Understanding the technology behind how a thermocouple or RTD works — and how they are used within a process loop — is useful. (For more on that, see “Monitoring Process Temperatures with Thermocouples" in the May 2021 issue.) In this article, I will show common process applications that are controlled with the help of thermocouples and RTDs.
The largest manufacturing industries in the United States are petroleum, steel, automobiles, aerospace, telecom, chemicals, electronics, food processing, consumer goods and lumber. Thermocouples are used in almost all stages of these myriad industries.
Temperature Control in Food Processing Applications
While thermocouples often can be found in small-scale, low volume, commercial restaurant-grade appliances, they also can be found as part of the food production process. Typical industrial process cooking equipment that employs temperature-sensing probes include ovens, warmers, fryers, toasters and grills.
Temperature sensors for the food industry need to provide consistent temperature readings to ensure precise cooking, frying or heating and protect food from being over- or undercooked. In addition to cooking equipment, it is not uncommon to find thermocouples in food packaging equipment.
Deep Fat Fryers. Fryers contain a reservoir of oil into which wire baskets are lowered or through which a wire conveyor is drawn. Most systems use electric heaters — either mounted around the outside or immersed into the tank. The thermocouple is immersed in the oil through the side of the tank or suspended over it. Occasionally, the probe is tacked to the frying vessel wall to reduce damage.
Pastry Ovens. Smaller pastry ovens require simple controllers — usually a nonindicating type. As the pastry oven chamber size increases, the features on the controller normally improve. Larger pastry oven systems typically use natural gas because of the lower operating cost, so controllers for these larger gas-fired systems will be either differential (deadband) or fully proportional (4 to 20 mA) type.
Cookers, Steam-Jacketed Kettles and other Electrical Heater Configurations. This type of cooking equipment usually consists of a kettle with an exterior jacket though which steam flows to heat the kettle contents. The sensor normally is inserted into the process, but it also may be strapped to the outside of the vessel. Stirring the process mixture in the kettle provides more even heating and a more consistent temperature. Electric heaters also can be used to heat kettles or tanks. Typically, they are mounted either on the outside of the tank, inserted through the side or hung from the top. The temperature sensor is placed in the process mixture.
Walk-In Type Cold Storage. Food manufacturers and some drug manufacturers frequently require a large cold room for storage of products before shipment. These systems normally need cooling only (on-off type), but they also may use heat in certain geographic locations. The temperature sensors are located in the storage space.
Chocolate Tempering. Chocolate tempering melters and automatic coating systems require more accurate temperature control than most industrial and food processes. Because tolerances of ±0.5°F (0.2778°C) are required, an RTD sensor normally is used. Typically, electric heat with an SCR or solid-state relay is best suited to this type of localized heating.
Other areas of the chocolate, fudge and candy market use low cost thermostats in melting and cooking kettles.
Shrink Film Packaging Systems. Heating tunnels with various styles of heaters are used to shrink plastic film around food packages. Sensor placement can be either in the airstream or on the heater. In addition, often a film-tacking assembly is placed before the heating tunnel. That equipment typically will utilize controller or other low cost temperature device.
Blister Packaging. Hard plastic protective covers are formed via vacuum-forming machines, which require a heat source to soften the plastic sheeting before a vacuum “pulls” it to the mold and forms the final shape.
Heated presses also are used to form blister packaging. Products are placed on cards, usually cardboard, covered with a “plastic bubble,” placed in a press and sealed. The press may or may not use heat. Wherever there is a heated surface in the system, there will be a thermocouple located on or in that surface to measure the temperature.
Thermoforming. In thermoforming, structured foam and plastic sheeting are heated and pressed into molded shapes to be used for food packaging. This type of package is common in egg cartons and fast food containers. Heat is used in the preheat section and can be used on the mold itself. Wherever heat is being applied in the process, a thermocouple is used to measure the temperature and feed that reading into the process control loop.
Horizontal Form/Fill/Seal Machines. These general-purpose packaging machines are well adapted to food products packaged in plastic film. Such normally solid items — pastry cakes, blocks of cheese, frozen items and the like — can be moved on a horizontal conveyor without separating. Heat is required in two places:
- The back fin seal, which is along the back of the package.
- The end seal bars, which form the package ends. This is also where the packages are cut apart.
Slip-rings normally are used on form, fill and seal (FFS) machines to bring power as well as the sensor or thermocouple leads out of the rotating seal bars.
