Reducing heat loss and insulation calculation

[asimpson]

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

 

[TD2K]

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.

 

[asimpson]

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.

 

[TD2K]

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

 

[IRstuff]

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_insulation_panels_-_vips

TTFN
faq731-376

 

[asimpson]

Inside temperature is maintained by electrical heating.

 

[IRstuff]

So, how did you determine there was a 2kW loss?

TTFN
faq731-376

 

[asimpson]

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

 

[IRstuff]

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

 

[urgross]

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