Properties to determine best gas for coolant medium
Properties to determine best gas for coolant medium
(OP)
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





RE: Properties to determine best gas for coolant medium
The Prandtl coefficient is not a property of the gas itself. It is one measure of the heat transfer condition.
Going the Big Inch!![[worm] worm](https://www.tipmaster.com/images/worm.gif)
http://virtualpipeline.spaces.msn.com
RE: Properties to determine best gas for coolant medium
corus
RE: Properties to determine best gas for coolant medium
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 ?
RE: Properties to determine best gas for coolant medium
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
RE: Properties to determine best gas for coolant medium
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.
RE: Properties to determine best gas for coolant medium
Going the Big Inch!![[worm] worm](https://www.tipmaster.com/images/worm.gif)
http://virtualpipeline.spaces.msn.com
RE: Properties to determine best gas for coolant medium
A homework problem ? Highly unlikely considering Corus' record.
RE: Properties to determine best gas for coolant medium
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
RE: Properties to determine best gas for coolant medium
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!![[worm] worm](https://www.tipmaster.com/images/worm.gif)
http://virtualpipeline.spaces.msn.com