Yet another fermentation cabinet

I've been reading the forum for a week or two, with particular interest in this section (I love DIY projects), even built the 2-gallon mini-MLT detailed on here a few days ago, so I thought I'd say hi and post my latest project.

Inspired by some of the fermentation cabinets I've seen on the web, I built my own over the past week.

It's about 17"x17"x36", and is designed to hold a single bucket or carboy, with a couple of containers of ice in the bottom section (probably 2-liter soda bottles). There's a spot for an 80mm computer fan between the two compartments in the back right corner, and some air holes at the other 3 corners. I'm still working on a temperature controller for it.

The latch pulls it shut pretty tightly against the weather strip so it should be moderately well sealed. All the foam is 1-1/2" thick. I'm anxious to get the temperature controller running, brew a batch, and see how well it holds a temperature, and how often I have to change out the ice, etc.

There's a bit more detail on the construction (and some bigger pictures) at my website:
http://eegeek.net/content/view/68/39/

Comments/suggestions welcome!

Nice build. I was thinking of creating a nice cabinet too when I got around to building this.

Just asking:
Why did you put the chiller chamber on the bottom and make the shelf for the 40ish pound bucket?

joebou4860 said:

Just asking:
Why did you put the chiller chamber on the bottom and make the shelf for the 40ish pound bucket?

Click to expand...

A valid question; my rationale was that if I put the ice on the top I'd probably end up with condensation from the ice containers dripping down through the air holes onto the carboy/bucket. Of course that could have been worked around, and probably wouldn't have hurt anything anyway, but I decided I'd rather just be dealing with wiping up a puddle in the bottom compartment only, rather than having to wipe it up from the top compartment AND all over and around the carboy/bucket.

Plus, the shelf is really strong - I weigh about 170 pounds and I STOOD on the shelf (and even hopped up and down a bit) when I was building it to make sure it was strong enough, and that was BEFORE I put the extra brace in the center of the shelf, and I hardly even noticed it flex. I would have used the same materials (plywood and 2x3 studs) even if I'd made it with the ice on top, because I already had the plywood and the studs were dirt cheap, so it didn't really cost me any added time or effort to make a sturdy shelf.

Most refrig/freezer combos put the cooling unit in the top of the machine because cold air sinks and by nature that is more efficient...
You could get around the condensation problem by using the drip trays that you can get from Home Deopt of Lowes for less than $1...

bottom makes sense, i'd think, because it would have better control. you'd only have the cold air rising when the fan was on, and otherwise the temperature in the top wouldn't just drop because of airflow. may be wrong or just nitpicking, but it seems like that makes sense

Melana said:

Most refrig/freezer combos put the cooling unit in the top of the machine because cold air sinks and by nature that is more efficient...

Click to expand...

I was thinking the same thing. Looking at the build, it could be rather simple to correct (turn it upside down).

I think either could have its advantages. In warmer weather when I'm trying to cool quite a lot below ambient temperature, having the ice on top would be a bit more efficient for the aforementioned reasons. Though efficiency isn't really a huge concern because the fan in it will use so little power, with all else being the same it would be an easy improvement.

In cooler weather, where I'm only trying to cool a little below ambient, I'd be a bit worried that the natural cold-air-sinking effect might be enough to cool more than I want it to, without the fan ever kicking on, as DeathBrewer pointed out.

Without actually testing it out, it's hard to say how significant the effect would be in either case. Either way, I don't expect it would be that important. But as Ryan_PA mentioned, it would be a simple matter for me to flip the thing over and try it either side up, to see if it makes any difference... I may do that at some point.

Melana said:

Most refrig/freezer combos put the cooling unit in the top of the machine because cold air sinks and by nature that is more efficient...
You could get around the condensation problem by using the drip trays that you can get from Home Deopt of Lowes for less than $1...

Click to expand...

Actually thats not the reason they are built that way. It is just more convenient to have it on top. The cold air is transferred by fan anyways so it doesn't matter where the freezer is.

I have a freezer on the bottom model and it sucks to get into the freezer...


If you were to put a fan on the shelf to suck air up through it you could adjust the temps with a controller.

