A field survey can determine how well a heater is operating and provide crucial information so a properly trained technician can adjust underperforming assets.
Take courses on heater efficiency from the Department of Energy (DOE), similar ones, or from a furnace vendor, and one on furnace maintenance to help with your survey. Before you begin, conduct a paper audit of heaters to find those that are inefficiently designed versus those that are inefficiently operated.
For a shortcut to only get an idea of how well your heater is performing, the two most important measurable heater variables are excess air and flue gas temperature. From this, you get heater efficiency to compare to industry best practices. For a real understanding of how well energy is used, measure useful energy versus energy produced.
Pull out designs for furnaces you want to audit. Best practice design depends on the type of furnace and what you’re trying to do. In this column, it’s difficult to provide absolute numbers. A stack temperature of 300º F may seem hot to some but excessively cold for others. Ask your furnace vendor what the best in your industry can do for the type of heater you have. Create a spreadsheet with your heater and the best for that heater type. Also, use a good furnace program, like PHAST (from the DOE), to calculate efficiencies. The difference between industry best and your design indicates losses built into your system by the design engineer. Again, as a shortcut, use design calculations to establish all other losses, such as radiant or wall losses, or use appropriate software for more detail.
Once you’ve completed the paper audit, conduct a field survey. At a minimum, if the heaters are well instrumented, use a thermal gun to shoot temperatures. Take a portable thermometer, a flue gas analyzer, and an infrared camera to get more accurate data or if the units rely on local readings.
Inspect the outside heater box. Stand back and look for visual signs of inefficiency. Is the insulation well maintained? Are there any discolorations in the paint? Are any holes visible or are any of the peepholes open? Can you see obvious leaks? Do you see heat radiating from the box? Do you feel heat as you walk around the box? Is the stack clear, cloudy or dirty? With a thermal gun or infrared camera, shoot the furnace and all auxiliary equipment. Concentrate on areas used for maintenance access, where lines go in and out, or that involve moving parts. Check process inlet and outlet temperatures. Then check other temperatures used to calculate efficiency. If there’s an air preheater or some sort of economizer, get all the temperatures in and out.
If you can analyze stack gas, attach an analyzer or take a sample. Unless you have a small stack, it’s nearly impossible to get a good representative gas sample without traversing the stack’s entire cross section. The more places you can measure stack gas, the better the final assessment. So, don’t neglect process side numbers and rely solely on your stack. Some furnaces will have flue gas readings inside the box’s radiant section. Differences between radiant and flue gas readings may point to air leakage or bad burner controls.
If possible, look inside your furnace. Observe flames from all angles, including the burner bottom. NOx control burners have specific patterns, so become familiar with the appearance of each type of burner. Although color and height may vary, look for uniform efficient burns. If there’s more than one burner, any burner that looks different indicates an inefficiency. See if flame heights impinge or lap on tubes. See if any flames are higher than others or if flame patterns jump around. Look for incomplete burns, burners that aren’t working, or anything unusual. Also, check insulation, tube temperature, scale or color, and look for any debris.
Note in your spreadsheet anything you physically see along with calculated field efficiency. Add the useful efficiency to your spreadsheet. Follow the process material you just heated, whether steam, oil or some chemical, and determine how much heat was transferred to your final process. Sometimes, you’ll suffer great and expensive heat losses outside the furnace.
So, with those four efficiencies: best practice, design, field and useful, you can calculate your loss in every step to help prioritize. Although you can automate most of this work, a visual inspection is essential to maintain an efficient system.
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Chemical Processing's Energy Columnist. You can e-mail him at [email protected].
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