Improving Heat Transfer in Heat Exchangers
In the chemical processing industries, heating, cooling and other thermal processing of viscous fluids are an integral part of the unit operations. In these processes, mechanical heat transfer predominantly occurs by screws or scraping vanes. But, thermal processing by conventional equipment with mechanically moving parts has some shortcomings, including abrasion, contamination, fluid leakage and abnormal stagnation of fluid. Furthermore, conventional heat exchanger designs may not be able to meet the requirements of process. For those applications, adding motionless mixer elements can help. A heat exchanger equipped with static mixer elements is free from the shortcomings mentioned, and it can handle viscous fluids at very low Reynolds numbers.
Heat Transfer RatesHeat exchangers with mixing elements installed have achieved heat transfer coefficients approximately three to five times larger than that of empty tubes. With the mixing elements, heat is uniformly transferred, and no local heating or cooling occurs. Fluid temperature is kept uniform around all cross-sections during all stages of processing.
No Moving PartsRevolving scrapers can provide some of the same heat transfer benefits as static mixing elements. But, heat exchangers fitted with revolving scrapers may consume more energy, suffer from mechanical difficulties and fluid leaks, and contaminate the product being processed with metallic tailings from scraping. Also, with revolving scraper-equipped heat exchangers, the heat transfer coefficient may decrease as fluid viscosity increases.
Reduced Film BuildupFor both heating and cooling applications, the static mixer element design reduces film buildup on the inside walls of the heat exchanger through its mixing action. The process fluid continuously is pushed from the center of the tube wall and back again. To allow for heat conduction from the tube to the elements, the elements are sealed into place where the edges are nicrobrazed onto the walls.
Decreased Size RequirementsDue to the increased inside film coefficients and heat transfer rates provided by motionless mixer elements, heat exchanger designs utilized with static mixers can be smaller than conventional shell-tube heat exchangers.
In summary, heat exchangers utilizing static mixer elements can provide the following process advantages:
- Improved overall heat transfer coefficients.
- Increased inside film transfer coefficients.
- Continuous self-cleaning action.
- Plug flow eliminates temperature gradients.
- Reduced size and space requirements.
Static mixer elements have been used in many applications, including polymers such as nylons, polyesters, silicones, adhesives and sealants; plastics processing operations such as extrusion and injection molding; energy applications such as petrochemicals and fuel oil; and foods processing operations such as dairy products, sugar syrups and cooling of sauces.