Nichrome wire heater optimisation

[gordon13]

Hi, I am designing a plastic recycler and I need advice on choosing gauge of some nichrome wire. I want the heater to reach around 220 degrees C and I will be powering it with a laptop charger (19v 7.1 amps). Temperature control will be achieved with a PID controller.

The nichrome wire will be wrapped around a 21mm outer diameter tube and 17mm inner diameter (I'm thinking copper). The heater has to be able to melt the plastic before it reaches a 3mm hole at the end of the tube.

I used Q=mCdT to find the energy needed to melt the plastic pellets as they travel down the tube at a given velocity, where Q is the energy required, m is the mass flow, and dT is the change in temperature required. To raise the plastic by 200 degrees it will need 628J/s

Using power equations I estimated the power dissipated by a coil given the length of the tube (0.1m), gauge of the wire, and voltage input. The power dissipated by my coil ends up being 120watts.

My question is, do the values for energy_needed and power_dissipation I calculated above seem realistic bearing in mind this will be be a small unit? (the tube is only 10cm long and I'm winding a coil around it once). To me they seem quite high somehow, but then again I'm not sure :/!

Also I want to optimise the design of the coil.
Is it simply a matter varying the gauge of the wire until the power_dissipation = Q or am I overlooking something?

I've attached my spreadsheet if anyone is interested.

Thanks!

 

[anorlunda]

Yes, 120 watts sounds reasonable. But only if the energy lost through the outer tube is zero. It will have to be well insulated to make that true.

I will be powering it with a laptop charger (19v 7.1 amps). Temperature control will be achieved with a PID controller.

Huh? What will the output of the PID controller do? The only way to control a laptop charger is to turn it on/off.Also for safety, you need a way to shut off the power immediately if the mass flow stops.

 

[jrmichler]

To provide a couple of real world data points:
1) A standard hot glue gun, professional grade, is rated 80 watts. It draws a little over 100 watts cold, which drops to about 30 watts when hot. Standard hot glue is injected at about 130 deg C.

2) High temperature hot glue is injected at 196 deg C. A 3M Hot Melt Applicator TC (shown below) designed for 0.44 g/sec flow at 196 deg C is rated 150 watts, and draws 140 watts according to my Kill A Watt power meter. You have four times this flow and a higher temperature. You will need more than 140 watts.

And some practical realities:
1) Winding Nichrome wire on a copper tube will create a short circuit.
2) The gun shown above costs $200. For that price, you get something that is safe to use. It is safe electrically (won't electrocute the user), and thermally (won't burn the user), and safe operationally (won't overheat and burn the product). It may be hot enough to melt the 220 C plastic.
3) If you can adapt a commercially available hot glue gun to your application (they make other models), it will be far cheaper than designing and building your own. Here is a link to 3M hot melt applicators: https://www.3m.com/3M/en_US/company-us/all-3m-products/~/3M-Hot-Melt-Applicators/?N=5002385+3291079629&rt=rud. There are other manufacturers.
4) Controlling temperature requires a temperature sensor, a controller, and a relay to control the heater.

 

[Baluncore]

To raise the plastic by 200 degrees it will need 628J/s

628 J/s is 628 watt.

Does that include the solid to liquid phase change, or just the thermal capacity of the solid feed stock?

What type of plastic is being processed?

How is the feed stock pushed down the tube. Is it continuous or start stop? It is usual to employ a 3 stage screw, the first stage draws in the feed stock, the second stage mixes and melts, then the third stage meters the flow. A stainless steel tube surrounds the screw. The ID of the screw is increased as air is lost from the feed stock and melt density increases along the tube.

The tube surrounding the screw has an external heated jacket in three or more sections, so individual parts can operate at optimum temperature. If the flow needed is start-stop the screw is used as a piston, the screw moves slowly back as the melt reservoir fills, then the reservoir volume is injected and the cycle repeats.

By allowing slightly less than 1 mm clearance, leakage back past the middle section of the screw, the plastic is heated by mechanical work in addition to heat from the jacket. That also mixes the plastic.