Snack Foods Packaging. This segment of the food market includes packaging products such as potato chips, candy bars and pretzels. Two machine formats handle the majority of this market — horizontal and vertical form factors — and both use similar sealing techniques. These include the back seam, which seals the ends of the package and separates it from the next package. Solid snacks such as candy bars and individual packs of crackers are sealed on a horizontal FFS machine. The horizontal machines use rotary seal bars with the sensor and power leads brought out through slip-rings.
Products that are sold by volume usually are packaged in a vertical FFS machine. The vertical format takes advantage of gravity to pull the product into the package. The vertical machines use fixed back-seam and end-seam seal bars that close, pull the package down, open, move up and close again repetitively.
Regardless of format, temperature-sensing probes and heaters are located in the seal bars. The more sophisticated machine models integrate all machine functions into a single microprocessor with a screen to display readings, settings and menus. Even in such systems, thermocouples or RTD sensors are used for measuring the temperature readings.
Pizza Ovens. Pizza ovens can range from conveyor systems to batch ovens and can use a variety of heating fuels. Typically, all use a Type J thermocouple and a simple controller to maintain the temperature.
FSMA Requirements. Section 415 of the Food, Drug & Cosmetic Act requires “domestic and foreign facilities that manufacture, process, pack or hold food for human or animal consumption in the United States to register with FDA.”1 Such facilities also must comply with the requirements for risk-based preventive controls mandated by the FDA’s Food Safety Modernization Act (FSMA) as well as the modernized Current Good Manufacturing Practices (CGMPs) of this rule. The rule requires food facilities to have a food safety plan in place that includes an analysis of hazards as well as risk-based preventive controls to minimize or prevent the identified hazards. While there are many requirements for a food safety plan, one key element of preventive controls is process controls.
Process controls include procedures that ensure that the control parameters are met. Process controls can include operations such as cooking, refrigerating and acidifying foods. They must include parameters and values (e.g., critical limits) as appropriate to the nature of the applicable control and its role in the facility’s food safety system. Obviously, where the process variable being measured is related to temperature, thermocouples are more than likely to be involved.
Temperature Control in Plastics Processing Applications
Several manufacturing processes are used to make plastics goods. For simplicity, only the most common methods are discussed in this article.
Injection-Molding Machines. Most thermoplastic materials are molded by the injection-molding process: the polymer is preheated in a cylindrical chamber, to a temperature at which it will flow, before it is forced into a relatively cold, closed mold cavity. High pressures are required to feed the molten polymer into the mold, and these typically are achieved by a reciprocating screw. After the polymer melt has solidified in the cool mold, the screw rotates backward to ready the next charge of polymer for the next cycle. In the meantime, the mold opens, and the finished product is removed.
Temperature sensors are located throughout the injection-molding equipment: from the heated barrel to the mold itself as well as in the mold-cooling medium. Typically, the sensors are Type J thermocouples. The controller type varies with the barrel size, but most plastics machinery requires PID control.
Extruders. In much the same way that injection-molding machinery uses a heated screw feed, extruders use the same principle to melt plastic and force it through a die that give the material its final shape. Many continuous shapes can be achieved. Some systems consist of a screw enclosed in barrels, around which are mounted multiple band heaters. Large extruders use cast-in heaters that include two halves that bolt together; these usually include both a heater and a cooling water or oil channel.
Temperature sensors — usually Type J thermocouples — are mounted against the outside of the barrel using a spring assembly. Controller type varies as the size of the barrel increases. This is primarily due to the large amount of friction generated in larger machines, which require cooling.
Thermoforming Machinery. In the thermoforming process, sheets of plastic, foam or any other heat-sensitive material are heated until pliable in a preheating rack and then conveyed into one of two types of presses to form the final plastic shape. The first type is a mold composed of two heated halves. They close, forming a package such as an egg carton. The second type is a vacuum-forming operation. In these systems, the one-piece mold is positioned under the preheater. Small holes in the face of the mold are connected to a vacuum pump. Once the sheet is pliable, the mold is moved into position. A vacuum is drawn that pulls the pliable material into the desired form.
In both types of thermoforming machinery, thermocouples typically are located in the molds. The controllers are usually proportional only.
Pellet Dryers. Moisture creates problems during plastics processing. It can cause poor or inconsistent quality in the final product. For this reason, dryers are used to remove moisture from the plastic pellets before they are melted. These systems typically consist of an air heater, a blower and a proportional-only controller with a Type J thermocouple in the airflow.
