In high temperature applications such as those found in chemical or gas processing, heat transfer in industrial boilers, furnaces and ovens is achieved via direct firing into tubes. As one would expect, monitoring the temperature — by measuring the temperature of the tubes inside the industrial furnace, boiler or oven — is critical. Accurately measuring the tube temperature allows the processor to monitor temperature trends and detect signs of coke buildup on the heat transfer surfaces.

The two most common methods of measuring the temperature of the heat transfer tubes are:

• Indirectly, using infrared thermometers with a line-of-sight view of the tubes via a peek window.
• Directly, by placing temperature-sensing elements in contact with the tubes themselves.

This article will focus on direct temperature measurement.

When measuring the tube temperature directly, many applications use tubeskin thermocouples. Understanding the design concepts inherent to tubeskin thermocouples can help you optimize their performance in industrial furnace applications.

Tubeskin thermocouples are one type of temperature sensor used for measuring the surface temperature of the tubes within heat exchangers, boilers and other vessels. The thermocouple is installed on the surface of the tube. It measures the temperature of the tube wall to determine tube life.

A Closer Look at Furnace Heating

Inside the industrial furnace, a series of tubes contain heat transfer fluid, which is heated and circulated throughout the plant to provide heat to equipment such as heat exchangers. In the first part of the furnace, also known as the radiant section or firebox, the tubes are heated by direct radiation from the burner flame (figure 1). Above the firebox is the convection section, where the tubes are heated by convection for additional heat recovery.

FIGURE 1. In the first part of the furnace — also known as the radiant section or firebox — the tubes are heated by direct radiation from the burner flame. Above the firebox is the convection section, where the tubes are heated by convection for additional heat recovery. Photos credit: SOR Inc.

The temperature of the heat transfer fluid is determined indirectly by measuring the temperature at the tubes’ surface on the side where the highest degree of heat transfer is occurring. This commonly is referred to as the hot side of the tube; the opposite face is commonly known as the cold side.

As the furnace is used, the heat transfer capability of the tubes degrades; eventually, they need to be replaced. This decline in heat transfer performance can be attributed to normal usage. It also can be caused by:

• Scale buildup on the outside of the tubes.
• Coke formation on the inside of the tubes.

Under normal operating conditions, the high temperatures accelerate the oxidation process inside the furnace, causing layers of soot and scale to build up on the outside of the tubes. This buildup creates an insulative layer that reduces the heat transfer effectiveness from the furnace to the tubes. Similarly, coke buildup on the inside of the tubes negatively impacts heat transfer from the tubes to the fluid inside.

When either of these insulating barriers forms, the furnace overheats as it attempts to bring the tubes to the required temperature. This overheating damages the tubes and prematurely shortens their lifespan. Fortunately, coke and scale can be removed to extend the tube life.

Using tubeskin thermocouples to measure the tube temperature allows the operator to observe performance trending data over time. Signs of declining performance provide an early warning of overheating so that preventive measures can be taken.

To obtain the most accurate temperature readings, the tubeskin thermocouple must measure the temperature directly at the surface of the furnace tube. It is important to avoid allowing the ambient temperature inside the furnace to influence the thermocouple accuracy. Photos credit: SOR Inc.

Tubeskin Traits, Achieving Accuracy

As it relates to construction, most tubeskin assemblies share some common traits. First, they typically use a Type K thermocouple because it offers a higher temperature ceiling compared to RTDs or other thermocouple types. Second, although the sheath material is specified based on application criteria, the upper temperature limit of the sheath material selected must be able to withstand the intense heat to which it will be subjected continuously. Third, the sheath material selected must have high corrosion resistance to protect against temperature-accelerated oxidation. It also must withstand exposure to the flue gas, the composition of which varies depending on the type of furnace. Fourth, most tubeskin thermocouples use heavy-wall mineral-insulated (MI) cable to limit degradation of the conductors inside and to provide better durability. Finally, further robustness is achieved by using thicker elements with larger conductors that have a longer lifecycle in harsh environments. This robustness comes at the cost of reduced flexibility, however.

