When it comes to impedance systems, not all systems are made equally. Yes, all impedance systems heat by passing current through a pipe. Yes, they all have a set temperature they can maintain. But what about the amount of time it takes the system to heat from startup? What about the ability to increase the heat of a product as it flows through a pipe? What can each system do? Or, as usually asked in the initial design phase, what does this system need to do? That simple question opens the door for many possibilities when it comes to the design of an impedance heating system.
An impedance system is a wonderful method to maintain temperature in a pipe, but it can also be used very effectively as a method to heat a pipe from ambient and also to increase the heat of a product flowing through a pipe. The performance needed from the system is a very crucial part of the initial design phase. A system that needs to only maintain temperature or provide freeze protection will require different control and power than one that must increase the heat of a product as it flows through the pipe.
Generally, if someone is looking for a heating solution they know what their heating requirements are, and it is just a matter of engineering a system that meets the needs of the user. First, let’s look at simple temperature maintenance.
Temperature Maintenance/Freeze Protection
Temperature maintenance and freeze protection systems are nearly identical. With freeze protection, the impedance system only needs to keep the product temperature above a certain value (the freezing point of the product). An example of this might be an external transfer line.
A temperature maintenance system does the exact same thing, with the exception that the product enters the heated pipe section at a given temperature and the system maintains that temperature through the length of the pipe (temperature out = temperature in). A vegetable oil that needs to be maintained at some temperature for proper mixing would be an example.
For maintenance/freeze protection systems, we look at the heat losses as the product moves through the pipe. This value is determined by evaluating the expected difference between the ambient and the product temperatures and the amount and type of insulation used. With this information, a simple system can be designed to offset the thermal losses, ensuring that the product will maintain the desired temperature throughout the pipe configuration. A system designed for temperature maintenance will be extremely efficient, and exert the lowest possible load on the plant’s power system, but will be the most limited. Generally, this system will use a simple on/off control scheme, and require the least amount of control hardware. On the downside, if the system is ever turned off, what happens? Unless the line is drained, the result will be a line full of product that has cooled. If the product will flow at ambient, cooling is not a significant problem, but if it becomes excessively viscous at ambient, then another method of heating the pipe to operating temperature will need to be found or the pipe will need to be cleaned out. An alternative impedance system design is to upsize the system to add warming/heating capabilities to a temperature maintenance system.
An impedance system can be designed that will not only maintain the product temperature in normal operation, but also has the capability to warm the pipe up to operating temperature when required. This can be a necessity for facilities that suffer from power losses or unplanned shutdowns that cause the product temperature in the pipe to drop lower than desired. For this type of system, the first consideration is the temperature maintenance needs, as described above. The second consideration is how much power is needed to warm the system from ambient to operating temperature with a pipe full of product. Once the power requirements are known, the final consideration becomes the time required to reach operating temperature. A system can be designed to heat from ambient to operating temperature in anywhere from a few minutes to several days. The design is dependent on the requirements of the customer and the temperature limits of the product. This design configuration results in a transformer several times larger than the transformer required for temperature maintenance only, with a higher secondary current, and cabling and connections designed to match. While the normal power usage for this configuration will be close to a standard maintenance system, the increased current required for raising the temperature will result in slightly higher losses in system components outside the pipe, and slightly lower efficiency.
Additionally, if the system is on/off control, it will have a larger impact on the plant power supply when on than a temperature maintenance system, with components designed to handle the larger load, even though it will have longer off times. A solution to reducing large electric load swings is to utilize silicone rectification controls (SCR). SCR control smooths out the power supply load swings but does increase the cost of system control components by approximately 10%. All of this is dependent on just how quickly the system needs to raise the temperature and by how much.
Product Temperature Increase
The final type of impedance heating system, and the most uncommon, is designed to raise the temperature of the product flowing through the pipe so that the product temperature at the pipe output is higher than the temperature at the input. While uncommon, this application can be easily accomplished with an impedance system. It has the highest power requirements, with the largest impact on the facility’s distribution system. In addition to the standard considerations of heat losses, it is frequently necessary to add pipe length in order to have time to raise the product temperature appropriately. This is mostly dependent on product flow rates.
As a system is designed, it is important to keep in mind the performance requirements. Does the system need to thaw or reheat product, or is maintenance enough? Does it need to raise the product temperature? Impedance system designs can be very flexible, with pros and cons to consider for every application. In the end, the designer and the customer need to ask themselves what the key performance objectives of the impedance pipe system are? What does the process need, and what would be of the most benefit? By asking these questions, the needs can be pinpointed, and a system designed that will provide successful reliable and cost-effective operation over many years.
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