The source of heat is commonly steam condensing in the vessel jacket. Practical considerations of importance are:
1. There is the minimum of air with the steam in the jacket.
2. The steam is not superheated as part of the surface must then be used as a de-superheater over which low gas heat-transfer coefficients apply rather than high condensing coefficients.
3. Steam trapping to remove condensate and air is adequate.
The action of the agitator and its ability to keep the fluid moved across the heat transfer surface are important. Some overall heat transfer coefficients are shown in Table 6.1. Save for boiling water, which agitates itself, mechanical agitation is assumed. Where there is no agitation, coefficients may be halved.