Sizing a Heating Element by Batch Size

I came across some info when reading through a hot water heater element guide that might be helpful in determining a heating element size for an electric kettle.

Here is what I came up with:

((Batch Size x 8.25 x 1.0)(Temp Change)) *.293 = Element Wattage

Where:
8.25 = Weight of 1 Gallon of Water
1.0 = Specific Heat of Water
Temp Change = Difference in Tap Water Temp and Desired Temp (ie, boiling temp)

.293 is converting the product from BTU to Watts. This is based on 100% Efficiency and a 1 hour time interval.

Here is an example.
Batch Size 3 gallons
Tap Water Temp = 51 degrees
Boiling Temp = 212 degrees
Temp Change = 161 degrees

Anther way to look at it is:

(Batch Size x 8.25)(Temp Change)
_____________________________ x 60 Minutes = Time to Bring to Boil
(Element Watts/.293)


(3 Gallons x 8.25) (161)
___________________ x 60 Minutes = 56.37 Minutes to bring 3 gallons to a boil
(2500 Watt/.293)

Don't know about you numbers but one thing I did pick up on was you seem to be using metric and imperial units (eg boiling temp 101 degrees [centigrade] & 3 gallons)
Your conversion factor might sort that out (I dont think so though becuase it would need to add/subtract the offset between the two scales) but my suggestion would be to use deg f and check your conversion factor is still valid - or use all metric units since you want to calculate watts anyway. And just one more suggestion to make it easy, include the time in your formula as well that way you just stick in volume, temp rise and time then out drops the wattage.

Sayin that using my memory I have (in metric)
Cp = 4181.3
Volume = 11.3 litre = 11.3 kg (will be a bit more for wort)
Temp start = 68 deg C
Boiling = 100 deg C
Temp rise = 32 deg C
Element size = 1650 W

Watts = ( Cp . mass . temp rise ) / time(seconds)
or
Time(minutes) = ( Cp . mass . temp rise ) / ( Watts . 60)
Time = ( 4181.3 x 11.3 x 32) / (1650 x 60) = 15.3 minutes

So after all that you were pretty close... theoretically

5500 watts. DOne

samc said:

http://gnipsel.com/beer/software/calculators/electric-heat.xls

Click to expand...

Dang, that's a gem. Thanks