×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

metals and frost

metals and frost

metals and frost

(OP)
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

RE: metals and frost

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.

RE: metals and frost

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

RE: metals and frost

Does surface roughness have an effect?

RE: metals and frost

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.

RE: metals and frost

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.)

RE: metals and frost

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

RE: metals and frost

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

Thanks for the responses

RE: metals and frost

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.

RE: metals and frost

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources