To provide a better idea of differences in the energy demands of different exhaust air treatments, we will use the example of an imaginary die casting foundry in Changchun. die casting machines create highly greasy and polluted smoke. Changchun has a median daytime temperature of -4.4 degrees during the winter period (October to the end of April), while the desired indoor temperature is 17 degrees, or around room temperature. The manufacturing hall of the foundry has a total volume of 78,000m3 and is equipped with 20 die casting machines in several sizes (from 640 tons up to 2500 tons). The management has now several options for air treatment. 

1st Method: Conventional Exhaust Air Ventilation System 

A conventional exhaust ventilation system works very simply: the waste air is led outside through complex exhaust ducts and chimneys at the roof of the building and fresh air from outdoors is led inside through the inlets. This method ensures clean air at the workplace but at high follow-up costs. 

Allowing fumes to get dispersed in the hall first creates a nonessential increase in exhaust air, which then needs to be purified. High volumes of air always mean high energy consumption. Furthermore, the same amount of air led outside has to be replaced by fresh air from outdoors, which has to be temperated first before being led inside. All this will cause high amounts of energy consumption and operation costs for the company. 

Practical examples show that generally the ambient air inside the hall has to be exchanged with outside air by a conventional ventilation system 10 to 12 times per hour to keep the indoor environment clean. In our example this means an air volume exchange of at least 781,200m3 per hour, plus high pressure loss and high heating costs. This results in an annual operation cost of RMB 19,056,770 and carbon dioxide emissions of about 3,500 tons per year.

Photo © KMA Umwelttechnik

2nd Method: Exhaust air filtration system in a recirculating air mode 

Today´s most energy-efficient principle of exhaust air treatment is the recirculating air mode in which the exhaust air will be filtered by electrostatic filter units. After that, the purified air will be led back into the production hall via an integrated ventilation system which works with a frequency converter. 

Whenever the filter is in operation, the energy-efficient ventilator starts working and creates air movement, so that the exhaust air and later purified air is led to the right direction through the filtration system up to the air outlet of the filter. 

In this system, each die casting machine is equipped with specially adapted extraction hoods. These extraction devices capture fumes efficiently and directly from the die casting machines, preventing fumes from being dispersed into the production hall and therewith, no high air volumes for the exhaust air treatment. 

In our example the exhaust air volume decreases to 215,400 m3 per hour. Low air volume always means less energy consumption. Each die casting machine is also equipped with one decentralized compact filter system. 

These are electrostatic precipitators, which ensure highly effective separations of smoke, dust, and fine mist whilst using little energy. In our model case the exhaust air filter system with a recirculating air mode leads to very low energy consumption. 

Hence, the foundry in Changchun now has operation costs of only RMB 1,098,791. That’s more than 90% less than the costs of using a conventional ventilation system. Additionally, carbon dioxide emissions are reduced to 416 tons per year. 

Although this principle is used in many modern and new-built foundries already, the demand for exhaust air mode systems still exists in some production facilities. For these cases there is an energy-efficient exhaust air alternative.

Photo © KMA Umwelttechnik GmbH

3rd Method: Centralized exhaust air filtration with heat recovery system 

On some winter days in Changchun the ambient median temperature goes down to -16°C, which also brings down the temperature inside the hall. Many foundries using a conventional exhaust ventilation system have to heat up the production halls, but due to the costs, this is quite an untenable solution. 

In most production processes high amounts of heat and energy are created which can be used for follow-up processes. The centralized exhaust air filtration system, equipped with an integrated heat recovery system, enables an energy-efficient heating or cooling of the hall by performing in an exhaust air mode. 

In this system, multiple machines are connected to one centralized large exhaust air filter system. The waste air from the machinery is extracted through exhaust ducts and led to the centralized filter system, which is equipped with electrostatic precipitators as well as integrated in-tube heat exchangers. 

After filtering the aerosols out of the exhaust air, the heat pump, which is connected to the filter system, extracts the energy out of the purified air. The energy is transferred in the condenser of the heat pump to a heating water circuit. Afterwards, the heating water is led to a second heat exchanger, which is installed in the incoming air duct. With this method the incoming air is heated up in an energy-efficient and economical way before flowing into the production hall. 

With a highly efficient heat pump, the foundry in Changchun increases the temperature inside the hall up to room temperature just by using the heat recovered out of the exhaust air. With an energy demand of 352 kW per hour, the heat pump creates almost 1,500 kW per hour for follow-up processes. 

In this scenario the foundry in Changchun would have annual operation costs of RMB 4,235,717, going along with an annual carbon dioxide emission of almost 1000 tons. These energy savings are less than the foundry would achieve with the recirculating air mode, but the savings are still much higher than those of a conventional exhaust ventilation system.

Take action on air pollution 

Energy consumption in foundries depends heavily on the energy input of the exhaust and supply air systems. High energy consumption always causes high amounts of CO2 emissions. 

By the use of energy-efficient exhaust air filtration systems the air is cleaned and significant reductions in carbon dioxide emissions can be made. 

Die casting Foundries and other metal processing entities can easily improve their companies‘ “Carbon Footprint” and make an important contribution to climate protection. Therefore, a change in thinking for each factory is worthwhile.