- Freeze Protection
- Process Temperature Maintenance without Steam Exposure
- Process Temperature Maintenance with Steam Exposure
- High Temperature Heat Tracing
- Long-Line Heating
Electric heat tracing is used in many process industries to maintain process fluids at the desired temperatures. It is important to understand the requirements of the application as well as each heat-tracing technology’s capabilities and limitations before selecting a specific heat-tracing technology for the application.
Manufacturers have developed various heat-tracing cable technologies that are suited for certain applications. The range of electric heat-tracing technologies available on the market vary with respect to their maximum maintain temperature and their maximum circuit length capabilities.
At a high level, the technologies are categorized as series resistance or parallel resistance. The parallel-resistance technologies have advantages such as being able to be cut to length, field terminable, flexible and usually available off the shelf. The series-resistance technologies have advantages of being able to be used for longer circuit lengths from one power source.
A look at five types of heat-tracing applications will demonstrate some of the key characteristics and requirements that can help users select the most appropriate technology. These are most commonly encountered in the industrial sector and usually have unique needs.
The main purpose of a freeze-protection system is to prevent fluids such as process water, drain water and fire water lines from freezing in pipes. A plant does not necessarily need to be located in the extreme, remote cold regions of the world to experience freezing temperatures. Temperatures can easily drop below 32°F (0°C) overnight and put the process in jeopardy if the pipes are not properly insulated or heat traced.
The main purpose of a freeze-protection system is to prevent fluids such as process water, drain water and fire water lines from freezing in pipes.
Parallel self-regulating technology is well suited for this application. Self-regulating heating cables incorporate a heating element made of polymer mixed with conductive carbon black. This special formulation creates an electrical path for conducting current between the parallel bus wires along the entire cable length. In each heating cable, the number of electrical paths between the bus wires changes in response to temperature fluctuations. Simply put, as the surrounding temperature decreases, the heating cable increases its current flow to provide heat to the pipe, tank or vessel. Conversely, as the temperature increases, the heating cable reduces its current flow to provide less heat because it is not needed.
This self-regulating behavior is important for freeze-protection applications because it enables energy-efficient and cost-effective solutions for freeze protection. For freeze-protection applications, typically tight temperature control is not required, and a group of electric heat-trace circuits is controlled by a single ambient-sensing controller or thermostat. Once the ambient temperature falls below the preset temperature, the heaters are fully powered. The self-regulating heaters adjust their power output in response to their immediate ambient conditions and save energy. The self-regulating behavior also results in more uniform pipe temperatures than other technologies.
Self-regulating cables can be cut to length, so the wattage is not affected by the cable length. The cut-to-length feature is important because the pipe length in the field may be different than originally designed. The cut-to-length feature enables a true off-the-shelf offering for urgent needs.
Although most water lines are maintained at 35 to 40°F (2 to 4°C), for freeze-protection applications, you want to be able to choose a self-regulating cable that can maintain process temperatures up to 150°F (65°C) and can withstand intermittent exposure temperatures up to 185°F (85°C).
With process temperature-maintenance applications, there is usually a need for viscosity control of certain fluids such as fuel oils, acids and fertilizers at higher temperatures beyond freeze-protection applications. These maintain temperature requirements typically range from 140 to 230°F (60 to 110°C). Self-regulating technology can still be used in this case. However, because the maintain temperatures are higher, the heat losses are high in these applications. Hence, it may make more sense to choose a cable that meets the temperature requirement and provides more power output — up to 10, 15 or even 20 W/ft. This will allow users to have fewer runs of cables while producing more heat for your application.
In some freeze-protection and process temperature-maintenance applications, the heat-tracing cable is exposed to higher temperatures than the maintenance temperature when steam is used to clean the pipes. In these applications, the cable not only needs to meet the maintain temperature requirements but also the exposure temperature. Although the steam-cleaning temperature exposure typically is for short term, it can damage the cable.
In other freeze-protection and process temperature-maintenance applications, the process normally operates at temperatures higher than the maintain temperature. In such cases, the heating cable needs to withstand these normal operating temperatures over the longer term.
In these applications, self-regulating technology is a preferred method of heat tracing. In addition to the benefits already cited, in these applications, using self-regulating cables results in lower cable-sheath temperatures, which is critical for hazardous area applications. Users should be able to specify a cable with maintain temperatures up to 250°F (120°C) and maximum exposure temperature of 420°F (215°C). The key is for the cable to be able to withstand the higher steam temperatures.
There are some high maintain temperature applications — sulfur, various fuel oils and lubes, waxes, resins and certain plastics, for example — where the fluid must be maintained at temperatures greater than 250°F (120°C) and the process could run up to very high temperatures such as 500°F (260°C). For such applications, power-limiting heaters could be a good choice. These heaters have some of the same characteristics of a self-regulating cable such as flexibility, parallel circuitry and cut-to-length construction. In addition, they have higher maintenance and exposure temperature capabilities.
Some applications such as asphalt or bitumen have a requirement for very high maintenance temperatures. Mineral-insulated (MI) technology is well suited for such applications because the cable can withstand maintain temperatures up to 1022°F (550°C) and exposure temperatures up to 1200°F (650°C). MI construction consists of conductors embedded in a highly dielectric magnesium-oxide insulation surrounded by a seamless metal sheath. This allows the cable to be rugged and to withstand harsh environments and cold climates. Power outputs can go as high as 61 W/ft.
Long-line heat-tracing applications are required when circuit lengths from 1,000 feet (300 meters) up to several miles are powered from a single power point. They are needed to heat long pipelines for freeze protection or for viscosity control and temperature maintenance of certain fluids. Applications include:
• Transfer lines between processing plants.
• Storage facilities needing to transfer product to and from tank farms.
• Loading and unloading facilities at piers for ocean transport vessels.
• Loading and unloading facilities at depots for rails and trucks.
Manufacturers have developed specialized technologies that can be used to heat trace long circuit lengths from a single power source. To choose the appropriate technology, you still need to determine the process requirements, including:
• Circuit lengths.
• Power outputs.
• Type of fluids.
• Pipe materials.
• Insulation type and thickness.
• Field labor costs.
• Temperature requirements within the project.
The long-line heat-tracing systems typically are engineered systems. Many variables can be customized to provide the most cost-effective and reliable solution for a specific application.
Skin-effect heat-tracing systems (STS) are well suited for extremely long pipelines. STS are custom engineered heat-management systems that can be designed for circuit lengths up to 15 miles (25 kilometers); power outputs up to 49 W/ft (150 W/m); and to withstand maintain temperatures up to 390°F (200°C) with exposure temperatures up to 480°F (250°C).
Self-regulating products also are available that can be used for long-line heating, but they are limited to a few thousand feet of circuit lengths. Polymer-insulated cables can also be used up to 12,000 feet of circuits. They offer advantage of field terminations, flexibility and high maintain and exposure temperatures.
The key point is to avoid limiting your options by specifying one particular type of technology for all applications. Understanding what your application requirements are will help you choose the most suitable heat-tracing technology for your specific application.