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| Author | Topic: estimating power requirements |
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Alfredo Member Posts: 4 |
posted January 20, 2006 06:25 AM
   
How can you estimate power requirements of a PVC and PE pipe extruder, mechanical (motor load)and heating? Thank you very much IP: Logged |
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Tom C Moderator Posts: 683 |
posted January 20, 2006 08:56 AM
Motor load can be estimated by using specific energy requirements as in Kw-Hrs/kg. Heating and cooling demands are best estimated with extrusion simulators. Heat transfer coefficients and models are not very reliable however. Tom C IP: Logged |
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Alfredo Member Posts: 4 |
posted January 20, 2006 09:19 AM
Thank you Tom Regarding motor consumption, how can you calculate an aproximate estimate of specific energy consuption, kw per kh/hr? As for heating, I suppose you can estimate theoretical heat input required + losses + heat regain, żis that ok? We are on the first stage of an energy audit. Thanks again IP: Logged |
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Tom C Moderator Posts: 683 |
posted January 20, 2006 09:28 PM
The materials you are using will use a lot of motor power, likely 0.25 kw-hr/kg. If the materials are highly lubricated this number can be lowered significantly, but the process might not work either. In the case of these materials heater are typically used to help get the process going, but then cooling is required to keep the material from overheating. Therefore part of the motor energy goes into the air or down the drain. Typically for your process 80% of all energy comes from the motor. Tom C IP: Logged |
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Alfredo Member Posts: 4 |
posted January 23, 2006 06:30 AM
Thanks Tom for your information, its a good starting point. The aproximate 80% of energy from the motor means that heating requirementes is aproximately 25% of motor power (0.0625 Kw per kg/hr); is that for twin screws or single screws? Heating in single screws comes mainly from friction, while in twin screws it acts more as a positive displacement pump, is that ok? thanks, Alfredo IP: Logged |
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Tom C Moderator Posts: 683 |
posted January 26, 2006 07:56 AM
Your questions are far too complex than can be answered here You should get a copy of Chris Rauendall's book on extrusion and attend a few seminars on the subject. An understanding of extrusion, in order to do some worthwile engineering, takes quite a bit of effort to obtain. Tom C IP: Logged |
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Stephen J. Derezinski Senior Member Posts: 11 |
posted January 29, 2006 01:58 PM
Your screw design will also affect the power consumed. If it is a poor design, then there could be excessive power needed along with excessive cooling, both of which will have costs. This could easily happen if you intend to process different products with the same screw. Otherwise, figure in the cost of changing screws for every product change. As Tom said, it does get complex easily. Steve Derezinski, Ph.D IP: Logged |
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Stephen J. Derezinski Senior Member Posts: 11 |
posted January 29, 2006 04:31 PM
Re: Jan 23 note: Most of the heating is a result of viscous shear of the polymer both in single and twin screw extruders. However, the strain field that produces the viscous shear in the single screw extruder is different and less complex than that in the twin screw extruder. Steve Derezinski, Ph.D. IP: Logged |
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Stephen J. Derezinski Senior Member Posts: 11 |
posted January 29, 2006 04:42 PM
Re: Note of January 20: If you are fortunate enough to have data for an existing machine, then you can calculate the heat transfer coefficients that prevail with your process. Then, use the same model to predict what a new machine would do. This can remove allot of amibguity in using the correct heat transfer coefficient. The same thing can be done for melting rate and solids conveying. All key factors for designing a new screw. Steve Derezinski, Ph.D. IP: Logged |
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griffex Senior Member Posts: 36 |
posted January 29, 2006 11:55 PM
It's not all that bad. First of all, stop worrying about power consumption, especially with PVC. That is the least power using of all major plastics (see below). Add up the cost of the kW actually drawn by the motor plus the heaters and you'll get maybe 1-2% of the raw material cost -- maybe more in other countries, but do it anyway. The lessons here are (1) spend time and effort in using as much scrap as possible in ways that replace new material, and (2) tight tolerances to allow lower aim thickness. I know this view is uncomfortable because we are constantly worried about our own personal power costs -gasoline, home heating/cooling, etc., but do the math before buying into this distortion, and please let me know if you get anything different. Helpful numbers: 0.001163 kWh = 1 KCal, enthalpy (heat content) of PVC at 180 C vs room temperature is around 0.065 kWh/kg, and same for HDPE at 210 C is about 0.19 kWh/kg. Figures from Rao & O'Brien, Design Data for Plastics Engineers. You can't waste much heat with PVC because if it overheats it "burns" and the product is useless. With PE it's almost as difficult to waste heat -- if it is too hot it won't size easily. As for most energy coming from the motor, this depends on machine size and twin-vs-single. Little single-screw machines put more heat in thru the barrel, and the bigger the extruder the more the motor is important. Twins put in more thru the barrel, which is more controllable and is why the twins keep melt temp lower, with consequent savings in formulation cost. I hope this is all useful. Allan Griff Consulting Engineer algriff@griffex.com IP: Logged |
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Alfredo Member Posts: 4 |
posted January 30, 2006 08:26 AM
Thank you all for your comments, they are very valuable and I apreciate very much. There is no data for any of the equipment and I guess that most of them have suffered modifications; I am just introducing a data collection system. I know theoretical calculus is not easy, but some benchmarking always helps a lot at the beginning, and gives you are rough idea of where you are standing. IP: Logged |
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