The mechanism of power consumption in a Votator†-type scraped-surface heat exchanger

✍ Scribed by A.M. Trommelen; W.J. Beek

• Publisher: Elsevier Science
• Year: 1971
• Tongue: English
• Weight: 793 KB
• Volume: 26
• Category: Article
• ISSN: 0009-2509
• DOI: 10.1016/0009-2509(71)80036-2

✦ Synopsis

In scraped-surface heat exchangers (SSHEs) the power consumed by rotating the shaft with blades can be high, in which case the effective heat transfer rate will be reduced. A study has been made to ascertain the mechanisms governing power consumption and to determine the influence of design and operating conditions on its magnitude.

Power consumption was measured under various operating conditions of a SSHE, by recording the torque and the rotational speed of the shaft. For most stirrers the power number, PO, is a unique function of the rotational Reynolds number, Re,. This is not valid for SSHEs. For the same value of Re,, PO increases with decreasing viscosity. This is caused by two phenomena, viz:

Due to viscous dissipation the temperature of the liquid between the edge of the blade and the wall increases and therefore the viscosity decreases.

The clearance between the edge of the scraper blade and the tube wall is dependent on the operating conditions and the physical properties of the fluid.

A model incorporating these effects has been derived for the power consumption. The parameters in this model have been determined from the experiments. The values of the parameters thus determined were in agreement with the results of the flow studies. An empirical equation for power consumption, which is more readily evaluated, has also been derived.