Basic physics question and mashing

[RandyAB]

So I insulated my stainless mash tun with 2 layers of reflectix and I wanted to see how well it held heat. I transferred 10 gal of water at 156 deg and watched the temp drop over an hour. It lost about 2.5 degrees which I thought was pretty good. My question is will those numbers be better or worse or the same with the mash? I know conductivity is going to increase with density but will that translate into a bigger temp drop over time with mash vs plain water?

 

[BigEd]

I'm guessing the added mass of the mash will probably improve the heat loss numbers.

 

[joex444]

It depends on the specific heat of barley. The pot loses a certain amount of thermal energy so to convert to temp loss you need specific heat.

I'm not sure what that actually is but water has a high specific heat so my guess is barley is lower meaning it would perform worse.

The net specific heat of your mash is the weighted average of the components.

 

[MaynardX]

My question would be how hot/cold was your mash tun before you transfered that water? I know that my keggle lost a lot of heat during a 5g batch in 25deg ambient temp. (condtions of my last batch of beer).
Anyone have any suggestions on how to insulate a keggle to keep this from happening reguardless of ambient temperature?

 

[RandyAB]

My question would be how hot/cold was your mash tun before you transfered that water? I know that my keggle lost a lot of heat during a 5g batch in 25deg ambient temp. (condtions of my last batch of beer).
Anyone have any suggestions on how to insulate a keggle to keep this from happening reguardless of ambient temperature?

I didn't start measuring the temp drop until the mash tun had stabilized after addition of the hot water. I added water at about 180 deg and it stabilized in the tun at about 156 deg. Ambient temp in the garage is 41 deg.

The answer I really want is will the mash perform better in terms of heat loss than pure water? I guess I'll find out definitively when I do my first batch in the next week or two.

 

[erkwist]

Your mash should hold heat better than that just due to the added thermal mass of all the grain. the grain will hold heat better than water.

 

[theredben]

Just to make sure, are you preheating the tun with a quart of boiling water for a few minutes?

 

[Brewing Clamper]

All things being equal, in a physics way of speaking, the grain does not have anywhere near the heat capacity of water; therefore, it will lose heat faster than just water...

 

[erkwist]

Heat capacity is not the issue, conductivity is. Adding grain will help the water hold the heat better.

 

[Brewing Clamper]

If you have the same volume in both scenarios, when you add grain you actually have less water, therefore less heat. The grain does not hold heat better than water. Technically speaking, with the same volume of water vs. water+grain the water alone would have more heat than water+grain assuming you're not heating up the grain to mash temps.

 

[MalFet]

Your rate of heat loss is determined primarily by the surface area of your tun and the conductivity of the tun relative to the absolute quantity of heat capacity of the contents of the tun.

A lot of this question depends on how you picked the volume in your test. I am assuming you preformed the test with a volume of water equal to your strike volume, not equal to your strike water plus your grain. I think others are making the opposite assumption, and they're right if you are doing it that way. When you add grain, the volume of water in your tun doesn't change, but the volume of air does. Adding grain will slightly increase your heat capacity.

That said, the addition of the grain will be a relatively small change. A bigger factor in planning your temperature schedule will be the time spent stirring...presumably you didn't do that with the water, and if your system needs that you'll lose some heat there.

 

[Northcalais40]

Theories abound, time for the experiment. I'm with Malfet.

As an aside, you (the OP) may have done this, but with reflectix you get better insulation with an air space between the layers. I do this by duct taping a thin strip at the edge, and down the middle. Basically doubling the thickness with a 1" strip. These standoffs give you an air space and better R-value.

 

[Northcalais40]

Forgot: You could also make a hat for the mashtun. Cut a disc out of reflectix (or rigid foam if you can eliminate the foam crumbs) and float it on top. I bet it would make a huge difference. I have a chunk that I use as a thermometer boat so I can drop the therm. in and put the lid back on and check it after it stabilizes.

 

[RandyAB]

Thanks for the replies fellas. Most of the answers deal with how much heat is in the mash tun at the beginning whereas I'm really interested in the rate of heat loss for whatever heat was there no matter what its initial magnitude.

Malfet. The volume I used was about 10 gal of water which is water + grain volume during the mash.

Northcalais 40. Thanks for the tip on Reflectix.

As an aside, I left the water in the tun overnight and 15 hours later it had lost 37 degs or 2.5 deg/h which seems fairly constant and consistent with what was observed the first few hours. The rate of heat loss as measured by temperature decline seems to be linear and independent of temperature (absolute amount of heat) in the tun at any particular time.......shheeeesh I need a homebrew

I'll repost when I do my first mash and let you know how the numbers compare.

 

[pvtschultz]

The grain will act like the fibers of fiber glass insulation and the water will act like air in this scenario. What hold heat in better, a wall with no insulation or the same wall with insulation? The answer is pretty obvious and is the reason that we fill the spaces of home walls with insulation. By preventing the air (water in this case) from moving due to natural convection, the heat transfer rate of the tun turning mashing will be lower than it was during your experiment. Go ahead and give it a try and try to prove me wrong. What's the worse thing that could happen, you make a batch of beer?

 

[RandyAB]

What's the worse thing that could happen, you make a batch of beer?

Mmmmmmmm........how true.

 

[MalFet]

Thanks for the replies fellas. Most of the answers deal with how much heat is in the mash tun at the beginning whereas I'm really interested in the rate of heat loss for whatever heat was there no matter what its initial magnitude.

I got that you are interested in heat loss, but the rate of heat loss is determined in part by the amount of heat in the vessel. It's not possible to talk about one without the other. pvtschultz makes a good point that indicates why this is the case.

In any case, you'll be fine I suspect. I use a direct fire rims, but in reality I barely need to turn it on. I'm sure I'd get almost equivalent results if I just let it sit.

 

[RandyAB]

I...but the rate of heat loss is determined in part by the amount of heat in the vessel.

If the rate of loss is dependent on the amount of heat in the vessel then why is the decline in temperature linear? If the rate changed as heat was lost then decline would be a curve when plotted as temp vs time.

It doesn't really matter. I think that the mash will hold heat better than the water alone as pvtshultz suggests. Unfortunately I won't find out since I'm not about to leave my mash sitting in the tun for 12 hours......

 

[MalFet]

If the rate of loss is dependent on the amount of heat in the vessel then why is the decline in temperature linear? If the rate changed as heat was lost then decline would be a curve when plotted as temp vs time.

This is becoming increasingly moot to the issue at hand, but temp/time definitely plots as a curve.