A separate door for the ice compartment might make sense so when you change the ice you do not lose all the cool air around your carboy.

This is painful seeing all these cool closets.....gonna have to build one soon.

bwest said:

A separate door for the ice compartment might make sense so when you change the ice you do not lose all the cool air around your carboy.

Click to expand...

True, but I figured it wasn't important enough to justify the extra construction complexity to me - I had enough trouble building ONE door and getting it to seal properly when closed, let alone having to build TWO .

This cabinet has a whole lot of liquid for the amount of air space - 5 gallons of liquid holds a lot more heat than the amount of air in the cabinet, so a short change in air temp isn't going to make any real difference. Based on some back-of-the-envelope calculations using the specific heat and densities of air and water, and the dimensions of my cabinet, I calculated that letting some heat in momentarily and then closing the door again and letting it settle back to equilibrium, would only result in changing the liquid temperature 1/735 as much as the change in the inside air temp, and that's not even counting any of the cooling from the ice.

evandude said:

would only result in changing the liquid temperature 1/735 as much as the change in the inside air temp.

Click to expand...

you have either done WAY to much mathmatical calcs OR.... I need to call BULL$#!^
Good job on the build I am sure you can put it to good use.
Cheers
JJ

jaybird said:

you have either done WAY to much mathmatical calcs OR.... I need to call BULL$#!^

Click to expand...

Off the back of the envelope and onto the internet I suppose

Q=m*c*dt
Q=heat energy transferred, m=mass, c=specific heat, dt=temperature change

if I assume that all the excess heat energy of the suddenly warmed air is absorbed by the liquid to reach equilibrium - ie, ignoring outside effects - then two of these equations can be written and equated:
mw*cw*dtw=ma*ca*dta (where 'a' and 'w' denote air and water)
rewritten: dtw = (ma/mw)*(ca/cw)*dta
ca/cw = 1.012/4.1813 (ratio of specific heats)
mass = density*volume, and the ratio of the densities(rho) is 1.225/999.1
5 gallons of water is 0.01892706 cubic meters, and the volume of the box (14x14x27 inches) is 0.086720343 cubic meters, so the ratio of va/vw is 4.58.
dtw = (ca/cw)*(rhoa/rhow)*(va/vw)*dta
dtw = (1.012/4.1813)*(1.225/999.1)*4.58*dta
dtw = (1/735)*dta

I just noticed one reason it's not quite right, I used the volume of the empty box for the air volume, without subtracting the volume of the liquid. But correcting that just makes the air temperature change even LESS significant in my cabinet.

Someone with some (any) better knowledge of physics or thermodynamics or whatever can feel absolutely free to point out any issues with it, as I said it's all back-of-the-envelope. The equation and all the density/specific heat values were from wikipedia.

evandude said:

Off the back of the envelope and onto the internet I suppose

Q=m*c*dt
Q=heat energy transferred, m=mass, c=specific heat, dt=temperature change

if I assume that all the excess heat energy of the suddenly warmed air is absorbed by the liquid to reach equilibrium - ie, ignoring outside effects - then two of these equations can be written and equated:
mw*cw*dtw=ma*ca*dta (where 'a' and 'w' denote air and water)
rewritten: dtw = (ma/mw)*(ca/cw)*dta
ca/cw = 1.012/4.1813 (ratio of specific heats)
mass = density*volume, and the ratio of the densities(rho) is 1.225/999.1
5 gallons of water is 0.01892706 cubic meters, and the volume of the box (14x14x27 inches) is 0.086720343 cubic meters, so the ratio of va/vw is 4.58.
dtw = (ca/cw)*(rhoa/rhow)*(va/vw)*dta
dtw = (1.012/4.1813)*(1.225/999.1)*4.58*dta
dtw = (1/735)*dta

I just noticed one reason it's not quite right, I used the volume of the empty box for the air volume, without subtracting the volume of the liquid. But correcting that just makes the air temperature change even LESS significant in my cabinet.

