Self-Regulating vs. Mineral-Insulated : When is using MI more efficient?
In an electrical heat tracing system, self-regulating (SR) heating cables and mineral-insulated (MI) heating cables are the two representative types, with SR cables being the more commonly used.
Our question is: which cable is better from the client's perspective?
If both options are available, the client will likely want to reduce costs. Between SR and MI cables, which one is more cost-effective? The answer depends on the situation.
In this discussion on SR vs. MI cables, we will analyze which type of cable is more effective in specific environments.
Let me introduce an inquiry I received in 2023 for the ‘S* Port & Marine Project.’
Pipe length: 1,343m
Pipe size: 10”
Maintain temperature: 180°C
Minimum ambient temperature: 4°C
Insulation material and thickness: Mineral Wool, 100T
Supplied voltage: 400VAC 3P
Heat Trace Design for Temperature Maintenance
Given the criteria, the heat loss value is as follows:
In an offline meeting, the client expressed a preference for using SR cables. Now, let’s compare the scenarios of using SR cables versus MI cables.
<Case of using SR Cable>
To maintain the pipe temperature at 180°C, we need to select an SR cable with a maintain temperature and withstand temperature higher than 180°C. I designed the system using SR cables from E* company, which have an output of 75W/m at 10°C.
Construction Cost
The output of an SR cable decreases as its temperature increases. Although the nominal output is 75W/m at 10°C, it drops to 18W/m at the maintain temperature of 180°C. (The reason for this will be discussed in detail in an upcoming article.)
To compensate for the heat loss of 113.5 W/m, we would need more than seven times the length of cables (> 113.5/18 = 6.31) to maintain the pipe temperature at 180°C.
Additionally, SR cables have a circuit length limitation due to the current-carrying capacity of the bus wires (nickel-plated copper, 1.23mm²). The maximum length of SR cable installation depends on the switch-on temperature (which takes into account the minimum ambient temperature) and the circuit breaker size. With a 32A breaker, one circuit of the selected cable can cover only 36 meters. To cover a 1,343m pipeline, more than 38 circuits are required (> 1,343/36 = 37.3). The installation diagram would be as follows:
Given this design, the cost of the distribution panel, additional junction boxes, and power cables must be considered, which would significantly increase the construction cost.
SR cables regulate heat by changing their molecular structure. However, like a rubber band that loses elasticity when overstretched, SR cables may lose their original heating capacity over time if used at high temperatures for extended periods. This means the client will eventually need to replace the SR cables after several years.
<Case of using MI Cable>
MI cables are made from inorganic materials, allowing them to withstand temperatures up to 800°C.
Since the output is based on the resistance value of the conductor, we can select the appropriate model to generate the required output.
By using MI long-line heating cables, we can design the system as follows. For load balancing, we will use a three-line ‘Y’ connection.
(How to choose the MI cable model for the given environment will be discussed in another article.)
Only a threefold ratio is required, and distribution panels are needed only at both ends of the pipeline.
In terms of maintenance, MI cables maintain a constant output over time, so there is no thermal degradation that necessitates cable replacement. Additionally, MI cables are mechanically stronger than SR cables, resulting in lower maintenance needs.
Conclusion
For the above project, MI cables are more suitable than SR cables. But does this mean MI cables are generally better than SR cables? MI cables are better when the pipeline is long and requires high-temperature maintenance.