Ventilation question (what else?!)

[glass_affair]

Thought I had it all planned out ...Then I had a HVAC guy come in and now I need more help. I am not sure that what the HVAC guy is suggesting is ok. I am most grateful to those who take the time to look at my problem.

I have a wall mounted vent hood with sides that come down to the table top. The face opening is 36”X 38.5” giving me a 9.8 sq ft opening.

Using the formulas that Mike posted, this would mean a fan with a minimum 784 CFM.

My first concern is that using an 8” duct – the velocity is only 2,246 FPM. Isn’t this a little on the low side? Using a more powerful fan would correct this – however how much is too much? I have read Mike’s post that it is okay to exceed the planned CFM by 10-15%. What if it was 20%? ie 1000 CFM. This would give me a velocity of 2,865 FMP. Isn’t that better?

The other concern I have is the length of the duct work my HVAC guy is proposing. It would be almost 20 feet with two 90 degree bends, one at either end of the 20 foot span. He is suggesting this so that I would be able to use an existing window for make up air and still have my 10 feet of distance between intake and exhaust. The other reason for this long length is so that the noise from the fan can be contained in the room next to my studio (keeping the fan indoors but not in the same room as where I will be working). All good reasons but I am not sure if it is okay to have a run this long.

The static pressure with a 784 CFM fan would be 0.308 . Not knowing much about this, is this amount of SP considerable? When I figured the SP for a 1000 CFM fan it was 0.413 .

I am leaning towards the 1000 CFM fan – because considering the static pressure, I am thinking that the total CFMs are going to be a lot lower anyways. Please, please, please, tell me I am on the right track!

Oh, one more thing…. Would you ever consider a refurbished squirrel cage blower? Or would you recommend to buy new only. Here is a link to a seller who has both - the refurbished ones are on ebay for only about $65.

http://www.theblowerstore.com/ The new one I was thinking of is: BLOWER # 6 LAU DD10-10A / 800 - 1200 cfm . I have just emailed asking for the SP numbers.

If I did go with a used one, how would I know that it is working properly? Can you test the CFMs of a system after installation? Soooo many questions!

Thanks very much!

[MikeAurelius]

The higher the velocity, the more noise generated. That's why I recommend velocities in the 2000 to 2500 FPM range, unless you are using a LOT of frit, powders or do substantial fuming.

The SP number is important because if your fan cannot handle high(er) SP's, it will quickly overheat and burn out.

I would NOT go 20% over - remember that you have to replace every CFM you exhaust.

Refurbed fans are just that - cleaned up usually, but have an old motor, perhaps an old set of bearings. Buy new, then you won't have to replace the fan after a short period of time.

[glass_affair]

Thank you Mike, I'll take your advise and buy new.

I received a reply from the manufacturer about the SF - his answer:the cfm rating is @ 1" WC . I'm not sure what this means - can you tell me if this would be okay with the SP numbers that I came up with :

The static pressure with a 784 CFM fan would be 0.308 . Not knowing much about this, is this amount of SP considerable? When I figured the SP for a 1000 CFM fan it was 0.413 .

Mike, you didn't comment on my concern over the 20 foot length of ducting. Does this mean that it isn't a problem as long as the numbers work?

thanks very much for helping.

[MikeAurelius]

It means the blower is rated 800-1200 CFM at 1" SP.

Don't worry about the .308/.413 SP numbers, as long as the blower/fan you select can put out the required CFM at that SP.

The 20' of duct length is built into the SP number - that's why I didn't comment on it. If you cut the duct back to 5', you SP number would be like 0.03"

[glass_affair]

Hate to be a pain and repeat myself, but I really am trying to make sure I understand what you're saying, so....

It sounds like it would be safe to use this fan (800-1200 CFM at 1" SP) operating at low speed (800 CFM) with most of my bead making and then use the high speed 1200CFM for frit, fuming, enamels etc. Correct? Thanks for your patience!(and of course your advise!).

[MikeAurelius]

The fan you pointed out is actually a 3 speed fan, so I'm going to guess that it is 800/1000/1200 CFM.

Yes, I'd run 800 CFM for your "normal" work set up (based on the 784 CFM calculation), then boost it to 1000 CFM (not 1200) for the other stuff. You will probably have to open additional windows/doors etc to handle the higher CFM for that period of time.

[glass_affair]

Thank you sooooo much! You have given me much needed peace of mind!