What is a dynamic scraped surface heat exchanger?
Dynamic scraped surface heat exchangers (DSSHEs) were developed to deal with problems encountered when cooling or crystallizing viscous fluids. What kinds of problems?
One way to think of a DSSHE is as a pipe inside a pipe. The inner pipe conducts a fluid that has to be cooled to be processed. The outer pipe carries a fluid that carries heat away from the inner pipe so the contents of the inner pipe can cool.
- If the product being cooled in the inner pipe remains a liquid, it obeys Pascal’s law. Pressure applied anywhere in a confined incompressible fluid is transmitted equally in all directions throughout that fluid. But the fluid isn’t confined, so it flows in the direction of lower pressure at the same time it is being cooled
- If the product in the inner pipe is cooled enough that it begins to crystallize, however, the pressure of the liquid forces the crystals to accumulate on the inner walls of the inner pipe. These crystals insulate the hot liquid inside them so that heat exchange becomes less efficient and the temperature of the product increases.
The moving scrapers of a DSSHE prevent this problem. They prevent crystal accumulation on the inner wall of the inner pipe. They allow the efficient transfer of heat into the coolant fluid of the outer pipe. Of course, heat exchangers do not have to be cylindrical pipes. They just need to have a coolant chamber surrounding a conducting chamber. And sometimes dynamic scrapers can be replaced with static fins.
When do you need a DSSHE?
There are two situations in which DSSHE’s are an optimal choice in heat exchangers.
- Cooling highly viscous fluids. As a viscous fluid loses heat in the heat exchanger, the ring of liquid closest to the wall of the heat exchanger becomes even more viscous. Hotter fluid in the center of the inner chamber flows faster than the cooler fluid, limiting heat exchange.
- Removing waxes from oils and oily mixtures. Waxes and oils usually have about the same boiling point, but the wax will usually have a much higher freezing point than the oil in which it is contained. In these processes, scrapers can be an efficient method of removing the wax from the oil.
Sometimes it is best to run a test to determine if crystallization is the most efficient method of cooling a mixture before scaling up the process. Sometimes a static plate and fin heat exchanger is sufficient for an industrial process. The choice of heat exchange technology will involve consideration of cooling cycle and residence time, solvent demand (including how much of a solvent can be recycled), and available methods of separation.
Let MMI Tank Works help you with all of your heat exchanger needs.
DSSHE’s are cost-effective for many food processing, oil refining, agricultural, and water treatment processes. Let MMI Tank Works provide you with a cost-effective build. Take advantage of our 46 years of steel fabrication experience to help you determine whether you need a DSSHE. MMI Tank Works can also repair your old heat exchanger, retrofit it for your new facility, or transport it across the United States to your new plant. Call us for all of your heat exchanger needs.