Energy in a steam autoclave

[shardik]

In a steam autoclave (pressure cooker)at a set pressure/temperature, for a set length of time, will the same amount of energy transfer to say 6 objects vs 3 objects, all of same size, mass, materials, etc... Basically can I "heat" something quicker if there is more steam vs mass to heat?

 

[BigInch]

Yes. If you apply more steam at an equal or higher temperature and pressure, you will heat your objects faster, since there should be more heat from the increased steam flow being delivered to your objects. Of course it will take more time to heat the greater quantity of steam, so it might be a bit of a trade off.

Raising an object's temperature depends on the material's "heat capacity" property, which is a measure of how much heat Btu or Calories are required to raise the temperature of a material of an object by 1 degree. If using English System, the units for heat capacity of a material will be Btu/Pound/ºF.

A typical steel has a heat capacity of 26 Btu/lb-F.
Therefore, if you have 10 lbs of steel, It will take 26 x 10 x 1ºF = 260 Btu to heat the 10 lbs of steel by 1 ºF, or 26 x 10 x 100 = 26,000 Btu to heat the objects by 100 ºF

Heating objects also depends on a few other factors, such as how the heat is actually transferred to the object, ie, by convection, conduction or radiation, ie, immersing in hot water, contact with another hot surface, or by blowing hot air on it, or by sitting it in the sun, etc. or a combination of all three, as well as the amount of surface area exposed or available that the applied heat can actually cross the material interfaces and be absorbed or transferred to your objects, but all of that involves quite a bit of more complex calculations. The above method will give you an approximate solution. If you need a more detailed solution, you will have to grab a good thermodynamics textbook... or a consultant.


Going the Big Inch!

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[JStephen]

"It depends"

If heating capacity is such that you can maintain the same pressure and temperature of steam, then there ought to be little difference in the heating time, whether you have one object or a dozen being heated.

If heating capacity is such that pressure and temperature drop with the additional items, then yes, it's going to take longer.

To apply it to a home pressure cooker, if you're heating grains of corn, it won't matter if you have 1 grain or 5, time will essentially be the same. If you are heating 5 cans of corn, then you should see a slower heat-up time for the additional mass.

 

[BigInch]

JS, thanks for making that clear. I see I mentioned that more heat would be required for more mass, but kinda' side-stepped the issue about the increase in rate of heat application that would be needed to do so in the same amount of time. In my example, 10 pounds of steel heated with steam at at least the same temperature and pressure you used to heat 1 pound would take almost 10 times the time to get the 10 lbs to the same temperature as the 1 lb piece.

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[Zapster]

Also consider the quality of the steam. I believe that saturated steam will have a much higher heat transfer coefficient than super heated steam. So, look carefully at the heat transfer coefficient for the quality of steam you can provide verses what you have now.

 

[shardik]

Thanks everyone for their reply. I am looking at the data for "heat" up time now. Basically what I am looking at is curing rubber in a plastic "mold" inside an autoclave.