How Can We Measure Load Profiles?

July 8, 2003
Readers help a reader solve this control problem. Next month's problem: Should We Go Paperless With the EPA?

A Reader Writes:

We sinter batches of metal parts in vacuum furnaces at temperatures up to 2,300 F and atmospheres of hard vacuum, hydrogen partial pressure, and two-bar nitrogen cooldown. The furnaces have a single Type S control thermocouple which holds up well, but we need uniformity at high temperatures within 2 F. We would like to track temperatures throughout the loads for quality control and preventive maintenance. Has anyone found an accurate, reliable, rugged method for load profiles in vacuum furnaces?

--From April 2003 CONTROL


Go Infrared

Infrared sensors would be a reliable way to monitor these types of temperatures with the needed accuracy. The 4-20 mA signal outputs from the infrared sensors could be connected to a 4-20 mA input module on the controller. Using an open I/O system would offer many choices of bus types and modules. In addition, PID loops can be used to control the temperature and vacuum. Using Ethernet controllers and a data transport utility, these components can be connected to a supervisory PC for data logging, process control, batch/heat numbers, etc.

Tracy Lenz, Senior Application Support Engineer

Wago, Germantown, Wis.

Simulation Will Satisfy

There are two issues to deal with in the defined temperature measurement problem. One is the accuracy of the selected temperature measuring device(s) and the other is correctly locating the sensor(s) to accurately provide the desired information.

...Using multiple sensors and averaging the outputs can address sensor accuracy. This is not a new concept, as we were doing this with pneumatic devices in the past. As stated in the problem text, a Type S thermocouple is used in the existing control scheme and is deemed suitable for the application. The simplest solution from a measurement accuracy standpoint is to determine the number of Class A (high accuracy) sensors to employ to achieve the desired measurement accuracy. The formula to correlate measurement accuracy with number of sensors is: Sq.Rt.(Sum of accuracies)2/N where N = number of measurements. In this case, if a single sensor had 1% accuracy, averaging three sensors would improve the measurement accuracy to 0.57% and averaging 10 sensors would improve the accuracy to 0.32%. Obviously there are diminishing returns to adding sensors, and practicality is also an issue.

The short answer is that in all probability, if three Type S T/Cs can be mounted in appropriate furnace locations, the desired result should be achievable. Alternatively, an optical temperature measurement device capable of monitoring multiple points in the furnace may be an option. We do have some experience with these devices in high-temperature applications such as cement, smelting, and induration processes we have dealt with.

The second issue, sensor location, is more complex. Available ports for sensor mounting, shape of vessel, product surface area and type of charge, fuel used, location of heat generators (induction, burners, or elements), heat rates, etc., all play roles in developing an effective solution.

There are two alternatives to multiple measurements. One is to employ inferential techniques using derived measurements such as rate of rise, time above certain temperatures, integrated error over time, etc.

The second alternative is using thermal and fluid dynamic modeling software to model the process. We employed this approach on one of our recent pyro process projects with very positive results. The software was able to pinpoint the best location for specific process measurements, provided insight to improving control of key process parameters, and predicted process behavior under a variety of operating conditions.

Bob Medower, Mineral Processing Specialist

Lew Gordon, Control Guru


Augusts Problem:

Should We Go Paperless With the EPA?

We are considering changing from paper-based to paperless reporting to the EPA for continuous emissions monitoring (CEM) compliance. We are dealing with just five stacks, and Im not sure it makes sense, but the new plant manager wants us to be state-of-the-art. What problems have people had with paperless CEM? Are the benefits significant?

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