With most plants now searching for ways to minimize energy waste, one of the most common questions I hear is whether there’s enough heat in an oven or furnace exhaust stream to make it worth recapturing. Obviously, everything has to be approached on a case-by-case basis, but here are some facts, figures and guidelines that may help you make an informed decision.

Heat Content of the Exhaust Gases. The amount of heat you may be able to recapture depends on two things -- the temperature of the waste gases, and their mass flow.

Temperature is obvious -- the hotter the gases, the more heat they contain, and the easier it is to set up a viable heat-recovery system. In the furnace business, an old rule of thumb says it becomes economically feasible to apply heat recovery when the exhaust gas temperature is 1,400 to 1,600^oF (760 to 870^oC) or higher. As energy prices have skyrocketed, though, that temperature threshold has decreased.

There are plenty of economically successful installations, however, that don’t abide by this guideline because the weight flow of exhaust gases is high, so the amount of potentially recoverable energy is also high, in spite of modest exhaust temperatures. For payback and ROI purposes, what matters is the number of BTUs you can pull out of the exhaust stream, not its temperature. It’s the combined effect of temperature and flow rate that matters.

Where Do We Use It? Some things you almost hesitate to bring up, because people start wondering if you take them for idiots, but this question has to be asked: Have you identified a use for this recovered heat? If not, move on to the next promising project.

How Much Heat Is in the Exhaust Stream? To figure this, you need to know its temperature and flow rate. Temperature is fairly easy to determine; flow rate can be found several ways.

1. Convert the average velocity in the exhaust stack to flow using this relationship:
   
   Flow (cfm) = Velocity (ft/min) x Duct Cross-Sectional Area (ft²)
   
   To get a good average, measure the velocity at several points across the stack. Recommended sampling procedures can be found in a number of texts and in some manufacturers’ literature.
   
   
2. If you have an exhaust fan, determine its rotational speed and horsepower consumption. With that data and the exhaust temperature, you should be able to figure flow from the manufacturers’ fan curves or tables.
   
   
3. Estimate it from the firing rate of the combustion system using this equation:
   
   scfh exhaust gases = (BTU/hr/1,000) x (11 + [% Excess Air/10])

Remember that your firing rate is not the nameplate rating of the burners -- it’s the rate they’re operating at under normal production conditions.

Once you have the flow and temperature of the exhaust gases, use the graph to find their approximate heat content. This is strongly affected by the amount of water vapor the gases contain. The closer to stoichiometric ratio the combustion system operates, the higher the water vapor content, so I’ve shown curves for stoichiometric operation and 100 percent excess air. If your process operates at even higher amounts of excess air, the heat content will move closer to the air curve.

How Much of This Heat Can I Recapture?

Enlarge this picture The heat content of your exhaust gases is strongly affected by the amount of water vapor the gases contain. The closer to stoichiometric ratio the combustion system operates, the higher the water vapor content.

This depends primarily on the effectiveness of the heat-recovery device. Effectiveness is defined, in rough terms, as the percentage of heat in the exhaust stream that can be recovered. Exchangers can be built for effectiveness levels of 85 percent or more, but generally, that comes at a higher price. For estimating purposes, start around 50 percent.

Whatever effectiveness you select, do a “gut check” on the exhaust temperature exiting the exchanger. If it’s too low, you may have to contend with condensation in the heat exchanger and stack. You can check this quickly with the graph.

Say you have a combustion system that operates at about 100 percent excess air. Exhaust gases are 800^oF (427^oC) and you plan to use a 30 percent effective exchanger. From the graph, 800^oF gases contain almost 20 BTU/scf. Remove 30 percent of that heat, and you’re left with 14 BTU/scf. Re-enter the left side of the graph at that value, read across to the curve and down to a temperature of about 500^oF. That’s the exhaust gas temperature leaving the exchanger.

How Low Can the Exit Temperature Be? Like everything else, this depends on individual conditions. You want to maintain a high enough temperature that subsequent cooling in the exhaust stack won’t lead to condensation of the water vapor in the gases. You’re at greater risk with a tall, bare metal stack on a winter day than a short, well-insulated stack in the heat of summer. As a rule, however, you should use care if the predicted exit temperature falls below 350 to 400^oF (177 to 204^oC). If condensation looks like a potential problem, either lower the effectiveness of the exchanger to raise the exit temperature, or deal with the condensation with corrosion-resistant stack materials and a condensation drain. Bear in mind that the effectiveness of heat exchangers tends to increase as flows decrease, so you have to design to avoid condensation at minimum flow.

These are only general guidelines to help determine if heat recovery is even viable. Always consult with a reliable exchanger manufacturer for a final determination.

In conclusion, there are no rules of thumb, as far as I know, to guide you in these matters, so you’re going to have to use common sense.