Ideally, we can measure energy reductions in terms of the amount of energy required to produce the products we manufacture. In the case of heat-treating applications, we can measure these reductions in BTUs per pound of production or in units of energy per unit of production. We can obviously reduce energy consumption by producing fewer products, but the true goal should be to reduce the products’ energy intensity with the potential to allow us to produce more products using the same or even less energy.
One method to achieve lower energy intensity is through more efficient combustion-control systems. One combustion-control system methodology that has a proven ability to reduce energy intensity has been utilized in North America for the last 25 years and in Europe for the last 40 years. It is known as pulse-firing, or impulse, control. This article will review the technical attributes of pulse firing, particularly as applied to heat-treating applications with current and new burner technologies.
Pulse-Firing ControlPulse firing was originally designed to control high-velocity burners during heating cycles when the combustion-system input needed to be reduced in response to the heat demand. Rather than being controlled by a single-zone air valve and a pressure-balance ratio regulator, each burner utilizes its own individual high-cycling air valve and a rapid-response ratio regulator. Much like the modulated ratio control system, the rapid-response ratio regulator receives a burner air-pressure signal through an impulse line connecting the regulator to a point immediately downstream from the high-cycling air valve (Fig. 1).
Both of these specialized components are designed and tested for the demands and cycling of a pulse firing control system. The pulse-frequency can be as high as 10 cycles per minute. Rather than controlling the energy input of the burners by modulating between high and low output, the pulse system operates the individual burners at a high/low or high/off sequence. The energy input is thereby controlled by varying the on and off times of each burner.
The efficiency of pulse-firing control can be demonstrated by examining the kinetic-energy principle: K = 1/2 MV2. Since kinetic energy puts the furnace gases in motion, the higher the kinetic energy, the greater the stirring effect. The stirring effect directly relates to the convective heat-transfer rate that is so important to improving heat transfer to the load and creating temperature uniformity within the load.
Figure 2 compares the kinetic energy of a conventionally modulated combustion-control system to the pulse-firing control system. Comparing the kinetic energy increase at a 50% heat demand, Figure 2 demonstrates that the pulse-firing control system provides two times the relative kinetic energy of the conventionally modulated system. This attribute is extremely important when using high-velocity burners.
Additional Benefits of Pulse-Firing ControlPulse-firing combustion control also provides other advantages to the combustion-system performance. Since each burner is independently controlled in a high/low or high/off mode, the furnace zones are selected electronically and not by hard piping. This enables the control strategy to be determined by the furnace load, and it can compensate for varying load shapes and densities. The burners within a specific zone of control respond to a temperature-controlled heat demand by “pulsing” in a high/low or high/off sequence in turn. For example, if a zone contains four burners and the heat demand signal is 50%, each burner in the zone is at high fire for 50% of the cycle time and at low fire (or off) for 50% of the cycle time. The burners are sequenced so that alternately two burners are at high fire and two burners are at low fire (or off).
Since the pulse-firing control operates the burners on high/low or high/off, burner calibrating is simplified to two adjustments. One adjustment is at maximum firing rate and the other is at minimum firing rate, which greatly simplifies burner maintenance. Proper burner adjustment and maintenance can save energy by maintaining the ideal air/fuel ratio and eliminating unneeded excess air.
Pulse-firing control is easily adapted to accommodate preheated combustion air with the use of a second biasing regulator installed immediately downstream of the ratio regulator and controlled by an electronic preheated-air compensation system. This system also contributes to maintaining proper air/fuel ratio and therefore maximizes the increased efficiency of the preheated combustion air.
Finally, the pulse control system fully utilizes the flame and heat-transfer characteristics of a high-velocity burner while minimizing the need for excess air as evidence in a fuel-only modulated control system. Fuel-only control systems are employed to maintain velocity at lower heat-input requirements of a modulated system. The fuel-only control system depends on excess air to maintain temperature and velocity at lower firing rates, which diminishes the system’s energy efficiency.
Case StudiesThe following four case studies demonstrate the energy-intensity reduction made possible by pulse-firing control.
Case 1 – Stainless Steel Wire
A batch furnace used to anneal stainless steel wire was fitted with pulse-firing control. The high-velocity burners employed were configured in one zone with three burners firing over the load and six firing under the load. The temperature uniformity requirements were: at 1150+/-10°F, at 1600+/-20°F and at 1950+/-25°F. This combustion system achieved all uniformity requirements while providing a 20% energy-intensity reduction.
Case 2 – Centrifugal Castings
A batch-type furnace used to heat treat centrifugal, spun castings was fitted with pulse-firing control and six high-velocity burners configured into three zones with one burner top fired and one burner bottom fired. The first efficiency benefit realized was that the time to temperature of the load was reduced from 3 hours to 1.5 hours. Temperature uniformity at 1800°F was +4/-1°F while providing an energy-intensity reduction of 30%.
Case 3 – Copper Billet Pusher Furnace
A continuous pusher-type furnace used to heat copper billets was fitted with pulse-firing control of 14 high-velocity burners. Billet heat-up time was reduced from 7 hours to 5 hours while maintaining a temperature uniformity of +/-10°F at 2000°F. An energy-intensity reduction of 20% was achieved.
Case 4 – Bell Furnaces for Titanium
Two identical top-charge bell-type furnaces were each fitted with two high-velocity burners configured tangentially near the top of the bell. These furnaces were used to harden titanium rings. One furnace was single zone with conventionally modulated control using excess air in the soak portion of the cycle. The second furnace, fitted with the same type and capacity high-velocity burners, was pulse fired and did not require the use of excess air during the soak portion of the cycle. Temperature uniformity in both furnaces was +/-10°F at 800°F. The pulse-firing controlled furnace achieved a 30% reduction in energy intensity as compared to the modulated controlled furnace.
Other Pulse-Firing Control Applications and BenefitsHigh-velocity burners achieve maximum entrainment of the furnace atmosphere when operated at high or maximum velocity. Laboratory testing has documented a reduction in NOx production during this maximum entrainment condition.
Other modern burner designs, namely self-recuperative burners in direct-fired and radiant-tube applications, benefit from pulse-firing control because of the ease of burner calibration and enhancement of the burner’s energy efficiency at the maximum firing rate. Typically, these types of burners incorporate pulse-firing control.
ConclusionOur future may certainly hold new regulations for reduction of carbon emissions. Regardless of our opinion of the effects of CO2 in the atmosphere, reducing energy intensity is good policy and good business. We look to new technology to provide the vehicle for these efficiency gains. Pulse-firing combustion control has demonstrated significant reductions in process energy intensity while offering the added benefits of improved temperature uniformity, reduced cycle times and lower NOx emissions. It is important that our industry understand the complexities and benefits of improving our energy intensity and not focus mainly on the more simplistic goal of energy reduction. IH
For more information: Contact Sherri Stom at Hauck Manufacturing Co./Kromschröder Controls, PO Box 90, Lebanon, PA 17042; tel: 717-272-3051; e-mail: email@example.com. The author is the president of Hauck Manufacturing Co./Kromschröder Controls.
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