conduction from under water

[cdxx139]

http://www.eng-tips.com/viewthread.cfm?qid=388496

Hi I am cross posting from HVAC from the advice of a fellow engineer.

I am looking to calculate the heat loss of a dry dock in water to heat the inside the dry dock, when the OAT = 25 degF, the water is 50 degF and indoor conditions are to be heated to 60 degF. I calculate the Q=UAdT and was told that may not be the right equation because I have sea water fluid on one side of the steel and air on the other, where my equation is for air on both sides?

So I opened up the old thermo book, and was looking at conduction, but the k value is looking at the steel material. So it doesnt care what fluid is on either end.

Or should I be looking at convection? I am treating it as steady state because the dry dock isn't moving, but since the waves are, maybe I should calculate the convection as well?

Just looking for some guidance.


knowledge is power

 

[georgeverghese]

What is OAT ?

Can we post a simple sketch with relevant details?

 

[crshears]

I'd ASSume OAT means outdoor air temperature...

My instincts say to include air convection currents within the controlled environment, conduction through the drydock steel plating, and seawater convection currents that will develop concurrent with the delta T of 10°F between the seawater immediately proximate to the drydock hull and that further away...and these currents will be present whether there is wave action or not.

BTW what scheme of portable roofing is in place to seal the conditioned space around the hull from the ambient atmosphere above?

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[Compositepro]

This question was first posted in the HVAC forum. If any one wants to contribute please post over there.

 

[MintJulep]

Q=UAdT has the in-built assumption of convection to or from air on both sides of the material in the middle.

You will have:
Convection from air to steel.
Conduction through steel.
Convection from steel to water.

Convection will either be forced or natural, depending on relative movement between steel and fluid.

 

[IRstuff]

There is a subtle difference in the formulation of the equation between convection and conduction:

k*A/L for conduction -- since conduction is a bulk phenomenon wherein length is a critical parameter; k is treated as a constant, but is somewhat dependent on temperature

h*A for convection -- since convection is a surface phenomenon wherein length is essentially fixed by the convective media as a the depth of the the boundary layer; h is highly dependent on geometry and media velocity. Natural convection is the case where there is no external forcing of velocity, and it's solely dependent on geometry and buoyancy of the hotter media relative to the cooler media.

In any case, your problem statement seems to be incomplete. Is the dry dock completely enclosed? Are the heaters in direct contact with the walls? Are there circulation fans? Are you trying to heat the walls or the enclosed space?

Assuming the latter, then you have
heater --> convection (forced?) --> walls
-->(1) conduction through wall --> convection into water
-->(2) conduction through wall --> convection into air

TTFN
faq731-376

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[cdxx139]

thank you for the suggestions, and I apologize for the cross posting, I thought it was ok, if people were aware of it, i will let the HVAC group let them know it will be continued here.

OAT = Outdoor Air Temperature. I can work on a detail if still not legible.

The dry dock is completely enclosed. below and sides with water, and above with ambient air. I am concerned with the hull parts under water.

It seems I need to focus on the convection between the water and steel, because the equation used does not do that.

knowledge is power

 

[IRstuff]

Well, you appeared to be posting am HVAC question, but HVAC is typically narrowly focused on specific applications involving buildings, and your boundary conditions wandered off into the heat transfer realm, hence the suggestion that you concentrate here. Obviously HVAC is basically heat transfer, but it's specialized. See:

http://www.engineeringtoolbox.com/convective-heat-transfer-d_430.html

for a start. Note that rather gigantic range in natural convection transfer coefficient for water/liquids.

http://s3.amazonaws.com/suncam/npdocs/119.pdf

http://www.tlv.com/global/TI/steam-theory/overall-heat-transfer-coefficient.html

There's lots of sites that discuss liquid convection.

TTFN
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[chicopee]

About presenting a sketch of the dry dock cross section in water so that we can advise on a course of action.

 

[cdxx139]

Ill do that, thank you for the interest

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[cdxx139]

Hello all, I have attached a sketch of the floating dry dock section I am heating to 60 degF.

I am still looking through the references provided above.

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[IRstuff]

Almost all the way there... How is it heated; space heaters? fans? air speed?

TTFN
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[cdxx139]

Yes, we are planning electric unit heaters. The space is 80 feet (into the page from the sketch provided). The unit heaters have fans that will throw the air 50 feet, so I will have two on both ends (for a total of 4 in each section), so the airflow is parallel with the 80 foot sections, and it creates an rectangular circular pattern around the 80 foot by 16 foot wide space. The unit heaters usually push out air at 1500 fpm (from memory)

Thnak you for your interest.

knowledge is power

 

[chicopee]

It is still an incomplete sketch as the walls of the dry dock have no thicknesses and no properties such as concrete or steel panels. Also is the dry dock opened at the top to atmosphere? Will there be boats within the flooded dry dock?

 

[racookpe1978]

How are you keeping the outdoor air from circulating around the (submarine?) in the drydock.

When gluing the USN rubber anti-acoustic tiles on the nuke subs in the mid-80's, we had to enclose the scaffolding with plastic/cavas sheets to eliminate the humidity (and keep the rain and snow off) from ALL areas around the hull to get close to the required air temperature.

Heat transfer through dry dock walls - They're Hollow, empty Tanks about 6 feet across with very very little webbing and ribs bridging the two walls completely! - is water to outside tank wall to static air moved by natural convection (inside the tank) to inside tank wall to static air (inside the dock) across the dock "air space" (also only natural convection moved) to hull-of-submarine (or boat).

You CANNOT make this scheme work if you are not closing off both ends of the dock and covering the entire heated dock air space. Been there. Been tried. Doesn't work.

 

[cdxx139]

Thank you Chicopee, there walls are 1/4" steel. I am heating the area and it is completely enclosed in steel and watertight.

Racook - sounds like you have worked on these. THis is my first, so I still working out the lingo. I am heating the safety deck of a floating dry dock. It is water tight and surrounded by steel. From the sketch, a portion of it is submerged in water, and a portion above water.

We are not heating the dry dock portion where the submarine or ship is, that will be open to the elements on the Gulf of Mexico.

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[racookpe1978]

Gulf of Mexico eh?

Clarify (redraw!) your sketch of the drydock.
Hint: Water level, empty tank wall thickness of the ""dock" - figure the steel itsel is only 1/2 to 5/8 thick nominal. But, from outside wall to inside wall on each side of the dock = ??? feet.

Yes, both ends of the dock may be closed off, more often, the dock when floated carrying the sub or boat inside, has on ly the lower connecting flat bottom in the water. Very, very little time is spent sitting with the whole dock submerged. This lets the dock be kept clean (no marine bio's to muck the inside with and cover all of the parts and plates) and be available to move the dock blocks and rails for the next ship to come in.

In the Gulf, I do NOT believe you will have a heating problem, but a massive steel wall-sitting-in-95-degree-humid-air-with-sunlight-on-steel-heating-the-plates-up-problem 9 months of the year. And 3 months of bitterly cold 30 mph winds at 30 degrees freezing the steel plates the rest of the time.

 

[cdxx139]

I drew the port dock only. It is 16 feet wide. The (starboard?) dock is 200 feet away and identical (mirrored).

I am realizing that the amount of time submerged is minimal.

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