Thermal processing is an essential part of many production processes. Whether cooking, pasteurizing or sterilizing, or heating or cooling a range of products, it is a safe bet that many manufacturers will be using a heat exchanger to carry out their thermal processing requirements. Yet, with such a variety of applications possible, it is important that you select the right heat exchanger for your individual requirement.
A number of heat exchanger types are on the market, including plate, tubular, corrugated tube and scraped surface. Each is suited to a particular application. A smart first step when selecting a heat exchanger for a process heating application is to think carefully about your process, including:
- The nature of the materials to be heated or cooled.
- The objective of the process such as heating or pasteurization.
- Any restrictions of the environment where the heat exchanger is to be used.
The driving force for heat transfer is the difference in temperature between the two substances. (In most cases for heat exchangers, the two substances will be fluids.)
In the case of a smooth tubular heat exchanger, the temperature of two simple fluids changes as they pass through the heat exchanger. One of the reasons for making corrugated tube and scraped-surface heat exchangers is that they are suitable for fluids and materials with complex properties, such as viscous and non-Newtonian fluids, or for materials containing particles or sediment. You should, therefore, always be mindful of the material to be processed before selecting your heat exchanger. It is a good idea to seek professional advice from manufacturers and their agents to help with the selection process.
Once the correct type of exchanger has been chosen, processors must then make sure that the model supplied is correctly sized for the job. In other words, the specifier should be sure that the heat exchanger selected offers the right amount of heat transfer for the fluids being treated and at the throughput required. The heat exchanger must have a large enough heat transfer area for the specified fluids and their specified inlet and outlet temperatures. Most calculations should also factor in variables such as whether the heat exchanger operates using counterflow or parallel flow.
Determining when laminar flow becomes turbulent flow is critical to equipment performance.
Breaking Down Thermal Barriers
Another important factor controlling heat transfer is the resistance to heat flow through the various layers that form a barrier between the two fluids. There are effectively five of these layers:
- The inside boundary layer formed by the fluid flowing in close contact with the inside surface of the tube.
- The fouling layer formed by deposition of solids or semi-solids on the inside surface of the tube (which may or may not be present).
- The thickness of the tube wall and the material used, which will govern the resistance to heat flow though the tube itself.
- The fouling layer formed by deposition of solids or semi-solids on the outside surface of the tube (which may or may not be present).
- The outside boundary layer formed by the fluid flowing in close contact with the outside surface of the tube.
The values for the second, third and fourth items can usually be supplied by the client based on experience, while the designer of the heat exchanger will select the tube size, thickness and materials to suit the application. The partial heat transfer coefficients — that is, the resistance to heat flow resulting from the first and last items listed previously — depends both on the nature of the fluids and the geometry of the heat transfer surfaces themselves.
One way to prevent the buildup of these layers is to increase the speed at which the fluid passes through the heat exchanger so that turbulence is formed and the boundary layer breaks away from the surface of the tube. This is the point at which so-called laminar flow (with the fluid passing through in smooth layers, where the innermost layer flows at a higher rate than the outermost) becomes turbulent flow (where fluid does not flow in smooth layers but is mixed or agitated as it flows).
The speed at which this occurs is influenced by many different factors, but in order to quantify it for the purpose of specifying heat exchangers, engineers use the Reynolds number (Re). This 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 this zone as much as possible. Tube deformation such as corrugation helps to increase the heat transfer performance once the fluids have entered the turbulent flow area (Re greater than 2,000). This is the main reason for using corrugated tube heat exchangers.
Using a heat exchanger to handle difficult materials containing particles or sediment needs careful consideration.
As with any kind of science, the mathematics and understanding of thermal dynamics is continuing to evolve and improve. However, much of the literature commonly used to build calculations and model heat exchanger performance can be up to 80 years old and does not always reflect the most recent science. Also, while there is scientific literature for the behavior of fluids in smooth and corrugated tubes, there is little published data on scraped-surface heat exchangers. Using experience and the most recent scientific data available, some heat exchanger makers have produced software programs to perform the calculations. These programs can be used to calculate the necessary size of heat exchangers. This software is already producing some interesting results and giving new insights into how best to design tubular and scraped-surface heat exchangers that provide the very best levels of performance.