To obtain the most accurate temperature readings, the tubeskin thermocouple must measure the temperature directly at the surface of the furnace tube — but without influence from the ambient temperature inside the furnace. This is accomplished by welding the tubeskin thermocouple directly to the furnace tube (via a weld pad, for instance). Additionally, the tip of the thermocouple must be positioned on the hot side of the tube, closest to the flame. If it does not fall on the hot side, the measured temperature will be affected by ambient conditions and the distance away from the flame.

Even with the tip welded to the hot side of the tube, additional measures can be taken to ensure the best accuracy. Because furnaces usually lack electrical interference or noise, tubeskin thermocouples use a grounded junction to provide direct surface contact. This provides improved accuracy and faster response times. Another common practice is adding a heat shield. This technique insulates the tip of the thermocouple from the ambient environment and isolates it to the tube’s surface.

Routing Within the Furnace

The inside of an industrial furnace can be a complex maze, with many twists and turns. After the thermocouple tip is welded to the tube, the remaining sheath length is taken back to the furnace wall, where it is fixed in position by the process connection.

Expansion loops or coils play an important role in the operation of tubeskin thermocouples. Photos credit: SOR Inc.

The path taken by the tubeskin — from the tube to the point of entry at the furnace wall — is referred to as its routing. From the sensor tip on the hot side of the tubes, the sheath is routed to the cold side of the tubes. This protects the sheath from the flames on the hot side.

After reaching the cold side, the sheath often requires additional bends so it can be routed to the furnace wall. This is an important consideration one must keep in mind: A lack of additional sheath length for bending in the field may result in a suboptimal routing path or make installation impossible.

It should be noted that if the tubeskin thermocouple being installed is a replacement for an existing tubeskin thermocouple, it cannot be welded at the same position. Additional sheath length may be needed to move it to a new position. As previously mentioned, tubeskin thermocouples use thicker elements to improve durability; however, complex furnace designs and routing may warrant using a smaller-sized element to provide better flexibility and, thus, easier bending.

An important feature of tubeskin thermocouples is the incorporation of expansion loops or coils. As the furnace is cycling up, temperature fluctuations cause the tubes to move via expansion and contraction. Because the tubeskin thermocouple is kept in a fixed position by the weld on the tube and the fitting at the process connection, the movement can cause the sensor assembly to snap if relief is not built into the design.

This is where expansion loops come into play. Expansion loops are added by bending the sheath into tight concentric coils or other such shapes. Normally, they are shaped by the tubeskin thermocouple manufacturer. Once installed, when the tubes move, the expansion loops act like a spring and allow the coils to absorb the tube movements.

Tubeskin thermocouples often use a heavy-wall mineral-insulated cable to limit degradation of the conductors inside and to provide better durability. Here, a standard-wall 0.25" mineral-insulated cable is compared with a heavy-wall 0.375" mineral-insulated cable. Photos credit: SOR Inc.

Inside Out

The last consideration for tubeskin thermocouples in industrial furnaces is the installation process. Due to the large size of the expansion loops, they are unable to fit through the point of entry at the furnace wall. This makes it impossible to insert the tubeskin assembly from the outside into the furnace. In the same way, the fitting used to secure the tubeskin assembly at the process connection cannot be premounted on the sheath.

As a result, tubeskin thermocouple assemblies must be installed from the inside of the furnace and out through the furnace wall. The installation process starts by feeding the back-end of the tubeskin assembly out through the point of entry on the furnace wall. The sheath inside the furnace then is bent as needed to route it to the cold side of the furnace tube. Then, the tip is welded in place on the hot side of the tube. If weld clips are utilized to help secure the sheath in place, they are welded afterward. Finally, the fitting is added to the back-end of the tubeskin assembly and affixed at the process connection.

As with most temperature sensors, when specifying a tubeskin thermocouple, several factors should be considered. Because of the nature of tubeskin thermocouples’ custom designs, it becomes even more critical to look through the entire design process (from routing to installation).

A properly installed tubeskin thermocouple not only provides temperature readings but also trending information. This data can be used to extend the life and performance of the furnace tubes, which can help the plant save time and money. A basic understanding of tubeskin thermocouples and careful planning are key steps to successfully measuring temperature in an industrial furnace application.