Over the last few columns, I’ve beaten the energy conservation drum pretty hard. But energy costs remain fairly high, and many companies are still struggling with their impact on profitability, so please indulge me one more swing of the drumstick. This time, I’ll look at some ways to quickly assess efficiency and energy savings in process heating equipment.
To better understand, consider the impact of an efficient combustion operation on oven or furnace efficiency, which is defined as the percentage of total heat input to the process that finds its way into the product itself. As a simple example, if a process load absorbs 450,000 BTU out of 1 million BTU released into the oven, the efficiency is 45 percent. The remaining 55 percent of the input was consumed as various losses, and these generally fall into two classes -- fixed and variable.
Fixed losses tend to be constant regardless of an oven’s production rate because they’re driven primarily by the operating temperature of the process. They include heat losses due to conduction through wall, floor and roof insulation; heat carried out by cooling water, airstreams or continuously running conveyors; and heat conducted out of the oven or furnace proper by objects such as conveyor roll necks extending through the insulated walls.
By contrast, variable losses are driven primarily by the production rate of the oven or furnace. By far, the most significant of these is flue gas or exhaust loss, the hot mixture of air and combustion products that exits through the stack. As production volume increases, heating systems have to be driven harder, so exhaust losses are proportionally greater. The amount of heat escaping out the stack is a function of the weight flow and temperature of the exhaust gases.
Available heat is a term I’ve used frequently in the past. As a reminder, it’s the sum of the heat to the load and the fixed losses, or, if you want to look at it another way, the total heat input to the process minus the exhaust gas losses. It’s frequently expressed as a percentage of the total input. In the example above, if the total loss of 55 percent consisted of 15 percent fixed loss and 40 percent exhaust loss, the available heat would be 60 percent.
Available heat is often confused with process efficiency. It isn’t process efficiency, but the two are closely related. The heat to the load equals the available heat minus the fixed losses. Put another way, as the available heat percentage goes up or down, the efficiency will follow closely (figure 1).
Understanding this helps spotlight priorities in energy conservation. To maximize efficiency, you want to maximize available heat, and to maximize available heat, you want to minimize exhaust losses. That requires lowering the exhaust gas mass flow, temperature, or both.
Minimizing mass flow of exhaust gases usually means minimizing the amount of excess air that enters the oven or furnace through the combustion system or as secondary, dilution or leakage air drawn in by the exhaust fan or stack draft. There are limits on how low you can go with excess air, especially in low temperature ovens or processes that drive off solvents or moisture from the product. In my experience, though, most plants have room for improvement.
Lowering the exhaust temperature can be done a number of ways. The most efficient is optimizing the heat transfer from the combustion gases to the load. It’s also the most challenging to do, so the next step is capturing some of the heat in the exhaust and recycling it back to the process that generated it. The third option is to use the waste energy to heat some other process or provide comfort heating.
Reducing energy consumption has a secondary benefit -- the reduction of the amount of CO2 escaping into the air. For every million BTU of natural gas you conserve, you reduce CO2 emissions by about 120 lb. Figure 2 can be used to see the fuel saving and CO2 reduction benefits from raising your process efficiency. Start with your current, unimproved efficiency at the bottom of the graph, read up to the improved efficiency and then across to the percentage reduction in energy use and CO2 emissions. For example, an increase in efficiency from 45 percent to 60 percent will cause a 25 percent decrease in energy use and CO2.