Ties for Temperature

Rapid thermal response, accuracy and surface-mount simplicity are some of the benefits of using thermal-ribbons as piped-liquid temperature measurement devices.

Accurate temperature sensing of liquids flowing in pipes is essential to process control and energy management systems. The traditional approach is to use probe-style sensors and thermowells inserted into the fluid stream. These immersed sensors can be costly and difficult to install, especially when retrofitted to existing facilities. Thermal-ribbon flexible RTDs (resistance temperature detectors) make an economical alternative to thermowells, offering surface-mounted simplicity with no loss of accuracy.

Thermal-ribbons are thin, flexible resistance thermometers. They contain flat wire-wound elements laminated between layers of electrical insulation. This design improves thermal response in three ways:

• Flexible sensors conform tightly to sensed surfaces, leaving no air gaps to block heat transfer

• Thin electrical insulation reduces the thermal gradient between the sensing element and sensed surface; response is rapid and self-heating negligible

• The element winding senses over an area to reduce point measurement errors

To see how these benefits apply to fluid measurement requires an understanding of the thermal profile of fluid systems.

Thermal profiles

What is the temperature of a liquid flowing inside a pipe? It depends on where you sense it. Assuming the fluid is warmer than the ambient air, the highest temperature exists at the center of the flow. Temperature gradually declines as you move to the outside wall of the pipe then drops off sharply through the insulation.

Figure 1 shows the temperature cross-section of a well-insulated pipe. Although the maximum reading is at the pipe’s center, all points inside the insulation, including the pipe wall, are at nearly the same temperature. Why then the difference between immersion and surface sensing?

The problem is conduction. When you introduce a thermometer into a system, it conducts heat away from its own sensing element to the outside environment. The element actually detects a temperature somewhere between true fluid temperature and the ambient air.

A typical thermowell/sensor installation is depicted in Figure 2. The metal thermowell and connection head conduct heat from the sensing area to the air outside the pipe insulation. A large gradient appears along the well. In addition, errors can result if the thermowell tip does not penetrate far enough into the fluid stream.

The major drawback to thermowells is their installation complexity. Pipes must be drained, holes drilled, fittings welded, and threads tapped. In many instances, such as retrofit installations, the cost of hiring specialized personnel and altering plumbing leads us to look for a simpler solution. Why not install a thermometer on the pipe’s surface?

In Figure 3, the thermowell is replaced with a surface sensor comprising a spring-loaded probe clamped to the pipe, with a junction box for leadwire connections. This arrangement is almost guaranteed to produce errors because ambient air directly cools the pipe surface through the opening in the insulation.

Heat also flows through the probe into the junction box, which presents a large surface for radiant and conductive heat loss. The sensing element transmits an uncertain mixture of fluid and ambient temperatures. A better approach is needed for surface sensing.

Figure 4 shows a thin, flexible thermal-ribbon inserted beneath the insulating blanket. Two leadwires form the only possible route for heat loss, and the thermal profile is now the same as in Figure 1.

As long as the pipe has sufficient insulation, its outside surface temperature agrees with the fluid inside. Insulation is a must since exposing the thermal-ribbon to outside air will reduce its accuracy.

(The thermal profiles shown are approximations. Actual values depend on pipe size, fluid temperature, flow rate, insulation rating, and other variables.)

Installation issues

When installing a thermal-ribbon, it must conform tightly to the sensed surface and be well insulated from surrounding air. Attachment options for silicone rubber thermal-ribbons include pressure sensitive adhesive (PSA), stretch tape, and RTV cement. (Polyimide thermal-ribbons, typically specified for aerospace use, may have different instructions.)

Pressure sensitive adhesive offers easy press-in-place installation, but is restricted to flat surfaces and temperatures below 177°C. PSA is factory applied and must be specified when ordering a thermal-ribbon.

To use PSA, insulation must be removed, all dirt and oil cleaned from the mounting area, and a wire brush or sandpaper used to expose shiny metal. The protective backing paper is then detached from the thermal-ribbon, which is then pressed in place applying pressure to remove voids. Leadwires should be secured so they do not pull on the thermal-ribbon body. Once attached, the installation can be replaced or installed.

Self-adhering silicone rubber tape with two-way stretch is used for quick installation on cylinders. Because there is no adhesive layer between the thermal-ribbon and the pipe, silicone rubber tape provides optimum thermal response. The thermal-ribbon is positioned on the pipe, flat side down, and the tape wrapped around it and the pipe a minimum of 1½ turns before cutting the tape and pressing the end in place.

RTV cement is a room temperature vulcanizing cement suitable for curved surfaces and temperatures to 392°F (200°C). The flat mounting surface of the thermal-ribbon is first coated with cement, keeping the adhesive layer thin but void-free for best heat transfer, and then quickly positioned on the pipe, applying uniform pressure to remove bubbles.

Ideally, the thermalribbon should be clamped in place for at least two hours and the pipe not heated for 24 hours.

As for leadwires, care must be taken to install these so they do not conduct heat away from the thermal-ribbon or pull it loose from the pipe. The leads should thus not be allowed to hang free where they might be subject to tugs and pulls. Instead, tape to pipe hangers or mounting brackets. And the leads should run along the pipe a few inches before bringing them out through the insulation. This way, the wires will be at the same temperature as the sensor and will not cause conduction errors.

Interfacing thermal-ribbons to control systems requires careful accounting for leadwire resistance in long signal wire runs. A 3 or 4-wire compensating circuit may be required.

Minco offers two-wire transmitters – for use with RTDs and thermocouples – to provide a linear 4-20 mA current signal, immune to noise and leadwire resistance, so preserving signal accuracy over extreme distances.

Sensor comparison

But is thermal-ribbon performance truly comparable to immersed sensors? Shown in the table are test data comparing a thermal-ribbon against a thermowell installation under the following test conditions:

• Thermal-ribbon mounted to the top of a 7.6 cm steel pipe with stretch tape, then covered with 7.6 cm of fiberglass insulation.

• 30 cm long brass thermowell probe assembly longitudinally installed beneath the thermal-ribbon. (Brass chosen for the thermowell material because it conducts heat seven to eight times better than stainless steel.)

• A bare tip-sensitive probe inserted into the fluid as a control.

• To test time response, flowing water abruptly switched from 10°C to 65.6°C.

As the results show, a thermal-ribbon reacts to temperature changes more quickly than a probe inside a brass thermowell. An even larger improvement would be expected in comparison to a stainless steel thermowell. And after the system reaches steady state, all three sensors give identical readings. The surface mounted thermalribbon is effectively sensing true liquid temperature. (These conclusions were verified by an independent manufacturer of energy management systems, who conducted a side-by-side comparison in a working installation.)

In summary, thermal-ribbons work well to measure liquid temperature in pipes because they include flexible sensors that conform tightly to the surface. There are no air gaps to block heat transfer; thin electrical insulation reduces thermal gradient between the sensing element and sensed surface; and the element winding senses temperature over a sensed surface area. Rapid thermal response, accuracy and surface-mount simplicity give thermal-ribbons an economic advantage over traditional probe-style sensors and thermowells.

Figure 1

Figure 2

Figure 3

Figure 4

Typical thermal-ribbon installation

Thermal reaction chart

Based on information from Minco (www.minco.com).