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Thread: In-line or centrigugal blower ? Which is better ?

  1. #1
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    Default In-line or centrigugal blower ? Which is better ?

    To exhaust the space which includes the engine room and rear-deck, I have a pair of centrifugal blowers installed on the transom of my twin-engined cruiser.

    I was thinking of installing a pair of blowers up-front, right by the engines, to augment the air-flow from the louvers (plus air-boxes) installed on the hull sides.

    On a comparable air-flow rate (CFM) basis, the in-line blowers are about a quarter of the price and consume a half/third of the power , in relation to their centrifugal counterparts.

    Can anyone please advise as to any (hidden ?) attributes of centrifugal blowers that are not readily apparent on the spec sheet ?

  2. #2
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    Hello carioca,
    If you want to displace air from a room/cabin into open atmosphere, my vote will be for an exhaust fan. Centrifugal blowers are required if a bit greater suction and pressure is required to overcome the friction of duct-work or similar arrangements.

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    Hello Syed,
    Nice to see you back and many thanks for the input !

    I need to blow air over the engines to cool things down a bit and have the exhausting centrifugal blowers pump out the heated air.

    Given that turbo-diesels create a low-pressure region in the space surrounding them, in-line blowers/fans would seem to fit the bill pretty well, as per your reasoning.

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    I'd never given the subject a moment's thought.
    http://www.aeroflo.com/product.htm

    Huge choice.
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    Quote Originally Posted by Syed View Post
    Hello carioca,
    If you want to displace air from a room/cabin into open atmosphere, my vote will be for an exhaust fan. Centrifugal blowers are required if a bit greater suction and pressure is required to overcome the friction of duct-work or similar arrangements.
    This is my understanding as well. Centrifugal blowers will deliver their rated CFM against a significant pressure differential. Inline fan type blowers (which I have on my boat, btw) are significantly hindered by the friction of long or convoluted duct runs, but are well suited to moving air from one side of a wall to another.
    No adversary is worse than bad advice.

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    I know very little about venting motor spaces, but I have dealt a little bit with fans. A single CFM figure is all but useless except as a basis of comparison with fans of a very similar design. What you really want is a graph or chart that shows the CFM at different static pressures. This will tell you how quickly the power of the fan falls off as resistance from ductwork increases. Applying this information in a strict way to an engine compartment would not be easy (how much resistance to air flow is created by all the stuff in the compartment?), but such graphs or charts would at least give you a feel for what you are dealing with. A fan where the CFM's fall off very quickly with increased static pressure is going to be less effective in most real world situations than a fan that does better against static pressure.

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    All I've ever seen for venting ERs is centrifugal blowers, but like PI, I'd never given it a moment's thought. Of course, most ERs are vented through ducts.

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    Thanks everyone for the tips.

    Ventilation systems assisted by blowers used in automobiles
    are designed to work with the windows rolled up, that is, supply a satisfactory flow of air against static pressure, which is the point made by Bruce Hooke.

    Does anyone know what type of blower is normally used in this application ? Centrifugal or in-line ?

    Actually just one model of 12 V DC centrifugal blower on Surplus Centre´s site is specified for CFM in open-flow as well as for two levels of static pressure. Boat chandlers´ sites don´t provide any such detailed info, excepting that their stuff is salt-water proof.

    Given a low-pressure region in the vicinity of running turbo-diesels, and 4" dia. 250 CFM in-line blowers installed on bulkheads immediately in front of these, blowing air down via 30cms. of 4" dia. ducting......... should do the job ?

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    Automotive blowers are squirrel cage/centrifugal style.
    250CFM sounds pretty high for a 4" inline fan.
    Residential bathroom vents,as cheesy as they are, are generally between 50-100 CFM,on 4" duct.
    Kitchen vent fans,for mortals,are about twice that, on 5-6"
    Rich folks kitchen vents sometimes approach 900CFM on 8'-10"duct.

    IMHO, if you get too much airflow,you will pressurize the engine room and drive fumes into living spaces.
    We did it by pointing the engine room vent cowls forward,for "better ventilation".
    When we were running or at anchor,the breeze blew down the vent hoods and seriously stunk up the boat.
    When we turned them to face aft,the Bernoulli Effect took hold and Voila,no more engine room stink.
    R
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    Boyle,
    What you refer to as an exhaust fan I call blower. Both may blow air in, or extract it out of a space, depending on which way it is installed. Thanks anyway for the surplus centre address which I have now bookmarked - a great deal of very good stuff, very attractively priced.

