Winding Copper wire / strip on Aluminium body

[Navaneetha]

Hello All,

Greetings. I am new to this forum. I am working for a heater project, in which we use plastic (PA66) over molded on aluminum (6061-T6) body for melting corrosive liquids (similar to sea water). The Aluminum body is heated by means of Positive temperature coefficient material (PTC) from a 12V battery. The problem we face is, the quantity of heat is not sufficient to melt the stated quantity of liquid in a specified time. We have constrains like not to change the size of the outer surface of heater and supplied voltage. But room to play inside the heater. Hence i am looking to use copper strip placed inside slots provided in Aluminum body or to wind copper wire wound around Aluminum body and then over molding by plastic. In these solutions, i have bottle neck like whether these copper strip will really contribute for extra heating of Aluminum body for the given voltage ? The copper material having thermal conductivity (400 W/m-K) will contribute for effective heat transfer but the resistivity is less (1.72e-8 Ohm-m). Will this solution work ? Also, do we need an insulating material between copper strip and wire in between Aluminum body ?

 

[sophiecentaur]

You need to separate the Copper and Aluminium really well or you will get serious corrosion problems under those circs. I am not sure what the copper is supposed to achieve?
But why not use simple resistance wire as your heating element?

 

[Navaneetha]

Hello Mr.Sophiecentaur,

Thank you for your reply. Do you mean direct contact between copper and aluminum cuases corrosion in a normal environment ? In our case, Copper wire wound around Aluminum body will be plastic overmolded by Nylon which is resistant towards corrosion. The Copper (400W/m-K) with high thermal conductivity than aluminum (250W/m-K) will have better transfer heat rate from Copper to Aluminum. Also, inorder to save cost and weight of the system, we are inneed to go for 1:3 ratio for Copper:Aluminum.

 

[sophiecentaur]

Seawater will cause copper / aluminium interfaces to corrode very quickly so you need to separate them well. This could have an effect on the total thermal resistance. (Certainly, a layer or corrosion could provide a high thermal barrier.)

But what limits the heating rate is basically going to be the Power of the Heating Element, the volume of liquid to be heated and the rate it is circulating. Conduction within the heater, whether it is simple or composite construction will only affect the core temperature - which may not be very important.

To get a good answer about this, you need to know something about the temperature gradients across the various interfaces (particularly across the plastic sheathing, I should imagine). I think the difference in conductivity for those two metals may not be very relevant to the overall thermal performance.

 

[Navaneetha]

Mr. Sophiecentaur,

I have attached the cross section of the new and existing heater which i mean. The heat transfer follows: Copper -> Aluminum -> Nylon -> Seawater. The maximum power input that can be provided is 100W from 12V battery. Currently the heat output from PTC in existing heater is not sufficient to heat specified amount of liquid, hence i am looking for the new heater solution. Do you think will this be a bad solution and let me know the concerns involved in this ?

 

[sophiecentaur]

It isn't clear what is what in those two diagrams. Is one of those horizontal bars the resistive element? What is the current path?

But I don't think this is relevant, compared with the Power flow situation. Why should you think that the sea water temperature would be particularly affected by the particular arrangement of the metals?
I think you need to go back to square one in your thinking and consider the following basics:
The seawater will be losing heat to surroundings and, in a steady state, will reach a temperature at which the heat supplied equals the heat lost, per unit time. What is the mass of seawater? Is the container insulated? Is there a steady flow of water?
If you are stuck with 100W of available power then, as I said before, the final temperature of the seawater will be governed by the rate of heat LOSS from the water and not the thermal conductivity of the heater element; the core temperature will just end up at a different value. (You could, for instance, use a 100W filament bulb, with a filament temperature of 3000C or a thick heating element with an internal temperature of 200C and the sea water would end up at the same temperature)