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Brewing Topics => Brewing Discussion => Topic started by: switzead on February 11, 2010, 08:06:58 AM

Title: Evaporation Rates
Post by: switzead on February 11, 2010, 08:06:58 AM
I have been getting very high evaporation rates so I did a little experiment the other day.  I put 7 gallons of water in my pot and cranked up the heat.  It took a little while to get that much water from approx. 70 degF to boiling, probably 45 minutes.  After it began boiling I let it go at full heat as if I were brewing for 15 minutes then measured my volume.  Based on that volume reduction in 15 minutes my evaporation rate was 28% per hour, VERY HIGH, right?  I continued to let it boil for another 15 minutes, measured, then another 15 minutes, measured and finally for another 15 minutes and measured.  The evaporation rates for the last 3, 15 minute intervals was on average 15% per hour.  This tells me that I am loosing liquid during the 45 minutes it takes to get to full boil.  That all being said, is it ok to leave the lid on the brew pot until it comes to boil and then remove for the boil time.  I know you are supposed to boil with lid off but I have never read anything about covering until you get to the boil.  Thanks.
Title: Re: Evaporation Rates
Post by: UselessBrewing on February 11, 2010, 08:24:29 AM
Technically speaking you should you should leave the lid off. However it may not make much of a difference at that point and may decrease the amount of time to get the boil going.

You would want to leave the lid off because the steam is expelling unwanted compounds like oxidation on the equipment and Dimethyl Sulfides (DMS). But these are more prevelant when you have a full rolling boil.

So I'm on the fence on this one.

Put a lid on it and see how much faster it begins to boil than your previous test. You may be saving propane/gas by putting a lid on.

Anyone else have any thoughts on this?

Title: Re: Evaporation Rates
Post by: BobBrews on February 11, 2010, 08:47:57 AM
Time for another experiment. How long does it take to bring 70 degree water to a boil covered? and then uncovered? If it only saves 5 minuets than is it worth chancing (DMS)? DMS leaves with steam but does it leave with vapor before it's visible? Droplets forming on the cover drip back in. does DMS recirculate or change to something else?
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Title: Re: Evaporation Rates
Post by: MaltLicker on February 11, 2010, 07:20:10 PM
I do heat my strike and sparge water in the boiler, covered, to expedite heat-gain and reduce evaporation of the plain water.  I use cheap 12" foil pizza trays.  3-pack is like $2. 

But from the mash run-off it should be ~140F, correct?  Once the boiler is full of wort, I do cover until it starts to foam over, and then I remove it to eliminate risk of boil-over.  DMS is supposedly created above 140F, so any I create up front is boiled off over the next 90 minutes.  I do not cover again until it is fully cooled to pitching temps and I've whirlpooled. 
Title: Re: Evaporation Rates
Post by: CR on February 12, 2010, 10:16:37 AM
My range top only puts out so much heat. It's a reall nice dual burner Wolf but I'm using LPG.
 I takes me about 20 - 30 minutes to get 6 or so  gallons of wort to a full boil.
But the wort is already at mashout so it's about 175 Degrees F (less some for the handling)  already.
 I  leave the lid on while I'm getting it up to boil and then I usually have the lid just sort of mostly covering the pot with maybe a 1.5 inch gap.

Over an  80 minute boil I will lost a lot of water.   I accept the loss because I do my primary in a glass carboy and I need the head space.
Then when racking to  secondary I add more boiled water.  It's usually about a gallon or  1.5 gallons I need to add.
I find that I am very happy with the result and the addition of the extra water at racking does not harm the product.