In my previous two columns, I've explored the relationship between air and gas flows and their pressure drop through system resistances. I showed that two resistances in series - one fixed and one variable (a valve) - are required to control those flows.
But that's only part of the challenge. In addition to controlling the air and fuel flows individually, you need to tie the two control systems together to set and hold repeatable air-fuel ratios for flexibility, efficiency and safety while the burner system cycles between high and low fire. Before I get into the mechanics of it, I'll look at the different ways combustion air-fuel ratios can be controlled.
Unlike domestic appliances, which are simple on/off devices, most industrial combustion systems operate over a range of heat inputs. This allows them to respond to varying heat loads while maintaining good temperature control. The ratio of a system's maximum to minimum heat input is called its turndown. For example, a burner with a high fire output of 1 million BTU/hr and a low fire rate of 200,000 BTU/hr has a turndown of 5 to 1. The air-fuel ratio has to be set and maintained at two points - high and low fire - in addition to all firing rates in between.
On-Ratio ControlThe classic method of setting air-fuel ratios is on-ratio control (figure 1), where the air-to-gas ratio is set at nearly correct, or stoichiometric, ratio at high fire and low fire, and the control system is expected to maintain the same ratio in between. If, for example, you're burning natural gas, the correct ratio is about 10 volumes air to 1 volume gas, so the controls are set to give you a 10:1 ratio at high and low fire and, you hope, in between.
On-ratio control produces the highest flame temperatures and efficiencies, but it's not well-suited for many applications, including most low temperature processes. Low temperature processes frequently have heat requirement turndowns of 20 to 1, 30 to 1 and even higher. With on-ratio control systems, burners are hard-pressed to operate at heat output turndowns higher than 10 to 1 - it's difficult to control flows accurately and repeatably over wider ranges. (Remember, because of the square root law, to control air and gas flows over a 10:1 turndown range, you have to control their pressure drops over a range of 100 to 1. Most valves and control systems simply aren't up to the task.)
Excess Air ControlThis problem -- stretching the burner's turndown without resorting to exotic, expensive control systems -- led to the development of excess air systems. With these systems, the burner is set up at or near correct ratio at high fire, but it is allowed to operate with more and more excess air as it's throttled to low fire. The simplest and most popular way to do this is to lock in the combustion airflow at a fixed rate and vary only the fuel flow (figure 2). Thousands of these systems are used on ovens, dryers and other low temperature processes.
You're probably wondering why this would improve heat turndown. After all, if the fuel valve is the same type as you use with an on-ratio system, won't fuel turndown be the same? You're right -- but at reduced firing rates, all that excess air acts as a heat sink, absorbing surplus BTUs and carrying them out the stack. In effect, you've handicapped the system, adding a heat load to extend the turndown. All that excess air also contributes to uniform regulation of temperatures. The result? A higher effective turndown and sensitive temperature control, but it comes at the cost of higher fuel consumption. After all, you're deliberately wasting some of the heat to get the turndown you want.