The definition of oven or furnace efficiency is pretty straightforward -- it’s the thermal energy put into the product, divided by the total thermal energy consumed to carry out the process. To do an energy efficiency calculation, then, you need to know the amount of product heated within a certain time period, what temperature it reaches, and how much energy it absorbs getting to that temperature. You also have to calculate or estimate the process losses -- the energy conducted away through furnace insulation, heat carried out by material-handling equipment, cooling water or air and atmosphere gases, radiation out openings and heat stored in the oven structure.
With fuel-fired ovens and furnaces, you also have to figure in the heat carried away by the exhaust gases, and that’s usually done by measuring their temperature and the excess air they contain. Excess air, in turn, is generally derived from the oxygen content of the exhaust gases.
So why is figuring efficiency such a tricky proposition? Because of one variable not mentioned above -- when you take your measurements.
Look at figure 1, which represents some of the things going on during a typical batch oven or furnace heating cycle. At the beginning, the exhaust gas temperature is low because the oven and its load are cold and absorb heat like sponges. This is true in spite of the fact that the fuel input through the burner system is high and excess air is usually at a minimum. As the load approaches setpoint temperature, the temperature control system begins to throttle the burners to low firing rates. As a rule, burner ratios are set for higher amounts of excess air as firing rates decrease, so the O2 in the stack climbs. Finally, you reach the soak portion of the cycle, where the product and oven become saturated with heat and the combustion system settles down to its minimum firing rate and (usually) maximum percent excess air.
So here you have the three biggest efficiency factors wandering all over the map during the operating cycle. Heat storage and wall, opening and cooling media complicate the picture. Some variables are high at the beginning of the cycle while others increase as time goes on. As a rule, though, they don’t make enough of an impact to upset the overall trend: High efficiency at the start, when the load is absorbing large amounts of heat and the exhaust temperature and excess air are low, then declining to its minimum efficiency during the soak, when the load absorbs very little energy and exhaust temperatures and excess air levels are high. Obviously, a set of tests taken at any one point in this cycle could give you a false picture of your efficiency.
This is the time to look at the process as a movie, not a snapshot. You need to know the total amount of product heated and energy consumed over the entire cycle to get an average heating efficiency, which is what really matters. This is why individual fuel meters on furnaces and ovens are such a great asset -- they measure the total energy consumed over the cycle without forcing you to collect repeated temperature and excess O2 readings or worrying about whether you did them at the right times.
“Great,” you say, “All I have to do is hook up a gas meter and check my production logs, and my work is done. No need to mess with O2 readings and stack temperatures.”
Not so fast. Although the meter and production data allow you to capture your cycle efficiency, how do you know it’s as good as it could be? For that, you and your instruments need to pay a visit to the stack, and getting useful stack data takes more effort than you might imagine. To do things right, you really need to get readings at high and low fire. The degree of difficulty depends on whether your temperature control system is high/low (or on/off) or proportioning. Figure 2 shows the difference in how they operate. High/low and on/off systems have only two firing rates, so you’ll be able to check both in one visit, no matter where you are in the cycle, other than at the beginning or end. But, if the time interval at one input or the other is brief, you may not have enough time to draw a stable flue gas O2 sample.
With proportioning systems, you don’t have the luxury of wandering out to the oven any time you please. Most of the cycle, the burners are operating at some intermediate rate, which is unlikely to reflect either high or low fire settings. You have no choice but to take one reading early in the cycle, when the combustion system is running flat out, and another late in the cycle, when it has settled down to low fire.