Energy recovery at Sotrem

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 540°C. Sotrem designs most of its furnaces to meet very specific characteristics that suit its needs.

Although rules of thumb are frequently used to dimension the furnaces and their burners, judgment is required in their use. Every rule is based on certain assumptions, such as the production rate and the furnace’s dimensions and insulation. If the system considered differs from the conditions initially posed, applying a rule of thumb may result in a significant error. For special operating conditions, or when more precise results are necessary, a thermal balance is necessary. A thermal balance consists of calculating the thermal loads required and adding the losses to determine the energy necessary to operate the furnace.

A furnace’s thermal losses can be summarized in four main categories: losses up the chimney, which are the most significant, losses through the walls, losses by radiation, and heat accumulated in the furnace structure. To increase a furnace’s performance, the research must first be oriented to the greatest losses. In order to reduce losses up the chimney, we need to try to recover the heat. A furnace’s efficiency thus largely depends on losses up the chimney. We know that the energy available for the process during gas combustion is inversely proportional to the temperature at which the furnace will have to operate for a given quantity of excess air.

20-2-3-graph1-en — Sources: Copyright 1983, GTE Products Corp. Towanda, PA 18848 USA

For example, the 4,000,000 Btu/h furnace operates at a temperature of 540°C (1,000°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 furnace’s 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 65°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 * (T_i – T_f)

E: Energy in Btu
c: Specific heat of water = 1 Btu/lb°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 = 39°F
Tf: Final temperature of the water in the tank = 149°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). Thanks to such savings, Sotrem was able to turn a profit on the installation costs within six months. It could also count
on Gaz Métro’s assistance program for introducing energy efficiency measures, which offers $0.25 per cubic metre saved.

Daniel Gendron, Eng.
Technical Adviser
DATECH Group