Using an inexpensive automotive O2 sensor in each tube exhaust and a single ratio trim controller to check each sensor periodically via a multiplexing circuit would be one way to achieve relatively inexpensive feedback to operators that certain burners need attention.

In 2006, as part of its “Save Energy Now” program, the U. S. Department of Energy funded “Energy-Savings Assessments” of process heating applications in many industrial plants. The assessments are conducted by engineering professionals who have completed the DOE’s training to become BestPractices Qualified Specialists. I was responsible for conducting assessments at several plants.

At the plants assessed, up to three typical heating devices were selected for energy-balance studies to teach plant personnel how to continue the process on their own. The expected outcome was identification of operating conditions wasting energy and informing the plants about technology upgrades to make the processes more efficient.

One striking discovery was the amount of energy wasted for simple lack of tuning and maintenance. Ovens and furnaces in eight of the plants I visited were operating below their potential simply because burners were firing with too much excess air. The amount of gas that could be saved with proper tuning amounted to 3.75 percent of the plants’ entire gas bills.

Projected savings could only be figured on equipment actually studied; we weren’t permitted to assume other units were running the same way. In reality, though, they probably were, and if so, the average potential reduction in plant-wide gas consumption is 6.5 percent! Not all these processes are high temperature, either, where small excess air variations have a big impact on efficiency. Clearly, if industrial manufacturers are going to put a significant dent in energy consumption and emissions, we have to figure out what’s going wrong and how to deal with it.

Combustion ratio control systems come in all flavors, from simple, manually set valves to microprocessor-based systems with feedbacks for air and gas meters, stack oxygen, fuel properties and ambient air conditions. The interesting thing I learned was that technical sophistication did not guarantee efficient operation. There was no correlation between the correctness of the ratio and the cost of the control system.

The problem seems to lie in the nature of the pressure and flow control equipment used to set and maintain fuel-air ratios. Even the most sophisticated systems depend on components like butterfly valves and pressure regulators to carry out control instructions. Unlike modern electronics, which are binary devices, they behave in an analog fashion, which means they go out of adjustment or fail a little bit at a time. Deterioration is so gradual you don’t notice it until it becomes very obvious, and by then, a lot of energy has gone to waste.

In short, drift is inevitable unless the system has built-in self-correction. High-end ratio controllers do, but they’re not immune either. Heat, dirt and long periods without recalibration will reduce their long-term accuracy. They can slow the rate of ratio drift, but they can’t stop it altogether.

Chasing the Payback

Unfortunately, high-end systems are too complex and costly for small applications. Their capability to handle a wide variety of combustion systems and operating parameters prices them out of the market for smaller, simpler applications. Users must make do with the same mechanical or pressure-balanced ratio controls they’ve been using for the last 75 years, simply because there is no middle ground.

We need a family of ratio controllers incorporating features like oxygen analysis and metered flows, but at a cost justifiable on small-and medium-sized applications. Specifically, this calls for:
  • Basic ratio monitors and controllers without all the capabilities not needed on smaller applications. Sacrifice versatility and adaptability for “focused” features.
  • Simple, easy-to-understand setup and operating procedures.
  • Willingness to accept good, but not necessarily state-of-the-art, accuracy and response speed, as long as this doesn’t pose any safety hazards.
Consider a possible control system for a multi-burner radiant tube heat-treating furnace using flue gas oxygen feedback (figure 1). It might use an inexpensive automotive O2 sensor in each tube exhaust. A single ratio trim controller would check each sensor periodically via a multiplexing circuit. Because excess O2 settings usually differ between high and low fire, the O2 monitor would be empowered only when the control zone is at high fire, in the case of a batch furnace, or at its normal steady-state firing rate, in the case of a continuous furnace. To further reduce complexity and cost, the controller would not actuate any ratio-trimming valves at the burners - it would simply advise operators certain burners need attention.

Despite its limitations, it would be a huge improvement over present-day systems, where ratios only get adjusted after someone notices black smoke pouring out of the tube exhaust. Half a loaf is better than none.

The Human Element

The promise of lights-out manufacturing and other automatic, self-governing processes has lulled us into thinking human intervention will soon be a thing of the past. In truth, humans are needed more than ever. Low-tech combustion controls can’t monitor and correct their own operation, and we have learned high-tech systems can’t be set and ignored, either. High-tech controls can be visited less often but require someone with a higher level of skill to adjust them. Unfortunately, industry has lost most of the skilled operators and technicians who used to keep these systems running efficiently. We have to train a new generation.

Recycling Savings

No improvements will last unless the financial culture of the company permits it. If all savings must flow to the bottom line in the interest of “shareholder value,” projected savings will not materialize, and eventually, efficiency and emissions performance will be as bad as they originally were. Saving energy is not a self-sustaining activity unless some of the savings are recycled to keep the effort going. In the words of Goethe, “Seed for the planting must not be ground.”1

Tuneups do not produce permanent improvements, nor do sophisticated controls. The only way to keep combustion equipment operating at peak efficiency is through regular rechecking and adjustment. For a company with a $1 million annual fuel bill, the average savings of 6.5 percent mentioned earlier will produce a $65,000 reduction in energy costs - more than enough to support a full-time combustion technician, armed with the diagnostic tools needed to keep ovens and furnaces running efficiently, and still have something left for the bottom line. If energy consumption isn’t sufficient to support a full-time technician, contract service might be the answer.

Note: This column is condensed from Dick’s presentation at the Controls & Sensors Conference, sponsored by Industrial Heating & Process Heating in Cleveland, Ohio, May 2, 2007.