The best answer to this question is that under the absolute best of circumstances the blend time for two impellers will be half the time for one impeller and the pumping capacity for two impellers will be twice the pumping capacity for one impeller.  First, both of the impellers need to be axial flow (pitched-blade or hydrofoil) impellers, not radial flow.  Then, the stated spacing looks good.  However, to be somewhat conservative, a recommended blend time for two impellers should be about 60% of the blend time for one impeller and the pumping capacity about 1.8 times the pumping capacity of one impeller.  Without specific tests for the conditions involved, these estimates should provide basic guidance for the process.

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