Best Fan Type to Push or Pull Air?

Discussion in 'Hydrodynamics and Aerodynamics' started by CatBuilder, May 20, 2012.

  1. CatBuilder

    CatBuilder Previous Member

    I am looking ahead to fan types for a couple of applications I don't want to go into full detail on yet (so as not to create thread drift).

    I'm looking to find what the best fans shapes are for pulling air through a heat exchanger (like an air conditioner's evaporator coil) and for pushing air through that same heat exchanger.

    Ideally, I'm looking for high CFM, low amp draw and ability to push or pull the air through the heat exchanger.

    What type of fan geometry is best (and most efficient) for doing this?
  2. Stumble
    Joined: Oct 2008
    Posts: 1,913
    Likes: 73, Points: 48, Legacy Rep: 739
    Location: New Orleans

    Stumble Senior Member

    It really depends on the application, how much preassure is needed, the amount of particulate in the air, and what you are shooting for... See and Blowers.pdf for a detailed explanation of different fan types.

    Typically on a boat engine room fans should be positive preassure, meaning they force air into the engine room, with either a matching exhaust fan, or free exhaust. This keeps the engine room slightly pressurized minimizing the amount of dust and water droplets that will land on the engine.
  3. CatBuilder

    CatBuilder Previous Member

    Thanks, Stumble. This is for HVAC. I think they like to use squirrel cage fans, but I wasn't sure...

    I guess it's a blower I'm looking for... based on what I'm starting to read in the link you gave.
  4. DCockey
    Joined: Oct 2009
    Posts: 5,229
    Likes: 634, Points: 113, Legacy Rep: 1485
    Location: Midcoast Maine

    DCockey Senior Member

    Answer depends on:
    - Flow rate
    - Pressure drop characteristics of the heat exchanges
    - Other factors causing resistance / pressure loss
    - Geometry and space available
    - Fan technology available. Shrouded axial flow fans with molded blades that have good airfoil sections are much more efficient than axial flow fans with sheet metal blades.

    For automotive condensors and radiators fans behind the heat exhangers are more efficient than the same technology fans ahead of heat exhangers.
  5. CatBuilder

    CatBuilder Previous Member

    That's the info I was looking for, thanks. It was also in Stumble's link. I needed a starting point to understand which type of fan was more efficient.

    Incidentally, why do they use cheaper, propeller axial fans to blow through automotive radiators? Lower cost? Geometry of installation?

    I think the type of centrifugal fan I need for Hvac is called a crossflow fan. Apparently, the stream of air is more uniform and they are much more quiet.
  6. DCockey
    Joined: Oct 2009
    Posts: 5,229
    Likes: 634, Points: 113, Legacy Rep: 1485
    Location: Midcoast Maine

    DCockey Senior Member

    Geometry, available space, and minimizing need to power the fans are reasons axial flow fans are used for automotive cooling. The transverse dimenisions of the heat exchangers are much larger than the length of the space available ahead of or behind the heat exchangers. And minimizing front end length is desirable for both asthetic and technical reasons. Also it is desirable to not need to run the fan all the time but to be able to use "ram air flow" instead when the cooling requirements are lower and the vehicle is moving.

    I'm skeptical of the typical efficiencies shown for axial flow "propeller" type fans in Stumble's reference. My guess is those are for fans with simple sheet metal blades as shown in the photo in the document. High efficiency automotive cooling fans use molded fans with airfoil sections and tip rings. The blade planview shape and twist are chosen to maximize efficiency and minimize noise. I'm sure there are examples of automotive fans in use today which are less efficient designs but what I described above was in production use twenty years ago.

    I don't think axial flow fans are necessarially less expensive than centrifugal fans. It depends a lot on the materials and manufacturing methods used for both. Automotive HVAC fans typically use centrifugal fans. On the other hand AC compressor & condensor modules which are located outside of buildings typically use axial flow fans.
  7. yipster
    Joined: Oct 2002
    Posts: 3,486
    Likes: 97, Points: 58, Legacy Rep: 1148
    Location: netherlands

    yipster designer

    guess its a heavy application? for cooling smaller stuff you may look at small pc fans, 12v, low consumption, cheap and they come upto 10 inch

  8. CDK
    Joined: Aug 2007
    Posts: 3,324
    Likes: 148, Points: 63, Legacy Rep: 1819
    Location: Adriatic sea

    CDK retired engineer

    The size of the radiator and the fact that most air movement comes from driving, anything else than a propeller would be impractical. But most car heaters have a radial fan: higher pressure, smaller size and less noise.
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.