Define the rate of air flow in a fermentation chamber
Define the rate of air flow in a fermentation chamber
We need some definition here for air speed in a fermentation chamber of "n" cubic feet with a fan of "y" cuft/min. I know that we have engineers and physics majors who can lend some numbers to this. Fans are no problem. They can be had in size from an inch and a half square to four inches square and the speed can be varied with the applied voltage.
Ross- tightwad home cook
Ross, you are asking a very good question but the answer is going to depend on the humidity of the chamber and the differential of the humidity of the incoming air. Basically, if you have really dry air outside the chamber you will want less airflow. If your chamber has a humidifier that is cranking out the humidity you will want greater airflow. I don't think it is as simple as moving X% of the chamber's volume per minute -- though that works well for koi pond water filters. 
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It is easy to determine the number of air changes per unit time but I was addressing the question of air circulation within the chamber. If you are in a closed room the air flow within the room will depend entirely upon the mechanical movement of the rooms occupants. An individual walking across the room will cause some air circulation, a small fan will cause the air to circulate.
The recommended air circulation speed in a chamber is one to three mile per hour or 1.5 to 4.5 feet per second.
The recommended air circulation speed in a chamber is one to three mile per hour or 1.5 to 4.5 feet per second.
Ross- tightwad home cook
Ok, got it.
I was thinking about my comment about the koi pond filter -- it works because it is a closed system. What if your curing chamber was similar, a closed system? Move the air out of the chamber at at the appropriate speed (3 mph), condition it (temp and humidity) and pump it back into the chamber. Could it be so lucky to solve all of the problems?
I would be concerned about growing pathogens in a closed system. I don't know squat about biology but I seem to recall some wicked stuff likes to grow in enclosed environments.
I was thinking about my comment about the koi pond filter -- it works because it is a closed system. What if your curing chamber was similar, a closed system? Move the air out of the chamber at at the appropriate speed (3 mph), condition it (temp and humidity) and pump it back into the chamber. Could it be so lucky to solve all of the problems?
I would be concerned about growing pathogens in a closed system. I don't know squat about biology but I seem to recall some wicked stuff likes to grow in enclosed environments.
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More like a cellar with no outside access. A simple fan will keep the air moving and help to prevent stagnation in the room. It never takes much but it serves to equilize the humidity throughout the room. Otherwise you start growing things in the corners and behind stored stuff.
Ross- tightwad home cook
Ross,
I think what you're after is circulating the air inside at some reasonable velocity (enough to keep the interior well-mixed), and venting a small portion to control humidity. Assuming adequate interior circulation, measure the humidity anywhere in the chamber (except near the inlet) or in the exhaust air. What you're really after is humidity control, not air flow control. Right?
But concerning air flow:
At any given cross section of the chamber, average air speed can be calculated by dividing whatever the fan is rated for, cubic feet per minute, by the cross sectional area of the chamber minus the cross sectional area that the meat and hardware takes up. Air speed will vary throughout the chamber.
For example, in a 1.5 cfm fan and a 2 ft by 2 ft by 4 ft vertical smoker, bulk flow would be (1.5 ft x ft x ft/min) / (2 x 2 ft x ft) or 0.375 ft/minute.
If I load the chamber with 30 links of sausage, each 1.5 inch diameter by 8 inches long, which comes out to 30 * (pi * (1.5/2) * (1.5/2)) / (12 * 12) or 0.368 square feet of sausage [it's up flow, not cross flow] [don't forget to convert from cubic inches to cubic feet!][don't forget to multiply by the number of sausages!], then a decent approximation for the area is (2 x 2 x ft x ft) minus 0.368 sq.ft. for the sausage, or 3.6 sq.ft. and the average air velocity past the sausages is 1.5/3.6 = 0.42 ft/minute.
...which suggests that, unless you way overpack the chamber with sausage (like a can of Vienna sausages?), the air velocity will be reasonably close to the fan rating divided by chamber cross section.
How you arrange the sausage in order to adequately circulate the whole volume of the chamber is another problem. There are many solutions. Hopefully you can keep it simple!
Russ
I think what you're after is circulating the air inside at some reasonable velocity (enough to keep the interior well-mixed), and venting a small portion to control humidity. Assuming adequate interior circulation, measure the humidity anywhere in the chamber (except near the inlet) or in the exhaust air. What you're really after is humidity control, not air flow control. Right?
But concerning air flow:
At any given cross section of the chamber, average air speed can be calculated by dividing whatever the fan is rated for, cubic feet per minute, by the cross sectional area of the chamber minus the cross sectional area that the meat and hardware takes up. Air speed will vary throughout the chamber.
For example, in a 1.5 cfm fan and a 2 ft by 2 ft by 4 ft vertical smoker, bulk flow would be (1.5 ft x ft x ft/min) / (2 x 2 ft x ft) or 0.375 ft/minute.
