×
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

Reducing heat loss and insulation calculation

Reducing heat loss and insulation calculation

Reducing heat loss and insulation calculation

(OP)
Hi all.

I have a heated body roughly 35 cm cubed. (14" cubed) with a steady surface temperature 50-60C and it seems to be loosing heat at about 2 kW in 20C air. I wish to reduce this heat loss with insulation. I am considering an insulated jacket of exp. polystyrene and I am considering a cylindrical outer shape to reduce surface area.

Is the cylindrical outer case a valid idea?

How would I model the thermal losses?

I am afraid I am not much of an analytical engineer. However I only a rough idea in order to confirm I am going in the correct direction.

Many thanks

RE: Reducing heat loss and insulation calculation

Is the air still, moving, how quickly?

Your temperatures, dimensions and estimated heat loss works out to about 13 BTU/hrft2F. Seems high to me unless you have the air really moving.

RE: Reducing heat loss and insulation calculation

(OP)
Air is still. Some natural convection. Assuming a 8.17 ft^2 , 13 equates to 106 BTU/hr or 31 Watts. Seems very small for a body with surface area 8.17 ft^2 at 60C in 20C air.

RE: Reducing heat loss and insulation calculation

2 kW which you said was the heat loss is 6830 BTU/hr

RE: Reducing heat loss and insulation calculation

Standard values for htc and emissivity result in about 420W loss with your given surface area.

Thick enough styrofoam should drop that considerably and does not require any particular shaping.

You haven't stated how long you want to keep the object hot, or how much heat you want to lose.

Why not just put the thing in a styrofoam cooler?

One extreme option is based on Ultra-R vacuum insulation panels, which are aerogel insulation panels under vacuum with an external structural walls. VIPs are almost as good as a vacuum Dewar.
http://www.pacificseabreeze.com/products/vacuum_in...

TTFN
FAQ731-376: Eng-Tips.com Forum Policies

RE: Reducing heat loss and insulation calculation

(OP)
Inside temperature is maintained by electrical heating.

RE: Reducing heat loss and insulation calculation

(OP)
Sorry I made an big error.

Body is heated by electric heater and maintained at temperature. To maintain this temperature 2kWh/24 hr. of energy are consumed. This is a mean power consumption of 83 w or 284 Btu/hr.

It must be very frustrating when wrong units and are given.

However my main question was in relation to the shape of the outer surface, cube V cylinder.

Could anybody recommend insulation modeller or calculator for heat loss so I could run through a few options? Assuming I can get the figures right.


Many thanks

RE: Reducing heat loss and insulation calculation

Minimizing area is certainly good, provided you have sufficient insulation in the first place.

there are calculators here http://www.novelconceptsinc.com/calculators.htm that might be coerced into doing the calculations, but I get about the same efficiency for a slab fin vs. pin fin.

This site has Nusselt numbers for different geometries: http://www.egr.msu.edu/~somerton/Nusselt/

btw. based on your original dimensions, the heat loss is relatively benign, ~1/5 of what I had calculated earlier.

You'd need about 2 inches of Styrofoam to make a significant dent in the heat loss, given your new number.

TTFN
FAQ731-376: Eng-Tips.com Forum Policies

RE: Reducing heat loss and insulation calculation

I would recommend looking up conduction shape factor for a cyclinder based upon the length, internal radius and external radius of the insulation cylinder. Once you have the conduction shape factor (no forced air so no boundary layer concerns) you should be able to use

q = S*k*deltaT

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