Application Specific Temperature Sensor Designs

In document ABB Temperature Handbook (Page 111-127)

Hot Gas Measurements in a Furnace

A temperature sensor measures changes in gas temperatures very slowly due to the poor thermal conductivity of gases. In order to reduce large errors due to thermal radi-ation (cooled walls), which may exist in blast furnaces, vacuum temperature sensors are utilized. The hot process gasses are drawn off using a vacuum created with com-pressed air.

Fig. 3-38: Vacuum temperature sensor in a blast furnace

Temperature Measurements in High Pressure / High Temperature Reactors In these applications temperature sensors with in- and outside ceramic thermowells and used. The thermocouple wires are sealed by a pressure tight connector as they exit to the connection box. To protect against aggressive fluids which might influence the thermocouple characteristics (e.g. sulphur in Claus Processes), an inert purge gas is introduced through a fitting. This creates a positive pressure in the thermowell. The purge flow can be regulated or increased using an additional outlet connection.

Purge gas will only flow if its pressure is greater than the process pressure. Only a very small purge flow is usually required. Applications include the manufacture of chemical products which require the addition of high pressure/temperature elements for the reaction (synthesis reactors, fertilizer production, etc.).

Connection for vacuum meter

Compressedair

Fig. 3-39: Purged thermocouple in a high pressure reactor

Temperature Measurements in Particle Loaded Gases

For the pneumatic transport of granulates and powders a temperature measurement is often required in order to monitor the temperature to assure that the ignition limit is not exceeded. The temperature sensor, which is inserted in the flow stream is subjected to a high degree of abrasion. It is possible to counteract abrasion by installing armor coated thermowells (e.g., with Stellite, see Fig. 3-40), low wear tips made of solid ma-terials, eccentrically drilled thermowells or by installing an deflecting impingement rod ahead of the thermowell. This temperature sensor design is used in wood and coal pro-cessing, cement and glass industries and in coal fired power plants.

Fig. 3-40: Armor coated thermocouple in an abrasive gas stream

Temperature Measurements in Flue Gas Channels

Filter systems in smoke stacks are very sensitive to overheating. Therefore it is impor-tant to recognize a temperature increase very quickly.

Since a horizontally installed, thin sheathed temperature sensor is not sturdy enough and a minimum insertion length is required, a special design is required. The tempera-ture sensor in this design has a support pipe upstream of the measuring element and which bent at a right angle to guide the flow.

Fig. 3-41: Fast responding temperature sensor in a flow channel

Multipoint Temperature Sensors for Temperature Measurements in Large Tanks In chemical processes the temperatures in large volumes are often monitored. Since the temperature distribution in a large tank may not be uniform, multiple measuring locations are necessary, which are distributed in a representative manner throughout the volume. Since most tanks only have a single opening at the top, multipoint sensors are used. They have a number of measuring locations within a single thermowell. Mul-tipoint sensors with lengths up to 20 m (65 ft.) and weighing more that a ton are not uncommon.

Good heat coupling is established in thermowells by the contact between the measur-ing element and its inside wall. Individual designs for explosion and pressure proof ap-plications are possible. They are used for status monitoring in liquid and solids storage tanks.

Fig. 3-42: Multipoint temperature sensors in storage tanks and process reactors

Temperature Measurements in Metal Melting and Salt Baths Using Angled Thermocouples

These temperature sensors are used primarily to measure temperatures in non-iron metal melting furnaces and salt baths for hardening. For vertical installation in open vessels an angled design is used so that the connection head and connection cables can be mounted outside of the radiating surface at the top of the furnace. Suitable materials made of thermal shock resistant ceramic are used for thermowells, as well as metal. Since the thermowell for direct immersion in the molten materials is stressed to the maximum, it is considered to be a consumable part. Its durability can be increased, if in this region, an additional protective sleeve is installed over the thermowell.

For waste incineration furnaces, rotary kilns, fluidized bed furnaces and air heater applications, thermowells made of silicon carbide, metal ceramic or porous oxide ceramic are particularly well suited because of their high temperature resistance, hard-ness and abrasion resistance together with their resistance to acid and alkali vapors.

These temperature sensor are then not angled, but are designed as “straight thermo-couples“.

