FIGURE 1. Use this chart to calculate the relative fuel consumption per unit of product as a function of loading. The three curves represent the nominal straight-line efficiency of a properly tuned unit operating at 100 percent loading.

With energy prices throwing everyone’s manufacturing cost projections into the trash, efficient operation of ovens, furnaces and other fired heating equipment has become a hot topic again. Over the years, I’ve discussed many of the factors affecting energy consumption and what you can do to improve the situation, but I’ve done it in sort of a random fashion. I’m going to go through it again, but this time in an organized, systematic way to give you a framework for analyzing and improving your process heating operations.

There are three basic steps in getting optimum energy efficiency from your process heating equipment, and you should take them in this order:
2. Investigate upgrades or improvements to your existing equipment to increase its efficiency.
3. Investigate more radical ways to improve energy efficiency, including replacement of the existing equipment.

There’s a lot of appeal to tearing out that wheezing, grubby old oven and replacing it with a shiny new one, but you owe it to yourself to give it one last chance to prove its worth. For one thing, you can do it more quickly. Second, it will cost you a lot less money. Third, it can be done with a minor disruption to production. I’m talking about two things here -- scheduling and maintenance.

Scheduling practices are often overlooked because we have an “I want it, and I want it now” mindset when it comes to running product through heating equipment. This attitude has been reinforced by adoption of the just-in-time philosophy, which discourages allowing in-process work to sit still on the shop floor. This often leads to running undersize lots of material through heating equipment, often starting them up to run just a few pieces and them shutting them down again. Unfortunately, this beats you up on efficiency. Thermal efficiency is defined as the amount of energy absorbed by the load, divided by the total amount of energy consumed by the process. For example, if the load takes 400,000 BTU/hr out of a total heat input of 1,000,000 BTU/hr, the thermal efficiency is 40 percent. At reduced production rates, there’s less material being processed, so the heat consumed by the load decreases in proportion. In the example above, the load will consume only 200,000 BTU/hr at 50 percent throughput.

Exhaust losses will decrease because the firing rate can be reduced. Conveyor losses may decrease if the line speed can be reduced. Unfortunately, heat lost through the walls and out openings will stay essentially constant, because they’re dependent on temperature, not production rate. When you factor in everything, the total amount of heat required will decrease, but the heat consumed per weight or unit of product will increase.

How much? It’s tough to generalize, but if I make a few reasonable assumptions about the operation of the furnace or oven, I can generate a chart like figure 1, showing the relative fuel consumption per unit of product as a function of loading. There are three curves representing the nominal straight-line efficiency of a properly tuned unit operating at 100 percent loading. By the way, this chart applies only to continuous processes -- I’ll discuss batch processes next.

To use the chart, determine what percentage of full load the oven is operating at. Locate that number at the bottom of the chart and read up to the curve representing the oven’s nominal full-load efficiency. Then move left to get the fuel consumption multiplier. For example, assume your oven has a nominal efficiency of 40 percent, but you’re operating it at only 30 percent of capacity. This gives you a fuel multiplier of around 1.6. Now, suppose your fuel cost is 10 cents per pound of finished product when the line is running at 100 percent capacity. At 30 percent loading, the fuel cost has climbed to (10 x 1.6), or 16 cents per pound. As you can see, the penalty becomes more drastic the lower the production rate, and lower efficiency ovens and furnaces are more severely affected.

Batch processes tend to do worse than continuous processes because of heat storage losses -- this is the energy required to bring the unit up to operating temperature, and all or part of it is lost when the unit is allowed to cool down. The more often the oven is cycled, the more often it is necessary to replace this lost heat. The length of the shutdown period comes into play, too -- the longer it is, the less heat is retained in the oven or furnace, so more fuel has to be burned to bring it up to operating temperature the next time.

The moral? Operate your heat processing equipment as close to 100 percent of capacity as you can, even if it means holding some production back until you can accumulate a full furnace load.

Next time, I’ll look at maintenance.PH