Heat Transfer In Agitated Jacketed Vessels: The principles are straightforward, but their application is tricky

Agitator-equipped vessels with jackets for heating or cooling are drawing increased attention in the process industries. Apart from a growing importance in biotechnology (box, p. 94), such vessels are widely used in a variety of other process applications. Accordingly, engineers can benefit from a working knowledge of how heat-transfer and temperature-control principles apply to such vessels.

The rate of heat transfer to or from an agitated liquid mass in a vessel is a function of the physical properties of that liquid and of the heating or cooling medium, the vessel geometry, and the degree of agitation. The type, and size of the agitator, as well as its location in the vessel, also affect the rate.

These values of the agitator parameters are set by the given mixing task (such as suspending or dispersing solids or gases, emulsifying an immiscible liquid, or fostering chemical reactions), usually before their effects upon heat transfer are considered. But if during operation, the course of the process proves to be governed mainly by the heat transfer, then such variables as log mean temperature difference and heat-transfer surface area will usually take on more significance than the agitation variables. In either case, the mixing can affect only the heat-transfer resistance on the inner vessel wall, which (as pointed out in Equation [6]) is but one of the resistances that determine the overall heat-transfer coefficient.

Many jacketed vessels are reactors, so exothermic or endothermic effects must be taken into account. Furthermore, in many applications employing jacketed vessels, successive batches of material are heated (or cooled) to a given temperature, so the heat transfer is unsteady-state.

Quick review sets the stage

In a vessel containing an agitated liquid, heat transfer takes place mainly through conduction and forced convection, as it does in heat exchangers. So, the starting point for heat-transfer calculations involving such vessels is the resistance or film theory that applies to exchangers. The heat flow and the calculation procedures may best be explained by building step by step upon the basic film-theory equation:

(1)

This equation is not available electronically. Please see the January, 1999 issue.

where the heat-flow rate per unit area is in (for instance) Btu/(h)(ft superscript 2), the driving force is the temperature difference in degrees Fahrenheit, the...