical examination, but rather the judicious use of manufacturer data and basic formulas for trade-offs. Remember the core principle: where layout permits, selecting a larger pipe diameter for critical gas supply paths and circuits with high gas consumption is often the most economical investment to enhance the overall system response speed and energy efficiency.