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Generator cooling air venting

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egansen
egansen Member Posts: 31
I am in the process of installing an older Onan air cooled generator for back up power on my property in a shed.  I am planning on ducting the engine cooling air through wall.  Inside the cooling air duct will also be a 2" threaded steel pipe for the engine exhaust mounted concentric.  This way I only need one hole through the wall.  Based on my research the anticpated temperature of the cooling air coming out of the engine will be 140 degrees above the temperature of the room. So in my case I'm assuming the air temp will be around 240 degrees in a 100 degree room without taking into account the additional heat from the exhaust.  There is also a separate exhaust fan to help keep the room cool and a large fresh air inlet to supply air to the room while the equipment is operating.

The wall the pipe will go though is 2x6 wood frame construction with the studs 16" on center.  There is 5/8" fire rated drywall on the inside of the walls and on the celing and ribbed steel siding on the outside.  There is fiberglass insulation between the studs.  The duct is 10" diameter.  What I am trying to figure out how to properly put the pipe through the wall and how far I need to keep it from the ceiling as it runs 6' horizontally before going through the wall. This is sort of an odd situation because the air temp will be hotter than what would be expected for the air from a forced air furnace but cooler than what would be expected for the chimney on a wood stove.

Another thing that I am trying to figure out is what to do for flexible couplings for the 10" pipe to allow for vibration and movement of the generator while running.  Given the temperatures involved I'm guessing that a typical canvas flexible coupling would not perform well.  Could I use a short segment of semi flexible duct to allow for movement?  The exhaust will also have separate flexible couplings to handle movement of the generator.

I also have installed a CO detector in the room as well.

Thoughts and thanks for your suggestions!!

Comments

  • hot_rod
    hot_rod Member Posts: 22,382
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    What is the reason for exhausting that air? Also you need some air into the space for the engine intake air.
    Maybe just a screened louver down low? Hot air out, combustion air in. Then exhaust out the side.

    The red silicone roof boots should handle the exhaust temperature, or just a galvanized steel version.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 8,157
    edited January 2023
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    Does the Air Cooled generator have a fan that forces the air across the engine and generator in order to cool it? How many Cubic Foot per Minute (CFM) of air is required to cool the engine and generator in order to make this operation viable? If the exhaust duct thru the wall is too small then the engine and or the generator may over heat.

    Where is the air that will exhaust thru the wall coming from. If you say "the shed". then how much air is in the shed? A typical 10' x 10' x 8' tall shed has 800 cubic ft of air in it. If the fan requirements above are say 200 CFM, then that shed has only enough air in it to last 4 minutes until there is a complete vacuum inside the shed. That is assuming the shed is air tight... LOL.

    So where is the shed getting the air from, to replace the air leaving thru the duct in the wall? Have you made provisions for that air. And don't forget about combustion air required to operate the engine that turns the generator.

    Is generator designed for outdoor use? If so then enclosing it may not be as easy as building a shed.

    Good luck with this project.

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

  • Jamie Hall
    Jamie Hall Member Posts: 23,554
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    I would be very much opposed to putting an air cooled generator in a closed shed. The cooling air requirements are just too much, especially approaching full load. If a weather cover is needed, then I would use a shed with one wall completely open, with the generator placed near the opposite wall. It would take pretty extreme conditions for rain to reach it, and snow doesn't bother them.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    EdTheHeaterMan
  • egansen
    egansen Member Posts: 31
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    I will try my best to answer your questions.  The 12.5 kw LP powered generator has a crankshaft driven squirrel cage fan that sucks the cooling air across the engine and discharges it.  According to the manual it will move 1600 cfm of cooling air at rated speed.  It also moves 126 cfm of cooling air for the generator (not ducted outside at this time) and 64 cfm of air for combustion.

    I arrived at the 10" size for the cooling air duct by measuring the rectangular area of the discharge and increasing it to a standard round duct size taking taking into account the area lost by adding the 2" exhaust pipe in the center. Total length of the pipe will be approximately 8'.  Provisions will be made to prevent hot air from being recycled into the air intake.

    The exhaust pipe from the engine is 1 1/2" and I increased this to 2" a foot from the exhaust manifold to reduce back pressure.  It has 3 90 degree elbows and an approximate length of about 10' not including the muffler.  There is also provisions for any condensate that forms in the piping to drain.

