Once the total volume of an individual vessel has been established the detailed dimensions or mechanical design can be performed. Much of this is based on "Rules of Thumb" built from years of experience. A summary is provided below for a standard Stirred Tank Reactor (STR).
Perhaps the most important factor in the mechanical design of a bioreactor is the height to diameter or aspect ratio as this determines many of the other important bioreactor characteristics. There are relative advantages of increasing either the height or diameter.
(a) Increasing height
i. increases the hydrostatic pressure
Advantage - increases solubility of oxygen
Disadvantage - increases solubility of other gases which may be toxic, e.g. CO2
ii. allows more agitators
Advantage - better power development
Disadvantage - can lead to poor mixing due to compartmentalisation
(b) Increasing diameter
i. lowers the superficial (linear) gas velocity
Disadvantage - poor mixing, oxygen transfer
ii. reduces wall surface area (per unit volume) available for heat transfer
Disadvantage - leads to temperature control problems