Help with my ventilation - building condensation and I'm surprised

[martianpc]

I need some advice on how to not get condensation build up.

I'm getting dripping from the top of the hood as well as from the inside of the duct work. I find this very surprising considering its eight inch duct with a five foot run and only one 90-degree elbow. The fan is a vortex s line 8 inch.

Any ideas on what I could be doing wrong? I have a window open on the other side of the basement to provide plenty of make-up air. Is it possible that my kettle is just too far away from the hole at the top of the hood?

Don't mind the ugly appearance of the hood I'm still working on making everything look nice as I build the brewery.
Thanks!

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[Wizard_of_Frobozz]

Overall, that fan should be providing a lot of airflow, even with a decent amount of static pressure. It could be the distance between your kettle and the hood. Experiment with that to see if it helps.

A couple of questions/things to think about:

1. Is it possible that the duct is not sealed, so the condensation on the top of the hood could be coming from the inside the duct, and leaking out?
2. Is the fan installed correctly? We've all done silly things like putting something in backwards. There's a reason the first thing tech support asks is "is your computer plugged in?"
3. How long of a duct run do you have, and how many elbows in it? The specs on the 8" S-Line fan say it should have a lot of cfm's, even with a good amount of static pressure, so this is not likely part of the problem, but it's worth checking.
4. How big is your heat source? Since you posted in the electric brewing forum, I'm assuming you are using electric heat. Kal's site says you need wattage/17.6 for airflow. As long are you aren't using something really big, that 8" S-Line should be moving enough air...

You can measure the actual airflow with a cheap anemometer to see if the fan is providing the expected cfm's. Measure windspeed in feet/min at the 8" duct, and multiply that by the cross sectional area (3.14 * r^2) of the duct to get cubic feet/min (r needs to be in feet, not inches, so 4/12 or .3333 for your duct)

 

[TungstenBeer]

I tried to think over and over about what could be happening... and came up with nothing (other than installing the fan backwards as was mentioned by the other poster).

Subscribing to the thread because this makes me nervous as I'm about a month or two away from installing the exact same setup!

 

[shortyz]

thats a big fan, only thing to keep that condensation off there might be to insulate the aluminum. how cold is the room your brewing in?

 

[martianpc]

Overall, that fan should be providing a lot of airflow, even with a decent amount of static pressure. It could be the distance between your kettle and the hood. Experiment with that to see if it helps.

A couple of questions/things to think about:

1. Is it possible that the duct is not sealed, so the condensation on the top of the hood could be coming from the inside the duct, and leaking out?
2. Is the fan installed correctly? We've all done silly things like putting something in backwards. There's a reason the first thing tech support asks is "is your computer plugged in?"
3. How long of a duct run do you have, and how many elbows in it? The specs on the 8" S-Line fan say it should have a lot of cfm's, even with a good amount of static pressure, so this is not likely part of the problem, but it's worth checking.
4. How big is your heat source? Since you posted in the electric brewing forum, I'm assuming you are using electric heat. Kal's site says you need wattage/17.6 for airflow. As long are you aren't using something really big, that 8" S-Line should be moving enough air...

You can measure the actual airflow with a cheap anemometer to see if the fan is providing the expected cfm's. Measure windspeed in feet/min at the 8" duct, and multiply that by the cross sectional area (3.14 * r^2) of the duct to get cubic feet/min (r needs to be in feet, not inches, so 4/12 or .3333 for your duct)

Amazing answer, thanks so much for the thoughts and time to write them!

I ordered the anemometer, mostly just out of curiosity. To be clear to everyone, I get plenty of suction, when I place a bucket lid over the hole it takes two hands to get it back off. I can feel my hand being sucked up into the hole if I place it in front.

My room is not cold, it is about 70 or so degrees. As to the heater power, it is a 5500 watt element and was run on 100% power.

I just did a second test and moved the kettle more centered under the hole and while I got a very small amount of water formed around the opening, the amount was almost non existent. Unfortunately during a brew I will not be able to place it directly underneath the hole because it is almost directly between 2 kettles.

 

[Wizard_of_Frobozz]

Amazing answer, thanks so much for the thoughts and time to write them!

I ordered the anemometer, mostly just out of curiosity. To be clear to everyone, I get plenty of suction, when I place a bucket lid over the hole it takes two hands to get it back off. I can feel my hand being sucked up into the hole if I place it in front.

My room is not cold, it is about 70 or so degrees. As to the heater power, it is a 5500 watt element and was run on 100% power.

I just did a second test and moved the kettle more centered under the hole and while I got a very small amount of water formed around the opening, the amount was almost non existent. Unfortunately during a brew I will not be able to place it directly underneath the hole because it is almost directly between 2 kettles.

No problem! Psychrometrics are not easy to grasp sometimes. I deal with it professionally, and I've had people insist to me that the condensation forming on the outside of a cold glass is water leaking out of the container!

