The electricity saved on a large extrusion line could power a number of Chevrolet Volts.

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The electricity saved on a large extrusion line could power a number of Chevrolet Volts. An extruder uses a lot of electricity, and movements toward conservation and lean management require that we take a hard look at the energy efficiency of these machines. But to tackle electricity usage by the extruder you first have to understand just how an extruder uses it.

Electricity enters the extruder at the screw drive, the barrel heaters, and the controls. Energy leaves the extruder almost entirely as heat—in the polymer melt, barrel-cooling fluid, feed-throat cooling fluid, gearbox cooling fluid, or lost to the room air. By knowing where and how much heat is leaving the extruder you can go about minimizing the losses and thereby improving the energy efficiency. This is called constructing an “energy balance.”

The majority of the electricity used in single-screw extruders enters at the screw drive, and most of that goes into heating the polymer through shear and friction. Polymer transport, pressurization, and mixing consume a much smaller portion of “juice.” As the biggest electricity user, it’s important to see how effective the screw drive is in transferring energy to the product. That energy balance consists of matching the kilowatt-hours (kWh) used by the drive system to the heat transferred to the polymer. For amorphous polymers, that is simply the temperature increase in the polymer from the feed temperature to the melt temperature, as follows:

(Output) x (avg. specific heat) x (temperature rise) x (0.00029) = kWh

Crystalline polymers require an additional element called the heat of fusion, which is the energy required to convert a polymer from a solid to a liquid at the crystalline melting point. In other words, this is the energy required to break down the crystallinity of the polymer but results in no increase in temperature:

{[(Output) x (avg. specific heat) x (temperature rise)] + (heat of fusion)} x 0.00029 = kWh

Ignoring the heat of fusion can introduce serious errors when processing polymers like HDPE or nylon, where it can account for a major portion of the total power for heating and melting the resin.

To obtain total power usage—the electric input measurements to the extruder—requires you to connect a kilowatt-hour meter to the main distribution. The motor power is measured at the drive input leads. These are usually available for loan from your power company or can be purchased for about $2000. Sounds expensive, but on a number of large extruders this can pay for itself many times over in a single year. For instance, a change of 5% in energy efficiency on a 6-in. extruder running HDPE would provide an hourly savings of approximately 15 kWh. On a 24/7 basis this equals $10,000 to $15,000/yr in savings, depending on your electricity cost.

In addition to the drive conversion efficiency, a complete energy balance requires examination of the cooling of the barrel, feed throat, and gearbox. Finally, the losses from the barrel to the environment (room) have to be investigated. I have found many extruders can be improved as much as 20% when all aspects of the energy balance are considered. That translates into an annual savings of $40,000 to $60,000 per extruder.

Extruders with a good energy balance also typically run better, with a more uniform melt quality, because the flow of energy is consistent-i.e., intermittent heating/cooling cycles are eliminated.

 

About the Author

Jim Frankland is a mechanical engineer who has been involved in all types of extrusion processing for more than 40 years. He is now president of Frankland Plastics Consulting, LLC. Contact jim.frankland@comcast.net or (724) 651-9196.