Dryer exhaust dimension rule of thumb

[David]

I have IDC 30lb dryers. They have 6" exhaust. People routinely complain the dryers don't dry, even though they operate at 180 degrees.

All the dryer's exhaust converge into a large manifold. I'm thinking that the manifold could be too small.

I know the make up air should be approx 1 sq.ft. per dryer and I have that in a large metal louvered panel.

Is there a formula or rule of thumb for calculating the size the exhaust?

(Please don't suggest I go through the roof. I can't.)

David

[Anonymous]

Manifolding is a very detailed engineering calculation. I have done this for wet chlorine gas pipelines. To do it correctly you need to sit down and calculate the exact lenght of each duct and the equivalent lengths of all the bends and elbows. Then you need a manometer to check the pressure as you need to be careful you don't create back pressure. You also need to be sure you feed each dryer into the manifold so it does not try to blow back into another pocket. That said, you best bet would probably to find a good HVAC contractor and pay them to calculate it all for you. You may find that if you cannot make the duct large enough that you will need to install a induced draft fan/blower to help you overcome friction losses.

The other approach is just to install a five foot diameter header - its called over engineering :)

[David]

Thanks Kirby, uhh, I think! :)

I do have room to install another duct that will exit the back wall. I may just split the dryers into two manifolds and try that.

I would like to replace the dryers with Huebsch/SQ as I already have many parts/spares for these dryers and I know them much better. But of course, if these dryers don't dry well because of back pressure, then neither will the others, so some over engineering may be just what I need.

[pete f]

If you can't go thru the roof, can you go thru a wall?
If the dryers are back to an outside wall then that is an option.

I have 6 stacks, 2 singles on a trunk line that is about 36" diameter. I would have to measure it. The length is around 20' Dexters, 2 SQ singles, all run fine. What size and how many dryers is your current config? If you buy new SQ's I am sure they will engineer it for you.

[MichaelCa]

David, Alliance wants 1.5 sq. ft. of makeup air, not 1 sq.ft. as ADC wants.
I would check what IDC (?) wants.

Also, you need to take into consideration the length of your run - is it long ? or an unusually high number of turns, or even degree of turns ? (more than 2 will meaningfully impact you)

[David]

If you can't go thru the roof, can you go Thur a wall?
If the dryers are back to an outside wall then that is an option. yeah, that is my only option.

What size and how many dryers is your current config?
10 IDC 30lb single pocket. Approx 5 feet straight out the back of the dryer is a square (36x24) collection duct. The duct runs parallel along the wall as each dryer is ducted straight into the duct. From the last dryer, it is approx 8 feet, with 6" joggle around some electrical.

In my reply to Kirby, I suggested I could just install a similar size duct and put half (5) of the dryers on the new duct, effectively cutting the shared manifold in half. It probably would allow me to install stacks later on if business demands it.
If you buy new SQ's I am sure they will engineer it for you.
I'm sure they would, but I don't think I'll ever be able to buy new stuff!
David, Alliance wants 1.5 sq. ft. of makeup air, not 1 sq.ft. as ADC wants.
I would check what IDC (?) wants.

1.5 hmm, I'll have to measure opening again. to see if I have 1.5 per. That might be a factor.

[pete f]

Seems you have decent outflow, or at least not really a factor..what does your flame look like on the burners? Maybe it is incomming air?
Any easy way to check is to go behind the dryer, observe one running, attaualy do it on a busy time, then open the door to get to the dryers and see if the flames change much. Stand at the front of that door and see if you can feel any air flow comming thru.

[David]

Stand at the front of that door and see if you can feel any air flow comming thru.Seems I've lost my ability to think simple! Thanks Pete!

Ok I'll try again. Maybe I should cut an inspection hole in the duct and see if the things full of lint!

[MSKLAUNDRY.]

IDC's are known for weak blowers (low CFM's) and as such you will likely have lint buildup in your ducts near the dryers outlet.

[David]

Maybe it is incomming air?

I knew I had measured and calculated the makeup air when I bought the store last year. I guess I can't multiply either! :P I remeasured again today and discovered I only have half the size I need. I assume, for a six inch exhaust duct, 1 square foot per dryer. I have ten, so I need approx 10 square feet. I have a 24x32 opening, making a total on 5.3 square feet.

I pulled the duct off the first dryer, and the last dryer. I could find NO standing lint. I checked the duct where it exits the building and, again, did not find any lint.

I think I have found my problem. Not enough makeup air.

After I get another hole cut, I'll try to remember to repost to report the results.

Thanks for your suggestions.

[CharlieS]

Don't forget that your louvers also subtract from the makeup air. That's the reason you see 1.5 or even 2.0 sq ft per vent by some manufacturers.

I have 2 4' x 8' opening servicing 32 dryers. It seems like a lot, but when all the dryers are running, there's a pretty strong breeze coming in these openings.



Charlie

[pete f]

I am not sure what type of building you have, or what type holes you can put in it. 1 cinder block sideways is about 3/4 sq ft of air. On one side I have a space maybe 4x8' of them. I do have a wide mesh screen towards the bottom to keep pests out. Rain has not been a factor. Painted they look OK.

[David]

Don't forget that your louvers also subtract from the makeup air.

I plan to install a larger than needed louver in case the business grows enough to add a couple of stacks plus any loss from louvers.

I have a cinder block rear wall where the current louver is located, so enlarging this should be an easy fix. (famous last words.)

The only potential problem with enlarging this area, is my water heater is between the opening and the dryers, so the risk of freezing pipes might be a factor. (I was told this store was setup by a coin laundry dealer who had been in the business for 20 years. Scary!)

[MichaelCa]

I was faced with the situation of needing 35% more opening (for M.U.Air) than what i had - so, as i prepared to carve open more of my Landlords' 1905 woodframe building, i observed the narrow decorative frame/moulding on the edge of the existing openings: these openings were once actually windows in the ancient past.

Something made me measure the total surface area that this edge moulding covered, and, voila !! ...it was 25%. Off went all the mouldings.

The 2nd part was examining the screen louvers which were actually just very heavy-gauge screen with small diamond-shaped openings - I substituted that for very light-gauge (galvanized) 1/4" square open screen.
That easily made up for the remaining 10% and then some.

Anyways, you may have different needs according to your weather (our coldest days in winter are in the 40's), but definitely calculate how much surface area is being taken in by what appear to be minor
distractions.

[CharlieS]

In Virginia, I would be worried about freezing pipes next to the open louvers.

On the other hand, my hot water heaters are located directly in front of two smaller louvers for air and ventilation. I don't cut off the hot water heaters at night, nor the circulators, because of this.

Charlie

[David]

I installed a new louver, above the original louver. I increased the size by a factor of two. I installed larger than I thought I needed, to plan for the future.

I was finally at the store when all ten dryers were running, and the customers all said the dryers dried fine. They each took about 30-40 minutes, which is typical.

I inspected the exhaust manifold and found NO lint buildup at all. The only thing is that the cover on the outside of the building is capped at a 45 degree angle. I may modify that to allow the air to exit straight out.