Energy Management in Plastics Processing - Part 6
A series of energy efficiency
worksheets by Dr. Robin Kent for the Carbon Trust to help the plastics
industry reduce costs through efficient use of energy.
UK Government Environment and Energy Helpline 0800 585 794
Extrusion is not only a final forming process for products but is also an intermediate process for other processing techniques such as injection moulding, blow moulding and film blowing. The efficient operation of extrusion screws is therefore essential to much of the plastics processing industry. The process is highly dependent on electricity and most of the energy used is directly related to machine operation. For profile extrusion the energy used to drive the extruder itself is 50% of the total and the remaining energy is used for items such as ancillaries and utilities.
Industry surveys show that a typical company should be able to reduce energy usage by 10% without major capital outlay.
Variation in energy use in extrusion
The initial cost of energy efficient extruders may be higher but they will give rapid returns on the extra investment. Options such as high efficiency AC motors and Variable Speed Drives have good payback for both new purchases and when replacing motors and drives.
Whatever the age of the machine, it is essential to get the right extruder for the job and the screw diameter and design should be checked to make sure they are right for the polymer and product.
- Tip - Using large extruders for small profiles is wasteful.
- Tip - Total efficiency (including energy efficiency) is best operating at the design conditions.
- Tip - Set the extruder to run at its most efficient speed (usually maximum design speed) and control the screw speed to give an extrusion rate as close to the maximum as possible and still produce good product.
Motors run most efficiently close to their design output - a large motor at part load is less efficient than a small one at full load.
- Tip - Size and control the electric motor to match the torque needed by the screw.
Optimising the extruder speed maximises the heat from mechanical work and minimises the amount of electrical energy needed. Provided the downstream equipment does not limit the output, the energy consumption can decrease by nearly 50% by doubling the rotational speed of the extruder.
Accurate temperature control is needed for good extrusion - excess temperatures are wasted energy. The polymer needs to be kept close to the optimum processing temperature.
- Tip - Barrel insulation has a payback of under 1 year and also reduces Health and Safety issues and air current fluctuations.
- Tip - Check the controls to make sure that the heating and cooling are working efficiently together.
Standby’ operation can use significant amounts of energy in utilities through barrel heaters, cooling water, calibration vacuum and lights.
- Tip - Find the minimum ‘standby’ settings and set an operator routine to always leave machines in this condition.
- Tip - Can you turn off barrel heaters and cooling fans between runs?
- Tip - Can you turn off cooling water on idle machines?
Energy use can be used as a diagnostic tool to identify deterioration of the machine condition and the need for maintenance.
- Tip - Increasing the frequency of maintenance involves effort and cost but can lead to significant energy savings.
The main opportunities for energy saving in ancillaries are in minimising the demand for utilities, such as vacuum and compressed air. The electric motor drives are generally small so replacement with efficient motors is only likely to be cost effective when motors fail. Specifying energy efficient features at the design stage will give rapid paybacks on any additional costs.
The first step is to get the extruder right - if the extruder is at the optimum conditions the need for downstream cooling and calibration will be minimised. For utilities the approach should be to ‘minimise the demand and then optimise the supply’.
- Tip - Find the maximum acceptable extrudate temperature after cooling and set the maximum cooling water temperature to achieve this.
- Tip - Check that cooling water is not circulating through idle calibrators.
- Tip - Check that cooling water is treated, chilled and distributed efficiently.
- Tip - Check that compressed air is not supplied to idle machines.
- Tip - Check that compressed air is generated and distributed efficiently at the minimum pressure needed by the process.
- Tip - Check that the vacuum supply is the minimum needed and that it is generated and distributed efficiently.
- Tip - Check that the vacuum supply is switched off when it is not needed.
- Tip - If replacing electrical motors then match the size to the actual demand and fit energy efficient motors.
‘Tweaking’ of machines by setters and operators causes more lost time and energy than any other cause.
- Tip - Get the machines set right, record the settings and do not change them unless absolutely necessary.
Increased maintenance involves additional effort and costs but can lead to significant energy savings.
- Tip - Set up a total productive maintenance (TPM) programme to keep all machines and systems in top condition.
Energy efficiency will save you money – start an energy management programme today and reap the benefits of improved profits by cost effective investment and management.
The "Energy Management" series is designed to give plastics processors an insight into how to manage a valuable resource.
Download the complete series as an Adobe Acrobat file.
Last edited: 11/03/10
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