Non recirculating Dehumidifier design 2

[inventdv]

[highlight #73D216][/highlight]Looking for some help with this design it’s a dehumidifier for a paint booth non recirculating, what it will do is output dehumidified air that gets immediately evacuated by an exhaust fan.

The ideal cfm across an evaporator is 400 cfm(what I read) so I'm aiming for this or even 500, but I made some calculations with my current blowers, the opening is 9” x 21” and flows 13ms (measured) and thus I calculated over 3000 cfm into the evaporator shown on pic, how can I reduce this flow over an evaporator whilst keeping my same cfm output into the booth, at first I was thinking maybe if I increase the evaporator size it may but the air flow in m/s is the same. Now I'm thinking what if I use smaller multiple evaporators before the blower (blower sucking through evaporator) and having each one spilling into a main duct. The same can be done after the blower but I'm thinking the air would be dong strange things trying to push through multiple ducts?

 

[MintJulep]

I'm sorry, but nothing in your post makes any sense.

 

[inventdv]

which part dont you understand ?

 

[MintJulep]

None of it. Allow me to elaborate.

" output dehumidified air that gets immediately evacuated by an exhaust fan." Then the system has no purpose except to waste energy. Because the dehumidified air has no opportunity to do anything useful.

"The ideal cfm across an evaporator is 400 cfm" As a universal statement, this is not correct.

" so I'm aiming for this or even 500" If you have a target that you believe is "ideal" (even though your belief is incorrect), it does not make sense to then exceed that ideal by 25%.

" the opening is 9” x 21” and flows 13ms (measured) and thus I calculated over 3000 cfm" If I guess that 'ms' in context means 'm/s' for meters per second, not the usual milliseconds then it still makes no sense to mix SI and IP units.

"how can I reduce this flow over an evaporator whilst keeping my same cfm output into the booth" It is not possible to reduce something and simultaneously keep it the same.

" if I increase the evaporator size it may but the air flow in m/s is the same" Increase the evaporator size above what? To what? m/s is the same where?

Are you perhaps confused about the distinction between volumetric flow and velocity?

You seem to understand the relationship between volumetric flow, velocity and area, so it doesn't make sense that you are confused about a sub-set of this relationship - empirical evidence not witstanding.

 

[inventdv]

ok what im trying to have is dehumidified air comming into a spray booth and i need help with the air flow across the evaporator. This is a paint booth so all of the air that comes in has to be evacuated, this air needs to be dehumidified.

[highlight #3465A4]"The ideal cfm across an evaporator is 400 cfm" As a universal statement, this is not correct.[/highlight]

this is what i read online if its not correct you can chime in.
the specs i list does not have to be spot on im more concerened about the evaporator having the best airflow as possible to avoid issues with icing up or having too much superheat

" the opening is 9” x 21” and flows 13ms (measured) and thus I calculated over 3000 cfm" If I guess that 'ms' in context means 'm/s' for meters per second, not the usual milliseconds then it still makes no sense to mix SI and IP units.

sorry this is meters per second measured with the meter that measures windspeed.

"how can I reduce this flow over an evaporator whilst keeping my same cfm output into the booth" It is not possible to reduce something and simultaneously keep it the same.


ok what i really mean here is i want the output of the blower to still be at the current 3000 cfm or so but obviously i dont want that 3000 cfm blowing across mt evaporator
does this make better sense ?

 

[MintJulep]

"The ideal cfm across an evaporator is 400 cfm"

So, an evaporator intended to cool a part of a shopping mall has an ideal flow of 400 cfm?
An evaporator intended to cool a refrigerator has an ideal flow of 400 cfm?
Every possible evaporator, for every possible application, has an ideal flow of 400 cfm?

This is clearly absurd isn't it.

There is a rule of thumb of 400 cfm/ton to size the air flow for residential cooling. But this would not be applicable to your application.

Numerically, 400 is kinda sorta close to a rule of thumb velocity limit for wet coils to prevent condensate carry-over. That would be applicable to your application.

Don't use rules of thumb unless you understand the assumptions that make the valid, and their limitations.

" i want the output of the blower to still be at the current 3000 cfm or so but obviously i dont want that 3000 cfm blowing across mt evaporator" Once again, something cannot be different and the same simultaneously. So no, it still does not make sense.

You have 3000 cfm out of your blower. Apparently you want some lower flow through your coil. Where does the rest of the flow go?

 

[inventdv]

Don't use rules of thumb unless you understand the assumptions that make the valid, and their limitations

ok this is why im asking , if i am not correct just let me know

You have 3000 cfm out of your blower. Apparently you want some lower flow through your coil. Where does the rest of the flow go?

the question really is how can i have the same 3000cfm output without having too much airflow over my evaporator ?
i as thinking using multiple exaporators on the intake side ?
again obvously the volume plays a role this is why im asking and tryed to explain

 

[MintJulep]

Are you perhaps confused about the distinction between volumetric flow and velocity?

 

[inventdv]

maybe i am,

im tryin to get some help sounds as if you are knowledgeable enough to understand what i need, are you going to help or just rand and insult me ?
let me know and i wont waste time anymore

 

[GT-EGR]

Reading your initial question and taking your analysis at face value - you have a 3,000 CFM blower but only want to dehumidify 400-500 CFM across an evaporator?

Just put some distance between the blower outlet and the dehumidifier, and install a branch, and divert that extra 2500 CFM somewhere else likely just back to the same place your fan inlet draws from.

Keep in mind that blowing 500 CFM through your evaporator may not be a factor of safety above 400 CFM. The air will not be as dry, but you will have more of it, so it really depends on what minimum air dewpoint you need vs how much overall dry air movement you need.

