Cellar Ventilation / Exhaust System

[PMR]

Hi all--

We're in construction on a new cellar that's fairly small, walled off from other parts of the brewery and packed with fermenters. We'll be installing make up air fans and exhaust fans to help ensure Co2 levels are safe for our employees, but I wanted to see if anyone could verify that I'm calculating this correctly.

The square footage of the space is 1066 s.f., and the cubic feet is 14,924 (14 ft. ceilings). The fermentation capacity of the space is 420 BBL (7 x 60 BBL FV's), and our average attenuation is 15 Plato over 5 days.

I want a "worst case scenario" design, so the assumption would be that all fermenters in this space would be at their peak of fermentation simultaneously (which will never happen since it takes 24 hours for us to fill up each tank). I came up with a maximum of 72.5 lbs. of Co2 being released per hour, which I believe equals 634 cubic feet of Co2 per hour. We'd want to keep Co2 levels to 5000 ppm or below (OSHA low end Co2 concentration safe for 40 hour work week) as there will be keg washing in this area, so it's possible that a worker would be in here for 40 hours a week.

To get the Co2 levels with 1 air change per hour, I'm multiplying the cubic square feet by 500 ppm (a bit above average for outside air-- equals 7,462,000 parts of Co2) plus 634,000,000 parts of Co2 (634 cubic feet of pure Co2 times 1,000,000 to get the ppm equivalent), for a total of 641,462,000 units of Co2, which averages to 42,981 ppm. Since I need it to be 5,000 ppm or less, there needs to be 8.6 air changes per hour (42,981 ppm / 5,000 ppm = 8.59).

Does this look reasonable? We'll get a Co2 monitor just in case, just want to see if this is how you'd all go about it.

Thanks in advance for the help!

Cheers,
Patrick Rue
The Bruery, Orange County, CA

[jwalts]

Hey Patrick,

I've never sized a displacement system, but maybe this can serve as a sanity check for your math. Assuming you have one 420-bbl fermentation, your OG is 17 P (starting with no alcohol simplifies the calculations), your gravity drop is linear (which we know isn't true, but it'll be more than offset by the assumption that all fermentations are peaking at the same time), and wort/beer mass is constant:

3 P/day x [1 day/24 hrs] = 0.125 P/hr
OG (SG) = ((0.00000008 x 17 + 0.00001216) x 17 + 0.00387324) x 17 + 1 = 1.06975
Initial density = 1.06975 x 258.7 = 276.74 lb/bbl
Initial mass = 276.74 x 420 = 116,231 lbs
Initial extract = (17/100) x 116,231 = 19,759 lbs
Water mass = 116,231 - 19,759 = 96,472 lbs
Gravity after 1 hr (SG) = ((0.00000008 x 16.875 + 0.00001216) x 16.875 + 0.00387324) x 16.875 + 1 = 1.06921
Density after 1 hr = 1.06921 x 258.7 = 276.6 lb/bbl
ABV after 1 hr = 0.516 x (17 - 16.875) = 0.0645 %
Alcohol volume after 1 hr = (0.0645/100) x 420 x [117,347.8 L/bbl] = 31,790 mL
Alcohol mass after 1 hr = 0.789 x 31,790 / [453.59 g/lb] = 55 lbs
Remaining extract after 1 hr = 116,231 - 96,472 - 55 = 19,704 lbs
Extract to CO2 in 1 hr = 19,759 - 19,704 = 55 lbs
CO2 mass produced in 1 hr = 0.489 x 55 = 26.9 lbs

I'm guessing you assumed that degrees Plato always equals percent extract, which isn't true when alcohol exists. You may have chosen 72.5 lbs/hr of CO2 for a good reason, though, so let's roll with it. Using the ideal gas law for room temperature and atmospheric pressure:

CO2 volume produced in 1 hr = 72.5 x [453.59 g/lb] x 8.314 x 293.15 / (44.01 x 101,325) x [35.315 ft3/m3] = 634.7 ft3

Your number looks good to me there. Assuming that CO2 produced by fermentation displaces the same volume of air (i.e. atmospheric pressure is maintained) results in a CO2 concentration of 1,000,000 x 634 / 14924 = 42,482 ppmv. Again, our numbers are pretty close.

