Getting Ideas for Energy Management Opportunities

2.3.9 Heat treating

In heat treatment (such as heating, stress relieving, tempering, high-temperature annealing), major energy losses come from:

  • Heat losses through the outer shell of the heat treatment furnace due to inadequate refractory lining;
  • Poor burner performance (type, fuel/air adjustment);
  • Negative pressure in the furnace/oven;
  • Air infiltration; and
  • Inadequate heat recuperation from flue gases.

Furnace lining
Increasingly, replacement of firebrick with ceramic pyrobloc modules is gaining favour for their excellent insulating properties, low heat storage and low heat loss, fast thermal cycling and reduced maintenance requirements. Thick firebrick lining wastes energy due to its large mass and the ability to develop cracks. To illustrate the low thermal transmission of these materials, the outer shell temperatures of 260–320°C with firebrick lining can be reduced to 40°C with the modular ceramic insulation lining. The ceramic fibre properties may eliminate the need to keep the idle furnace on low fire. This eliminates extra fuel consumption because the ceramic fibre-lined furnace can be rapidly restarted and brought up to temperature. As well, ceramic fibre-lined furnaces have lower maintenance requirements and muffle sound; their operation is quieter.

Burner performance
The burner fuel efficiency can be improved in many ways, some of which have already been described in Section 2.3.3, “Melting” (page 51) and in a few examples of different approaches shown below.

It is in any company's strategic interest over the long term to control the consumption of energy, hence its cost.

Air infiltration
The infiltration of outside air is a major problem. It has to do both with maintaining the positive pressure in the furnace
and with keeping the insulation and seals around the furnace door in good repair. Normal wear and tear on the furnace will cause gaps in seals around doors and cracks and crevices to appear, which will contribute to major energy loss. Preventing cold air ingress will maintain the desired furnace atmosphere, improve temperature uniformity by avoiding chilling of parts of the load, and save fuel by not having to heat infiltrated air.

Even a correctly designed furnace can develop negative pressure. This is because the flue openings and chimney dimensions are designed to maintain correct pressure under full load conditions. On small loads, with lower heating requirements, the furnace will have negative pressures.

One way around this is to control the opening of the damper in the flue by a pressure transmitter located on the side of the furnace near its hearth. The flue damper is controlled by a modulating control and butterfly valve which, in turn, is controlled by a pressure controller in the furnace control panel. Keeping positive pressure will prevent cold air infiltration as well as ensure that the furnace operates as efficiently as possible.

Infiltration: Air heated for no useful purpose at an approximate cost of $25,000/year!

To illustrate the extent of air infiltration: Take a furnace with a 4.9 m × 3.7 m (16 ft. × 12 ft.) door and a gap of a mere 3.8 mm (0.15 in.) wide, with a furnace draft of –8 Pa/m (–0.01” WC/ft) and negative furnace pressure of – 30 Pa (–0.12” WC). The rate of infiltration through the combined opening of 1250 cm2 (i.e., 1 m × 12.5 cm or 200 sq. in.) is 2400 m3 (85 000 cu. ft.) air per hour!

Controlled atmospheres
Controlled atmospheres, designed to offset the decarburizing effect of oxygen, are expensive to maintain. The generation of endothermic or exothermic protective atmospheres consumes a lot of natural gas or propane. A few examples are mentioned below. The alternative is to purchase bulk liquid nitrogen gas. In a small foundry that cannot afford its own generators, the expensive purchases of nitrogen gas must be carefully managed and its use reduced through conservation measures.

An alternative for newly planned treatment facilities may be the consideration of vacuum heat treatment processes.

Heat treatment examples
Pulse firing
A foundry in Quebec heat treats parts up to 1000°C and holds them at this temperature for several hours. In a major retrofit of the annealing furnace, it employed a pulse-firing system, ceramic fibres, furnace pressurization and microprocessors to regulate the burners. Each burner is fired at a predetermined frequency and order at only one selected rating. The result is a high turbulence inside the furnace (i.e., high convective heat transfer to the parts) and minimal temperature variation between zones (as low as 4°C). The gas consumption dropped to 31% (!) of the original levels, and the retrofit investment costs were paid back in 10 months. The annealing furnace works reliably and contributes to increasing efficiency, productivity and product quality.

