Energy Management in Rubber Processing - Part 3 - Compounding
The third in a series of energy efficiency worksheets by Dr. Robin Kent for
the Carbon Trust to help the
rubber industry reduce costs through efficient
use of energy.
UK Government Environment and Energy Helpline 0800 585 794
Energy is a large cost to compounders but there is a range of simple activities that can reduce energy use and costs dramatically.
Good practice is inexpensive and reduces all costs - not just energy costs
Power-versus-time profile for conventional single stage mixing cycle
Energy saving for pre-heat systems focuses mainly on warm rooms. These are likely to need more careful management than the more compact and probably well-insulated autoclaves.
Savings of up to 50% can often be achieved by good insulation of walls and ceilings and draught-proofing of the door.
Spacing out the bales and arranging successive layers at right angles can increase the surface area by more than 100%. Spacing out reduces the capacity but increases the heating rate and this decreases the need for large volumes. Savings can then be made by reducing the warm room size (using insulating panels) and reducing the standing loss.
Tip: The important thing is the throughput of the warm room, not the volume capacity.
Warm air stratifies in any room and temperature differences of up to 15oC between the floor and the ceiling can result. Using fans to move the air will substantially reduce heat losses.
Tip: Thermostats should be properly maintained and able to achieve control with a minimum dead band.
Tip: To prevent heat loss, preheated bales should be repacked into a close-fitting mass when removed from preheating.
Transferring waste heat from other site operations (e.g. flash steam, heat from air compressor cooling operations, heat from mixer/mill cooling) to the warm room will reduce heating costs.
Tip: This will involve piping costs and will only be economical for factories operating on a three-shift basis.
Tip: Microwave preheating eliminates standing losses but has a high capital cost and will only be economical if the site is already using it for other processes and has spare capacity.
Tip: Use waste heat to preheat extender oil and reduce the viscosity before pumping. If extender oil is a large proportion of batch volume, it may be better to cool the oil so that it acts as part of the process cooling system. This can be useful where high batch temperatures result in multi-stage operations and the extra cooling can enable a switch to single-stage operation.
Controlling additions is the key to good compounding and accurate weighing systems will quickly pay for themselves through improvements in product quality and consistency and in energy savings through a reduced need to rework off-specification material.
Using detailed energy (kWh) and power (kW) recordings of production allows much closer control of batch conditions and also the optimisation of additions timing. Incorporating this information into batch controls also contributes to product quality and consistency. The savings from optimised energy usage can give payback periods of less than one year.
Tip: The same information also allows batch comparison and can be used for preventive maintenance systems by monitoring changes in machine performance in terms of kWh/kg batch (on-load).
Tip: Reliable measuring systems are essential to allow project energy savings to be verified to justify further investment. The capital costs of better monitoring systems can often be justified on the grounds of energy saving alone. In many instances, switching off equipment when it is not in use for significant periods (as during shift changes and meal breaks) can be highlighted as an easy method of saving energy.
Tip: Better measurement on mills, related to throughput and separated into on and off-load kWh, will reveal other opportunities for savings and short payback investment.
Energy for motors in mixing, pumps, dust extraction etc is a major cost and using variable speed drives and motor controllers/soft starts can significantly reduce energy use in internal mixing and milling. Worksheet 6 deals exclusively with motors and drives but some areas for improved control are:
Water cooling pumps and cooling tower fans both use significant amounts of energy. Using good and flexible control systems to vary flow rates and keep cooling water supply temperatures constant not only reduces energy use but also improves product consistency and quality.
Festoon coolers need high volumes of air crossing the rubber sheet. Designs using counter current flow and minimum airflows are best but the main savings come from ensuring that fans are switched off when not in use.
Tip: Use a photocell at the inlet to the festoon cooler to sense when product is no longer entering the box. Use this to activate a timer to shutdown the fans as the sheet passes through the cooler.
Tip: For dust extraction systems, minimise the volume of air extracted at the point of use by employing high velocity/low volume collection systems.
Tip: Cooling water distribution should be as clean as possible to prevent the formation and deposition of rust and scale on cooling surfaces.
Tip: Insulate extender oil systems and use either steam or fossil fuel-based heating systems rather than electricity.
The "Energy Management" series is designed to give plastics processors an insight into how to manage a valuable resource.
Part 1 - Reducing energy costs - the first steps
Part 2 - The rewards
Part 3 - Compounding (This Section)
Part 4 - Moulding
Part 5 - Extrusion
Part 6 - Motors and drives
Part 7 - Compressed air
Part 8 - Buildings