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What is the boil-off rate of a saturated cryo liquid inside a vessel?

  1. Aug 9, 2013 #1
    Note: this is not a homework question; this is strictly conceptual in understanding the equations at work in this scenario.

    The tank is vacuum-insulated, the liquid is saturated, and the vents have just been opened in order to vent down the ullage pressure. My question is simply how do you calculate the rate of boil-off of the liquid (i.e. how much liquid mass evaporates and discharges as gas until it is cooled well below saturation and the ullage pressure is 0 psig) and more imporatantly how much liquid gets used up during that time?

    Thanks for any input-
  2. jcsd
  3. Aug 9, 2013 #2


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    The amount of liquid that boils off is called "NER" (Natural Evaporation Rate) in the industry and is measured as a percentage of product boiled off from the full tank over a period of 1 day. So if you have a 100 gallon tank and you boil off 1 gallon in 24 hours, the NER is 1%. NER is specified at atmospheric pressure and using a specific type of cryogen.

    NER is difficult to calculate from basic principals of heat transfer from the design, but easy enough to test. To test, you maintain the vessel at less than 1 psig and measure the amount of gas that comes out over time. Generally, nitrogen is used for atmospheric tanks (ie: nitrogen, oxygen, argon) and the actual product for other tanks such as hydrogen or helium. When calculating the boil off rate, you should also consider that given some liquid is boiled off, that volume of liquid is now taken up by saturated gas, so not all the product that is boiling off comes out of the vent. Some is left in the tank to maintain tank pressure, but NER is a function of what actually boils off.
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