Ducting (pressure drop) = smaller CF? Question!

[JayBee]

I need to get a new fan and filter pretty quickly for my DR120 (1.2 x 1.2 x 2m) - I think I am going to get a 125L (5inch) that is 350cmh fan. However the 5 inch moutainair filter is only 295cmh.

I was thinking that because there will be a little ducting including a bend I was the pressure drop from the ducting would mean the 295cmh carbon filter is ok for this fan? OR should i go for the 150/6inch filter that goes up to 580cmh? Since this is in a tent saving space and weight is a plus.

Thanks All!

JB

 

[Pistol Pete]

You Will Encounter Problems If You Do Not Match Up Your Fans And Filters !!
A 125 mm Fan Uses A CF360
Use A Stronger Fan And Expect The Activated Charcoal Not To Be Able To Absorb Odours
You'll Be Blowing Faster Than The Carbon Filter Is Tailored To
You Could use A Fan Speed Controller To Lower Fan Speed , But IMO It's A Inefficient Way Of Going About It
PP

 

[doc]

im agree the 5 inch mountain air filter will cope this space eazy! mine is connect to rvk 150l ! is yours 1 the small 5 inch or the long 1? mine is long 1! not sure what is cmh of this 1! is very good filter , im like it :)

 

[JayBee]

Sorry, maybe i was not clear. This issue is fan is 350cmh. Filter is 295cmh.

However I know that because i am using ducting to conect the filter to the fan, and the duct will have a bend, that there is be some pressure drop. So I was thinking because the filter will not be attached directly to fan but will be Carbon Filter in tent, then ducting to top of tent with fan on top outside tent, this ducting will create pressure drop so 295cmh filter will be ok to match with fan that is 350cmh. Is this correct?

Thanks!

 

[JayBee]

Another questions would be, if filter is inside tent, and fan is outside, on top of vent. With ducting connecting the two I wonder if anyone knows what the pressure drop would be for a 125/5inch rvk 350cmh?

JB

 

[eyes]

simple. get a variac speed controller.will control the speed of your fan to match the filter.actually run your filter at less cfm and it will not tax the filter and allow for proper contact time of smell.this speed controller will not have your fan humming like the cheapos.hope this helps.problem solved.

problem 2.anytime u place bends or length in duct you cause static pressure.Nobody can tell you what the drop in cfms will be .get a cfm meter to measure the proper cfms at the end of your duct.

 

[thefabman]

there will be no significant pressure drop unless you are putting alot of bends or running a long ways. Most inline fans are rated for CFM and Pressure over a 50' run, and each bend being equal to 5' of ducting. Your fan is only rated 55 cfm higher than you filter. You should be right on track to pull across your filter just fine. Also these fans are designed to pull, not push a long distance.

Just my opinion from 25 years in the HVAC business...

 

[JayBee]

Thefabman, thanks for the great info! Each bend = 5' ducting is a great rule to know and should be posted in FAQ. Also did not realize rating is over 50' run, also good info to know.

So just to confirm, i will be ok with the filter being 295 and fan 350. I should note i am talking cubic meters per hour, so in CFM is actually only about 30 difference.

Another question for you fabman- I want to use passive intake with this set up (5inch extractor), however I need to put some sort of filter on the passive intake to help with light, dust, bugs, etc. I am planning to use bit of carbon fiber, if I use eight inch passive intake opening, will this increased size make up for the loss of flow because of the carbon fiber (like the carbon pads used in kitchen exhaust hood).?

Really appreciate your input and expertise Fabman!

Thanks!

JB

 

[hoosierdaddy]

I agree with fabman 100%. Almost a perfect match, actually.

With a 5" exhaust duct size, you would want about 10% larger opening at intake to have sufficient air to satisfy the system. However, with the filtering material, you will have restricted that flow somewhat, and there is really no formula available to tell you how much you restricted it.

What I would suggest you do is first figure what size passive intake you really need, assuming it's not covered. A 5" exhaust will require a 19.6" intake +10%.
Now, since you are covering the intakes with a somewhat restrictive material, the holes need to be bigger to allow the sufficient air. I suggest double the needed size.
You could start out by just cutting the appropriate sized holes first, allowing the 19.6" of air in. Then run the box and see what you have temp wise. This can serve as a baseline.
Now cut the holes to allow the 39.26 inches of air in, and cover them up. Now see where you are temp wise. If hotter than your control temp, cut another port.

 

[JayBee]

I dont understand about the intake, 5" plus 10% = 19.6?? Never heard of someone with such a big intake.

Thanks!

JB

 

[thefabman]

JB--With a passive intake 8-10 with a filter will suit you fine. I am not sure the exact pressure drop with the carbon filter, but if you plan 25% loss for most filters you will get it right...ie intake through filter to be 25% larger...

 

[doc]

im use the rvk125 and 5 inch mountain air c/filter , try to keeps bends long and sweeping, not right angle bends! normal ducting is porus and this alone will reduce the cfm of the fan! im like to use molded bends for ducting and use the insulated ducting! my filter is connect straight to fan, no ducting, but ducting on the exhaust side! is kept short as poss with the lightest of bends, works well!

if you have probs check out www.growell.co.uk and look there at there filter fan packages, it show to you correct filter for your rvk125!


mine is all good

 

[JayBee]

right on doc. are you running the A or L? On growell i only see 125A but i am going to be getting the 125L.

Question though, did the mountainair come with a prefilter?

thanx-

JB

 

[hoosierdaddy]

I dont understand about the intake, 5" plus 10% = 19.6?? Never heard of someone with such a big intake.

Thanks!

JB

A 5" intake duct has an open surface area of 19.63 square inches.
If the exhaust duct opening is a 5" hole, then the intake needs to be 10% larger than the exit hole, to provide the sufficient amount of air.
SO, if you cut a 6" hole, instead of a 5" for intake, you will then have 28.27 square inches of opening, which is well over the 10% additional opening area needed.

Hope that made things clearer.

 

[JayBee]

Thanks Hoosierdaddy that does help. I gotta admit that the fabman has a slightly easier to understand style, but do appreciate your thoroughness :)

Best regards,

JB

 

[hoosierdaddy]

I am not giving you style, just facts.
Airflow dynamics are carved in stone, and there is no style involved.
You needed both of our info to have something to work with.