Impeller tip speed classes: larger-diameter marine propellers typically operate at which rotational speed range in bioreactors?

Introduction / Context:
Impeller selection balances mixing intensity, shear, and power input. For a given tip speed target, larger-diameter propellers turn more slowly than small impellers. Knowing the typical rpm range helps in preliminary design and scale-up checks.

Given Data / Assumptions:

• Marine propellers are axial-flow impellers commonly used for low-to-moderate viscosity broths.
• Larger diameter implies lower rpm for a given tip speed (u_tip = π*D*N).
• Excessive rpm increases shear and may damage cells.

Concept / Approach:
To achieve practical tip speeds (for example, 2–6 m/s) in large tanks, rotational speeds are modest. Ranges of 20–150 rpm are typical for large-D propellers, whereas several hundred rpm is characteristic of small lab-scale impellers. Very high rpm (approaching 1000+) is associated with tiny impellers or non-bioprocess mixers.

Step-by-Step Solution:
1) For fixed u_tip, N scales inversely with D: N = u_tip / (π*D).2) As D increases in scale-up, N must decrease to keep shear and power in check.3) Practical ranges for large propellers fall in tens to low hundreds of rpm, i.e., 20–150 rpm.

Verification / Alternative check:
Vendor catalogs and scale-up case studies show marine propellers in production fermenters frequently below ~120 rpm at multi-meter diameters.

Why Other Options Are Wrong:
150–300 rpm and 400–800 rpm are typical of smaller D or lab systems; 1150 rpm is unrealistic for large bioreactor propellers; 5–15 rpm is too low to provide adequate mixing and O2 transfer in most cases.

Common Pitfalls:
Transferring lab rpm directly to plant scale without accounting for impeller diameter and tip speed constraints.

Final Answer:
20 to 150 rpm