Getting Ideas for Energy Management Opportunities
A casual look will show many sources of waste heat: in melting furnace exhaust, ladle preheating, core baking, pouring, shot-blasting, castings cooling, heat-treating, quenching, ventilation exhaust, etc. To use waste heat profitably at a desired temperature level, its temperature must be raised by suitable equipment.
Many opportunities for heat recovery in the foundry industry are not yet utilized. It is a complex subject, best handled with the help of a competent consultant. The current volatility of our economic situation, particularly with respect to rising energy prices, should make this an interesting subject to explore. The steps to take are:
- Perform a heat balance: compare (quantify) the heat demand and potential waste heat supply;
- Assess how easy it is to access the waste heat, and the distance involved in bringing it to the point of use;
- Assess the type, heat content and condition of waste heat sources;
- Determine the degree of heat upgrade required;
- Short-list equipment/process options; and
- Perform economic evaluations to select the optimum solution.
The principal parts in a heat recovery system are:
- Waste heat source (e.g., exhaust hoods, furnace ducts, hot water from furnace cooling);
- Heat exchanger (in case of high exhaust temperatures, a waste heat boiler could be employed; note: a feedwater system would be required);
- Heat distribution system; and
- Heat recipient (e.g., in case of a waste heat boiler employment, a steam turbine with generator (or air compressor) and condenser, ventilation air, combustion air, raw material drying oven preheater, etc.).
A wide range of equipment for heat reclamation is available. The waste heat sources with the highest temperatures are the off-gases from melting and heat-treating furnaces. Waste heat from treating furnaces is the cleanest, steady temperature source with no particulates or corrosives. Other high-temperature waste heat sources include pouring and core baking, but the intermittent nature of waste heat generation or the relatively small streams make economic utilization difficult.
Additional sources of waste heat in a foundry include:
- Furnace dust collectors;
- Reverberatory furnaces;
- Cooling water;
- Sand-cooling drums;
- Burning off mould patterns (see “Lost foam casting,” page 65);
- Induction furnace preheaters;
- Copper-melting furnace;
- Process water; and
- Cupola start-ups and shutdowns.
The possibilities for waste heat utilization include the following:
Preheating combustion air
We have already demonstrated the profitability of preheating combustion air. Even after preheating the combustion air, there may be enough residual thermal energy to be used economically for another application, e.g., to provide building heat or low-temperature process heat. The extraction of heat from the furnace flue gases is usually done in a recuperator. The many types available on the market use a variation of the three basic types: counter flow, parallel flow or cross flow. About 65% of the waste heat can be recouped.
Preheating of process gases
This may be applicable to heat treatment furnaces using protective atmospheres, as well as to the furnace itself, for preheating the fuel gas, with corresponding (and quite substantial) natural gas savings. Gas-to-gas heat exchangers of many configurations are used for this purpose.
Preheating raw materials and parts
For raw materials entering the process with possible water and oil contamination, preheating helps in driving the moisture and oils away and increases the temperature of the material for melting, thus saving energy required for melting.
Similarly, the thermal load required for heat treatment of parts is lessened by their preheating.
Any hot fluid in a foundry can be used as a heat source through the utilization of liquid-to-liquid or liquid-to-air heat exchangers.
Consider investigating the use of water jacket heat exchangers from sources such as electric arc furnaces or cupolas. The reclaimed heat could provide domestic
hot water and water coil-based space heating. Additionally,
in cupolas, water-based heat reclamation would reduce the need for evaporative cooling and scrubbing of the gases, leading to other substantial energy savings.
The literature is rich with successful case histories of imaginative and innovative waste heat utilization, ranging from domestic to aquaculture applications.
Some of the uses include:
- Space heating;
- Supplementing drying of refractory wash coatings;
- Ventilation make-up heating;
- Water heating;
- Process cooling; and
- Absorption cooling for air conditioning.
Other EMOs
Housekeeping
- Determine and eliminate as many waste heat sources as possible.
- Reduce the waste heat temperature.
- Perform regular and preventive maintenance of the foundry systems.
- Maintain the calibration program for monitoring and measuring instruments.
Low cost
- Consider returning the recovered heat to the operation from which it came, as a priority, since such systems usually require less control and are less expensive to install.
- Improve operational controls.
- Re-use hot exhaust air for preheating/drying of raw materials.
- If air compressors are water cooled, look for ways to recover heat from the cooling water and/or for recycling the water for use elsewhere (Note: many companies reported successful and profitable heat reclamation from, e.g., screw compressors).
- Use the reclaimed heat from air compressors for preheating of ventilation make-up air.
- Use the captured heat from melting furnaces to preheat incoming ventilation air (or combustion air, if natural-gas fired).
- Use the heat from the heat exchanger for the furnace cooling to heat the foundry buildings (locate the exchanger inside).
- Use the waste heat from filtered, clean air leaving dust collectors as heated make-up air. Make sure you have adequate controls in place to ensure the integrity of the filters/baghouse and that there are no toxic components in the air.
- If you have a high-quality waste heat source (i.e., with a high temperature differential), yet the potential place for its utilization is at some distance, consider transferring the heat to a liquid, which could be pumped to the destination easily.
- Consider storing waste heat (via liquid or eutectic salts) if there is a delay in its application.
- Consider using waste heat to drive air-conditioning units.
Retrofit; high cost
- Consider investigating the newly developed hybrid (absorption/compression) heat pump (by the Institute for Energy Technology of Norway) for upgrading waste heat from 50°C to a temperature of 100°–115°C. The heat pump can both produce heat well in excess of 100°C, as well as supply cooling, typically for air-conditioning applications. This is an important consideration for foundry ventilation systems.
- To recover energy from a high-temperature flue gas containing dust and particles (e.g., from a cupola furnace), consider using a waste gas tube boiler or water tube boiler for waste heat recovery, newly developed by the Norwegian Association of Energy Users and Suppliers (KNE).
- In galvanizing foundries, consider reclaiming waste heat from zinc bath flue gas by preheating the flux mordant and degreaser baths as well as the dryer.
- In galvanizing foundries, investigate fitting the pre-treatment reducing-atmosphere furnace (with its preheating, glowing and cooling sections) with gas burners with integrated heat recovery units. The radiation elements are equipped with heat exchangers in which the combustion air is preheated by the element’s own exhaust gases.
- Consider replacing cooling towers with a heat transformer that upgrades the temperature of waste heat to the required level, to produce saturated steam. In the next stage, an existing boiler may superheat the steam to prevent condensation in the distribution system, and augment steam supply (e.g., equipment by Finish manufacturer Rinheat-Ahlstrom).
- Consider converting high-temperature flue gas heat from melting and holding furnaces into superheated steam for steam turbine power generation.
Additional information: Apart from a number of current monographs, a comprehensive technical manual, Waste Heat Recovery (M91-6/20E) is still available from NRCan at (613) 947-6814. It provides a good overview of the subject. Other monographs in the Energy Management series are listed in Appendix 5.7.