How much BTU is needed ??

Question

So if a vat has 200 gallons of water how much BTU is needed to sustain the water at 140 degrees for 240 minutes??

Answer 1

The amount of energy needed to SUSTAIN the water at temperature depends on the shape of the tank, construction material, insulation on the tank, temperature in the area of the tank, wind speed in the area of the tank, and in unusual cases radiative heat loss might be significant.  In vented tanks evaporative heat loss might matter.

Think about it.  To sustain the temperature, you just have to add any energy that is lost.  How much is lost from the vat depends on all the cooling effects.  If the tank is very skinny and tall, then there is a lot of surface area for heat loss.  If it is very cold around the tank and the wind is blowing that will cause lots of heat loss.

To heat 200 gallons of water to 140 F (from say, 40 F) is a straightforward calculation (assuming no heat loss while heating).

200 gallons * 8.34 lb/gal * 100 deg F * 1 BTU/lb-F =

about .2 million BTU

There are techniques for estimating heat loss from tanks.  At the very least you need tank outside surface area and the outside temperature (in addition to target temperature and total gallons inside).

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After reading your additional information it seems that you only have three choices.

1.  make some guesses about heat loss.

2.  make a measurement (heat it up and then give it an hour or so with no heat added, figure the heat loss per minute and go from there)

3.  get tank geometry information and make an estimate based on surface area

Answer 2

Unknown. You have a heater keeping the temperature constant against external losses, but I don't know what the losses are.

Tell me what all the heat losses are and it's possible to do the calculation, otherwise not. all you can do is measure the power into the heater and calculate it that way.

Answer 3

The relation is Q = m * Cp * dT

Q = Heat (BTU)

m = mass flow rate

Cp = Specific heat of water

dT = Change in temperature