metals and frost

[EngForm78]

Why does frost form on metals easier than other materials like plastic. Is there a stronger chemical bond between the metal and water? Does the specific heat have an effect?

Thanks

 

[metengr]

Basically, it has to do with thermal conductivity, which is the ability to transfer heat. The thermal conductivity for metals is greater in comparison to most non-metallic materials. Thus, during conditions where the metal and the plastic are exposed to the same cold medium, the metal will cool or transfer heat at a greater rate promoting the formation of frost on the surface. The plastic will have a thinner layer of frost. The total amount of frost for the metal and plastic will depend on the relative humidity of the gas medium. Remember, heat flows from high temperature to lower temperature until an equilibrium condition is reached. The rate of heat transfer for the material and relative humidity are key to frost build-up.

 

[EdStainless]

specific heat, thermal condctivity, and emissivity all play a role.
I suppose you are talking about your car sitting out at night?
While the metal holds more heat it also takes more heat to warm it. In addition it will change temperature more quickly because of its higher thermal conductivity.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[Ashereng]

Does surface roughness have an effect?

 

[metengr]

Yes, it would, because in addition to thermal conduction within the material you also have convective heat transfer occurring between the solid surface (metal or plastic) and the fluid medium (gas). Convective heat transfer is rather complex, unlike conduction, and is dependent on surface roughness and films.

 

[NickE]

Also IIRC condensation follows standard rules of neucleation and growth. Therefore defects in the surface will have a lower energy barrier to attract enough atoms to cause a droplet to grow. I would assume that frosting (the opposite of sublimation, I'm sure theres a specific name for gas->solid phase changes) also follows this.

(actually I've noticed scratches in my paint and such do get frost first, and more extensively.)

 

[rmw]

Each grain, particle, scratch, what have you of surface roughness, no matter how minute, would act as a fin on the surface of the base metal with respect to heat transfer.

rmw

 

[EngForm78]

This question is in regards to designing a component that will not frost from being outside.

Thanks for the responses

 

[ornerynorsk]

I'm in Minnesota, and I have never seen a material that will not collect frost. Everything, from plant matter to asphalt to glass and pottery to cardboard. May I ask, what is the purpose of your application that would necessitate a frost free existence, and is there some other method (airflow, heater coil) that would be acceptable to prevent frost buildup.

 

[Ashereng]

You didn't mention what you are protecting, or where outside is (conditions?), but heat is a common and good solution to eliminate frost. Eg. car defroster is heated air.

Another method is to dry the air before contacting the surface (humidity control). A slight variation is to blow dry air over the surface to keep moist air off and/or to reduce the humidity of the wet air. Eg. humidistat recommendation for lower humidity in lower temperatures.

Separation of the protected surface from the outside air is another method. Eg. double/triple pane windows in homes.