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| Author | Topic: Barrel heater plumbing |
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rawelk Senior Member Posts: 16 |
posted December 16, 2006 02:30 AM
   
I was surveying a 4-1/2” NRM extruder using conventional aluminum cast-in heaters and distilled water cooling, and observed several plumbing routing decisions that I don't understand. Zone 1 was plumbed in the manner I'm familiar with – the two halves were teed on either side, fed with water on the feed throat side, and water returned from the head side. On zone 2 water was fed into one half on the feed side, exited to the second half on the feed side, and flowed to the return manifold from the head side of the second half. Zone 5 and 6 were plumbed with water flowing into the feed side on one half, “jumped” across to the second half at the head side, and returned to the cooling reservoir from the feed side of the second heater. I've been looking around for definitive guidelines regarding barrel heater plumbing with no success, and am posting the question here. IP: Logged |
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Tom C Moderator Posts: 811 |
posted December 16, 2006 10:16 AM
I don't know that there is a standard way to plumb the zones. Some considerations are; The system is not always pressurized, so therefore water can potentially pool in the lower heater. Some even mounts the heaters left-right instead of up-down I believe becasue of this reason and becasue the lower half will tend to come off the barrel due to strap stretching by weight. Having one half much colder than the other half is bad becasue it can cause the barrel to bend enough to cause screw wear or damage. The BZ1 routing will result in preferntial cooling of the lower heater half. If one of the heaters plugs with scale then the cooling will still work, but will be very uneven as described above. The BZ5-6 routing has the best chance of providing even cooling in the zone. ------------------ Tom Cunningham www.ExtrusionTechnicalServices.com IP: Logged |
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rawelk Senior Member Posts: 16 |
posted December 19, 2006 08:37 PM
Tom, I should also have said the pictured views are looking from the bottom up (as though one were on their back looking upward at the heaters). The halves are aligned such that the electrical terminal split is at 12 o'clock, and piping split is at the 6 o'clock position, so gravity would tend to drain both heater halves. Having said that, your point about what happens when one heater half loses flow in the paralleled (zone #1) example remains valid, although, outside of the blocked flow condition, it seems to me this would be most consistent - both heater halves going from 'cool' to 'warm' in the same rear-to-head direction. However, since this flow condition cannot be guaranteed without additional gizmos, and your point about barrel warp makes sense then the question becomes which of the series examples would be the better choice. Here, I'm not so sure .... the BZ5-BZ6 example goes from 'cool' to 'warm' on the one half in the rear-to-head direction, then 'warm' to 'hot' in the head-to-rear direction. That is, the rear side of the heater halves will match up 'cool' to 'hot', with the head sides both 'warm'. The BZ2 example, on the other hand, goes 'cool' to 'warm' in one heater half in the rear-to-head direction, then 'warm' to 'hot' in the second half in the rear-to-head direction (matching 'cool' to 'warm' across the rear side, and 'warm' to 'hot' across the head side). I'm thinking this should yield the lower radial temperature gradient of the two series arrangements. Funny thing - I've never paid much attention to the plumbing end of things (except to replace a bad fitting here and there); who'd 'ha thunk there were interesting questions here, too? IP: Logged |
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Tom C Moderator Posts: 811 |
posted December 20, 2006 08:05 AM
You make a good argument. I think thermocouple placement and barrel cooling strategy also plays a role. I once programmed a PLC which had a 20 second period for cooling. Of that 20 seconds, 2 seconds was the maximum on time for cooling (that was considered 100% cooling output). Additional cooling could be provided, if needed, by opening the flow valve to the zone. This was done on a twin screw extruder and the idea was to keep the zone in a consistent "flash to steam" mode. It worked fairly well, but in our example the area around the inlet would be much cooler than all other areas. Therefore BZ2 might be the best choice for reasons you have stated. Good Luck.
------------------ Tom Cunningham www.ExtrusionTechnicalServices.com IP: Logged |
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rawelk Senior Member Posts: 16 |
posted December 23, 2006 11:45 AM
Agree on both points (cooling strategy, and TC placement), but now you've set me off to thinking ... The nominal TC location is along the top of the barrel, and centered within the barrel zone heater element (i.e. - if a heater is 14" long then the TC is located 7" from either end). The extruder OEM drilled this TC mounting hole, and (as long as the end user doesn't change heater geometry) there isn't any reason to relocate it. Or is there? The thought I'm having is, what happens if you install a radically different screw, for instance, one where a mixing section was added to the end of the first stage compression. This type of modification must end up shifting heat balance. Also, the more I think about it, it seems to me there is a difference (whether significant or not remains to be answered) with the directionality of water flow - cool to warm in the throat towards head direction, or vica versa. I'd reckon there would be more of an effect on zones with a steep longitudinal temperature gradient (say, zone 2 compression stage). IP: Logged |
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Tom C Moderator Posts: 811 |
posted December 23, 2006 09:02 PM
How about this to complicate matters; Split the inlet water to opposite ends of each half, and then collect from opposite ends. ------------------ Tom Cunningham www.ExtrusionTechnicalServices.com IP: Logged |
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rawelk Senior Member Posts: 16 |
posted December 26, 2006 04:56 PM
Sounds like you are describing a contraflow parallel arrangement, which would match up as "cool-warm" at each end of the heater from one side to the other. I think this exhausts the plumbing possibilities for a single pair of barrel heaters, although we have yet to touch upon the permutations available when more than one set of heater halves constitute a barrel zone IP: Logged |
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Tom C Moderator Posts: 811 |
posted December 27, 2006 04:11 PM
No I have not seen the plumbing aspect discussed elsewhere. Seems like a good grad student engineering study. I wonder if it could be computer modeled because of water phase change issues. I would not be surprized if there were gross temperature gradients locally, but I doubt if they would cause much difference in an extrusion process itself. I believe the key element towards good extrusion control would be temperature stability in the zone overall. With the water cooling process this can sometimes be difficult to acheive. ------------------ Tom Cunningham www.ExtrusionTechnicalServices.com IP: Logged |
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Stephen J. Derezinski Senior Member Posts: 58 |
posted December 27, 2006 07:07 PM
It appears that case I is a purely parrallel flow heat exchanger arrangement to allow the solids conveying to approach melting as quickly (in length) as possible over the entire barrel diameter. Case II is a less aggressive parrallel flow configuration to allow melting to proceed easily, but still maintain control of the boundary temperature. Case III is a "more aggressive" partial counter-flow arrangement in an attempt to maintain an average constant barrel boundary temperature. There could be a case IV where both sides flow counter-flow (a direct opposite to case I.) However, this was not used for the given process and polymer. Possibly, at higher screw speeds this would be used. These arrangements were probably found by trial an error to make the machine operate well with a given polymer and process. ------------------ IP: Logged |
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rawelk Senior Member Posts: 16 |
posted December 29, 2006 01:13 AM
Hmmm ...
