Better temperature sensing and data gathering

Innovative controls, accessories, software and other techniques are enabling temperature instruments to reach in and help optimize many previously inaccessible applications. Here's how.

By Jim Montague

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Not much seems to change on the temperature front. Thermodynamics are physical laws, after all, and not guidelines or suggestions. Processes get hot and cold, and they're measured by thermocouples, RTDs, thermistors and other instruments just as they've been for decades.

What's different and evolving quickly are all the components, controls, software and other enhanced methods surrounding the basic roots of temperature measurement. These devices are allowing more and better temperature sensing and data gathering in many locations that used to be inaccessible, and they're gathering and organizing that information for much better archiving, analysis and eventual process optimization than was possible before. So just like pressure, flow and other immutable process variables, temperature isn't changing, but the world around it sure is.

Smart Heat Saves

For instance, Hydro Aluminum in Årdal, Norway, recently sought to adopt low-temperature Oxyfuel burner technology to increase its cold-metal melting capacity and save energy, so it worked with Linde Gas and with Eurotherm by Schneider Electric to provide an intelligent burner control system for four aluminum furnaces in its cast house (Figure 1). In general, Oxyfuel increases thermal efficiency because its furnace gases don't contain nitrogen like air fuel, so its radiant heat transfer is more efficient, and its exhaust gas volume and heat loss are reduced.

Hydro Årdal produces more than 125,000 tons of foundry alloys per year, and Oxyfuel combustion in its four furnaces is managed by Eurotherm's Foxboro T2550 process automation controller, a safety PLC to comply with regulations and an HMI, which operate in conjunction Linde's Gas Oxyfuel burners. As expected, Hydro reports it can replace 20,000 tonnes of liquid primary aluminum by remelting cold metal. Previously, the charge mix was 8 tonnes cold and 22 tonnes liquid metal in production line, and now the mix is 13 tonnes cold and 17 tonnes liquid metal. As a result, Oxyfuel helped the two primary foundry alloy production lines at Hydro Årdal achieve a 60% increase in remelting capacity, a 50% reduction in fuel consumption, a cut in CO2 and NOX emissions, and a reduction of waste dross to less then 2%.

See also: pH and temperature measurement and control tips

"By using low-temperature Oxyfuel, we can melt 50% more cold aluminum in the same amount of time as we did previously," says Wenche Eldegard, cast house manager at Hydro Årdal. "In addition, our propane consumption and CO2 emissions have been halved, which is also very positive."

Controls Optimize Acid, Cook Biomass

Just like a chef perfecting a recipe, today's controls can surpass simply babysitting their temperature instruments and applications to gather and analyze data, and more quickly reduce variability and improve their processes and the quality of their end products.

For example, Sterling Chemicals in Texas City, Texas, produces acetic acid by reacting carbon monoxide and methanol in the presence of a catalyst, and then purifying it in an associated distillation train (Figure 2). Greater efficiency was possible with a better catalyst, but this decreased reactor temperature control stability, and the existing PID-based controls couldn't prevent temperature excursions during upsets. Also, a possible advanced control solution was hindered because the reactor couldn't be bump tested due to its carbonylation exothermic reaction.

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