Someone with some (any) better knowledge of physics or thermodynamics or whatever can feel absolutely free to point out any issues with it, as I said it's all back-of-the-envelope. The equation and all the density/specific heat values were from wikipedia.

Click to expand...

Holy ****! The only thing I know is cold air falls down.

has anyone thought of using dry ice. This would end the dripping problem.

I want to build something like this some day.

Well, not sure about that. The dripping is from condensation, not from the ice itself (normally your ice is in sealed containers) so I'd expect you'd still get condensation with dry ice. And you'd have less to worry about with oxidation when using fermentation vessels that were slightly oxygen-permeable, because the evaporating dry ice would purge the whole chamber with co2. If the moisture in the air was also purged in the process, that might keep things dry though... but I don't know if that'd be the case.

You'd have to have a pretty steady, convenient, and cheap source of dry ice though - it's sure hard to beat walking over to your freezer and swapping out a jug of water for a jug of ice. I know you can make your own dry ice right from a CO2 tank, but I think that could get very expensive. A 20lb tank has (of course) about 20lb of CO2 in it, which means you'd get less than 20lb of dry ice out of it (maybe a LOT less) because the process won't be perfect. Supposedly dry ice has about 3.3x the cooling capacity of (water) ice (found via googling) so at most, a 20lb tank would give you about the same cooling capacity as 3.3*20 = 66lb of water ice, which is only about 8 gallons - and probably quite a lot less. I expect it would cost a LOT more to refill a 20# CO2 tank, than to freeze less than 8 gallons of water in your freezer, not to mention all the extra hassle.

Plus, I suspect you'd have some real problems with over-cooling the cabinet without the fan ever kicking on - at the very least, you'd most likely need to insulate the separator between the fermentation and ice sections of the cabinet.

Interesting thought though

evandude said:

dtw = (1/735)*dta

I just noticed one reason it's not quite right, I used the volume of the empty box for the air volume, without subtracting the volume of the liquid. But correcting that just makes the air temperature change even LESS significant in my cabinet.

Click to expand...

Looks right to me. Assuming no heat loss or gain, the equalibrium temperture would be about 0.7 degrees C higher than the beer temp. (this is using 1/940, the ratio after the volume of the beer is considered).

If your ferm chiller is airtight enough then condensation shouldn't be that big of a problem. Maybe if you were opening it too frequently it would allow for humid moist air to get inside which would cause a lot of condensation.

CollinsBrew said:

If your ferm chiller is airtight enough then condensation shouldn't be that big of a problem. Maybe if you were opening it too frequently it would allow for humid moist air to get inside which would cause a lot of condensation.

Click to expand...

I also plan to use Damp Rid in the cabinet to help dry out the air. Between that, and the fan circulating air a lot of the time, I think it should stay rather dry, but we'll see how it goes when I get to put it through its paces when I brew my next batch

IMHO my design would put a small trap door at the top of the unit that would be used to put in the frozen water bottles and take out the thawed ones. This way you don't have to open the larger door where the carboys are kept letting in all the warm air every time you replaced the bottles.
Since the colder 32-40F air will naturally fall to the bottom of the cabinet and force the warmer air up towards the ice to be cooled again you may not even need a fan. Of course the layout of the air vent holes and condensation drip trays will play an important part in the design's efficiency and maintenance.

Screwy Brewer

ScrewyBrewer said:

Since the colder 32-40F air will naturally fall to the bottom of the cabinet and force the warmer air up towards the ice to be cooled again you may not even need a fan.

Click to expand...

but then you don't have any automatic temperature control, which was one of the key reasons for building such a cabinet...

Funkenjaeger said:

but then you don't have any automatic temperature control, which was one of the key reasons for building such a cabinet...

Click to expand...

The frequency you change out thawed bottles and replace them with frozen bottles and the number of frozen bottles you use is really the temperature controller. I'm adding a probe type thermemeter to my design as well so I don't have to keep openening the chamber's doors to know what the temperature is inside.

I bought a 5 dollar indoor/outdoor thermometer with probe and have that outside with no mods to the thermostat.

and it's best to have separate access to the ice chamber to limit the opening of the main holding chamber.. IMO