Liquid Withdrawal Rate 1

[TangoCleveland]

We have a 3600 gallon LOX dewar and we need to replace the pressure building coil. In the interest of sizing the coil correctly, I want to know how much LOX we need to vaporize to support a liquid withdrawal rate of 3 gpm at 30 psig for 16 hours. I'm getting stuck on mass and energy balances. Our existing coil is helically finned copper with a total heat transfer area of 507 sq. ft. The new coil is longitudinally finned aluminum with a total area of 62.5 sq. ft., and a capacity of 2000 scfh. There has to be a straightforward way to do this; I can get all the physical properties I need. Any suggestions?

Larry

 

[iainuts]

Hi Tango,
You would start by assuming the following.
1. The tank is at some pressure (30 psig).
2. Tank liquid is saturated at tank pressure. This gives you all the properties of the liquid.
3. Gas has some temperature. Could be superheated, and in actuality it will be. This is a variable which will need to be input. This gives you all the properties of the gas.
4. The liquid withdrawn and whose properties are defined by #2 must be replaced by the gas added as defined by the properties in #3. Don't forget to consider the amount of liquid withdrawn for use in the PB coil, though that amount is generally very small.

- Now you have the gas flow rate needed to maintain the liquid withdrawl rate. Compare that number to the capacity of the PB coil you have.

The spreadsheet I have to do this gives me values for gas flow between 100 and 250 SCFH, depending on what temperature you assume the gas to be at as it comes out of the PB coil and enters the tank.

You can get more sophisticated, and assume the gas temperature coming out of the PB coil is much warmer than the existing gas in the tank, and it then mixes. I've done that for similar situations and there isn't much difference in the answer so I generally neglect that variation.

 

[TangoCleveland]

iainuts,
Thanks loads for the perfect answer. You nudged me in the right direction. Funny how we forget things we don't use very often, eh?

Larry

 

[iainuts]

No problem. Glad I could help.