Keeping What's Hot Hot
Finding ways to reduce energy usage in an energy-intense universe like heat treating is a challenge. Sometimes the answer lies in rethinking your setup.
Industrial heat treating may at first seem to be an unusual place to explore for energy savings. After all, heat treating is an area where energy must be expended and where workpieces must undergo processes that by definition use a great deal of energy. But in reality, the basic process of industrial heat treating makes it an obvious candidate for examination of ways to improve energy usage. The high use of energy resources to process a variety of parts through various stages of hardening, tempering and quenching raises a key question: How can the process provide an equal or superior part and still reduce the energy?
Curbing Your Appetite for Energy
The fact is that heat treatment must curb its appetite for energy. The more that processes are refined and automation is put into place, the more fixed costs are reduced, which causes energy costs to be moved into the spotlight. Both economics and the regulatory climate demand increased scrutiny of energy usage — economics because energy sources will only become more expensive over time and regulatory because environmental pressures will inevitably increase.
Global standards are part of this picture. Many larger European manufacturers (and indeed many U.S.-based as well) are embracing the ISO 50001:2011 bandwagon. Only a little more than three years old, 50001:2011, “Energy Management Systems – Requirements with Guidance for Use,” is not a governmental mandate … at least not yet. Rather, it is a system-wide framework, a set of guidelines. Following its directives, an industrial concern can put into place the tools and procedures necessary for a systematic approach for continuous improvement in the reduction of energy. Since careful analysis is a key element for improvement, ISO has recently developed a follow-on 50002 standard that articulates the principles and requirements for carrying out energy audits.
One Answer: More-Efficient Equipment
Where can one begin any high-level auditing of energy usage in heat treating? After all, if we cannot change the nature of physics, and thermodynamics can be pushed only so far, where can heat treaters begin to reduce energy usage? As designers and integrators of custom-built production equipment, our focus was to examine original approaches to innovative production equipment.
After a study of certain heat-treating processes, we observed that a major area of savings can lie in restricting the heating and cooling to onlythe part. In simplest terms, when green parts could be loaded onto hot trays directly inside the furnace for transport through the furnace, and then the hot parts can be unloaded from the hot trays from inside the furnace (without the trays leaving the furnace), considerable amounts of energy could be saved. The loading and unloading of the parts can be done by commercial programmable robots operating through powered, sealing doors. This concept makes sense, especially when the parts being processed are considerably lighter than the trays carrying them. Based on these observations, we developed a unique patented furnace configuration called “PIES” (Pusher Index Energy Savings).
Why is process equipment a rich mine for savings? Simply, because it has not received the attention it deserves. For example, typical of most discussions around energy savings in heat treating, the U.S. Department of Energy’s “Improving Process Heating System Performance: A Sourcebook for Industry” (Second Ed., 2007) focuses primarily on furnace design, fuels, combustion efficiency and controls, in addition to analytic and theoretical guidelines. Only slight mention is given to trays, fixtures and other moving parts without suggesting solutions that might represent significant energy losses.
Sterling Engineering typically focuses on the movement of “parts” in automation systems. When our clients expressed an interest in ways and means to save energy, we looked at these basics. Others have created great advances in furnace design, lightweight insulation, fuel efficiency and waste-heat recovery. What is left, we felt, are the basics of the process, which we address in PIES.
The PIES system as shown (Fig. 1) uses rectangular trays and has two rows (row 1 and row 2). Loading of green parts and unloading of hot finished parts are both accomplished at the same end of the furnace. Other tray configurations (e.g., square) are possible as well as having more rows.
A typical cycle is as follows:
• At the start, trays in row 2 have just cycled forward one tray position.
• A tray of hot (finished) parts is now at the unload-door tray position.
• Unload door opens, and automatic unloader handler grabs hot parts.
• Automatic unload handler exits furnace with hot parts and door closes.
• Hot empty tray (just unloaded) is cross pushed from Row 2 into Row 1 and tray is now at the load-door tray position.
• Load door opens, automatic load handler enters furnace and places green parts on the hot tray.
• Automatic load handler exits furnace, load door closes.
• Row 1 cycles one tray position forward, pushing tray to tray.
• Last tray of parts in row 1 is cross pushed from row 1 to row 2.
• Row 2 cycles one tray position forward, tray on tray.
• Cycle repeats.
In certain processes, such as hardening (austenitizing) – where atmospheres can be the same in all furnace zones, and especially where the work load on each tray is a fraction the tray weight – a PIES concept will save considerable energy.
Obvious Energy Savings
The energy savings with the PIES system are obvious compared to many conventional systems in which green parts are loaded onto cold trays. These trays with hot parts are both quenched. In the PIES approach, the trays remain hot inside the furnace.
Additionally, the transfer of a hot part to quenching and/or post-heat processes can be extremely rapid with the PIES approach. Robots can reach into the furnace, pick up the part(s), and in one smooth move immediately take it to an oil/water quench or downstream work centers (Fig. 2).
Rotaries: Good but with Limitations
Rotary furnaces can offer the same energy savings when the trays remain in the furnace. (Some of the savings are reduced in the designs that require the quick removal of the tray from the furnace twice, once for part loading and again for hot part unloading.) In general, rotary designs are excellent for a range of parts whose size and shape lend themselves to placement and processing within a segment of a circle.
Unfortunately, rotary designs can create dimensional constraints since the working surface of each tray in the furnace must fit within the circular segment. A PIES furnace typically would require less overall floor space for similar production requirements.
PIES: Any Part, Flexible Orientation
A PIES tray can handle full-size parts, and the parts can be oriented or positioned more freely than in rotary arrangements. PIES work trays are symmetrical end-to-end and in certain applications can be flipped to present a new surface, extending tray life. Tray life is extended when trays are not quenched. There are certain other advantages as well.
For a medium to large part, the envelope of the PIES system could be smaller than a rotary design for the same part. Since footprint and furnace casing/insulation are nearly always an element of concern, the smaller size has obvious advantages. In certain cases, it is relatively easy to expand a PIES furnace capacity simply by adding rectangular length to accommodate longer rows of trays.
Essentially, 100% of the hearth area can be utilized with the PIES design. Rotary furnaces with separate load and unload doors can typically have up to 30 degrees of empty tray position between the doors.
Competitive Advantage when Your Eye is on Energy
Whatever the source of energy savings, a company whose heat-treatment work-center managers give careful consideration to reducing energy usage will reap ongoing competitive advantages with every improvement. Every green part that requires less fuel to process than a competitor’s will create greater profit potential.