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Brew Kettle BTU Calculations

BTU Losses during Brewing

A lot of customers have asked over the years how many watts of power they need for their electric brewery. I've been able to give estimates based on the actual BTUs needed to heat water + 30% for loss, but my answer has never been exact. So, when I finally sat down and calculated real world losses I decided to use a converted beer keg or keggle because they make a great brew pot & a lot of 10 gallon brewers use them. Keggles hold 14.5 gallons of beer which makes them the perfect size for a 10 gallon batch.

To calculate BTUs needed and heat loss we need to start with the basic stats:

30# - Weight of a empty beer keg
16.5" - Keg diameter
23.3" - Keg height
12 gallons - Batch size these calculations are based on
0.12 BTUs - to heat one pound of steel 1 degree F
3.6 BTUs - to heat the keg 1 degree F
8.34 BTUs - to heat one gallon of water 1 degree F
1.5 sq ft - Water surface area of keg
8.38 sq ft - Side surface area of keg
1.5 sq ft - Bottom surface area of keg
3.41 BTUs - Heat generated by one Watt/hr of electricity

Then I found these standards in a published Engineering table:

This table lists BTU/hr loss per square foot in a steel tank with no insulation, assuming the temperatire outside the tank is 70F.

Assuming 70F Heat Loss in Btu per sq ft per hr
Temperature Evaporation Radiation Total Walls
90 80 50 130 50
100 160 70 230 70
110 240 90 330 90
120 360 110 470 110
130 480 135 615 135
140 660 160 820 160
150 860 180 1040 180
160 1100 210 1310 210
170 1380 235 1615 235
180 1740 260 2000 260
190 2160 290 2450 290
200 2680 320 3000 320
210 3240 360 3600 360

So, using basic stats & the Engineering Table I found, I can calculate heat loss in BTU / hr, assuming you are boiling 12 gallons of water.
The graph has the temperature across the bottom and the loss along the left side.
The first thing you probably notice is the fast growing yellow line - all the way to 9000 BTU / hr at 210 F.
This is also close to the BTU / hr you will need to maintain a boil.
The next thing you probably notice is the fast growing blue line - this is loss due to evaporation.
This also shows that keeping the lid on your hot liqour tank and boil pot will make a huge difference!

BTU Loss Through Top & Sides of a Converted Beer Keggle 

 

BTUs to Heat MASH Water

A typical 10 gallon grain bill for me is 20 pounds & I will heat 7 gallons of mash water to 170F and mix with my mash which will drop the temperature to less than 130F. I'll use my RIMS tube to bring my mash the rest of the way to 145F.

Using the weight of the keggle (30 pounds) + 7 gallons of water, I calculate about 61.98 BTU / hr to raise the temperature one degree F.
So, the total heat needed to raise the water and keggle from 60F to 170F should be 6818 BTU / hr.
Assuming no loss, here are the times for each heating element wattage to heat 7 gallons water to initial strike temperature.

Time to Strike Temperature

But let's look at the same graph with heat loss added in.
This chart is with the keggle lid off - the heat loss is about half with the lid on.
You can see that heat loss affects the lower wattage elements the most.
Almost no time is added to the 5500 Watt Element and the 1650 Watt Element is pushed out to 140 minutes!
In other words, you don't want to use a 1650 Watt element for 10 gallon batches.
And at 90 minutes you probably don't want to use a 2000 Watt Element either.

Time to Strike Temperature with Heat Loss

BTUs to Heat Sparge Water

A typical 10 gallon grain bill for me is 20 pounds & I will heat 9 gallons of sparge water to 170F to rinse the sugars from my grain bed.

Using the weight of the keggle (30 pounds) + 9 gallons of water, I calculate about 78.66 BTU / hr to raise the temperature one degree F.
So, the total heat needed to raise the water and keggle from 60F to 170F should be 8653 BTU / hr.
Assuming no loss, here are the times for each heating element wattage to heat 9 gallons water to initial strike temperature.

Time to Sparge Temperature 

But let's look at the same graph with heat loss added in.
This chart is with the keggle lid off - the heat loss is about half with the lid on.
You can see that heat loss affects the lower wattage elements the most.
Almost no time is added to the 5500 Watt Element and the 1650 Watt Element is pushed out to 185 minutes!
In other words, you don't want to use a 1650 Watt element for 10 gallon batches.
And at 120 minutes you don't want to use a 2000 Watt Element either.

Time to Sparge with Heat Loss 

BTUs to Reach Boil

For a typical 10 gallon I usually cut off my sparge at 12 gallons. This gives me enough volume for a 60 - 90 minute boil.

Using the weight of the keggle (30 pounds) + 12 gallons of water, I calculate about 103.68 BTU / hr to raise the temperature one degree F and my starting temperature is usually about 150F
So, the total heat needed to raise the water and keggle from 150F to 212F (boiling) should be 6428 BTU / hr.
Assuming no loss, here are the times for each heating element wattage to heat 12 gallons water from 150F to boiling temperature.

Time to Reach Boil from 150F 

But let's look at the same graph with heat loss added in.
This chart is with the keggle lid off - the heat loss is about half with the lid on.
You can see that heat loss affects the lower wattage elements the most.
And the 1650 Watt & 2000 Watt Elements never reach boil!
At about 174 degrees F for the 1650 Watt & about 187 fedrees F for the 2000 Watt Element the heater just keeps up with heat loss.
Ten minutes are added to the 5500 Watt Element and 20 minutes are added to the 4500 Watt Element.
The 3500 Watt Element is pushed out to 75 minutes to boil.
In other words, you can't use a 1650 or 2000 Watt element for boil.
You probably don't want to use a 3500 Watt Element either.

Time to Boil with Heat Loss 

My Take on This

I mentioned earlier that most of the heat loss is through evaporation. Go back to my BTU Loss graph and you'll see that evaporation loss becomes the largest heat loss past 180 degrees F.

Keeping the lid on your hot liquor tank and keeping the lid on your brew kettle until almost boil eliminates almost all of your evaporation heat loss.

The next step would be to insulate your hot liquor tank and brew kettle to reduce heat loss.

And finally, it's impossible to brew 10 gallon batches with a 1650 or 2000 Watt element and time consuming to brew with a 3500 Watt element. But a 4500 Watt or 5500 Watt element will work great.

 

 

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12021 South Memorial Parkway, Suite N-4
Huntsville, AL  35803
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