University of Minnesota
Minnesota Technical Assistance Program
http://www.mntap.umn.edu
612.624.1300

Energy Efficiency in Machining and Metal Fabrication Facilities

Energy use is relatively high in a variety of machining and metal fabrication facilities. A variety of processes require energy and can potentially be altered to reduce the amount of energy used. On this page, we offer suggestions for improving your facility’s energy efficiency.

Improvements

The following improvements are energy efficiency improvements you may be able to make in your facility.

  • Exchange heated process exhaust to heat building air.
  • Design make-up air units around processing areas with exhaust to withdraw recently introduced outside air rather than air that has already been heated.
  • Destratify air with fans and use smaller furnaces or spot-heating units where necessary on the shop floor to conserve the load on the facility furnace unit(s).
  • Utilize spot heating near operator stations or critical processes in larger facilities with large thermal loads.
  • Redirect oven heating.

Other Opportunities

A range of opportunities in a variety of process areas exist for energy efficiency improvements. For example, thermal savings can be found by making sure your burners are operating as efficiently as possible and your burner controls are working properly. Electrical savings can be found in a variety of projects that use powered equipment effectively, such as compressed air auditing; welding control; motor, fan, and pump optimization; and lighting improvements.

Burners

Efficient burners should be capable of maintaining stable flame throughout the firing range at low excess air. Many burners claim significant turndown capability, but if they cannot operate at low excess air, savings from turndown are smaller than suggested. Significant operating time above 7% O2 suggests a large conservation opportunity, but even below this level, a 1% reduction in O2 yields a 1% improvement in operating efficiency*. Larger efficiency improvements occur at higher excess air levels. The use of an economizer can decrease the impact of excess air reduction by a small amount.

Burner controls

In terms of improving burner controls, O2 trim typically has the biggest impact. O2 control is required by code to operate boilers at less than 3% O2 and can achieve levels below 1%. Trim control does require a burner that can operate stably within the desired control range.

Compressed air

Compressed air is an expensive and energy-intensive utility commonly used for air tools and control valves, as well as occasionally for drives, media blasting, cooling, and blow-offs. Common improvements include repairing leaks in piping and equipment, reducing the compressor output pressure, using a cold-air intake, setting up remote air receivers, correcting large system pressure drops, controlling the loading pattern of the compressed air system, and reducing inappropriate uses such as process cooling and cleaning. You may opt for more costly solutions which can include properly sizing and distribution of the compressed air system, improving sequencing controls, and installing variable speed compressors to handle variable loads. More information about compressed air opportunities is available in MnTAP’s Air Compressor Energy-Saving Tips fact sheet.

Facility HVAC improvements

Lowering the temperatures of unoccupied spaces during winter months can reduce your thermal load. Additionally, you can consider using radiant heating to keep people and materials warm while minimizing air heating. Other opportunities include lowering ventilation rates as needs decrease and using push-pull ventilation and unheated air for local process exhaust to reduce the demand for heated building air.

Heating system best practices

Heating system best practices include testing for leaks in steam traps and lines, repairing leaks, improving the thickness of insulation, repairing insulation as needed, improving condensate return, improving boiler feed water quality to reduce blow-down, and cleaning heat exchange surfaces to improve heat transfer.

Lighting improvements

Installing T-5 or T-8 fluorescence, reflectors, CFLs, occupancy sensors, and LED configurations in your facility can be a low-cost, rapid-payback opportunity.

Process controls  

Automating or refining controls can avoid over-processing and can shut unnecessary equipment off or move it to low energy stand-by mode. By improving these controls, MnTAP estimates that some facilities could save up to 9% of total energy use.

Process heat recovery

Process heat recovery is possible wherever waste heat is released, although the best recovery opportunities will be in cases where the waste heat temperature and total amount of heat is high. One difficulty with this technology is determining how to use the recovered heat; this will likely vary in each facility. Possible uses of waste heat include process or domestic water heating, product or raw material pre-heating, process or ventilation air heating, or possibly in boiler systems. The simplest heat recovery systems can use waste heat to preheat other operations directly, but it may also be feasible or necessary to upgrade waste heat with heat pumps or to store heat to allow for differences in supply and demand timing.

Process motors, pumps, and fans

Developing a motor replacement plan can be useful in helping you prepare for eventual motor failure by predetermining what type of motor you will use to replace a failed motor (NEMA premium, rewound, spare). In a failed motor situation, you will want to understand what other motors in the facility can be moved from less critical applications to cover for the failed equipment. Doing so can provide more time to install the best replacement rather than the fastest replacement. Properly sizing pumps, fans, and electric motors is important, as is matching supply and demand in variable flow applications. You should avoid throttling flow with valves or dampers as this can be costly.

*Steam System Survey Guide, Greg Harrel, Oak Ridge National Laboratory, 2001, ORNL/TM-2001/263

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