Heat Exchanger Considerations for Cooling Difficult Materials
The key to specifying the correct equipment for your heating or cooling demand is to study your process and material. They will define the type of heat exchanger to use.
Industrial applications have numerous requirements for heat exchangers and thermal transfer. Heat exchanger uses span the gamut: compressor and engine cooling, transformer temperature regulation, heat recovery from various processes, and treatment of effluent, digestate and other waste streams, to name a few. What is more, industrial heat exchangers often have to deal with complex or high fouling materials in challenging operating environments.
Effective use begins with understanding exactly what you want your heat exchanger to achieve as well as the nature of the material you will be cooling or heating. Knowing such information is vital to specifying equipment that will perform well for years. With such a variety of applications possible, it is important that you select an appropriate heat exchanger for your individual requirements.
Many types of heat exchanger are available: plate, tubular, corrugated tube, scraped surface, etc. Each type is suited to particular applications. To make an educated selection, consider the specifics of your process:
- The material to be heated or cooled.
- The objective of the process such as preheating or pasteurization.
- Any restrictions in the environment where the heat exchanger is to be used.
The key to efficient heat transfer is the difference in temperature between the two substances, which are usually — but not always — fluids. With a smooth tubular heat exchanger, the temperature of two simple fluids changes as they pass through the heat exchanger. The smooth flow within the material tube, however, means that anything other than simple Newtonian fluids may not be treated efficiently.
One way to overcome this limitation is to use corrugated-tube or scraped-surface heat exchangers. Such designs are suitable for fluids and materials with complex properties: viscous and non-Newtonian fluids as well as materials containing particles or sediment. You should, therefore, always be mindful of the material to be processed before selecting your heat exchanger. Consider seeking professional advice from manufacturers and their agents to help with the selection process if the fluid is difficult to process.
This scraped-surface heat exchanger handles difficult heat transfer applications efficiently.
Other important considerations include:
- The way in which materials flow through the heat exchanger.
- The design needed to allow for resistance to heat flow.
Resistance to heat flow occurs in the layers that form a barrier between the two fluids on either side of the exchanger’s walls. There are as many as five such layers:
- The inner boundary layer, which is formed by the fluid flowing in close contact with the inside surface of the tube.
- The inner fouling layer, which is formed by deposition of solids or semi-solids on the inside surface of the tube. (This layer may not be present in all fluids, but it is particularly found with digestate.)
- The thickness of the tube wall and the material used, which will govern the resistance to heat flow though the tube itself.
- The outer fouling layer, which is formed by deposition of solids or semi-solids on the outside surface of the tube. (This may not be present in all fluids.)
- The outer boundary layer, which is formed by the fluid flowing in close contact with the outside surface of the tube.
This scraped-surface heat exchanger uses a helical screw to prevent layer buildup.
The heat exchanger designer will select the tube size, thickness and materials to suit the application. The values used to calculate the fouling layers usually can be based on experience. The resistance to heat flow resulting from the inner and outer boundary layers — also known as the partial heat transfer coefficients — depend both on the nature of the fluids and the geometry of the heat transfer surfaces. These values also are determined by a combination of experience and mathematics.
The Importance of Turbulence
Boundary layers occur in tubes where the flow rate or fluid viscosity is such the fluid flows in parallel layers. Called laminar flow, in this flow regime, the fluid passes in smooth layers, where the innermost layer flows at a higher rate than the outermost. Such parallel layers do not encourage fluid mixing or exposing all of the fluid to the heat transfer surfaces. By contrast, with turbulent flow, the fluid does not flow in smooth layers but is mixed or agitated as it flows.
One way to prevent the buildup of boundary layers is to increase the speed at which the fluid passes through the heat exchanger. With higher flow, turbulence is formed, and the boundary layer breaks away from the tube surface.
The speed at which flow changes from laminar to turbulent is influenced by many factors. To quantify it for the purpose of specifying heat exchangers, the Reynolds number (Re) is used. This value is determined by the diameter of the tube, the mass velocity of the fluid and its viscosity. Reynolds numbers of less than 2,100 describe laminar flows while numbers above 10,000 describe full turbulent flow. Between the two values is an area of uncertainty called the transitional zone, where we see a general transition from full laminar to full turbulent flow.
In practice, engineers try to provide solutions outside of the transitional zone as much as possible. Tube deformation such as corrugation helps to increase the heat transfer performance once the fluid or fluids have entered the turbulent flow area (Re greater than 2,100). This is the main reason for using corrugated-tube heat exchangers.
Determining when laminar flow becomes turbulent flow is critical to equipment performance.
When Corrugations Are Not Enough
For many materials, the turbulent flow created in corrugated-tube heat exchangers is sufficient to prevent the buildup of boundary layers and maintain the efficiency of the heat transfer. For the most challenging situations — effluents, digestate from anaerobic digestion, viscous materials or those containing particles — further agitation or the physical removal of deposits from inside the heat exchanger tubes is required.
Two types of scraped-surface heat exchanger are available for such purposes. The simplest design uses a helical-screw mechanism or a scraper rod within the tube. In some cases, velocities of 300 rpm can be achieved. This provides high levels of shear and mixing at the heat transfer surface, which increases heat transfer rates. Such scraped-surface heat exchangers are suitable for challenging heat transfer applications such as those where the product has the potential to crystallize during processing or where aeration is required.
For more challenging materials, both a helical mixing spiral — which reduces the pressure drop in the tube — and a series of scraper blades are combined on the scraper rod. Together, these provide a continuous scraping action that mixes highly viscous products and reduces fouling. The design allows high viscosity products to be pumped with reduced backpressure and lower energy use. The design presents a relatively compact unit with a smaller footprint than traditional heat exchangers used for similar applications. Also, due to helical spiral, the heat exchanger can be run in reverse in many applications. This allows valuable products to be recovered before routine cleaning or product changeover occurs — without additional pigging or flushing systems.
In some applications, the material being cooled or heated has a high fouling potential but needs delicate handling to preserve fragile product integrity. While rotary scrapers are one solution, there are others. One is a stainless steel scraping mechanism that is hydraulically moved back and forth within each interior tube. This movement performs two functions. First, it minimizes potential fouling by keeping the tube wall clean. Second, the movement creates turbulence within the material. Both of these actions help to increase heat transfer rates. Together, they create an efficient heat transfer process for viscous and high fouling materials.
Also, the separate hydraulic action means that the speed of the scrapers is highly controllable. It can be optimized for the product being processed, so materials that are susceptible to shear stress or pressure damage can be handled gently. This helps to prevent damage while providing high levels of heat transfer.
Once the correct type of exchanger has been chosen, you then need to make sure that it is sized correctly. This will help ensure it offers the right amount of heat transfer for the materials being treated while providing the required throughput.
The heat exchanger must have a large enough heat transfer area for the specified fluids and their specified inlet and outlet temperatures. Most calculations also should factor in other variables such as whether the heat exchanger operates using a counterflow or parallel-flow arrangement. PC