need advice on finding a material

ramtough34

Hi guys,

I need a bit of help in making a decision on a particular material. Basically, i am doing a project where I will be heating water in a bottle. I have an idea of using a dynamo to provide the energy to heat the water. In order to be able to do this, I need some advice on a material that could be used to encapsulate the heating element.

Therefore, the material needs to be good at resisting high temperatures, but also allow the heat to be transferred to the water quickly and efficiently.

Another additional, issue is that the material must be Bisphenol-A free. I have looked at Polyimides such as Kapton tape, to wrap around the heating element, but am I correct in thinking that this contains Bisphenol-A!?

Any help or advice would be greatly appreciated.

edgepflow

What about SS316 or Incoloy? This is the sheath material for commercial heater elements.

ramtough34

Thanks, but I was more looking for a plastic / polymer, due to the form that it will have to be manufactured into...any ideas?

edgepflow

This could be a difficult application for plastic.

Kapton is going to have the highest temperature rating. You could consider Teflon PTFE and its derivatives, but they have low thermal conductivity. PEEK is another possibility.

Be extra careful determining the temperature the plastic will see.

ramtough34

The temperatue will not exceed 100 degrees on many occasions. So I am presuming that most plastics would be able to withstand the temperatures that are being asked of it. Do you agree?

Joseph Chikva

Polyimides do not contain Bisphenol but I also do not know nothing about admissibility of their usage in the the food.
Because they may contain another component not less toxic than Bisphenol made from nitrophenol or/and nitrochlorophenol.

And if the temperature not more than 100 deg C why you are not going to use the old kind polypropylene?

niwire

heating element made by nichrome wire will ok, having resistance property to many enviroment and also perfect material for heating appliance

AlephZero

Remember that the hottest part of the system will be the heater wire embedded in the plastic, not the temperature of the water. You can't heat water to 100C in a finite amount of time if the heater itself is at 100C (google for Newton's law of cooling if you don't understand why)

It's hard to be specific beyond that general comment, without knowing what heating rate you plan to use - the faster the heating, the bigger the temperature difference between the cable and the water will be.

I don't know anything about organic chemistry, but I would have though that an inorganic coating for the heater would guarantee no chemical contamination. You can get heating elements in many shapes and sizes, so unless you have a very specalized application you can probably find one "ready made" that will fit your design.

Joseph Chikva

Remember that we do not know any plastic running at temperature of heating wire.

Most usual electric insulation material there is ceramic and not plastic.
As I remember the task was to find plastic running well at 100 Celsius and allowed to use for food processing. And that plastic should not contain Bisphenol A.
I proposed polypropilene as that answers to both requirements.
If I remember correctly polyimide runs well at temperatures up to about 300 deg Celsius. But I never heard about their usage in food processing. This issue (application for food) should be investigated separately.
300 deg Celsius is also much lower than usual nichrome running temperature.

Thanks for but you are a little bit late. But I always open for criticism and grateful to people teaching me something that I did not know.

Enthalpy

Precisely because plastics don't transfer the heat well enough, standard electric heaters for water are made of good metal, with a resistive wire in them, red hot and hold at distance by ceramic spacers. I suspect (but didn't check) heat is transferred by radiation from the wire to the tube.

Even in normal operation, computation will probably show a very temperature at the heater, far from +100°C. Then you have places of bad contact between the resistor and the plastic, which would widen over time.

Polypropylene at +100°C for extended periods: I doubt. At least, it needs an precise variant.
PTFE is harmless at room temperature but gets ugly if overheated.
Very few plastics are allowed to touch food. Mainly PP and PE, PET (with bisphenol), some silicon rubbers, PTFE and maybe some parents - that must be nearly all.

-----

Why shouldn't you buy a banal power resistor used in electronics? They're insulated from their metal base, which you can screw (with some contact grease) in a tube of good healthy metal.

Joseph Chikva

Polypropylene runs well at 100°C and at little more.
One-time I worked as engineer at disposable ware making factory.
We made disposable glasses for cold liquids from polystyrene and for hot (tea and coffee) of polypropylene.

I never heard PET with bisphenol. As PET is product of poly condensation of ethylenglycol and terephtalic acid - no bisphenol.

uby

I can't imagine that this method of heating is more efficient than using microwave radiation to heat water!

Joseph Chikva

It is vain, as you should consider efficiency of microwave source and also what quantity of radiation remains unabsorbed by heated media.
And also purchasing and maintenance costs of heater in both cases.

Enthalpy

My mistake about PET and bisphenol, I mixed up with phthalate:
http://en.wikipedia.org/wiki/Polyeth...oxicity_of_PET
which is up to now a hypothetical risk which may let ban PET for food contact nevertheless.

As for the maximum operating temperature, it depends on the stress:
http://www.matweb.com/reference/defl...mperature.aspx
and yes, PP will suffice for a bottle.

Household microwave ovens cost nearly nothing and boast around 50% efficiency plug-to-RF; then the power not used is absorbed in a network that suppresses the reflected wave. But at least they won't kill a user as the unsound heater considered might.

Joseph Chikva

Resistive heater costs much lower, has near 100% efficiency.
And we talk about heating of water for heating of which the reflecting problem and consequently energy loss will arise.