Click to enlarge Figure 1. A typical cross-connected system with excess-air and pulse-firing capabilities

As combustion-control technology has evolved, the cost of fuel and improved product quality have contributed to product design. In the past, the most common combustion-control system has been the cross-connected design. In such a system, the air supply to the burner is modulated, and an impulse line connected between the combustion-air line and the gas regulator acts on the regulator and allows the gas to follow the air. As the control valve in the combustion-air line is modulated, the pressure in the pipe downstream of the valve is increased or decreased. The impulse line applies the pressure to the diaphragm of the regulator, opening or closing it.

This cross-connected system can be designed to operate a single burner, a zone or the entire furnace. The system can also be set up to operate at ratio or with additional (“excess”) air. As a downside, once the system is set up, it cannot switch between ratio and excess air without additional hardware. It also requires a skilled technician to set up and balance the system in order for it to meet temperature uniformity requirements. In addition, the system adjustments are entirely manual, with no feedback.

Click to enlarge Figure 2. Mass-flow combustion system with flow measurement and control valves on the air and gas line

Mass Flow

While the conventional cross-connected system offers a uniform temperature within the furnace, it is also highly inefficient. These systems are typically superseded by one based on mass-flow control that measures and controls the ratio of gas and air to each burner. The ratio is defined by the control system, and each burner gas and air line includes devices to measure and control flows to that particular burner. A mass-flow control allows the user to easily modify the air/fuel ratio for different product or process requirements. This plays a fundamental role in the safe and profitable operation of a furnace because the air/fuel ratio in the combustion zone directly impacts fuel combustion efficiency, environmental emissions and cost.

Efficiency

When trying to heat forging products to temperature, the combustion process is inherently inefficient. Heat released during combustion heats the combustion gases, which are comprised mostly of air. Approximately 65% of the energy consumed is used to heat this air. At 2250°F operating at 10% excess air, only 35% of the net heat is available to heat the product. In actual practice, a furnace is often set up to operate with greater excess air than needed to meet process-uniformity requirements, resulting in an even less-efficient process.

Click to enlarge Figure 3. Forge furnace with mass-flow combustion system

Achieving Uniformity

When setting up a combustion system, achieving temperature uniformity is often the primary goal. The challenge arises when trying to achieve uniformity over a wide range of temperatures, such as those used in forging different alloys. For example, the combustion setup to achieve uniformity at a forging temperature of 2350°F is different from the setup to achieve a forging temperature of 1700°F. At higher temperatures in a properly designed furnace, little excess air is required to achieve good temperature uniformity. Gases exiting the burner tile generally create sufficient stirring of the hot gases around the work zone to meet thermal-uniformity requirements. High gas velocities entrain the atmosphere surrounding the burner, creating a high degree of stirring within the chamber and a uniform temperature in the work zone.

Operating a furnace at lower-than-maximum temperatures requires less input as a result of the lower burner output reducing the amount and velocity of hot gases exiting the burner tile. Reduced velocity, which lessens entrainment and stirring, can significantly alter the thermal-uniformity characteristics of the furnace.

The technique most often used to increase burner-tile exit velocity is the addition of excess air. The additional air creates a larger volume of gases exiting the burner, increasing stirring and improving thermal uniformity within the work envelope. Although heating this excess air results in a loss of efficiency, this is a necessary trade-off to achieve the uniformity required by the forging process.

More Precise Control

Mass-flow control provides the most precise method of controlling the air/fuel ratio over a range of operating conditions. While the conventional cross-connected system is capable of precise control within the initial operating parameters of the system, it often has a limited range in which the system can operate and still meet process requirements. The conventional control type is an open-loop system in which there is no feedback of actual flows. When the control system calls for a change in temperature, the air control valve position is changed, resulting in a corresponding change in gas flow.

In contrast, the mass-flow system is a closed-loop design in which the flow is measured and is a part of the control loop. As the temperature controller calls for a change, the mass-flow system will respond similarly by changing the input to the furnace. As each burner has a control valve and mass-flow meter, the air/fuel ratio to each burner can be precisely controlled.

The mass-flow system can be set up to vary the air/gas ratio for particular temperature ranges or to vary the ratios linearly over a wide temperature range. This allows the user to customize the system for a particular process.  

Click to enlarge Figure 4. Air/gas ratio table used to optimize the furnace once calibrated

Reducing Fuel Usage

During initial setup for a furnace with mass-flow control, the furnace is calibrated to determine the optimum air/fuel ratio over its entire operating range. This data is then used to set up the furnace to minimize its fuel consumption. For each temperature range, the air/fuel ratio can be adjusted to get the best performance out of the furnace with the least fuel consumption, thereby also reducing its operating cost.

Conversely, when setting up the conventional system a service technician must adjust limiting orifices and regulators to achieve uniformity in the operating temperature range. This is often a compromise. At lower temperatures, more excess air is required to achieve uniformity, which can cause excessive fuel usage at higher temperatures. It also does not allow for adjustments on the fly and is prone to adjustments by unqualified personnel.

Conclusion

The addition of mass flow to combustion control offers multiple advantages when compared to a conventionally fired system. These include the maximization of combustion stability and the minimization of emissions and cost. Over the life of the furnace, the mass-flow system will lower long-term operational costs, reduce waste and conserve natural resources in most cases. In addition, while conventional systems are prone to modification, the mass-flow system has the capability to be locked within the software to prevent unauthorized changes.  

Author Pete Komrowski is a Senior Project Manager at O’Brien & Gere (Denton Thermal Products). Komrowski can be reached at Pete.Komrowski@obg.com. For additional information, visit www.dentontsi.com or www.obg.com.