Using better temperature control helps deliver better product quality. Two experts weigh in on the ins and outs of effective temperature control.

Temperature and process controllers can offer single-line or dual-line displays and PID autotuning.
Photo courtesy of Cal Controls, West Control Systems

Temperature control in an industrial process can be a tricky business. However, if handled properly, it can be the key to boosting product quality, which is a definite plus in today's competitive manufacturing climate.

To detail the specifics about temperature control, we tapped experts and colleagues Jim Cottrell, global marketing manager for Honeywell, and Bob Hoffman, also with Honeywell, for technical assistance. In a Q&A session, the two experts explained the finer points of temperature control.

Q. What effect does temperature control have in an industrial process application? More specifically, what results have you witnessed when proper temperature control is maintained?

Jim Cottrell (JC): Temperature can be key to the process. If your temperature controls are managing the process accurately, you conserve energy and produce a more consistent product. In refrigeration, a lower temperature than necessary means more expended energy. The goal is to select a temperature controller that balances the need for efficiency and repeatability.

Bob Hoffman (BH): Proper temperature control improves efficiency. Overall, most users' goals are to maximize their profit and ensure the reliability of their product. To do this, they have to consistently deliver a product that meets their quality standards. Keep in mind, controllers work within certain boundaries of temperature. For example, some controllers work within a range of ±3°F [1.6°C]. Others such as certain types of industrial controllers can regulate with ±0.5°F [0.27°C]. For applications that require a higher level of control, as in aviation parts manufacturing, a smaller range is needed. For a paper manufacturer, a wider range may be sufficient. Thus, greater efficiency is achieved with tighter temperature control.

Q. What happens with poor temperature control?

JC: Poor temperature control can result in lost energy, an inconsistent product and even equipment and safety concerns. Let's say the heat stays on too long. It can damage the product and the equipment, and even be potentially dangerous. Likewise, if cooling is left on too long, it can not only damage the product but could cause excessive wear on supporting devices. For example, a compressor can be overstressed, leading it to overheat, freeze up or fail.

We recommend using two controllers in the process. One is the primary controller regulating the process while the other is the high-limit temperature controller. This second unit shuts off or adjusts the temperature if a threat of overheating or overcooling is present. In terms of production, this dual unit approach keeps the process running and also helps sustain product quality.

BH: As Jim indicated, poor temperature control results in excessive wear of the system. Final controls such as heater elements, motors and SCRs may wear out much faster than normal.

Universal digital controllers can offer a high level functionality and reliability, including relay output and alarm relay.
Photo courtesy of Honeywell

Q. It seems clear that the type of controller you are using makes a significant difference. Compare basic controllers to advanced models. What advancements have developed?

JC: The basic models essentially act as an on/off switch, much like the thermostat in your home. Basic controllers work effectively for simple applications where a precise temperature is not required. An example of an application that may use a basic controller is the storage of food.

BH: The level above on/off control is proportional control. Proportional control algorithms adjust the output in proportion to the amount of power needed. For example, on/off control will only position an output at either 0 or 100 percent. Proportional control - using the difference between the input signal, setpoint and tuning values - will throttle the controllers until the desired temperature is reached and maintained. Proportional control works best when an exact temperature is required.

Advanced controllers use sensors to anticipate variables and make adjustments. The algorithm in advanced controllers measures data such as temperature, flow and humidity and uses additional algorithms to correct variances. For instance, with a refrigeration unit, the advanced controller may measure the temperature of the cooling fluid as well as the chamber temperature.

One type of advanced model is a cascade controller, which measures dual variables. For example, in food processing, a cascade control unit may monitor the temperature of food as well as the temperature of the hot water that is cooking. Cascade control is used in many applications, especially when a slower process and a faster process are both involved in production.

Another advanced-level unit is the feedforward controller, which measures the material at different stages in the process. In measuring temperature and flow, the flow is fed forward and any changes from the normal value are reset. For example, mashed potato producers may feed potatoes into their cooker, then through the processing line. The potatoes may cool as they move through the process, so the feed forward controller will raise the temperature to heat them back up.

Ratio control is another option in advanced temperature controllers. As its name implies, a ratio controller adjusts the variables according to the desired ratio. For example, when placing fuel and air into a combustion chamber, there is a combination of both materials that creates the ideal combustion. The ratio controller maintains this ideal level.

All of the advanced temperature controllers require the input of calculations to maintain the variables they are measuring.

Temperature and process controls can incorporate dual alarms and a universal controller design that accepts thermocouple, RTD, voltage and current inputs.
Photo courtesy of Love Controls, Dwyer Instruments Inc.

Q. Any other functionality that is beneficial in maintaining temperature?

JC: All of the instruments need to be tuned, or adjusted, to the process. Many controllers are adjusted manually by changing the temperature and assessing how the process responds. More advanced controllers offer features such as autotune (also referred to as adaptive tune or smart tune, among other terms) that can automatically tune the controller to the process dynamics. This feature can save time and make the startup process quick and easy.

Newer features include the ability to network the temperature controller or connect it to the Internet. This allows users to monitor the process from a remote location. This feature also has the capability of sending an e-mail to alert users of a change in temperature, functioning as a process alarm.

Some controllers are available with predictive features that can determine if thermocouple sensors are going to fail. Using unique algorithms, they can monitor the thermocouple junction and predict a failure before it shuts down the process. By issuing an alarm, the thermocouple can be replaced before it fails, saving valuable production time.

Q. Are temperature controllers sometimes integrated into a process?

JC: Several manufacturers provide controllers that can be integrated as part of the overall electronics as opposed to stand-alone units. These controllers typically are provided as a circuit board and embedded into the process.

BH: In pharmaceutical manufacturing, which requires a higher level of control, you may see many temperature loops tied into one system. This regulates the temperature of many different lines of products.

Q. What are the limitations of temperature controllers?

JC: Sometimes there is confusion between the temperature controller and the process itself. While temperature controllers are available with a variety of options and must be selected carefully, they cannot compensate for other shortcomings in the process. For example, if a heater is not properly sized and does not have the capacity to meet the demand for heat, upgrading the controller will not solve the problem.

BH: The user first needs to understand the process. You have to look at your inputs and outputs. Have you chosen a temperature controller that offers the accuracy and precision you need? With the move from analog displays to digital, digital instrumentation can provide much more precision.

Microprocessor-based controllers can combine a high degree of functionality and reliability.
Photo courtesy of Honeywell

Q. When choosing a temperature controller, what criteria should be considered?

JC: Of course, it will depend on the user's application. Repeatability and quality are key considerations. Accuracy is another. In some applications, temperature accuracy is critical. For example, with a laboratory developing a product, one degree of difference in temperature may denote the difference between success and failure.

The physical location of the product also is a consideration. Will it be indoors or outside? Some controllers are not manufactured for outdoor use and lack the ruggedness necessary. Selecting a watertight controller for food and beverage applications is best because the equipment is regularly washed down.

Taking this expert advice into consideration before implementing a temperature control device is essential if you want to create the best product you possibly can.