Consider scale-up of fungal fermentation from a $20 \mathrm{~L}$ model-type to $20,000 \mathrm{~L}$ prototype stirred tank reactor. The model-type and prototype have the same aspect ratio during scale-up. The impeller speed in the model-type is $500 \mathrm{rpm}$ and the scale-up criterion is constant shear.
The impeller speed in the prototype reactor will be $\_\_\_\_\_\_\_$ rm (Answer in integer).