Fig. 3-43: Angled thermocouple in a crucible

Resistance Thermometers with Extremely Short Response Times

For applications where control functions require that process temperature changes be recognized very quickly, special designs have been developed. The designs are such that the measurement resistor is sintered into the measuring inset tip with using a high heat conducting material. The measuring tip itself is designed as an adapter sleeve, which fits closely into the thermowell, and becomes part of the exchangeable measur-ing inset. As a result of the extremely good heat transfer possible with this design, response times τ0.5 of less than 3 seconds can be achieved (measured in flowing water at v = 0.4 m/s (1.3 ft/s).

Temperature sensors of this design are predominantly used in the primary circulating loops in nuclear plants, as well as in safety relevant applications for energy balancing in chemical systems, where the highest safety requirements must be satisfied, even during a failure condition. Process parameters include flow velocities up to 15 m/s (50 ft./s), pressures to approx. 175 bar (2,538.16 psi) at a maximum temperature of 330 °C (626 °F).

Fig. 3-44: Fast response temperature sensor in a reactor cooling pipeline

Temperature Measurements in Plastic Extruders

An exact knowledge of the product temperature during the extrusion process is an essential factor to assure the workability of the material and the quality of the end product.

The measurement is difficult because a built in sensor

• would interfere with the flow of the extrusion stream,

• must have a very rugged construction, since the processing pressures are between 300...500 bar (4,351.13...7,251.89 psi),

• would be greatly affected by exposure to the external heat jacket.

The design for this application is a massive sensor with a short length, in whose tip measuring locations at multiple steps are incorporated. Since it is not possible to pre-vent the effects due to external heat sources, a measurement of the temperature gradient allows a temperature determination to be made. In this way meaningful values for the temperature of the plastic mass can achieved.

Fig. 3-45: Extruder temperature sensor

Temperature Sensors for the Food and Pharmaceutical Industries

Temperature sensors for these applications must be designed in accordance with strict hygienic requirements. This means that the construction must not have any small gaps or dead spaces, where product or residue could be deposited in the sensor. The tem-perature sensor must be able to be cleaned and sterilized without being disassembled.

This property is classified CIP-Capable (Cleaning In Place) and SIP-Capable (Sterilis-ing In Place). The connection head must incorporate a high level of protection, in order to remain sealed when cleaned with a steam jet.

The measuring task requires very fast response times (< 3 s) at a high accuracy, so the product quality can be maintained within tight limits. High alloyed stainless steel mate-rials are used such as 1.4571, 1.4435 and 1.4404 (AISI 316Ti, 316L).

Fig. 3-46: Temperature sensor with ball type welded adapter for hygienic applications and installation at various angles

Temperature Measurements of the Tank Content with a Flush Thermowell All sided heat contact is not always possible with an insertion thermowell, because it may interfere with the process or cannot withstand some of the forces which may occur, e.g. in tanks with stirrers, the thermowell would interfere with the wall scraping stirrer, so the measurements must be made flush with the wall.

Special measures must be considered in the sensor design to assure that:

• the sensor is thermally decoupled from the wall,

• the contact area with the medium large enough,

• the measurement will not be affected by external heat jackets.

A suitable sensor design assures that the sensor element is in contact only with the interior of the tank and not with its mechanical mounting arrangement.

Fig. 3-47: Flush tank wall installation of a temperature sensor

Temperature Sensors for Heat Quantity Measurements

Since heat energy is very expensive, cost effective balancing is required with very precise measurements. The requirements relative to the design and allowable mea-surement deviations for heat quantity sensors are defined in the Standard EN 1434-2.

Because the accuracy requirement for the sensor pair is in the range of 0.1 °C (0.18 °F), it is very important, that in addition to the correct selection of the sensor, the relationship of the sensor mass to the installation length be considered in order to pre-vent any external influences from effecting the measurement.

Temperature sensors without thermowells with extremely short measuring resistors are used to allow an exact measurement to be made in the center, as required, of the usu-ally small diameter pipelines while minimizing the heat loss.

Fig. 3-48: Temperature sensors for the heat quantity measurements

Temperature Measurements on Surfaces

The surface temperature measurement has gained increasing importance. For a vari-ety of reasons (measuring location hard to access, sterility of the system, no distur-bance in the flow circuit, etc.) the direct insertion of temperature sensors into the pro-cess loop is often undesirable. For such applications, the non-contacting infra-red measuring methods are not the only ones used (see chapter 4). Surface temperatures are measured using contacting temperature sensors especially in applications where undefined or changing conditions relative to the emission coefficient ε may exist. A differentiation is made between two basic methods, a portable system (sensors positioned manually, touch sensors) and a system with sensors permanently mounted on the surface. For process systems, only the permanently mounted sensors are of importance.

