Increasing and maintaining temperatures in process piping is important to the operation of many industrial facilities. Selecting a heating system is not usually a quick and easy decision because there are short- and long-term cost and operability implications. Important factors when making this decision include:
- Effectiveness. Will the system perform as intended?
- Capital cost.
- Maintenance cost.
- Energy efficiency.
Traditionally, fully jacketed pipe and tube tracing (or pipe tracing) were the only widely accepted systems using fluids as the heating medium. As an alternative, a fluid tracing system for process piping offers advantages over traditional fluid heating methods. Its design addresses factors considered when selecting the heating system for piping.
Traditional Fluid Heating
When designed properly, fully jacketed pipe - jacketing the core pipe with a second, outer pipe and conveying heating medium in the annular space - is the most effective system for maintaining process temperatures in piping. Jacketed pipe offers the greatest heating surface area around the process pipe and offers direct heating contact between the process and heating medium. However, jacketed pipe also has some disadvantages:
- Relatively high capital cost.
- Potential for leaking heating medium into the process materials.
- Potential for leaking process materials into the heating medium.
- Time-consuming and costly to modify.
- High energy consumption.
Tube tracing - running a stainless or copper tube along the pipe, which conveys a heating medium - is typically a non-engineered system with relatively low capital cost. Unfortunately, this system is sometimes ineffective for maintaining elevated process temperatures due to poor heat transfer from the heating medium to the process.
Theoretically, there is a line contact between the tubing and pipe wall that facilitates a conductive heating path. All too often, this line contact is never attained during installation. The uneven contact between the pipe and tubing can result in convective heating rather than conductive heating, which leads to unpredictable performance. The convective heating is inefficient because it heats the air surrounding the tubing and the air then attempts to heat the process pipe.
Fluid Tracing System
Figure 1. The fluid tracing system for process piping heats the process effectively until it reaches the operation temperature.
One fluid tracing system that has been introduced can maximize heating and energy efficiency (figure 1). It is constructed with a conductive aluminum channel that fits over stainless or copper tubing. The high conductivity maximizes heat transfer from the tubing - conveying the heating medium - to the process pipe. It is intended to combine the flexibility and cost advantages of a tube tracing system with the predictable results of jacketed pipe.
Such a fluid tracing system converts standard stainless or copper tubing from inefficient convective heat transfer to conductive heat transfer using highly conductive aluminum as its main heating body. The heating surface area also is increased to as much as 3" per strip. The two enhancements increase two of the three parameters in overall heat transfer from the process to the heating medium.
Q = U • A • ΔT
Q is heat transfer, from heating medium to process, needed to overcome natural heat loss and/or to heat a process to a specified temperature.
U is combined heat transfer coefficient from heating medium to process.
A is the contact area between the heating medium and process.
ΔT is the temperature difference between heating medium and process.
Figure 2. The diagram compares the fluid tracing system, jacketed pipe and tube tracing methods.
From an operability standpoint, the main selection considerations are the ability to heat the process from an upset condition to its desired operating temperature, and the system’s ability to maintain that temperature during normal operation.
Consider raising the temperature of asphalt from 50 to 300°F (10 to 148°C) in an insulated 6" carbon steel pipe. Figure 2 reflects the effectiveness, or energy transfer (BTU/hr/ft), from the heating medium (in this case, 150 psig saturated steam at 365°F [185°C]) into the asphalt from the system, via jacketed pipe and a stainless steel convection tracer. This comparison also is an indicator of each system’s ability to maintain the desired operating temperature of 300°F (148°C) during normal operation.
Fully Jacketed Pipe. In jacketed pipe, the “UA” is maximized and more energy is transferred from the steam into the process than any other system. After the operating temperature of 300°F is reached, the jacketed pipe system continues to heat the process. This may be a disadvantage if a process has an upper temperature limit that cannot be exceeded. Due to the maximum energy transferred, the cost of the heating medium should be analyzed because it may be substantial.
Tube Tracing. In this example, the tube tracing is not effective at transferring energy into the process. The heat energy from the steam must transfer through stagnant air before entering the pipe and asphalt. Stagnant air is a great insulator (its “U” is very low), which prevents effective heat transfer. In fact, at 150°F (65°C), the tube tracing has reached equilibrium and has no more available energy to heat the process. This inability to transfer the energy into the process makes the system appear to be energy efficient. As reflected in figure 2, it uses less energy than any other system at every temperature. Once the maximum temperature of the process is met, all extra energy is lost through the insulation.
Figure 3. The figure compares the energy transferred to a process via the fluid tracing system and jacketed pipe.
Fluid Tracing System. The fluid tracing system heats the process effectively until it reaches the operation temperature of 300°F. The system is designed using software that models the heat transfer and allows the system to precisely control the amount of energy needed to meet the specific goal. In other words, the “UA” is specifically designed into the system. The fluid tracing is effective at increasing process temperatures as needed, yet is also energy-efficient. It does not continue to add unnecessary heat energy into the process once operating temperatures are met.
Normally, the fluid tracing system is designed to reach equilibrium at a temperature slightly higher than the target temperature unless designing specifically for heat-up or melt-out. The system can be designed to approach the performance of fully jacketed pipe if heat-up or melt-out is the primary goal (figure 3).
As the pressure to increase plant efficiencies and minimize downtime become more important, the need to consider innovative solutions is critical.