(batch calculations)The most efficient way to batch sparge is to collect 2 equal quantities of wort from the mash tun so to determine the total quantity of wort you must collect from the mash tun you will first need to know how much you want to end up with in the fermenter. (for example 19L/23L/50L) In the following explanation I will use the example of a 23L brew length.
To collect 23L of wort
in the fermenter it makes sense that you will need to start with more in the
kettle because you will loose some wort to evaporation during the boil and some
to the hops and trub in the bottom of the kettle when you run off.
I loose around 3 liters to the hops and trub, so I need to start with 26L even
before I boil. Only experience will tell you what you will loose, if you are
brewing a similar sized batch try starting with 3L, you can make adjustments as
you go along to compensate and make a correction next time you brew.
During the boil I will also loose wort due to evaporation, loss to evaporation is usually around 10% per hour so that's 15% in a 90 mins boil.
To find out what is needed at the start of boil to end up with 26L at the finish after 15% has evaporated, divide 26L by 85 then multiply by 100
ie (26/85) x 100 = 30.5L so we want to start the boil with 30.5L which means we need to collect 2 batches of 15.25L from the mash.
If you want to put it into an equation
Volume at Start
of Boil = 100 x (Vol
in Fermenter + Loss to Hops and Trub)
(100-Evaporation Rate%)
Now you know the volume you need to start with in the kettle to end up with 23L( in this case) in the fermenter, you also know the amount you want to collect when you sparge.
In case you missed it, it is 30.5L
*(the factor 85 comes from 100 minus the percentage you expect to loose ie 100-15%= 85. In another example, during a 60 min boil with a 10% loss to evaporation the factor is 90 instead of 85 )
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Sparging 2 (batch calculations)