Sotrem, an aluminium processing company, has reduced its annual operating costs by $34,000 through chimney heat recovery, a highly advantageous technological solution.
Sotrem operates in the national primary, secondary and tertiary aluminium processing industry. It offers ferrous and non-ferrous metal heat treatment services and recycling and recasting of industrial residues (aluminium).
In the case that concerns us, the tertiary process, the heat treatment consists of heating the pieces to obtain certain properties. The complete treatment process lasts about 17 hours at a temperature of approximately 540Â°C. Sotrem designs most of its furnaces to meet very specific characteristics that suit its needs.
For example, the 4,000,000 Btu/h furnace operates at a temperature of 540Â°C (1,000Â°F). Its available energy during combustion is about 65% for 25% excess air as indicated on the chart. Converted to an hourly consumption rate, we can say that, if the furnace operated at full capacity, we could possibly recover about 40 m3/h. If not, that energy will be lost as heat up the chimney.
Thus, out of the 37.89 MJ of available energy in 1 m3 of gas, 35%, or 13.3 MJ, will be discharged up the chimney. If the furnace consumes 250,000 m3/year, the energy equivalent of 87,500 m3 will go up the chimney each year.
In a context of constantly rising energy prices, any measure that improves a furnaceâs performance is welcome. The measures must be oriented to recover this energy and to serve other aspects of the process whenever possible.
In the case of Sotrem, the aluminum pieces are tempered in a 15,000 litre tank of water at 65Â°C at the furnace output to give them the desired physical properties. Originally, the hot water bath was heated by a boiler. Sotrem had the idea of recovering the energy discharged by the furnace chimney to heat this water. By means of an ingenious installation, which pumps water from the tempering tank to the furnace discharge outlet, it completely heats the water in the tank without the use of its boiler. The steel pipes through which the water circulates pass into the chimney and serve as a heat exchange between the water and the combustion products.
The following formula must be used to estimate the quantity of energy needed for the initial heating of the water in the tank:
E = c * V * d * (Ti â Tf)
E: Energy in Btu
c: Specific heat of water = 1 Btu/lbÂ°F
V: Volume of water in the tank to be
heated in ft3 = 530 ft3
d: Density of water = 62.32 lb/ft3
Ti: Initial temperature of the water
in the tank = 39Â°F
Tf: Final temperature of the water in the tank = 149Â°F
In the specific case of Sotrem, 3,633,300 Btu, or the equivalent of 288 m3 of natural gas, are saved during the initial heating of the water in the tank. It is then sufficient to make up the heat losses and the energy necessary to heat the fresh water intake evaporated during tempering.
Finally, implementing this measure allowed Sotrem to save $34,000 (70,000 m3/year).Â Thanks to such savings, Sotrem was able to turn a profit on the installation costs within six months. It could also count
on Gaz MÃ©troâs assistance program for introducing energy efficiency measures, which offers $0.25 per cubic metre saved.
Daniel Gendron, Eng.