posted March 12, 2008 09:54 AM            

The majority of the power is used through the motor, and delivered into the polymer through viscous dissipation. Much of that energy is wasted because of inefficient distribution of that heat into the polymer matrix during melting. Some energy is wasted moving the polymer around in long pumping sections. There have been many attempts to improve that situation over the years by using special screw designs. I would have to say that those screw designs work well for a narrower range of polymers than other designs, so they are not always a first choice for operations that want to use multiple resins.

Various twin screw types can make use of much more efficient melting modes than single screw. Twin screws issues are typically high initial capital investment and problems with pumping stability which often require the addition of discharge devices which then use up the other efficiency gains. (RPVC on conical twins is a special case, not addressed here. However the volumes of PVC processed does deserve a look at, but because of processing issues equipment choices are often limited.)

I don't agree that high speed single screw extruders offer an energy savings outside of some improvements in drive efficiency. That is because the melting mechanism is the same as in conventional extrusion. High speed extrusion often will result in a percentage of unmelted material delivered to the die. This saves energy, because the resin was not melted, but any extruder can perform the same trick if the process can tolerate unmelted resin in the output. High speed extrusion has the potential to be very wasteful of energy because of undesired increases in melt temperature. This is more likely to happen as the technology is applied to more viscous polymers.

Grooved extruders do have an advantage in efficiency because of a different melting mechanism employed and can provide high outputs. I would be curious to see someone tries high speed and grooved at the same time. I would watch from a distance.

Vacuum pumps don't really use that much energy after the vacuum is drawn. Typically there are no parts in contact, and the power goes into moving gas around. If the amount of gas is low, as in a vacuum, low amounts of power are used. There are potential efficiency gains with centralizes systems because of reduced mechanical losses. However my experience with centralized vacuum systems is that condensates in piping and leaks cause of lot of problems which may reduce energy efficiency.

One simple item that leads to wasted energy is screw and barrel wear. Melting and pumping are much less efficient n worn equipemtn, causing the polymer to be hotter at the same output rate. It might be interesting to do an energy audit and economic study on the value of screw and barrel rebulding.

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Best Regards,

Tom Cunningham

posted March 14, 2008 04:11 PM            

Welcome to the single screw forum.

This is an interesting topic and quite a lot of work has been done in the EEU with plastics processors from various sector groups.

I attended a seminar "Processing Plastics More Efficiently" last year, presented by John Colbert a RAPRA Research Technologist.

John has been researching energy usage by European Plastic Industry Sector Groups, with a view to provide them with the knowledge and means to reduce the energy consumption of the various processing methods.

You can sample the fruits of John's labours, by going to www.eurecipe.com If you submit your email addy you'll be emailed a copy of a European Best Practice Guide for the Plastics Industry, and if you register (no charge) you can download factsheets to help improve the efficiency of your energy use.

Now there should be an Interactive Toolkit available at the site, which will allow you to input details about your usage, energy charges and throughput and generate an evaluation of your energy consumption and guidance on where you may be able to reduce it.

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Steve Hodgson

posted March 18, 2008 08:44 PM            

This is indeed a nice source for information, very useful. And some ideas are quite new to me, e.g. groundwater cooling. There are also some simple suggestions like increasing the cooling temperature for pipe extrusion by a few degree (1 degree can save about 2-3% energy...). I believe that 60% of the PE/PVC pipe manufacturer have a standard set-up temperature, which are used for more or less all pipe sizes, regardless that some dimensions could be run at higher cooling temperature at same output.
I am particular interested in the "isolation" of barrels. I have seen hundreds of extruders, but to be frank I never met one with isolation of the barrel. It seems it is quite popular in the US.
Does anybody has experience with either new extruders equipped with barrel isolation (e.g. TCS System from RexMaterial), or simple cover sheets ? If you retro-fit an extruder with any kind of isolation sheets, how about the cooling which is usually done by blowers ? It sounds logical to me that the heat-up can be done much quicker and energy can be saved. Beside this what energy saving can be achieved ? Any experience ?
I am really interested in this topic (and other energy saving topics too). Thanks for your feedback !