If I load the chamber with 30 links of sausage, each 1.5 inch diameter by 8 inches long, which comes out to 30 * (pi * (1.5/2) * (1.5/2)) / (12 * 12) or 0.368 square feet of sausage [it's up flow, not cross flow] [don't forget to convert from cubic inches to cubic feet!][don't forget to multiply by the number of sausages!], then a decent approximation for the area is (2 x 2 x ft x ft) minus 0.368 sq.ft. for the sausage, or 3.6 sq.ft. and the average air velocity past the sausages is 1.5/3.6 = 0.42 ft/minute.
...which suggests that, unless you way overpack the chamber with sausage (like a can of Vienna sausages?), the air velocity will be reasonably close to the fan rating divided by chamber cross section.
How you arrange the sausage in order to adequately circulate the whole volume of the chamber is another problem. There are many solutions. Hopefully you can keep it simple!
Russ
Experience - the ability to instantly recognize a mistake when you make it again.
What we have above is technically called a "bulk velocity." It's a bogus number, unless everything you blow in blows out after going straight through. (This is one extreme, called "plug flow.") If there's any backmixing (as you would prefer to do), the number gives you a vague idea of overall turnover but has little to do with reality. There's a residence time distribution (don't go there!). All you care about in backmixing is that the concentration (humidity) is uniform. You can lower drying chamber humidity by increasing dry airflow into/out of the chamber, or raise it by slowing in/out flow.
Between extremes, there's an intermediate state, called "poorly mixed." Imagine looking into a mixer during operation. Some of the stuff goes 'round and 'round fast, and other parts go around slowly, and the trick is for the twain to meet. For solids and liquids, we all hope for well-mixed operation but rarely achieve it in a flow-through operation. In that case, we usually resort to batching the material and allowing sufficient mixing time (like viscous liquids, pellets, or sausage mixing). Gases, fortunately, are more forgiving, and "perfect mixing" can be pretty close to being true.
For gases, like in your fermentation chamber, the reactions are slow and a circulation fan ought to give a fairly uniform humidity throughout. In that case, you can use the circulation fan capacity to approximate internal velocity, and use inlet or discharge flow as your bulk velocity.
Usually what's important is enough circulating velocity to ensure that there's little or no concentration gradient at the surface of the sausage, and humidity control (bulk velocity) to ensure that the concentration difference between gas and sausage isn't too great (to avoid case hardening). It's a juggling act (ask me about burning up a catalytic reactor some time). Looks like you have a feel for the numbers, though.
Be sure to describe (with pictures) whatever your new contraption looks like. We always enjoy your inventions. (...and envy your wife's patience!)
Between extremes, there's an intermediate state, called "poorly mixed." Imagine looking into a mixer during operation. Some of the stuff goes 'round and 'round fast, and other parts go around slowly, and the trick is for the twain to meet. For solids and liquids, we all hope for well-mixed operation but rarely achieve it in a flow-through operation. In that case, we usually resort to batching the material and allowing sufficient mixing time (like viscous liquids, pellets, or sausage mixing). Gases, fortunately, are more forgiving, and "perfect mixing" can be pretty close to being true.
For gases, like in your fermentation chamber, the reactions are slow and a circulation fan ought to give a fairly uniform humidity throughout. In that case, you can use the circulation fan capacity to approximate internal velocity, and use inlet or discharge flow as your bulk velocity.
Usually what's important is enough circulating velocity to ensure that there's little or no concentration gradient at the surface of the sausage, and humidity control (bulk velocity) to ensure that the concentration difference between gas and sausage isn't too great (to avoid case hardening). It's a juggling act (ask me about burning up a catalytic reactor some time). Looks like you have a feel for the numbers, though.
Be sure to describe (with pictures) whatever your new contraption looks like. We always enjoy your inventions. (...and envy your wife's patience!)
Experience - the ability to instantly recognize a mistake when you make it again.
I am accumulating information so that when I again make a batch of fermemted sausage I won't be making dog food. Instrumentation and temperature control is not simple but with enough knowledge it can be inexpensive. Cheese aging and sausage aging have much in common and it is quite likely that a chamber for one can be used for the other.
I think that a refrigerator can be used as a source of chilled air and still be used as a refrigerator. Some small insulated ductwork connecting the fridge to the fermentation chamber with a thermostatically controlled fan and a return air duct, Could, I believe, control the temperature in the chamber while still keeping the fridge at 34 degrees F.
I think that a refrigerator can be used as a source of chilled air and still be used as a refrigerator. Some small insulated ductwork connecting the fridge to the fermentation chamber with a thermostatically controlled fan and a return air duct, Could, I believe, control the temperature in the chamber while still keeping the fridge at 34 degrees F.
Ross- tightwad home cook
