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treehuggerz (Civil/Environmental) (OP)
28 Mar 12 16:03
I have to calculate the heat duty for an immersion heater for a large water tank and am stuck.

The tank is 200,000 gal. The flow rate through the tank is 5000 gpm (so 40 minute retention time); the inlet water temperature is 32 F. The water temperature in the tank and outlet water temperature need to be raised to 40 F.

I can figure out how to calculate the tank heat loss but how do I calculate the heat duty to heat the inflow into the tank? I want to take credit for the heat transfer between the water already in the tank at 40F to the incoming water at 32 F, but am not sure how to proceed.

Thanks.
 
Helpful Member!  hydrae (Mechanical)
28 Mar 12 20:20
The heat required is based only on the delta T, flow rate, and losses, the volume inside the tank can be large or small it will make no difference once the process is steady state.

A smaller volume will have a short start up time but will have zero effect once the heating is steady state.

The definition of a BTU is the amount of energy to heat one lb of water 1°F, so at 5000 gpm at 8° delta T you will need 333,600 btu/min plus losses. (8.34lb/gal)

You will also need 13,344,0000 BTU for start up

Any credit you take for the water inside the tank heating the water coming into the tank needs to be replaced to reheat the water already in the tank since it gave up some of its heat to the incoming water

Hydrae


 
treehuggerz (Civil/Environmental) (OP)
29 Mar 12 14:02
Thank you. That helps a lot.

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