quote: perhaps, but I know this extruder was replumbed maybe a year or so ago when we changed from Asco solenoid valves to trial a different valve type (which, so far, holds up fairly well at elevated temperatures), and my guess would be the new plumbing had more to do with what fittings and braided steel hose lengths we had on hand than deliberate process decisions. I noticed the plumbing while investigating an instability problem. Reportedly, screw load and melt pump suction pressure would crash, and eventually cause a low suction pressure shutdown. We recently obtained a data acquisition system, and logging 15 channels worth of data at 10 samples/sec yielded interesting results. Several things we found could tie into the pressure crash symptom, although the proximate cause appears to have been material related (the day I hooked it up the granulator blades were replaced - dull as butter knives - and the symptom didn't show up during the subsequent week's worth of logging). Barrel temperatures were monitored during that time, and we found acceptable control on BZ1 through BZ4, but BZ5 and BZ6 were acting oddly. We use identical PID parameters for the barrel zones which, although it doesn't discount tuning, makes it less likely. Until we get the machine during a PM and check out it out more thoroughly further this deponent sayeth not For instance, there were occasional bursts of what appears to be electrical noise (in the 55000 second area on the following chart), and other problems discovered in the meantime that need to be investigated and nailed down before I get too concerned about plumbing routing as a causal factor. Steve, I'm very interested in your comments, and was wondering if you've seen barrel heater plumbing decisions based on processing capability, and/or situations where several different strategies had been employed in trial-and-error fashion to achieve control improvements? IP: Logged |
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Tom C Moderator Posts: 811 |
posted December 29, 2006 01:50 PM
Displaying suction pressure, amp draw, and zone control output on the same graph with zone temperatures is sometimes helpful with tuning. BZ6 seems to move very quickly and to be set much at a much lower temperature. Control seems good, but the thermocouple might be too shallow or too close to the heater halfs. ------------------ Tom Cunningham www.ExtrusionTechnicalServices.com IP: Logged |
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rawelk Senior Member Posts: 16 |
posted December 30, 2006 01:19 AM
Tom, Don't know why we run BZ6 down at 495°F, but the process group seems satisfied with the results, and, if they are happy, then so am I  Wish I could offer up heat zone % output, but don't have enough logger channels to devote to it, and, even if I did, would need to write an analysis program to read the data files, and convert time proportioned output back to percentages.
In any event, I do have a snapshot of other channels to go along with temperature trending. They are, from bottom to top -
Not shown, but later created a extruder screw power trend line by scaling % load to amps, % speed to armature voltage (we are running below base speed, so motor field weakening doesn't factor in here), and multiplying them - not 100% accurate, but it did show that screw power demand stayed almost constant for the first third of the trended time (where current rose, and speed slacked off), and rose slightly during the second third when current was relatively 'flat', but speed increased.
Was hesitant at first to believe discharge pressure trending, but I've since replaced the pressure transducer, and verified instrument calibration. Also measured raw mV output, and calculated the results for the 10K probe @3.33 mV/V and the instrument's excitation voltage (8.5 VDC) - it's real. This 70cc/rev melt pump has been in operation since approx. 1992, so my fairly strong conclusion is its worn out. However, one of the other notable problems was a burned-out 4800W, 8" long x 7" ID mica band heater in the downstream mixer. This section is 20" long, and used this and (if memory serves) an 1800W, 9-1/2" long heater, ergo, badly mismatched watt densities. I've ordered ceramic heaters spec'd for about 18 W/SqIn, which should provide a balance between start-up temperature rise, and processing considerations. I coould go on with other items of interest that popped up (but not in one sitting [This message has been edited by rawelk (edited December 30, 2006).] [This message has been edited by rawelk (edited December 30, 2006).] [This message has been edited by rawelk (edited December 31, 2006).] IP: Logged |
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Tom C Moderator Posts: 811 |
posted December 30, 2006 10:38 AM
What is the difference between suction pressure and head pressure? Was the gear pump run at constant RPMs? ------------------ Tom Cunningham www.ExtrusionTechnicalServices.com IP: Logged |
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rawelk Senior Member Posts: 16 |
posted December 30, 2006 06:56 PM
Pump speed was stable, and ranged between 26.23 to 26.27 RPM with a suction pressure setpoint of 850 PSI during the entire trended period. Here is a detail table showing pressures immediately before and after the sudden drop in pump % load at ~28000 seconds. Discharge pressure averaged about 30 PSI under suction pressure (-30 PSI differential) in the run-up to the load shift, and dropped even more (averaging 88 PSI below suction pressure) afterward. IP: Logged |
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