How to Save Energy/CO2, Water and Money in HPDC Foundries

When building a new foundry, the aspects of technical building equipment should not be neglected, as great savings potentials can be hidden here. The potential to be exploited is also shown in the case study of an HPDC foundry.

Light bulb against a natural background. — With the use of a heat recovery system, approx. 3,000 t CO2 can be saved over 10 years.

Some time ago, Project Engineering GmbH was commissioned to assess the concept of the existing planning for the cooling water system in a newly planned die casting foundry with 4 x 2,800 t casting cells. What we found was a 4 x 2,800 t die casting machine with a total heat output of 1250 KW. The temperature difference of the cooling water supply/return was set at 5 K, which requires a mass flow of 214 m3/h. This mass flow was supposed to be generated by unregulated pumps, the operating pressure was set to 6 bar. Disposing the heat should be done with an open cooling tower, which circuit is connected by a heat exchanger and equipped with an emergency cooling system. Heat recovery measures were not planned and not taken into account. This kind of setup is quite usual in many foundries and was established in times when energy was very cheap.

1. Heat flow and temperature difference

Heat flow between two fluids is represented by the following formula:
Heat flow (Q) = mass flow (m) * specific heat capacity (cp) * temperature difference (∆t)
The specific heat capacity (cp) can be taken as constant in this system, since it does not vary significantly in the temperature range considered. Therefore, the indirectly proportional factors mass flow (m) and temperature difference (∆t) remain as control variables.

Gifts for free: On the one hand, the temperature difference from "cold to warm" is generated so to speak free of charge by the casting machine which has to be cooled. On the other hand, we have to deal with the mass flow which has to be generated mechanically by power consuming pumps and thus generate costs.

Therefore it pays off and it is necessary to reduce the mass flow. This can be achieved by increasing the temperature difference (∆t) by changing the setpoints. Furthermore, it makes neither economic nor ecological sense to let the disposed heat go up unused into a cooling tower, while in winter, for example, the foundry is additionally heated. This is why the connection of a heat recovery system is recommended.

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Illustration that shows the original setup. — This illustration shows the optimized setup. (Source: Project Engineering GmbH/Timo Ehnert)

2. Resistances in the piping system vs. flow pressure

In the case of pump generated mass flow, the pressure in a hydraulic system is quadratic in relation to the mass flow. All resistances in pipes, valves, heat exchangers, etc. have a quadratic ratio in relation to the flow pressure. Therefore, it is important to keep the system pressure low by using well dimensioned pipes, valves and heat exchangers.

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3. Efficiency of a pump

Taking a look on various performance diagrams regarding power consumption/pressure and mass flow, we will recognize that pumps have a very tight optimal operating range. Therefore, it is necessary to select suitable pumps, to operate them at the "point of best efficiency" and to adjust them accordingly via speed controllers if the cooling circuit changes, e.g. if individual casting machines are switched off. Taking all this into account, the concept proposed and implemented by Project Engineering looks like this:

The temperature difference of the cooling water supply/return was set at 10 K (instead of 5 K before), which requires a mass flow of only 107 m3/h (instead of 214 m3/h before). This mass flow is generated by controlled pumps, the operating pressure was set at 4 bar (instead of 6 bar before). The heat is still disposed by an open cooling tower, however its circuit is connected to a heat recovery system and to the machine cooling circuit via heat exchangers. The heat recovery is carried out via the hall ventilation/heating system and has been designed frost-proof. Emergency heating and emergency cooling was implemented in the heat recovery circuit.

So the following four aspects are most important to achieve energy-efficient cooling:

1. Use maximums spread of temperature possible in cooling cycle
2. Use lowest pressure possible in your circulation system
3. Always operate pumps at the point of highest efficiency
4. Use variable speed pumps

Result

In this example the possible savings over 10 years add up to EUR1,000,000 [+ EUR680,000 in addition when using heat recovery] and approx. 2,000 t CO2 savings due to less electric power consumption + approx. 3,000 t CO2 savings through heat recovery system.

The investment costs for a new foundry building are roughly comparable for both solutions, as individual cost differences, e.g. smaller pumps vs. pump controls, are roughly balanced out.

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1. Heat flow and temperature difference