Elsewhere in this encyclopedia the thermal resistance due to the laminar sublayer is discussed (see Boundary Layer Heat Transfer). Because of the attendant drag forces associated with the flow of viscous liquid, the viscous sublayer is generally quite thick or, in many instances, turbulent conditions in the core liquid cannot be even attained without intolerable pressure loss and excessive pumping costs. One way of overcoming the problem is to remove, physically, the layers of fluid at the heat transfer surface and to mix them with the bulk fluid in the heat exchanger. In this way, if the fluid is being heated, heat is conveyed directly from the wall to the bulk liquid. The technique is particularly attractive for heat sensitive liquids, e.g. food or pharmaceutical products, because of the low interface temperature between the liquid and heat transfer surface for a given overall temperature driving force.
In the scraped surface heat exchanger, spring loaded rotating blades scrape the surface and effectively remove liquid from it. Alternatively, the blades move against the heat transfer surface under the influence of the rotational forces. At the same time as liquid layers are removed, any fouling substance deposited on the surface is also removed, thus ensuring that contamination of the process liquid is reduced to a minimum this can be crucial where taste and texture are important product qualities.
Figure 1 illustrates the principle of the scraped surface heat exchanger. The number of scraper blades shown is four, but any number of blades may be employed, although as the number of blades is increased the capital cost rises. Furthermore, a large number of blades are not necessary, since the time internal between successive scrapes is relatively short, i.e., the residence time of particles of liquid on the surface is low. The choice of the number of blades is an empirically based compromise between capital cost, acceptable speed of rotation and liquid viscosity. Because of the rotating parts, maintenance charges may be relatively high.
Although often classified with scraped surface heat exchangers, some exchangers have blades that do not actually touch the surface over which they pass, but move in close proximity to it. Such designs may be termed "wiped surface" heat exchangers, and may be preferred, where the wear of surface or blades cannot be tolerated from a mechanical point of view or because of contamination effects.
Scraped surface heat exchangers can either run full of liquid or the liquid may enter the exchanger as a peripheral stream. The former may be mounted either vertically or horizontally but it is usual for the latter, although not exclusively, to be mounted vertically so that the liquid flows downwards under the action of gravity. Where evaporation is taking place, for instance in a concentrating process, this is the preferred arrangement; the vapor being removed under vacuum from the top of the evaporator assembly. The residence time in the equipment may be quite low; desirable for processing heat sensitive liquids. Agitated thin film evaporators, as they are sometimes called, have been discussed by Salden (1987). In addition to the advantages already mentioned this author points out that the equipment is suitable for multipurpose applications and that evaporation to dryness is feasible where a solid product is required.
Attempts to correlate heat transfer in scraped surface equipment have been made and these make use of the usual dimensionless groups including Nusselt, Prandtl, and Reynolds Numbers common to many heat transfer correlations but also involving the so-called "rotary" Reynolds number defined as:
where D is the diameter of the exchanger body, N is the speed of rotation r.p.m., and ρ and η are the liquid density and viscosity, respectively. The aspect ratio of the exchanger and the number of blades may also be taken into account in the correlation.
Having regard to the complexity of the geometry particularly of the blade assembly, and the several interacting factors, design is more usually based on empirically determined parameters derived from experience. Scraped surface heat exchangers are, in general, used only for special applications.
Salden, D. M. (1987) Agitated thin film evaporators, Distillation Supplement Chem. Engr., Sept., 17-19.