Temperature Control in Metals Manufacturing and Fabrication
The furnace conditions to which the thermocouple will be exposed help determine the proper thermocouple choice for the application. When selecting the thermocouple, some of the conditions to be considered include:
- Temperature capability of the thermocouple wires.
- Temperature capability of the sheath or protective covering (metal or ceramic).
- The atmosphere in which it will be used (air, reducing, oxidizing, inert).
- The mounting configuration.
High temperature ovens form a separate category, and these furnaces use either gas or electric as their heat source. Single-point or ramp-and-soak controllers with either Type J, K, R, S or B thermocouples are the most common configuration. This market is fragmented with products that represent a significant capital investment. Some may be unique designs intended to meet the special metal-processing requirements. As a general rule, however, a typical system will have a sensor extending into the chamber’s air space, with open coil heaters mounted on insulators in the chamber’s walls.
Measuring the temperature of molten metal is difficult due to the high temperature and severe conditions encountered. For these reasons, Type K and N base metal and Types R, S and B platinum thermocouples are the only choice for contact measurements.
When base-metal thermocouples are used, the wires typically are large diameter 8 or 14 AWG solid wires with ceramic insulators as well as ceramic or metal protection tubes. The large diameter of the Type K or N wires degrade more slowly to allow time for measurements before the high temperature conditions degrade the wires.
Platinum thermocouple wires typically are on the order of 20 to 30 AWG solid. Unlike base-metal Type K and N wires that deteriorate due to oxidation or corrosion, the platinum wires become soft due to long-term annealing. They eventually fail due to grain growth.
Heat Treating. Heat treating is an industrial thermal metalworking processes used to alter the physical — and, sometimes, chemical — properties of a material. Heat treating requires that the furnace be held at a predetermined temperature for a precise amount of time in order to yield the desired depth of hardening. Such systems usually include a timer that is interlocked with the controller’s alarm to sound when the temperature is reached. Other than this special feature, the furnaces follow conventional designs.
Annealing Welds. Annealing is a metalworking heat treatment that alters the physical — and, occasionally, chemical — properties to increase the material’s ductility and reduce its hardness, making it more workable. It involves heating a material above its recrystallization temperature, maintaining a suitable temperature for an appropriate amount of time, and then cooling. Welded pipe lines that must be annealed to ensure rated strength and flexibility are a good example.
Typical annealing systems include a portable electric heater that is strapped around a weld, a thermocouple that is tack welded to the pipe, a proportional controller and a ramp generator for the setpoint. The temperature must be brought to setpoint and held without interruption; otherwise, the process must be restarted.
Temperature Control in Pharmaceutical Manufacturing
Package Machines. The most common package that requires heat in the drug industry is shrink packaging used for individual packages and packaging for quantities of an item to be shipped to distribution points. This type of package has become popular with the concern over tamperproof packaging.
System configurations can vary dramatically, but process controls for most systems usually consist of an air circulation heater, a thermocouple located at the exit from the heater, and a proportional-only controller. The heated air is applied to the package, which has heat-sensitive film loosely tacked in place. The film shrinks tightly when heat is applied, securing the package and making it evident if tampering occurs.
Sterilizers. Chambers for sterilization are common on pharmaceutical production lines. They typically use steam for sterilization. High accuracy requirements usually dictate the use of an RTD sensor and a proportional or PID-type controller. Modern sterilizers will use multiple RTDs in the drainline to monitor the sterilizer chamber and another RTD in the heating jacket. These will work together to provide precise process control.
Manufacturing Processing. Pharmaceutical manufacturing employs many different physical processes such as fermentation, heating, granulation, blending, filtration, chemical extraction, tableting, compression, coating and drying. Many of these processes involve the application or removal of heat, for which thermocouples will be used to measure and provide feedback to the process controllers.
Where temperature measurement is critical and high accuracy is a process requirement, RTDs will be used in place of thermocouples. As always, placement, construction and configuration of these thermocouples will be dependent upon the individual application.
There is an exhaustive list of industries and applications that need and use thermocouples and RTDs, so this article only scratches the surface. It should, however, provide a basic background of the technology and the ways in which temperature-sensing devices are employed to create better end products in the process industries.
1. Registration of Food Facilities: What You Need to Know About the FDA Regulation: Guidance for Industry, https://www.fda.gov/food/guidance-regulation-food-and-dietary-supplements/registration-food-facilities-and-other-submissions.