Consumption Curve
An oven or dryer’s most efficient operating point is at its design rated capacity.


“Can you spell efficiency?”

“Sure. E-f-f-i-c-i-e-n-c-y.”

“Okay, now what is it?”

“Uhhhh….”

Efficiency isn’t as clearly understood as many people would like to think. Part of the problem is that there are several ways to define it, and people’s choice of definition often reflects their unique perspective on oven and furnace operations. Maybe it’s time for us all to get onto the same page.

I’ll begin by defining overall thermal efficiency. As used by the process heating industry, it’s a percentage, figured by dividing the thermal energy put into the product by the total thermal energy expended by the process. So, if an oven heats 5,000 lb per hour of a product, and each pound absorbs 100 BTU, total heat-into-product is 500,000 BTU/hr. If the oven consumes 1 million BTU/hr to do the job, its thermal efficiency is 50 percent.

Pretty straightforward. The confusion arises when you introduce the term combustion efficiency because there’s disagreement about what it means. To some people, it’s the overall thermal efficiency of any process heated by combustion. To others, myself included, it means available heat. As I’ve mentioned before, available heat has a few definitions of its own, but the closest one defines it as the portion of the total heat input that actually stays in the oven or dryer. In other words, it’s the total heat input minus exhaust losses.

For some industries producing commodities under tight price constraints, efficiency is defined as the amount or cost of energy per unit of finished product. They know how much energy cost each pound or piece can carry into the marketplace and still produce the desired profit margin.

This can lead to some confusing situations - in one plant I visited, the production manager and owner had an ongoing argument about the efficiency of the gas heating equipment. The owner complained that energy efficiency was down and fuel costs were up. The production manager said that couldn’t be - they were operating at reduced capacity and everything was properly tuned up, so how could they possibly be using too much gas?

Actually, they were both right, but they were using different yardsticks. Overall gas consumption had decreased with production rates, but not as sharply, so the line’s per-pound consumption had gone up. This is a common characteristic of industrial heat processing equipment - look at the chart to see why.

This chart plots energy consumption per unit of production vs. production rate. There are curves for the product and what I’ll call fixed and variable losses. In addition, there’s a total consumption curve, the sum of the first three.

First, the product - as long as it enters and leaves the process at the same temperatures all the time, its heat consumption, per unit, stays constant.

Fixed losses, which include heat conducted out through the walls, carried outside by the conveyor, stored in the structure, and radiated out openings, remain constant as long as the oven runs at the same temperature and speed. As production rate increases, then, energy per unit decreases steadily. The same amount of energy is being spread over more and more production. This will hold true up to the design capacity of the oven. On the other hand, variable (exhaust) losses slowly increase as the process is cranked up to higher capacities. This happens mainly because the heating gases have a shorter residence time in the oven and are forced to exit at higher temperatures. Add heat to the load to the fixed and variable losses, and the sum, total energy per unit of production, decreases with a climb in production rate.

This trend continues until you reach the design capacity of the system, which is its most efficient operating point on a BTU/lb basis. Move beyond rated capacity, however, and the curve begins to reverse direction. Energy content per unit of production stays the same, but the decline in fixed losses begins to taper off and may eventually increase. Variable losses turn sharply upward once you pass the rated maximum capacity.

Why? Because the only way to push an oven, dryer or furnace beyond its rating is to move the product through it faster. Now, with less time to get the required heat into the product, you have to increase the operating temperature. Wall, conveyor, radiation and exhaust gas losses, along with heat storage, all increase, and of course, so does the total.

What are the lessons from all this? First, to avoid miscommunication within your organization and with your customers and suppliers, agree on your definition of efficiency. Second, if you’re tracking energy consumption per unit of production, the heating equipment’s most efficient operating point falls at its design rated capacity. The farther production rate deviates from this point, in either direction, the less efficient the equipment becomes.

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