Alloy Engineering Blog

Annealing Furnace Efficiency: Improving Gas Flow

Improving production and energy efficiencies in the steel and wire industry is of vital concern for both those in the industry, and the world in general. The longevity and performance of the equipment play a key role in realizing efficiencies. The design of energy efficient annealing equipment is one component in those improvements.

Consumer demand for automobiles and appliances right after WWII was at its height. The corresponding demand for metals in manufacturing called for production facilities and equipment that was durable and could withstand the hectic pace of production. By the 1970s, while the demand kept pace, the need for fuel conservation and eco-centric manufacturing called for more efficiency in production. Durability was second to efficiencies. Hydrogen replaced DE-OX gas in the annealing process, raising the cost of manufacturing but providing efficiencies at the same time.

Ideally, mills need solutions that provide flexibility, long- and short-term, and satisfy the need for higher quality product and shorter annealing cycles. Improved gas flow within annealing furnaces will help achieve this.

One solution is Alloy Engineering’s High-Flow™ System, which provides the benefits of continuous annealing without a major investment or the loss of operating flexibility common with continuous annealing. At the heart of the system is the charge support and diffuser base, coupled with a unique fan designed to ensure the flow angle of the exiting inert gas works together with the diffuser as a unit to minimize pressure losses for improved flow.

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Rebuilding for a more efficient furnace is, indeed, a bottom-up proposition. As refractory bottoms run their natural life cycle and need replacement, there are two options – cut off the old refractory and rebuild it, or replace it with a stool plate. A rebuilt refractory can take as much as 4 weeks to build and cure before the furnace can be back in operation. A stool plate concept eliminates at least half of that downtime.

A fan sits within the diffuser base, providing improved gas flow efficiency by minimizing pressure loss while providing maximum durability. The diffuser base supports the load and distributes the gas off the fan, maintaining the flow inside the furnace. Keep in mind that if you coordinate the design of the diffuser base with that of the fan wheel, you can achieve the most efficiency as they work together for optimum flow. The more rapidly the gas moves through the charge, the shorter the annealing cycle and the more uniform the product.

The integrity of the diffuser base is paramount. The longer it lasts and retains its shape, the more efficient the heating process and the more uniform the annealing. Diffuser bases built in one piece can lose their shape due to the thermal gradient between OD and ID. They become egg-shaped and highly inefficient. Heat for furnaces comes from outside and near the bottom; therefore the outside of the diffuser base is hotter than it is inside, where the fan is located. A two-part diffuser base (tip cage and outer section) minimizes the amount of thermal stress due to temperature differential.

An open charge plate allows the bottom coil (usually the slowest to heat) to be brought up to temperature more quickly and, working with the convector plates, provides a more uniform gas flow and heating to all coils in the load. The charge plate distributes the load over the diffuser base to help support the underside of the coil. By adjusting the ID of the charge plate or adding an orifice plate, the amount of flow can be controlled.

The convector plates sit between each coil and an open vane design facilitates heat transfer to the charge. They allow the gas to flow through the stack of coils to improve uniformity of heating and cooling. An angular vane design offers an open area where the heat can reach the inner portions of the coils and also provides directional flow to the gas steam.

Again, the continuity of the gas flow and annealing process promotes a more uniform product, and the very top of the uppermost coil can heat too quickly if the amount of gas going into the convector plates isn’t controlled. A radiation shield, therefore, is placed on the top coil, and the vanes on the lower surface of the radiation shield protect the hottest potion of the upper coil from overheating and facilitates a more even heat distribution throughout the system.

The goal is enhancing the uniformity of product by maintaining consistent, steady gas flow, and continual and uniform annealing. Whenever overall equipment cost can be reduced, as well – it’s a total win/win.