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How to Achieve Desired Temperature Control in Process Cooling
John DuBois PhD, Thermalogic Corporation

Preliminary Considerations

Successful control of your cooling process and selection of a suitable control requires a good understanding of the flow of heat in that process.

No matter how intelligent or elaborate a control, it cannot overcome the limits of basic thermodynamics in a system or process.

Regardless of any control system, heat applied to a process or generated within by exothermic reaction will cause a steady rise of temperature unless a path exists for removal of the heat energy.

Usually, provision for suitable flow of heat out of a process is not a difficult task and requires little more than balancing heat applied (in BTU/hour or similar units) with heat removal by passive conduction, convection, radiation, or active cooling over a suitable period of time.

Only after this balance is designed should a control system (electronic or mechanical) be considered to keep process temperature within desired limits during operation.

Selection of a suitable control system is then subject to another set of limitations. Accuracy, repeatability and speed of temperature control cannot be arbitrary. These parameters must also respect thermodynamic and practical limitations as well as cost.

Basic Heat Flow:

How fast can heat accumulate in your process? A common measure is BTU per hour (or equivalent units) and is available from heater specifications or knowledge of exothermic heat generation in the process.

How fast can heat be removed from the process? This may be a more difficult value to obtain but no less important. It can be obtained from conduction, convection or radiation charts, manufacturer specifications or calculated with knowledge of system dimensions, materials and environment.

Is the rate of heat removal greater than its generation? If so, then it will be possible to control the process temperature, otherwise even the most elaborate control system available will be unable to help.

Selecting a Sensor:

Sensor type will have a direct bearing on cost and performance. Select a sensor that meets your expectations for performance rather than specifying a particular type (thermistor, RTD, or thermocouple) simply because it is familiar.

The most energy efficient types are thermistor and semiconductor because they offer the best  system stability at lowest cost. If these types do not offer sufficient accuracy or range, then lower sensitivity but higher accuracy RTDs might be appropriate. Thermocouples offer extended temperature ranges and a fast response time, but they have the lowest stability and accuracy.

Sensor Placement:

Location of the control sensor is critical. There are two considerations in placing the sensor: temperature gradients and thermal lag. These effects, if not addressed, can cause temperature control to be erratic or occur at a significant difference between setpoint and important regions of a process.

Gradients arise because of the thermal resistance of materials. They can be minimized by careful selection of materials and attention to the path of heat flow but a control sensor should always be placed at the point which is most important to your process.

A temperature controller alone, no matter how sophisticated, cannot ensure an even, gradient-free temperature throughout a process. Gradients should be minimized by careful design of materials and geometry.

Thermal Lag:

Thermal lag is the time required for heat to flow through the process and from the process to the sensor element. It can cause erratic control if the lag occurs between the sensor and the system elements controlling heat flow.

Thermal lag can be partially offset by proportional control with integral and derivative (PID), but this approach will result in slower overall system response time as well as a higher controller cost. Addressing thermal lag in system design is a more effective way to achieve process temperature stability.

Design strategies to reduce lag time include close-coupling between the process media and the cooling device(s), using high-thermal-conductivity materials and minimizing the physical size of the system.

Temperature Control Specifications:

Expectations for cooling rate and process temperature must be realistic and within the capability of obtainable sensors and system construction. Arbitrary specification of temperature stability and limits can result in unobtainable results or unnecessary cost. Consider the importance of temperature precision and accuracy [see sidebar] in your system and relax these specifications as far as possible without affecting process results.

Temperature accuracy is important but how much accuracy is really needed? Unless you are dealing with a phase change, distillation threshold, or similar effect that has an inherently precise temperature it is unlikely that accuracy of one or even several degrees will affect process outcome.

Specifying accuracy better than 0.1 degree could drive control or system design cost and schedule well beyond acceptable limits. Be realistic about the accuracy and stability that your process must hold at set point.

Temperature Control Selection – Algorithm:

The most common methods of temperature control are ON-OFF (sometimes called “bang-bang”) and proportional with optional integral and derivative (PID).

If a process has low thermal lag time between the source and sink of heat and if the sensor is closely coupled to this path then on-off control will always yield the most stable and close to setpoint temperature. It will probably also be the lowest cost.

When either or both of these conditions (thermal lag and sensor coupling) are not adequately met then some form of PID control is appropriate. This selection should not be made without careful consideration because it will always result in higher cost and additional effort to obtain satisfactory adjustment of control parameters.

A proportional control can be adjusted to achieve smaller overall fluctuation of temperature from setpoint than on-off control but the average value of process temperature reached will still be somewhere within the maximum fluctuation obtained with the on-off control. This average value will depend on system parameters and cannot be significantly affected by the control.

Adding integral action (PI) will allow adjustment so the temperature fluctuation is small and the average temperature value reached will be close to setpoint. However on startup or change of setpoint there will be overshoot which can be reduced by parameter adjustment but not made arbitrarily small.

Finally, adding derivative action (PID)  will, in most cases, allow adjustment so that temperature fluctuation and average value are arbitrarily close to setpoint with little or no overshoot but at the cost of slow rise and fall time, which cannot be arbitrarily adjusted, on startup or change of setpoint. The parameter adjustments to achieve a desired temperature response may be lengthy, tedious and quite sensitive to system characteristics.

Temperature Control Selection – Display and Output:

Consider carefully how process temperature and set point will be displayed to the user. Even a high-quality controller and carefully designed system can appear to be unstable if the temperature readout is presented in 0.001 or even 0.01 degrees. Always specify readout precision in the largest unit that is meaningful to the process. One degree is usually adequate in nonscientific applications.

Control output to actuate active cooling devices will be specified according to the requirements of those devices. Common types are: mechanical or solid state relay contacts and analog voltage or current transmitters. Type of control of actuation usually has little or no effect on the quality of temperature control.

Summary:

To design or troubleshoot satisfactory control of process cooling start with system design, not control selection:

• Is there adequate cooling for the heat generated?
• Does heat have a low thermal resistance path to leave the process?
• Are system temperature gradients understood and minimized?
• Are precision and accuracy specified appropriately for the practical process requirements?
• Is the type of sensor appropriate for these specifications?
• Is the sensor placed at a location most important to the process?
• Is ON/OFF control satisfactory for the process and user?
• (Only proceed to Proportional, PI or PID if absolutely necessary.)
• Is the display precision appropriate to process and control stability?

SIDEBAR

Accuracy and precision have different meanings. These terms are often confused when specifying process control. The difference for both cost and performance is important.

Precision is how closely temperature can be read on a display or dialed for a set point. For example, a display of 102.3 °F is precise to 0.1 degree while a display of 123 °F is precise to only 1 degree.

Accuracy is how closely a reading conforms to the same reading calibrated and traceable to international standards measured at exactly the same point. For example, a process reading of 102.3°F that is accurate to +/-0.5°F could be showing an actual temperature anywhere from 101.8 to 102.8°F. A more precise temperature reading is not necessarily more accurate.

In general, high precision can be obtained more easily and at lower cost than high accuracy. However, neither precision nor accuracy should be specified beyond what is necessary for a process.