DYNAMIC SCRAPED SURFACE HEAT EXCHNAGER
Another type of heat exchanger is called "(dynamic) scraped surface heat exchanger". This is mainly used for heating or cooling with high-viscosity products, crystallization processes, evaporation and high-fouling applications. Long running times are achieved due to the continuous scraping of the surface, thus avoiding fouling and achieving a sustainable heat transfer rate during the process.
The most important technologies for indirect heat transfer use tubes (shell-and-tube exchangers) or flat surfaces (plate exchangers). Their goal is to exchange the maximum amount of heat per unit area by generating as much turbulence as possible below given pumping power limits. Typical approaches to achieve this consist of corrugating the tubes or plates or extending their surface with fins.
However, these geometry conformation technologies, the calculation of optimum mass flows and other turbulence related factors become diminished when fouling appears, obliging designers to fit significantly larger heat transfer areas. There are several types of fouling, including particulate accumulation, precipitation (crystallization), sedimentation, generation of ice layers, etc.
Another factor posing difficulties to heat transfer is viscosity. Highly viscous fluids tend to generate deep laminar flow, a condition with very poor heat transfer rates and high pressure losses involving a considerable pumping power, often exceeding the exchanger design limits. This problem becomes worsened frequently when processing non-Newtonian fluids.
The dynamic scraped surface heat exchangers (DSSHE) have been designed to face the above-mentioned problems. They increase heat transfer by:
· Removing the fouling layers,
· increasing turbulence in case of high viscosity flow,
· Avoiding the generation of ice and other process by-products.
The dynamic scraped surface heat exchangers incorporate an internal mechanism which periodically removes the product from the heat transfer wall. The product side is scraped by blades attached to a moving shaft or frame. The blades are made of a rigid plastic material to prevent damage to the scraped surface. This material is FDA approved in the case of food applications.