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I am currently making vacuum fryer, and was wondering this question.

Normally, when you apply heat for a long time in a completely sealed container, the gas will expand, and pressure will be built and eventually container will explodes (assuming no safety pressure valve)

In vacuum, since there is no gas, it should be safe, correct? No gas to expand, means no risk of sudden explosion, even for long time cooking (say 100 C for 60 hours)

Or i understand it wrong?

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  • $\begingroup$ a vacuum fryer en.wikipedia.org/wiki/Vacuum_fryer#Working_principle $\endgroup$
    – anna v
    Jan 6 at 14:06
  • $\begingroup$ Sous Vide sounds safer. $\endgroup$
    – JEB
    Jan 6 at 15:47
  • $\begingroup$ Re, "in a completely sealed container, the gas will expand" The gas in a rigid, sealed container cannot expand. $\endgroup$
    – Solomon Slow
    Jan 6 at 18:42
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    $\begingroup$ Re, "when you apply heat for a long time... pressure will be built" The ideal gas law, $PV=nKT$ does not have any dependence on time. If the volume of the container remains constant, and the number of gas molecules sealed inside the container remains constant, Then the pressure depends only on the temperature. Maybe there is some temperature at which the container will burst, but it does not matter how long it takes to reach that temperature. Depending on conditions, it may be that you can apply heat for a day, a week, a year, and never reach that temperature. $\endgroup$
    – Solomon Slow
    Jan 6 at 18:49

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If you just pulled a vacuum, disconnected the pump, and then applied heat, the chamber would quickly fill with water vapor and other gases from the cooking food. At that point, the same ideal gas law would apply, and you would have pressure that increases with temperature.

But, based on the description of what a vacuum fryer is, it seems like it's continually regulating the pressure during the cooking process. That's like a relief valve.

As an aside, at a fixed temperature, a pressure chamber doesn't keep building up pressure. If you keep a fixed 100°C, for example, you'll reach an equilibrium for that temperature.

Explosion occurs when you have continued input of heat. That's what happens when you have a pressure cooker on a burner. Temperature and pressure both rise until the chamber explodes.

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    $\begingroup$ thank you for more valuable insight! and yes you're correct, vacuum pump needs to be remain active throughout cooking process to make sure the pot remain vacuum. this might be different question, but is it safe for vacuum pump to keep active all time for that long, 60 hrs straight? or there is a risk of implosion? since it keeps pumping out even with 0 pressure. thank you again in advance $\endgroup$
    – mande
    Jan 6 at 14:30
  • $\begingroup$ @mande I don't think I'm qualified to comment on the safe limits of the pump. $\endgroup$
    – Alex K
    Jan 6 at 14:32
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    $\begingroup$ Often safety problems don't come from the issues you anticipate. They come from the ones you don't. It wouldn't hurt to put a relief valve in, like in an ordinary pressure cooker. $\endgroup$
    – mmesser314
    Jan 6 at 16:33
  • $\begingroup$ Re, "...continued input of heat..." If you put any object on a kitchen range, and you turn up the gas, the system (including the air, and the walls of the kitchen, and the walls of the house) will reach an equilibrium state: The stove continues to generate heat, but radiation and convection carry heat away from the object (and, from the kitchen and, from the house) at the same rate, and so the temperature of the object stops rising. $\endgroup$
    – Solomon Slow
    Jan 6 at 18:54
  • $\begingroup$ You're right -- though radiated and conducted heat from a cooking vessel are generally pretty low (that's why it's so easy to damage cookware by leaving it on the burner by accident). So the equilibrium temperature is going to be a lot higher, and at a hard to predict temperature. $\endgroup$
    – Alex K
    Jan 6 at 23:16

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