Heat Transfer into/out of an open topped tank

[friartuck]

Hi folks

I've posted this in the heat transfer forum as weel to see if they can help. I hope somebody can.

I have what I think is a fairly difficult problem (but I'm hoping you will find it easy)

I have an open topped tank (contains fish in a sea life centre)

The tank I understand is at about 15C and the space varys in temp from 15 to 27C.

The first question is, what will the heat transfer be between the two 'systems'.

I understand that the heat transfer through the tank wall could be simply:

UxAxDT i.e. thermal transfer coefficient x area x temp diff

alternatively I could use tables to give me heat transfer from plane surfaces and look up the heat transfer by convection and by radiation and add them together. (and if my sums were right, I hope the answers would be the same)


BUT, my next question is, what about the actual water surface. What heat transfer will I get there. Can I use the same heat transfer calc metods or is there some evaporation effect to take into account??

I think there are two concerns:

a. What effect is had on the tank i.e. how warm will it get or how cool will it get (the heat gain/loss will need to be catered for by a chiller circuit)

b. What effect is had on the space where the visitors occupy. Will they need extra heating or cooling to cater for the heat transfer.

I'm sure this is a fairly simple question to some of you, after all there are thousands of swimming pools out there


Answers on a postcard please

thanks in anticipation



Friar Tuck of Sherwood

 

[walkes]

Friar Tuck,

I think you may have a considerable radiant affect on the visitors to the space. Radiant heating panels may be a good idea to avoid increasing the air temperature around the tank and increasing the evaporation rate on the water surface.

But for maintaining the pool I would refer to the ASHRAE Applications handbook. It will give you the appropriate calculations for the evaporation component.

 

[TBP]

There's a residential/commercial steam & hot water boiler company in North America called Burnham. They put out a small publication called the "Burnham Heating Helper". (It's a pretty handy little pocket book.) They have a small section on swimming pool heating, and it includes a table. The delta-T is the temp diff between the air, and the water surface.

Delta-T in *F 10* 15* 20* 25* 30*
BTUH/sq ft 105 158 210 263 368

It looks like the worst case is your tank picking up about 210 BTU per hour per square foot of water surface. (The table is for in-ground pools, so the heat transfer through the sides is pretty minor.)

There are some other factors for wind velocity, but they won't apply to your indoor tank.

 

[friartuck]

cheers tbp

Friar Tuck of Sherwood

 

[friartuck]

One thing has occured to me.

If the room is 27C and the water 15C, using the 200BTU/hr per sq ft or about 630W/m2, this means that the size pool we have (36m2 water surface area) will gain 36x630=22680W---which does seems a lot.--after all radiators have similar heat exchange rates when using water at 75C and the room is at 20C giving a DT of 55C. We only have 12C differential.

Evaporation Effect
But what about the evaporation effect? The water may be picking up heat from the space (trying to warm the water) but as the water evaporates, won't this also have a cooling effect? and if so, what would it be?

Maybe this question is unanswerable!

Friar Tuck of Sherwood

 

[quark]

Friartuck,

You are right. Actually heat transfer takes place by convection, radiation and evaporation. You can approximate convection heat transfer coefficient assuming the pool as flate plate. There are simplified correlations for hot and cold plate with respect to laminar and turbulent flows and Holman discussed this in detail.

Dr. Willis Carrier worked on evaporation rates from open water sources with parallel and transverse air flows and this data is available from Dehumidification Handbook by Munters.

If you describe the system and its configuration in detail then I may be able to help you with approximate data and equations.

Regards,