Part Cooling

[jimschu]

I am trying to cool an aluminum part comming out of an oven at 250 degrees, down to 110 and I would like to know how to calculate the the cooling load and time required to cool down the part from t1 to t2.

what is the formular?

Thanks for your help.

 

[ClydeMule]

Q = m*Cp*DT = Quantity of heat to be removed. Such as BTU.

Divide that by how long you want the process to take and you will have a heat flow, such as BTU/HR.

Clyde

 

[magicme]

hi there
This answer will sound a bit stuffy because its mostly from notes I use for a course, but it'll make my point....

The Immersion of a Solid Body is an approximate calculation of the transient bulk temperature change of a solid whose exterior conditions change suddenly. An example would be a hot body dipped in a large vat of cooler liquid, or vice versa. The heat transfer into and out of the body is through surface convection and internal conduction, and it is assumed that the external conditions (fluid temperature and convection coefficient) do not change with time (though the body temperature changes).

The traditional equation for the transient temperature of a mass (neglecting internal conduction) is....
Tinternal = Tambient + (Tinitial - Tambient) * exp ( - h * surface area * time / mass / airCp)

That accounts for the fact that heat transfer is not constant as the body cools, so the temperature varies exponentially.

A modification of the traditional equation is used for this calculation. The modification is that the convection coefficient in the numerator of the exponent term is replaced by the Overall Heat Transfer Coefficient, U, which has the form U = 1/(1 / h + t / k). In this form, t is some representative thickness of the solid body from its surface to its center. This is somewhat of a judgement call by the user, as this one method attempts to approximate a wide variety of possible geometries.
The purpose of including the t / k term is to somewhat account for the internal conduction mechanism. Leaving out some attempt of accounting for this would imply a uniform temperature throughout the body, and would result in transient response that is generally faster than reality. Thus, the term was added to the exponent.
For critical applications, where thermal gradients inside the body are important, a more detailed approach needs to be taken. This approach works well in cases where only the transient bulk temperature of the solid needs to be calculated.

daveleo

 

[magicme]

sorry....that should be just Cp, not "airCp"

daveleo

 

[imok2]

Q = mass x specific heat x delta "T" x time = BTU/H

 

[sailoday28]

This is a transient problem and needs at least the following information to get a good handle on the temperature time distribution in the solid body:

Thermal conductivity, specific heat and density Which should be readily available for alumin

Shape of the material (brick, cube, cylinder, sphere, etc.)

Surrounding temperature -to determine outside coef of ht transfer.

A good heat trans text will have Gurnie-Lurie or Hottel or simmilar non-dimensional charts whic give non dimensional temp at various locations vs time.