Measuring the airflow will give you some data to work with, and that's always good for helping solve the problem but it sounds like you've found the root cause - too little of that airflow is pulling the steam from the top of your kettle. The key to using airflow in order to control condensation is making sure that the steam from your kettle is pulled up into the hood fast enough so that the moisture doesn't have time to migrate throughout your room. Water vapor will follow the "partial pressures" law and will move from high concentration to low concentration, even against airflow UNLESS you have enough air movement to overcome the migration of moisture. Sorry if that's more geeky than neccessary - I'm an engineer so I can't help it!

It's really not as hard as I make it out - ultimately, you want to focus the airflow stream so that it pulls the steam up into the hood. If you can't move the kettle closer to the hole, then move the hole closer to the kettle! You want less space for the steam to escape. You could try lowering the hood so there is less room above the kettle. Extending the sides of the hood downward would accomplish the same thing if you can't lower the hood itself. Maybe something on hinges that you could move out of the way when needed so they don't interfere with brewing operations? Another option would be to raise the brew kettle itself.

I would still try to find a way to move the brew kettle directly under the duct hole, since you have proven that to be effective. What's the 2nd kettle you mentioned? A HLT? If so, a HLT only needs to heat to 170F or so; it puts out a whole lot less vapor compared to a boil kettle. Maybe you could move it a little outside of the hood and cover it while it's heating. That would keep the moisture that goes into the air to a minimum and also help a little at keeping heat in your HLT. Just a thought anyway...

 

[BrunDog]

Condensation occurs when the air mass's relative humidity exceeds saturation (dew point). This is temperature dependent of course, so as a saturated air mass cools, condensate forms. If that mass were heated, it could accommodate more water vapor.

The steam from the kettle is basically saturated, so once that air cools, condensate will form. The key with ventilation is to combine the steam air mass with the ambient air mass (hence the big fans). This combined air mass is what your are concerned with. On one hand, the ambient air mass is drier, but it's also cooler. So what happens when it is combined with the steam air mass? In other words, which wins? Does the combined air mass become drier but yet cool enough for saturation to be exceeded, or does it become cooler but dry enough that saturation is not exceeded?

This is your challenge - you can't effectively change the ambient air temp or humidity. But probably as the ratio of steam air to ambient air is reduced, the probability of saturation goes down. It sounds like you have a big enough fan. But I would suggest you orient things in such a way that good blending takes place. In the warmer/more humid spring and summer you may not be able to mitigate condensation. In the drier winter, this problem may not exist. If you have to deal with condensation, adapt your system so it has a drain and drain the liquid out.

Edit: I would think, though I could be wrong that moving the kettle further from the hole would yield less condensation. Worth a test.

 

[Wizard_of_Frobozz]

Condensation occurs when the air mass's relative humidity exceeds saturation (dew point). This is temperature dependent of course, so as a saturated air mass cools, condensate forms. If that mass were heated, it could accommodate more water vapor.

The steam from the kettle is basically saturated, so once that air cools, condensate will form. The key with ventilation is to combine the steam air mass with the ambient air mass (hence the big fans). This combined air mass is what your are concerned with. On one hand, the ambient air mass is drier, but it's also cooler. So what happens when it is combined with the steam air mass? In other words, which wins? Does the combined air mass become drier but yet cool enough for saturation to be exceeded, or does it become cooler but dry enough that saturation is not exceeded?

This is your challenge - you can't effectively change the ambient air temp or humidity. But probably as the ratio of steam air to ambient air is reduced, the probability of saturation goes down. It sounds like you have a big enough fan. But I would suggest you orient things in such a way that good blending takes place. In the warmer/more humid spring and summer you may not be able to mitigate condensation. In the drier winter, this problem may not exist. If you have to deal with condensation, adapt your system so it has a drain and drain the liquid out.

Edit: I would think, though I could be wrong that moving the kettle further from the hole would yield less condensation. Worth a test.

Brundog,

I agree with the science behind what you are saying, but I would argue that in this case (having an exhaust hood), the OP is not trying to "blend" the moisture saturated air from the kettle with the drier air in the room. The function of the exhaust hood and fan is to pull the saturated air from the kettle out of the room entirely - ideally before the moisture has a chance to move out into the room. In order to do that, the design of the fan/hood/kettle combination needs to be such that the steam is sucked up into the hood fast enough so that none of it "spills" out into the room to condense on other surfaces. The science behind this can get pretty complicated, but it boils down to having a high enough airspeed around the steam plume to overcome the vapor migration from high to low concentration areas.

It's been my experience that the most effective way to remove moisture is to contain the source as much as possible. That's why I recommend keeping the kettle as close to the hood/ducting as possible.

Thanks and good discussion,
WoF

EDIT: here's a good reference that boils it down to the critical factors: http://www.engineeringtoolbox.com/exhaust-hoods-d_1036.html

Note that the airflow required is a function of the distance from the source to the hood squared!

 

[BrunDog]

I agree with you WOF that you don't want the steam spilling out into the room. That would certainly defeat the purpose.

