Heat Calculation

[alwilson]

Afternoon All,

I'm still struggling to maintain a stable and appropriate fermentation temp, I have an aquarium heater, and also the FV is in an insulated cabinet. Outside temperature is -1.

My problem is the heaters get it up to 18-19C and switch off, but the temp keeps on climbing, I presume through the thermal activity of yeast.

Now if I open the cabinet doors, the heaters struggle to get wort to 18/19.

So ultimately the cabinet is too energy efficiency, I'm going to remove the doors.

Does anyone know how I can calculate what size watt heater(s) I need to raise a given amount of wort above ambient temperature.

My heater is specced for 200L but I presume thats to get 200L of fish water upto the selected temperature for a much higher ambient temperature than -1

So, im after a formula please, that I can plug in Outside Temp, Volume of Wort, and desired Temp and hopefully it will tell me what kind of wattage I need to maintain this.


Cheers
Alex

[gregorach]

My problem is the heaters get it up to 18-19C and switch off, but the temp keeps on climbing, I presume through the thermal activity of yeast.

Steady on there... With a simple on / off control, there's always going to be an overshoot - just how much depends a great deal on sensor placement. How high does the temperature get?

[alwilson]

Overshoot i can deal with Dunc, but the last beer that went down the drain was 32C and so warm that it was creating a very small vortex in the FV from the yeast and trub movement... was a delight to see until I had to drain it.

[themadhippy]

im after a formula please, that I can plug in Outside Temp, Volume of Wort, and desired Temp and hopefully it will tell me what kind of wattage I need to maintain this.

nearly,you need to add in the time required to raise the temperature
E = m × c × θ
E is the energy transferred in joules, J
m is the mass of the substances in kg (1 litre of water weights 1 kilo)
c is the specific heat capacity in J / kg °C
θ (‘theta’) is the temperature change in degrees Celsius, °C
unfortunately there aint no c value for wort,waters 4181. That will give you the energy required,to convert to watts, P(watts ) = E(joules) / t(time in seconds)

[alwilson]

Cheers Hippy!

That kind of answers my question, well no it absolutely answers my question, but whilst going through your calcs, it raised another question.

Your above formula, suggests, for 200L of water, to raise it 18 degrees requires 830 watts of power (for 5 hours).

If im pumping wort (or water) into the FV at 17 degrees, in an atmosphere of 0 to 10 degrees Celsius, what wattage is needed to maintain 18-21

Though for that I presume I need to know what my heat loss is to atmosphere!?

[gregorach]

Indeed you will, and that's where things get horribly complicated...

[alwilson]

Ultimately, what I want to know, is if 2x 300 watt aquarium heaters, will maintain fermenting temps for 200 Litres, if I remove the doors from said cabinet.

I guess the only way of knowing is to throw the things in and find out!

[Dr. Dextrin]

If you turn the heat off and measure how fast it cools, then that can be used to calculate the heat loss. That will be at one particular temperature, mind, so either (a) make the measurement at your target temperature or (b) make a correction by assuming the heat loss is proportional to temperature above ambient (a reasonable approximation that follows from Newton's law of cooling).