Continuous quenching/tempering
A major metal-casting concern in Japan installed a continuous quenching/tempering process to treat metal parts in an oxygen-free, inert atmosphere. The process requires nitrogen gas generation. The exothermic reaction normally took place externally to the furnace and the heat was wasted. After the retrofit, the inner atmosphere generation was taking place inside a special radiation tube inside the furnace. The heat is captured and utilized in heating the metal parts. The inert atmospheric gas is generated by burning a gas/air mixture in a precisely controlled ratio. The flue gas is purified, reheated in a flue gas exchanger and reintroduced to the furnace, with 0% content of O2. Furthermore, the heat from the exhaust gas of the furnace is recuperated for product preheating. In addition, the heat from the quenching oil is recovered to warm the washing fluid used in the intermediate cleaning unit. The energy consumption was cut by 55%. The payback was less than one year.

Annealing with a fuel-based nitrogen generator
A foundry in Ohio implemented a fuel-based nitrogen generator (FBNG), with a twist. It produces the inert atmosphere required for both ferrous and non-ferrous annealing applications by halving the total energy required normally to produce and separate nitrogen by compression. In addition, it generates steam that can be used in other plant applications. The process received a Special Recognition Award from the U.S. Department of Energy.

In the FBNG process, natural gas and air are combusted. The combustion gases are cooled to remove most of the moisture. The gas is then compressed and passed through a catalytic converter, which removes any traces of oxygen. In addition, the NOX gases are reduced at that stage by 95%. A second converter removes residual water and CO to produce CO2, which is subsequently removed by a molecular sieve. The resulting atmosphere meets the required quality criteria. By helping to achieve excellent surface quality, it could eliminate a post-anneal acid cleaning step.

A broad range of atmosphere compositions can be produced to meet the needs of a particular process; hydrogen content range is 0–15%, while oxygen is only 5 ppm and combined CO and CO2 will not exceed 0.15%. Conventional systems contain usually upwards of 10 000 ppm (i.e., 1%) O2. The chief benefit of the FBNG technology is its low fixed operating costs. The payback period was 1.6 years.

Other EMOs
Housekeeping

  • Maintain the tightness of seals and insulation around the furnace door.
  • Maintain the calibration of monitoring and control instruments.
  • Maintain the proper adjustment of the burner controls.
  • Check the oxygen content in the furnace/oven and flue gases regularly.
  • Consult with your furnace/burner manufacturer on the best operating conditions.
  • Check the tightness of flue attachments to ovens to prevent the release of carbon monoxide into the foundry atmosphere (and hence the unnecessarily increased ventilation rates).
  • In controlled atmosphere operations, operate furnaces with minimal flow of the protective atmosphere. This is necessary from the cost point of view (less atmospheric gas needs to be generated) and because of heat losses.

Low cost

  • Consider an upgrade with high-efficiency burners.
  • Consider the use of regenerating burners in the heat-treating furnaces.
  • Consider repositioning of the burners to achieve higher turbulence inside the furnace for improved heat transfer and uniformity of heat treatment.
  • Add insulation to the outer surface of the furnace.
  • Consider improving the door seal by using ceramic fibre and stainless steel mesh in a channel arrangement, for a flexible seal.
  • Install automatic regulation of the furnace pressure through damper control.
  • Change the method of conveying product through the treating oven to facilitate rapid heat transfer to the product (e.g., exchanging wagons for open heat-resistant racks/platforms, etc.).
  • Preheat the combustion air, using any of the convenient waste heat sources in the foundry.

Retrofit; high cost

  • Consider retrofit or replacement of the heat-treating furnace if it is run:
    • On three shifts;
    • Kept at temperature over the weekend with no production going on;
    • With no excess air control; and/or
    • With firebrick lining and excessively high outside shell temperatures.
  • Consider replacing firebrick in heat-treating furnaces with pyrobloc modular ceramic insulation, which allows faster heat-up/cool-down and fuel efficiency.
  • If production requirements have been reduced, consider re-sizing/partitioning the furnace.
  • In a continuous heat-treating operation, consider using the hot gas from the exit vestibule and piping it to the entry vestibule to preheat the charge.
  • In batch furnaces, extract the heat from the exhaust atmosphere by suitable heat exchanger (e.g., heat wheel, ceramic regenerators) to preheat the incoming atmosphere gas.
  • On continuous treating furnaces using endothermic gas, consider using the exiting endothermic gas (with about one fifth of the natural gas heating value) in radiant-tube burners for preheating the parts.
  • Consider reclaiming heat from the steam generated by quenching operations.

 

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