    Ron,
    As per Bruce Hooke above, CFM specs are only as good as prevailing static pressure conditions. So little wonder that centrifugal blowers are fitted in cars.
    Thanks for the tip about engine room pressurization. The only louvers that my boat has are a pair on either side of the hull to feed the engines. Judging by the heat in the engine room, the latter could do with some more air in order to carry excess heat away to the centrifugal blowers already installed on the transom.
    For a quick glance at some brands, types, models and prices of blowers:
    http://www.defender.com/product.jsp?path=-1|6880|46775|732133&id=570334
    Last edited by carioca1232001; 03-31-2007 at 06:20 AM. Reason: change of word

  11. #11
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    One type of blower to consider is the tubeaxial fan. I have used them in a number of engine rooms and with an AC or DC drive the fans can be controlled easily. Installed in ductwork nothing beats them for CFM under different static pressures.

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    Graingers (http://www.graingers.com) and McMaster (http://www.mcmaster.com) have a huge selection of fans, some of which are rated for 12 volts. Graingers only sells wholesale, but McMaster sells retail. The McMaster catalog is hard to get (the only send it to high-volume buyers), but their website is very effective (much better than the Graingers website).

    Both Graingers and McMaster list detailed fan specs on their website. Other retailers may be able to provide more detailed specs if you ask, and sometimes, if you can identify the manufacturer then you can go to their website and get more detailed specs. Of course this tends to work best with name brand manufactures. Cheaper stuff made overseas and sold under many brand labels is going to be harder to get information on.
    Last edited by Bruce Hooke; 03-31-2007 at 03:38 PM.

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    dunno about the interior volume of your engine compartment and all the engine info etc.....but....when I installed the 37 hp yanmar in Tana Mari I had 28 inches clear around the engine and about 12 inches above. The air intake for the engine alone specified that at least a 4 inch diameter forced air intake had to be available for cooling and forthe fuel to burn, anything less would cause overheating and/or provide insufficient air for combustion..
    (I had a tight space, so I put in an air block that I could close and a remote control to dump an 60 cube bottle of CO2 in there in case of a runaway engine)
    I betcha...someplace there is a spec for your engines and it will include other info as to the proper volume for your size engine compartment.
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    One reason to consider using a marinized engine room fan is spark control. Diesel is a lot less prone to explosion than gas but better safe than sorry. You might be able to find an industrial fan that is sparkless for less than chandlery prices.

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    The sparkproof part is easy......you have two fans, one pulling air in and another blowing out keeping a low pressure in the engine room. On a bulkhead or inside planking make a boxto house the fan, cover the air intake with a copper/bronze screen, make sure it is grounded. The fan blows into the box, and at the other end a fan is sucking out the box through a hose to the outside, also covered with bronze/copper wire screen.......solves the problem. Use 6" fans.
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  16. #16
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    Quote Originally Posted by paladin View Post
    dunno about the interior volume of your engine compartment and all the engine info etc.....but....when I installed the 37 hp yanmar in Tana Mari I had 28 inches clear around the engine and about 12 inches above. The air intake for the engine alone specified that at least a 4 inch diameter forced air intake had to be available for cooling and forthe fuel to burn, anything less would cause overheating and/or provide insufficient air for combustion..
    (I had a tight space, so I put in an air block that I could close and a remote control to dump an 60 cube bottle of CO2 in there in case of a runaway engine)
    I betcha...someplace there is a spec for your engines and it will include other info as to the proper volume for your size engine compartment.
    Now I see a benchmark, viz., 4 ' dia forced air intake for engine-space cooling and diesel combustion, applicable to a 37 HP Yanmar.

    My MWM´s are 4-cylinder truck turbo-diesels that were DIY marinised. The 6-cylinder version of this engine is professionally marinised by MTU, but not the 4.
    A 6 was being overhauled this last week at the club. A simple gasket between the exhaust manifold and the turbo costs just US$ 200 ! Mine are copper-plated (over asbestos) and fabricated by a craftsman for just US$ 20 !

    So I may have to 'play it by ear' for obvious resons, athough it is relatively straight-forward to calculate the mass flow/CFM of the air consumption for these engines at full-load.

    Many thanks to you and all the others for sharing your experience and time.

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    Lack of sufficient will limit engine power output. Not much chance of black soot in the exhaust, though, as the fuel pump is driven by boost level. Not enough air, less fuel injection !

    Thanks for pointing out the engine-driven fan option.

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