For temperature measurements on the surface of bodies a basic knowledge of the tem-perature difference between the surface and the enclosed medium must be known.

Surface sensors operate within a defined temperature gradient range.

Errors may result when making surface temperature measurements due to effect of the sensor (interference) itself on the surface temperature (undisturbed).

When applying surface temperature sensors it follows that not only the actual errors in the sensor itself must be determined by a calibration, but also, the magnitude of the effect the temperature sensor has on the surface temperature itself must be deter-mined. The correct application of surface temperature sensors requires extensive experience in the field of temperature measurement technology. Requesting technical, application oriented recommendations from the temperature sensor manufacturer are recommended.

To keep the heat removal by the measurement element as small as possible, its mass should be a minimum. For small surfaces, thermocouples, because of their small mass with diameters of 0.5 mm (0.020“) are often used.

Sensor mounting methods vary for each installation. They can be mounted using sol-dering, welding, screwing or held in place by a spring. For larger cross sections, resis-tance thermometers are also used. They are designed as bottom sensitive types for the specific mounting arrangement (tangential/axial). They are either held in place by a pipe clamp or clamped using a metal plate screwed onto the surface.

Fig. 3-49: Measurements on a pipe surface

Pipe Wall Temperature Measurements in Heat Exchanger Pipes

In heat exchangers e.g., a liquid medium is pumped through a pipe bundle installed within a hot gas filled tank. Due to the large contact area, the medium approaches the temperature of the gas. Since the temperature and pressure in the pipes is usually high, near the material limits, monitoring the wall temperature of the pipes is necessary, in order to prevent over stressing the materials and possibly rupturing the pipes.

The design of a suitable sensor must assure good contact with the wall without, due its own mass and its contact with the hot gas, produce erroneous results. Since operating temperatures may reach approx. 560 °C (1040 °F), the use of conventional insulating materials is for all practical purposes excluded. The solution for this problem is a sensor with a mineral insulated cable with a V-shaped knife edge whose measuring section is bent toward the inner wall and welded to assure good contact with the pipe wall. In this design, the welded portion forms a cap over the measurement element and which is at the same temperature as the pipe wall. To compensate for the temperature differences, additional compensating windings are incorporated.

Fig. 3-50: Measurements on a pipeline in a heat exchanger

Temperature Measurements in Housings and Walls

In order to measure the temperature in solid bodies, the measuring element is posi-tioned in a hole drilled into the object to be measured. The hole itself and the measuring element disturb the temperature field, so that measurement errors result. The mea-surement error increases as the size of the hole increases in relation to size of the object and how different the heat conductivity of the inserted temperature sensor is from that of the object.

Guidelines for the ratio diameter/depth of the hole for temperature measurements in objects are:

• With good heat conductivity 1:5

• With poor heat conductivity 1:10 to 1:15.

The solution is a sensor consisting of two independent, spring loaded sheathed ther-mocouples, which due to their small mass form point shaped measuring locations which essentially assure an error free measurement. These temperature sensors are used, among others, in high, thermally stressed elements in power plants.

Fig. 3-51: Difference temperature measurements within a wall

Temperature Measurements in Bearing Shells and Housings

To measure the temperature of a housing a small hole is usually added with a minimum depth. This requires temperature sensor designs with very short, temperature sensitive lengths. They are usually pressed against the bottom of the hole by a spring to assure good thermal contact. Silver tips are also used to optimize the heat transfer. Since, e.g., there are enormous vibration forces present in Diesel motors, the measuring sensors must be designed with an extremely rugged internal construction coupled with the use of reinforced springs.

These temperature sensors are used to measure bearing temperatures in pumps, turbines, blowers and motors. For use in large Diesel motors in ships, type tests are also required by the Ship Classification Societies such as Lloyds Register of Shipping, German Lloyd and others.

Fig. 3-52: Temperature measurements in pump bearings

Temperature Measurements in Brakes and Railroad Train Axles

To monitor the brakes in high speed trains, temperature sensors with the following characteristics are required:

• Small, rugged design,

• resistant to high mechanical shocks,

• special measuring surfaces, which can be mounted as close to the rubbing surfaces (brake linings) as possible,

• fast response.

An appropriate design is a small, spring loaded sensor with a conical seat mounted in the brake caliper housing.

Fig. 3-53: Temperature measurements in a brake caliper

In document ABB Temperature Handbook (Page 111-127)