    The building is 9' X 13' with 8' celings.  It is fully insulated with fiberglass.  The concrete foundation has 1" styrofoam edge insulation and 2" styrofoam under the floor.  I have provided a 24" square louver mounted about 12" off the floor for air into the building and it has a hood to prevent snow and rain from getting sucked into the building.  I do not have the exhaust fan installed yet but am anticipating using a 12" or 14" thermostatic controlled fan mounted near the celing.

    When new the generator could be purchased with an enclosure for outdoor use but I would need to find or build one and that would not be easy.  Even if I did find an enclosure since it is air cooled there is no easy way to keep it warm and ready for operation in the winter hence me building the building.  The building also houses the transfer switch and panel for power distribution to the buildings on the property so it has multiple functions.

    The biggest thing I am trying to figure out is if I can run the 10" pipe through the wall with approximately 2 1/4" between it and the wood studs or if I will need to go to some form of dual wall pipe made for minimal clearance to combustibles.

    Thanks for your help as I realize this is not a typical heating and cooling question.
  • Jamie Hall
    Jamie Hall Member Posts: 23,554
    edited January 2023
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    Have you done the math? That air cooling fan, exhausting through your 10 inch duct, will be going at 33 miles per hour. That's a jolly good breeze. Perhaps more to the point, if that duct has any length at all, never mind bends, it's unlikely that the cooling fan will be able to provide the required cfm. I would earnestly suggest that you plan on using a 14 inch round duct or 12 inch square duct at an absolute minimum, and only then if the duct is perfectly straight and only a foot or two long.

    Your approach to sizing the duct was not unreasonable, but did not take into account that the exhaust from the fan on the generator is into free air, not a confined duct. The difference in pressure and hence reduction in air flow would be considerable.

    You will have to enquire of your local building inspector or fire marshall as to spacing from combustibles; where I am you would need double wall duct or a minimum of 2 inches clearance all around.

    Edit: on further thought, make that 17 inch round or 16 inch square. 14 or 12 is still too small.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    EdTheHeaterMan
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,764
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    Your probably going to want the generator warm and the battery charged when it isn't used. So I would use a little electric baseboard heater. I would pipe the exhaust outside and up out of the way with 2" pipe.

    As far as the combustion air and ventilation air goes I would size an intake louver to handle the total of combustion air and ventilation air and keep the louver up out of the snow.

    I would size an exhaust fan with motorized damper to handle the ventilation load (the combustion air goes out through the exhaust.

    I would have motorized dampers with end switches interlocked so the gen can't run without the louver open. Temp control to start the exhaust fan when the room warms up to 90-100 deg.

    For 12.7Kw (43000btu)

    5 degree temp rise on air would be 8000cfm you would have to move
    10 degree rise 4000cfm
    15 degree rise 2654
    20 degree rise 2000


    You have to get rid of that heat somehow I would forget about ducting and just have an intake louver and a exhaust fan.

    So if it ran in the summer at 100deg OA and you can take a 15 degree rise the room might get to115

    That's my take and crude calculations

    intake louvers have to be oversized due to the resistance of the blades and pest screens.


    Best bet: Contact Buckley Associates in Hanover, Ma. or Newington,ct. They sell fans, louvers and dampers and will probably help you out with the sizing.



  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 8,157
    edited January 2023
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    On a HVAC system that required 1600 CFM of air flow, I might select 16 x 16 duct for that much air flow in order to keep the noise top a minimum. Since this is for a generator that is most likely creating a significant noise when in operation, you may be able to use 12 x 12. But I like @EBEBRATT-Ed's suggestion of an exhaust fan on the wall with an air intake louver on the wall across from it. When the generator is on, operate the exhaust fan. Then you don't need to worry about what duct size is enough. As Jamie mentioned, the longer the duct, the larger it needs to be. The friction loss is measured per foot of duct length.

    You can get self opening/closing louvers to keep the shed from becoming a wind tunnel when the generator is off. Then you can store your lawn mower or priceless diamond collection in the shed without worrying about the weather leaking in.

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

  • pecmsg
    pecmsg Member Posts: 4,915
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    12.5 KW
    Thats a lot of generator WHY?

    An older Onan air cooled supplies dirty power so forget operating anything that has a computer connected to it. And today that's pretty much everything.