Tank heating 2

[Powerman81]

Hi all,

I need to prove theortically that natural convection will occur inside a water storage tank. The tank specs specs are
hight : 70 ft
Diameter: 27 ft
capacity: 285,000 gallons
Constant pressure isdie tank

The tank has 6 elecrical heaters (Thermostatically controlled). The heaters and thermostate are positioned horizantally 60 degrees apart at 6 ft above ground level. Temprature indication is provided is provided by a gage at height 1.5 ft above ground.

The problem is, when the heaters are energized natural convection occur in the volume above the heaters only while heat is trasfered to the volume below the heaters by conduction leading to temprature stratification in the tank.

I proposed a change to turn 3 heaters on(Cheapest way) to allow for elleptical circulation in the tank to decrease or eleminate temp stratification.

How can I prove this is true theortically without the aid of a CFD software. Or is there any other suggestions

I would apperciate any help

 

[IRstuff]

I don't see how you're going to get "elliptical" circulation with 3 heaters on.

Seems to me that you'd get better action by enclosing the heaters with open cylinders that extend downward, to force the displacement current to come from the bottom of the tank.

TTFN

FAQ731-376

 

[Powerman81]

IRstuff,

When heaters are energized, expansion of the water at the point of heat input casues the warmer water to rise along the heated half of the tank (where the 3 heaters are energized). As it rises it mixes with cooler water from the uneated half. Cooler water will descend along the unheatd half passed the unenergized heaters casuing an elliptical flow POSSIBLY.

With regard to your option of enclosing the heaters with open cylind... This would cost lots of money and displacement current would only extend a foot or more below the heaters. I need to go with the most econmoic option.

 

[Compositepro]

Natural convection will heat the water above the heaters. Switching on three rather than six heaters will not significantly change the situation. You must lower the heaters or, as IR suggested, install baffles so flow to the heat comes from the floor of the tank. Baffling only one heater may be sufficient to take care of stratification.

 

[MintJulep]

Won't work.

If natural convection is your only driver, then the water below the heaters will always be colder and denser than the water at or above heater level.

Add a circulating pump.

 

[IRstuff]

Won't work. The water rising has lots of bouyancy, but the water coming back down has none, and it will come down slower than the hot water rising. That's why there's stratification in the first place.

Your only hope with a passive solution is to try and get a chimney effect out of the hot water rising. Otherwise, active recirculation is needed.

TTFN

FAQ731-376

 

[MintJulep]

The water rising has lots of buoyancy, but the water coming back down has none, and it will come down slower than the hot water rising

That's got to violate the conservation of something or other law.

It won't work because a volume of water around each heater will get warm, become less dense the the water surrounding it, and move upwards.

The water surrounding that volume also gets warmer, and moves up. Etc. etc.

As each slug of warm water moves up, it transfers heat to the surrounding water, or at the surface to the air, or at the tank walls, to the wall and eventually to the outside.

Eventually each slug of water will become colder and more dense then the water around it, and start moving down.

As it moves down it gets warmer.

Since any slug of water starting out near a heater can never get colder than its initial temperature it will never sink below the heater. Stratification.

 

[Powerman81]

Thank you all,

 

[Compositepro]

Baffles around the heater can create a thermosiphon ("chimney effect"). Hot water will rise in the chimney and draw in cold water from the bottom of the tank.

 

[MintJulep]

Oooh, nice Compositepro.

 

[JoeTank]

"Thermosiphon" Dude, did you make that up? Cool word.

Joe Tank

 

[chicopee]

I don't believe that you do not get convection below the heaters. Perhaps not as effectively as above the heater but nonetheless you will get convection below the heaters.
To get only conduction thru water, the water depth will be a very shallow one.

 

[IRstuff]

http://en.wikipedia.org/wiki/Thermosiphon

TTFN

FAQ731-376