But you need to combine the steam with the ambient air inside the hood to reduce the probability of condensation. If you only drew the steam out, as soon as it is cooled even the smallest amount, it will create condensate. This would happen as soon as it enters the piping (maybe even from the negative pressure wave). The only way to prevent it is to draw enough ambient air, which combines with the steam, and that combination air being below the dew point given its RH and temp. Of course this blending is not a manual process - it happens in the hood/piping due to the volume of air being moved.

Anyway, might be a worthwhile experiment to try different kettle locations under the hood and see what happens. If the steam hits the hood surface, condensation will form - that is probably not avoidable. The hood should have a drain or trap to manage it.

 

[wilserbrewer]

You mention running the 5500w element at 100%, might you just be over boiling and creating more steam than necessary and pushing your ventilation system too hard?

 

[yowzers]

The condensation is occurring because your ductwork is cooler than the dew point of the humid air. The air is cooled considerably because of the large amount of mixing. You'd be better using a "snorkel" that can go down over the top of the pot and would not allow so much cool room air to mix in. Alternatively, you could heat the hood surface, but that would be a pain.

 

[TungstenBeer]

Hmm... is it possible the air is moving too fast?

 

[martianpc]

You can measure the actual airflow with a cheap anemometer to see if the fan is providing the expected cfm's. Measure windspeed in feet/min at the 8" duct, and multiply that by the cross sectional area (3.14 * r^2) of the duct to get cubic feet/min (r needs to be in feet, not inches, so 4/12 or .3333 for your duct)

I tested my f/minute with the anemometer. Got a max of right around 2400 at the opening of the duct hole. Am I calculating it right that is about 799 cfm? That sounds high.

 

[BlueHouseBrew]

Based on where the condensation is forming (only near the duct but not lower down on the hood), it appears that it may be the result of where the air is flowing, rather than how much air is flowing. There is probably a pocket of relatively stagnant/swirling air in that location. (Consider that if the flow were reversed, the part of the hood where the condensation is forming would not see much airflow.) Adding a baffle to the opening of the duct to direct some of the "suck" toward the side of the duct where the condensation is forming may resolve the issue.

 

[Weezy]

^^ I like it. Fluid dynamics issue, imho. This was my thought as well. Air is stagnating where there is condensation.

 

[martianpc]

Based on where the condensation is forming (only near the duct but not lower down on the hood), it appears that it may be the result of where the air is flowing, rather than how much air is flowing. There is probably a pocket of relatively stagnant/swirling air in that location. (Consider that if the flow were reversed, the part of the hood where the condensation is forming would not see much airflow.) Adding a baffle to the opening of the duct to direct some of the "suck" toward the side of the duct where the condensation is forming may resolve the issue.

Any suggestions for the baffle?

 

[Wizard_of_Frobozz]

Any suggestions for the baffle?

Turn the exhaust fan on without boiling in the kettle; use the anemometer to look for dead spaces where the airflow is low. Cut some cardboard and try a bunch of different designs. Once you have found the optimum design, use something more rigid like sheet plastic or stainless steel. Keep in mind that condensation WILL form on that baffle, so don't put it so it can drip back into the kettle.

I still think you should try raising the kettle/lowering the hood if you can. That should make the most impact, since airflow requirements are a function of that distance squared (see the link in post #8)

 

[WScottCross]

Insulating the duct will definitely help. That relatively cool (compared to the steam) metal will definitely cause condensation. Heating it would be better, but not really practical for our application.

 

[trav77]

You mention running the 5500w element at 100%, might you just be over boiling and creating more steam than necessary and pushing your ventilation system too hard?

+1 on this (?).

What batch volume are you doing? I only run my 5/10 gallon batches (15 gallon kettle) on 50-55% with a 5500 W element.

 

[Konadog]

Any suggestions for the baffle?

Could you get a screen made to fit inside your hood, fitted just inside the big end. This should help disturb the air enough so you don't have any dead spots inside.

 

[martianpc]

+1 on this (?).

What batch volume are you doing? I only run my 5/10 gallon batches (15 gallon kettle) on 50-55% with a 5500 W element.

I do boils from 6 gallons up to 18 gallons for a 15 gallon batch. I use slightly oversized 20 gallon kwttles. I don't need 100% often , usually about 75%. However I want to be able to run 100% without a condensation condensation problem when I am trying to boil off and reduce volume.

Could you get a screen made to fit inside your hood, fitted just inside the big end. This should help disturb the air enough so you don't have any dead spots inside.

Not sure what you mean. Do you mean inside the end of duct before getting to hood or a huge screen that fills the whole 3x6 foot opening for the hood.

 

[BlueHouseBrew]

I was envisioning something like a dryer vent cap -
http://www.luxurymetals.com/Stainless_wall_ventcap.3.jpg - except that you would attach it to the hood so that it only covered a portion of the left side of the duct. The idea is to direct some of the low pressure area to the spot where the condensation is. (It may be easier to envision this by picturing how the air would flow if you were pumping air in (instead of out) through the duct and trying to get it to flow over the trouble spot.) If condensation were to build up on the baffle, you may be able to cut/bend louvers into it to allow some airflow through/across the surface of it.