I'm not sure what you mean by "air charges", but here's how I'd proceed:

Max allowable CO2 = 5,000 x 14,924 / 1,000,000 = 74.6 ft3
Min CO2 to remove = 634 - 74.6 = 559.4 ft3/hr
Min flowrate of fans = 14,924 x 559.4 / 634 = 13,168 ft/hr

This approach has a couple of problems:

-If the fans displace cellar air with clean air at a 1:1 ratio, CO2 will accumulate at a rate of 74.6 ft3/hr. Perhaps a better approach would be to simply add 14,924 ft3 of clean air per hour.
-I'd wager the fans won't displace cellar air with clean air at a 1:1 ratio, meaning you'll need a higher flowrate.

Hopefully an HVAC expert can chime in.

Joe Walts
Ale Asylum

[PMR]

Thanks for your help, Joe! It's good to run it by someone, just in case I was completely off.

I found that with these calculations I need intake / exhaust fans that move 2300 cfm of air. I'll probably shoot higher and get something that moves 3000 cfm and put it on a VFD so we can adjust as needed. I'm curious what kind of force is needed to mix the Co2 with the surrounding air and get it out of the space (fans will be mounted on the roof)-- that's something I don't know how to calculate.

Once we get the system in place and a Co2 monitor, I'll post results.

Cheers,
Patrick

[dick murton]

I haven't checked the figures for CO2 evolution, and am certainly no H&V engineer, but just wanted to repeat the fact that CO2 is heavier than air, settles towards fllor level and all extraction must be at very low level. Air supply is often (but obviously not always) at high level, and from what I have seen tends not to point down so that the air in the room is stratified, i.e. fresher air at higher levels, more CO2 at lower levels.

Typically, to allow for people bending over to connect low level pipes, maintenance etc, the CO2 sensors will be placed about 18 inches abover the floor, though thesupplier / installer will be able to provide more accurate details.

[PMR]

Thanks Dick, good point. We'll probably need to put some ductwork on the make-up air to direct the force to the ground level.

[jwalts]

Patrick: I look forward to your results, and I have no idea why I read "changes" as "charges" or ignored atmospheric CO2 in my math. Homeboy needs some sleep.

Dick: I'd love to have access to finite element software and model CO2 settling/mixing over time, both for this sort of problem and to optimize tank purging paramaters. I know that CO2 introduced beneath air will form a blanket, but it's only temporary and the gases will mix fairly evenly if they're given enough time. When CO2 is being actively introduced, though, I suspect it's not worth worrying about and that you're absolutely right about placing CO2 sensors at low heights.

Joe

[revnatscider]

Speaking practically and not scientifically, could you vent your fermentation tanks outside of the cellar, possibly to the great outdoors? So instead of a hose to a sani-bucket, how about a hose through a wall, up a floor and outside to a sani-bucket? Might not be practical, but could save you some HVAC $$$

[Fullcourt]

Patrick, just to clarify your situation. I know that you are in Orange County, Calif. When you say a "cellar" do you mean a ground level room with tanks, or a below ground room with tanks, with the dirt behind the walls.

[Seangoloid]

You may want to take into account the fact that you are going to experience volume growth; meaning you will eventually need more than one person working 40 hours a week washing/filling kegs... just another little nuance variable to think about regarding your OSHA numbers...

[dick murton]

Dick: I'd love to have access to finite element software and model CO2 settling/mixing over time, both for this sort of problem and to optimize tank purging paramaters. I know that CO2 introduced beneath air will form a blanket, but it's only temporary and the gases will mix fairly evenly if they're given enough time. When CO2 is being actively introduced, though, I suspect it's not worth worrying about and that you're absolutely right about placing CO2 sensors at low heights.

Joe

I have never seen anything like this. I expect that a heat & vent engineer might have something like this, but suspect that the slightest air movement makes a nonsense of any theoretical calculations. CO2 being dnser than oxygen or nitrogen will always settle given calm conditions. However, you are right that there will not be a distinct layer like oil or water form, as the gases will try to move from an area of high concentration to area of low concentration, but are restricted bythe comparative molecular weights.

When placing the gas sensors, I expect there are recommended heights above the floor for the sensors, but if not, then I suggest about 18 inches. Any higher and they won't provide the safety element required when working at low levels, e.g. repairing a valve or pump. Any lower and they go off too readily and you should stop working in the area, perhaps unecessarily.