Also keep in mind that your 3,000 CFM may be different once you add ductwork and evaporator to it. So in the end size the connections correctly and you’ll have to play with your “2,500 CFM” bypass with the goal of getting your 400 or 500 CFM through evaporator.

 

[inventdv]

gt-egr
you are closer to what im asking
dont take my numbers of 400 and 500 across the evaporator as a set number as i was just told that its absurd.
what i have as a constant is about a strong 3000 cfm blower, what i dont want is for this to be toooo much for the evaporator to handle as if the air moves too fast it can cause an undesired effect.So i am assuming that i have to reduce the velocity or volume? of this air some how or maybe increase the area of the evaporator and that will reduce the overall volume?
thus not overloading the evaporator? Theory being if 3000 cfm comes out of a 9" x 21" opening then it spreads out and then
hits say a bigger evaporator the volume of air hitting this big evaporator will be spread out ?

my second idea was to use multiple evaporators BEFORE the blower so that the blower would be sucking from multiple evaporators, the output from the blower would be the same cfm but because it woulld be pulling from multiple evaporators this cfm would be shared among them

i dont know if this helps

 

[EnergyProfessional]

For some reason I think OP means 400 fpm (feet per minute), not cfm. But I agree with the comments, much more information is needed.

Usually a dehumidifier is already designed by the manufacturer. So I'd contact them for application guidance. You'll also need to understand psychometrics and need to account for the actual conditions and the desired outcome. Is this in Florida, or in Greenland? Does the air need to be bone-dry for that paint?

Rules of thumb are for an initial estimate, not actual design.

 

[GT-EGR]

Have to step back and try to figure out what it is you are trying to do and what is fixed and go from there.

You have a spray booth hood, and that hood came with a fan, and somehow you measured that fans air velocity and translated that it to 3,000 CFM?

Regardless of trying to get through this, your process to get to your result should be

- How much airflow at your paint booth do you actually need? Is it 3,000 CFM, how did you get that number - by some capture velocity calculations at the booth, or even better from the booth manufacturer. Figure this out first before trying to fit everything you do around some fan or coil velocity target

Once you have the actual booth airflow needed, that’s your airflow, then figure out how dehumidifier the air needs to be. Your evaporator coil can be a lot different and possibly need lower or higher velocities depending on if you are trying to bring the air temp down to 45 vs 75. Then you give a manufacturer your CFM, outside air temp, and target leaving air temp, and they can pick an evaporator coil for you

In parallel find fan curve (or pick a fan that gives the airflow you determined previously) that has enough static pressure capability to push or pull through the evaporator coil you selected plus a little extra for the other duct losses and entry/exit losses.

Hope this helps, beyond this you are basically asking us to design a paint booth ventilating system with no direct information and our speculation and guessing has too many variables which makes giving any helpful answer difficult.

 

[inventdv]

ok i said what im trying to do numerous times, im not asking you guys to design a paint booth ventilation system


You have a spray booth hood, and that hood came with a fan, and somehow you measured that fans air velocity and translated that it to 3,000 CFM?

never said i had a booth that came with a fan, what i said was i have a blower that will push intake air and that air has to be dehumidified, this is something i am putting together, my exhaust fan will remove this air, i have a specific question about the maximum or how to reduce the amount of air blowing over an evaporator to avoid overloading? i know there are numourous factors surounding this but i need a basic understanding of this airflow thing

How much airflow at your paint booth do you actually need? Is it 3,000 CFM, how did you get that number - by some capture velocity calculations at the booth, or even better from the booth manufacturer. Figure this out first before trying to fit everything you do around some fan or coil velocity target

the air flow i need in the booth is not the issue, this booth was built not bought, i got this blower cfm by using a velocity meter on the output of the blower and using the size of the opening to determine cfm output of the blower, again this cfm rating has nothing to do with the cfm rating i need in airflow in my booth

i guess if i cant get get pass this point i will just build something out and make mods

 

[MintJulep]

Myself and two other respected members here have taken our time to try and get you to answer some questions about what you need. Because what you have posted is not clear.

Three experienced professionals are asking the same questions.

You stating repeatedly "I've said what I'm trying to do" does not make your goal clearer.

Instead of answering our questions, or putting effort into understanding the hints that we have given you, you decided to become offended and accuse us of insulting you.

A pretty unwarranted sense of entitlement.

This is not how this site, nor the real world works.

Good luck with this project and the rest of your life.

 

[inventdv]

its just the wrong forum
it doesnt need overthinking

 

[MedicineEng]

inventdv:

Assuming that you need to exhaust 3000 cfm and that you want to keep the paint booth neutral, then you need supply 3000cfm as fresh air unless you recirculate some back in (which would be strange for a paint booth and you already ruled that out).
If you need all this air to be dehumidified, then you need to dimension a dehumidifier that is enough to accommodate such volume taking into consideration temperature and RH of your incoming air and the desired temperature and humidity of your supply air. Also, there are limitations on the air velocity that will pass across the chilled and hot water coils, which in the end you will define your overall coils area.

And take it easy with members that are trying to help as I'm sure that you will use their comments to reach your solution.

good luck.

 

[nuuvox000]

Q=vA
Q=volumetric flow rate (cubic ft per minute)
v=velocity (ft per minute)
A=Area (square ft)

Sounds like you read about the rule of thumb that cooling coils should not exceed 400 ft per minute? If you want a slower velocity over the coil (better for dehumidifying) then you need to increase the area (per the above equation). That being said, dehumidifying is not exactly an easy rule-of-thumb thing.

 

[Compositepro]

To achieve once through dehumidification of air for a paint booth you would need to use very large air coils, basically the whole wall opposite the outlet filter wall. There would be one fin-coil that cools the air to the desired dewpoint immediately followed by a second fin-coil that reheats the air. Air-flow would be laminar though the fin-coils and through the paint both.