During extrusion, the plastic is heated from the ambient temperature level to an elevated temperature level (usually >180 °C) so that the materials melts and becomes plastically deformable. After forming the product (film, tube, profile, rod, etc.), the product is cooled down to ambient temperature to achieve dimensional stability. Thus, thermal energy is “generated” first, which is “destroyed” again immediately after the forming process step. A use of waste heat is therefore obvious, but how, with what and for what purpose?
The extrusion process uses continuous (very cost-intensive) electrical energy to generate thermal energy in the extruder and in the electrical heaters (energy conversion). One part of this thermal energy is transferred to the environment as “waste heat”, but most of it is “stored” in the extrudate as thermal energy. In the cooling section, this thermal energy is extracted from the extrudate (the product is cooled). The energy is dissipated using classical principles of heat transfer and is transferred to a secondary medium (usually water, air, oil). This cooling medium is fed to a refrigerating machine or a free cooler for recooling, which dissipates this thermal energy to the environment in an electrically driven (cost-intensive) process.
This means that electrical energy is first used to “generate” thermal energy and then to “destroy” the thermal energy contained in the product.
The most important prerequisites for waste heat utilisation
In order for waste heat to be used sensibly, certain requirements must be met. The following list shows the requirements that have proven to be useful in plastics processing, even if other criteria certainly apply in other situations.
- The waste heat is bound in a concrete volume flow (e.g. cooling water circuit, oil circuit, air volume flow) and is not present as latent waste heat (e.g. warm surfaces).
- The waste heat has an interesting temperature level (from approx. 50 °C – the higher the better).
- There is a “heat sink” that fits the power requirement of the “heat source”.
Regarding point 1: Volume flow rate
It is often desired that latent waste heat is used as a heat source. Latent waste heat is present in extrusion plants in many different places, for example on the warm surfaces of the extruder cylinder, the extrusion tools, gears, drives or, for example, the cooling air of the extruder cooling fans. However, the utilisation of waste heat from the aforementioned sources is difficult and in most cases not economically feasible. To make waste heat utilisation realistic, the waste heat must first be converted into a specific volume flow that is suitable for feeding the heat to a subsequent process (e.g. oil circuit or water).
In case of a warm surface this would mean that a heat exchanger (e.g. a water-bearing system) has to be used. However, the use of a heat exchanger would inevitably lead to the surface being actively cooled (since heat is now released into the water flowing through it). This active cooling would now lower the surface temperature, so that (e.g. in the case of an extrusion tool) the heating zones installed there are switched on more frequently by the controllers. Logically, this would increase the electrical power consumption of the heating zones. Energy consumption would thus increase! This situation must of course be avoided.
Better possibilities for waste heat utilisation exist where the waste heat is already emitted in a specific volume flow. In extrusion, for example:
- Water cooling of cylinder zones or the grooved barrel (e.g. multi-screw extruder)
- Liquid cooling of drives, gears
- Liquid cooling of (compressed air) compressors
- Water-cooled extrusion tools
- Cooling water for product cooling (tool cooling, hydraulic cooling during injection moulding)
- Internal air cooling of tubes, hollow rods
- Waste heat from combined heat and power plants (engine cooling circuit, exhaust heat exchanger)
Regarding point 2: Temperature level of waste heat
The temperature level of the waste heat is another important criterion when checking whether a heat source is suitable for waste heat utilisation. In general, it can be stated that the higher the temperature level, the more likely the usability of the heat becomes. In general, there are various possibilities for waste heat usage depending on the temperature level for extrusion companies.
Temperatures below 20 °C, waste heat recovery is rarely advisable. In this case, care should be taken to ensure that the costs for cooling the waste heat are as low as possible. Cooling systems without compressors (cooling towers, free coolers) should be preferred over compression chillers, as their energy consumption is considerably lower.
If the temperature level exceeds 40 °C, sensible use in surface heating systems (floor-heating, wall-heating) is possible. It is important to bear in mind that the use of waste heat for heating purposes can only achieve seasonal energy savings, as we know that less heating is required in summer.
Warm air heaters and radiator heating systems as well as domestic hot water heating can also be considered above 55 °C.
In addition, from a temperature level of around 55 °C, the topic of waste heat utilisation for granulate preheating also becomes interesting, provided other conditions are met.
From a temperature level above 70 °C, heat-driven chillers (absorption and adsorption chillers) can also be operated, which must, however, be matched to the demand for commercial refrigeration. Electricity conversion processes only become interesting above 90 °C and here the rule applies, the higher the temperature level the more economical.
Regarding point 3: The suitable heat sink
The existence of a suitable heat sink is often one of the biggest problems in the implementation of waste heat recovery projects in plastics processing companies. In most cases, either the amount (thermal output) of the available waste heat source is so small that a sensible use is not economical, or the available thermal (waste heat) output is so high that no suitable heat sink can be found.
Possibility of estimating potential
To determine whether waste heat utilisation makes sense, it is advisable to first estimate the resulting savings potential. The most important value to be determined is the thermal output of the waste heat source.
The thermal output is calculated from the specific heat capacity, the temperature difference (delta T) and the mass flow (volume flow, density).
Example #1: Cooling water from an extrusion process to cool a pipe:
Flow temperature: 14°C | Return flow temperature: 16°C | Flow rate: 7,500l/h
Thermal power: ~17,4kW
Example #2: Thermal oil for gearbox cooling
Flow temperature: 75°C | Return flow temperature: 110°C | Flow rate: 100kg/h
Thermal power: ~0,5kW
Example #3: Air flow of a heating/cooling combination of an extruder
Flow temperature: 20°C | Return flow temperature: 125°C | Flow rate: 10m³/h | On-Off operation
Thermal power: ~0,29kW
Warm
is not always synonymous with “energy-rich”.
The examples above clearly show that not only the temperature level of a waste heat source may be taken into account as decisive for the energy content, but that the volume flow must also be taken into account. While the warm air from an extruder cooling fan or the warm oil from a gearbox cooling system is often perceived as an energy area, the thermal performance bound in the cooling water is often not realized at all. However, due to the high volume flows, the energy content there is many times higher, but due to the low temperature level it is difficult to use.
Once the thermal output of the waste heat is known, simple rough formulas can be used to estimate whether waste heat utilisation can be useful. The thermal output is converted into a possible annual saving in euros under the assumption that
- that the waste heat can be used with an efficiency of 75 %,
- an electrical energy source can be substituted (0,15 €/kWh electricity price) whereby of course
- the annual operating hours must be taken into account.
For a waste heat source with Q=50 kW thermal power this means:
K = 50 kW * 0,15 €/kWh * 5.000 h/a * 0,75
K = 25.125 €/a
This results in an annual savings potential of approx. 25,000€. Consequently, the measure may require a rough estimate of approximately 50,000 € in investment costs, provided that an amortization period of approximately 2 years can be assumed.
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