Enclosed hypercaric test vessel, chiller problem

[ginsoakedboy]

I am performing a preliminary thermal design for a test vessel.

The test vessel is required to have (sea)water maintained at 35°F. The test pieces will be suspended in this vessel to study the effect of long term exposure on coatings. The test pieces will be maintained at an elevated temperature ~300°F. Maintaining the seawater around this test piece at 35°F will give rise to a heat load of approximately 50,000 W.

There will be chiller-coiled tubing system with 30% glycol solution as the working fluid to take the heat away from the test vessel.

I have written down the equations for natural convection heat transfer from the test piece to the seawater. And, also calculated the OHTC (U) for the chiller tubing immersed in the seawater.

Assumed inlet temperature of chilled fluid to be 25°F and the exit temperature to be 50°F.

Also, the heat balance for the chiller fluid is set up.

My problem is that I cannot determine a good way to tie up the heat load to the heat being transferred away by the chilled fluid. The interaction between the ambient seawater and the chiller tubing is very complex involving natural convection, tubing geometry, and variable temperature differences.

This case is not a textbook heat exchanger so I cannot plug the traditional heat exchanger equations. Additionally, the seawater will be pressurized to around 8000 psi, so the chiller tubing cannot exceed 1" OD in size because of collapse loading concerns. This will constrain the maximum flow rate through the chiller tubing.

Can anyone guide me with the way to proceed? I need to determine amount of heat chiller-tubing system will take away and if I can even maintain the temperature of seawater at 35°F.

 

[MintJulep]

How do you get a 50F chilled fluid exit temperature with a 35F tank temperature?

That's just not going to happen unless you allow significant stratification of the seawater.

 

[ginsoakedboy]

I stand corrected. That was my initial assumption (as I mention this is all only on paper currently).

Say, the exit temperature is changed to 34°F. How should I proceed in that case?

 

[ChasBean1]

You already know the heat removal (your Q), so you should be able to just use:

Q = rho V Cp dT

Q is 50,000W (or 170,610 BTU/hr)
rho is about 66.8 lb/ft3 at that temperature
Cp is about .9 with the necessary glycol mix
and dT is 9°F (25 in, 34 out)

So you need 39 gpm of 25°F water. You might want to bump that up to a capacity of 60 gpm or something considering surface fouling, esp. with sea water.

 

[ginsoakedboy]

The exit temperature was modified for calculation purposes based on the previous reply.

The largest chiller tubing I can install is 1" OD x 0.7" ID. Will that be able to flow at 39 (or 60) GPM?

 

[ChasBean1]

You're right. That's screaming. 33 feet per second; typical upper design limit might be about 8 fps (~10 gpm limit). You can't have a larger inlet pipe and header it into a few 1" pipes?

Alternative is a higher delta T, but it would be difficult to have cooler entering water.

Possibly a liquid N2 application?

 

[ginsoakedboy]

I modified my calcs to change the entry and exit temperatures to 18F and 34F resp. I have to consider 45% solution of glycol in water for this.

Also, I have abandoned the idea of bent tubing in favor of fabricated 1.5" or 2" pipe.