Many aspects can influence the selection of the most appropriate control and monitoring solution for each project and application. The most effecting solutions are most often a blend of various combined technologies to achieve a balance between total installed costs (TIC) and long-term benefits associated with the entire heat management system, total operating cost (TOC), during the life of the plant.
However, if you are managing a project that requires fluid temperature maintenance or electrical heat tracing, but are not sure where to begin, this post is just for you! Here are the top three things to consider before selecting your electrical trace heating system:
1. Determine the control and monitoring philosophy
An overarching control and monitoring philosophy must be established for a project before any products can be selected. Types and methods of control and monitoring need to be chosen based on various aspects:
A) Process requirements (temperatures, flow path considerations, alarm requirements, upset conditions)
B) Maintenance strategy (simplicity, local or central monitoring, location of installation)
C) Power distribution parameters (location of panels, substations, cabling requirements)
D) Economical considerations (optimization of TIC, TOC)
It is also worth considering incorporating a variety of monitoring options into the system design. The use of monitoring of the circuit integrity increases the overall system reliability as failures in the heating and power distribution systems can be reported to operations and maintenance personnel locally or at a central location.
2. Choose the objective for your control systems
A variety of control systems provide for its own level of technical features and benefits, depending on the process requirements and the number of circuits. The objective of your electrical heat tracing system can be:
A) Frost Protection
Applied to fluids that must be kept above a minimum temperature - typically 5°C - e.g. for water lines and where moderate overheating of the fluid is not a major concern.
B) Broad Temperature Maintenance
Appropriate when the process temperature must be controlled within a moderate range. This is generally used for viscosity control to keep process fluids such as fuel oil flowing.
C) Narrow Temperature Maintenance
Applied to fluids that must be kept within a narrow temperature range to maintain viscosity and prevent fluid or pipe degradation. Typical examples include sulphur and acrylic acid lines.
3. Select the appropriate method of control
The choice of the controller depends on whether the system is required to be controlled on the basis of ambient or pipe/equipment surface temperature, which is dependent on the process requirements and, possibly, the equipment limitations.
There are three methods of control for Electrical Heat Tracing (EHT) systems.
A) Ambient Sensing Control
Uses a simple on-off algorithm based on ambient temperature. It is more energy efficient than just self-regulating control because the heating circuit is energized only when the temperature descend below the set point.
The control device can be either a mechanical thermostat or an electronic controller. Ambient thermostats are generally sufficiently accurate and reliable to provide and economical solution for most frost-protection applications.
B) Proportional Ambient Sensing Control (PASC)
Uses an electronic controller that senses ambient temperature and continuously matches the heat input to the predicted heat loss that occurs due to changing ambient conditions.
A pre-programmed algorithm calculates the cycle time that the heating circuits will be energized to maintain the desired temperature. PASC is suitable for all broad temperature-control and many narrow temperature-control applications. Compared to line sensing, the use of PASC can significantly reduce the number of circuits, as flow paths don't need consideration and can help reducing total installed cost of a project whilst reducing energy consumption.
C) Line Sensing Control
Is based on the pipe/equipment temperature. With this option, each flow path has a separate circuit controlled by a mechanical line-sensing thermostat or electronic controller. The control unit turns on the heating circuit when the pipe temperature descend below the desired maintain temperature.
Line sensing offers the most accurate control for narrow temperature band applications.
Total installed cost of line sensing systems can be considerably higher than systems based on ambient temperature, as the average circuit length of the EHT system is typically significantly lower based on pipe lengths and possible flow paths