Equilibrium??
Equilibrium??
(OP)
I just found this particular forum by accident, even though I have been using eng-tips for several months.
I have a "disagreement" with some colleagues.
We have a 27,000 gallon liquid nitrogen tank. As part of a test I pressurized it to 70 psig, opened the drain valve and monitored the temperature in the liquid line. I closed the drain line a couple of hours later. I left the tank pressurized and opened the drain the next day. I looked at the temperature and could not see any increase. A colleague says the low temperature is caused by liquid flowing across the thermowell and I say it's because the liquid has not equalized. In fact I think the liquid would probably take a couple of weeks to reach saturation temperature. If we fill the tank once or twice a day I'm not sure that it will ever reach the temperature associated with 70 psig.
Any thoughts?
I have a "disagreement" with some colleagues.
We have a 27,000 gallon liquid nitrogen tank. As part of a test I pressurized it to 70 psig, opened the drain valve and monitored the temperature in the liquid line. I closed the drain line a couple of hours later. I left the tank pressurized and opened the drain the next day. I looked at the temperature and could not see any increase. A colleague says the low temperature is caused by liquid flowing across the thermowell and I say it's because the liquid has not equalized. In fact I think the liquid would probably take a couple of weeks to reach saturation temperature. If we fill the tank once or twice a day I'm not sure that it will ever reach the temperature associated with 70 psig.
Any thoughts?





RE: Equilibrium??
If a tank that large is not stirred, it can stratify with a colder more dense fluid at the bottom. I've even seen in mixed hydrocarbons a statification. I've even seen non-condensibles come out of solution and raise a storage tank pressure in hydrocarbon service.
RE: Equilibrium??
RE: Equilibrium??
RE: Equilibrium??
For the sake of argument, let's say your 27,000 gallon LIN tank has an NER (natural evaporation rate) of 0.5%. It may be higher or lower than this due to age or other performance factors, but for a first stab, I’ll use this value.
A 0.5% NER on a 27,000 gallon tank corresponds to a heat leak of roughly 3250 Btu/hr. If LIN is saturated at roughly 20 psig and then pressurized to 70 psig (which results in the fluid subcooling) then assuming the tank is half full (say 14,000 gallons = 94,000 lbm of LIN) the temperature rise rate is aproximately 0.07 degrees F per hour.
If you’re refilling this tank once or twice a day, you’d be hard pressed to measure any significant difference in the temperature over any period of time.