Properties to determine best gas for coolant medium 1

[corus]

Which properties are best to compare which gas to use as a coolant medium? Is it the specific heat of the gas or perhaps the Prandtl number of the gas?

corus

 

[BigInch]

The temperature of the gas and the flowrate directed on the item to be cooled is probably more important than which actual gas to use.

The Prandtl coefficient is not a property of the gas itself. It is one measure of the heat transfer condition.

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[corus]

Thanks, but the temperature and flow rate of the gas can be assumed to be the same regardless of which gas is used. It is a measure of the heat transfer property that I wish to compare as to which gas gives the best rate of cooling given the asme conditions.

corus

 

[25362]

IMHO, I'd say neither. Pr numbers are of the same order of magnitude at equal temperatures. Apparently, it all depends on the flowing conditions at equal temperature and pressure.

When comparing air, ammonia, carbon dioxide, helium, hydrogen, and water vapor, all athe same T, P conditions, and using Sieder and Tate findings, I came to the following conclusions:

For equal Re, the dominant factor is the thermal conductivity. Thus, hydrogen would have the largest forced convection htc.

However, for equal gas velocities carbon dioxide and water vapor (depending on temperature level) seem to be the best in this group because they show a much larger Re, counterbalancing hydrogen's larger thermal conductivity. Again, all these thoughts are based on Sieder and Tate measurements and published graphs thereof.

If radiation from the hot body is also a factor, since diatomic gases are, more or less, diathermic I'd say carbon dioxide would have to be picked up as coolant among the others. Corus, do you agree ?

 

[corus]

Thanks 25362.
Since posting I had considered the calculation of the heat transfer coefficient as a function of the Nusselt number and conductivity, where the Nusselt numebr is a function of the product of Reynold's and Prandtl. Hence in comparing gases it would be a comparison of the Reynolds*Rrandtl, and the thermal conductivity. Conductivity may be the dominant factor, as you say. Radiation to the gas medium I hadn't considered at all. I'll have a look at these Sieder Tate references you quote.

corus

 

[JStephen]

The problem still isn't well defined. When you say flow rate is the same, do you mean mass flow rate or volumetric flow rate? If velocity is the same, I would think the heaviest gas would work better.

In real applications, the choices would be rather limited, I would think. Eliminate products that are flammable, poisonous, corrosive, unstable at the temperature, or expensive and see what's left.

 

[BigInch]

Exactly what I mean. I would be surprized if the best gas (in all around project economics and supply availability speak) would not be simply the coldest air you can get. Unless air cannot be used or this is some kind of a homework problem.

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[25362]

A homework problem ? Highly unlikely considering Corus' record.

 

[corus]

Sadly it's a little too late in life to be handing in homework for me, instead I get lines to write - but am paid for it!

This was a lazy question at the time as I hadn't at that time found all the material properties and worked out the numbers. Given the requirement for an inert gas, 25362's reasoning for CO2 and thermal conductivity as the main factor seems valid.
Many thanks.

corus

 

[BigInch]

Easy. I know.

Sometimes my dry sense of humor chimes in at the wrong time. Appologies.

IMO that's the only valid reason not to use air.

Going the Big Inch!

http://virtualpipeline.spaces.msn.com