engine room ventilation

Discussion in 'Propulsion' started by bam_yat, Nov 10, 2014.

  1. bam_yat
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    bam_yat Junior Member

    Hi, now i'm designing engine room ventilation (air inlet) for a boat with 30 knots speed.the air is from outside.i want to know is the any pressure exist in the ducting?now i have the cfm of the inlet.then what should i do?if i have the area needed for the inlet, how the pressure affect the calculation?
     
  2. TANSL
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    TANSL Senior Member

    What I do to calculate the ventilation of the engine room is considered that the total volume of air must be renewed 6 times per hour (or as often as you see fit). That gives you the necessary flow. Hope this can help.
     
  3. bam_yat
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    bam_yat Junior Member

    how about the pressure?
     
  4. TANSL
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    TANSL Senior Member

    Sorry if I do not look very harsh but I do not care about the pressure. Exhaust fans should have a similar flow to input fans, but nothing more.
    You also have to add the air consumed by the engines, main engines, diesel generator, etc.
     
  5. gonzo
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    gonzo Senior Member

    You want to minimize the negative pressure in the engineroom. One of the rules of thumb is half a square inch of air intake opening per HP. That is about 323 mm2. Depending on air temperature, you may need more ventilation. That value is what the engine needs to run at full power.
     
  6. rxcomposite
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    rxcomposite Senior Member

    bam- are you talking only of ventilation air to remove the heat the engine will generate or is engine air intake (the air required by the engine for combustion) included?

    There are two calculations needed.
     
  7. gonzo
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    gonzo Senior Member

    It is possible to route the air intake out of the engine room for lower temperature, and have blowers for ventilation. I assume that in your area the climate is from warm to hot.
     
  8. PAR
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    PAR Yacht Designer/Builder

    A lot depends on configuration, but I generally try to decrease pressure, very slightly in the engine spaces, with forced ventilation (fans). This keeps smells out of the living spaces, draws them trough the bilge and other interconnections and stuffs them through the engine space ventilation exhaust port. If you use the opposite approach and force air into the space, with an equivalent exhaust fan the smells inside the engine space will billow out, through the bilge and other interconnected areas, which is objectionable to say the least. The simple way to insure you've decreased pressure is to have slightly more exhaust fan capacity then inlet opening area and/or fan CFM.
     
  9. bam_yat
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    bam_yat Junior Member

    thank you TANSL.i will consider those machinery.

    rxcomposite-actually my question is about engine air intake only.maybe after this i need to calculate the exhaust fan too.if u can show me how the calculation method for both intake and exhaust, i think it will help me.

    gonzo-the 323mm2 is the intake area?
     
  10. missinginaction
    Joined: Aug 2007
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    missinginaction Senior Member

    Back a few years ago I was planning the vent system on my 27' Silverton v-drive restoration. Here is what PAR wrote back in September 2011 on another post (I don't think he'll mind a little cut and paste):

    "Often it's best to negatively pressurize the engine compartment. The easy way to do this is with a blower that sucks air out of the compartment. A 3" or 4" in line blower drawing compartment air, typically aft and out the transom or aft deck through clam shells or louvers.

    What this does is suck air through the bilge, down behind ceilings, etc. so that air is always entering the compartment, but most importantly not escaping into living spaces, such as the cabin. If your boat is a planning craft, you can arrange venting to do this naturally, without blowers, but you have to be at speed for it to work, meaning that in low speed or idling situations, compartment smells and fumes can leak into living spaces.

    The rule I use for minimum vent area is the HP divided by 3, which in your case is 44 sq. inches. If you want to be overly cautious then add 10%, making the vent area 48 sq. inches. I think 130 sq. inches is way over the top for your engine, no disrespect Tunnels, though you can't go wrong with this much venting either.

    The rule I use for minimum air flow (blowers) is HP times 3, then subtract 100. This would be 290 CFM (cubic feet per minute). These formulas I've used for many years and are in line with Dave Geer's recommendations too. His formulas are just like mine, though his figures will be slightly lower. Of course these are the minimums. You can't have too much compartment ventilation.

    If you brought your boat to me for this job, I'd install two 3" Rule 135 CFM in line blowers and send it through a 5"x10" louver or two 4"x6" vents. They'd be wired on a ON-OFF-ON switch, with one side slaved to the oil pressure sender. The blowers would come on automatically within a second or two of engine start, assuming you didn't already have the switch turned on and would shut down at engine off or by the switch. If desired, they could be wired to a delay, so they'd continue to run for a few minutes after engine off to vent fume build up.

    Tunnels is correct in that compartment cooling is usually woefully lacking. Engines last longer, run better and are more efficient with lots of air. Again you can't go wrong going big. My simple formulas provide more then enough air and in many cases a lot more then the manufacture's provide."

    I used this advise as a guide in setting up my own system. With a small 220 hp engine and about 200 cubic feet of air space in the compartment I used a couple of 105 CFM blowers to exhaust the space. As PAR pointed out you'll not get engine room smells in the cabin if you have a bit of negative pressure in there. A picture is worth 1000 words.......The blowers cost about $65 US each. I just set them up on a switch and run them for a few minutes before engine start. They stay on while the engine is running. According to the manufacturer they have about 1000 hrs service life. Since they are inexpensive I'll just replace them when they fail
     

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  11. rxcomposite
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    rxcomposite Senior Member

    Here is the tabulated approach from Volvo Penta manual. It covers engine from 255 to 356 Hp range. I have used this in the past and find it quite adequate.

    Note that it includes the air consumption for the engine which is much more than required for the ventilating air. This is a common mistake in engine room cooling. If the air consumption is inadequate, it will suck the ventilating air and cause overheating, or worse, vacuum in the engine bay. Engine air is usually routed by pipe direct to the air filter while engine vent is positioned so that the air will swerve to cover all areas and leave no hotspot for the engine.
    Not covered is the derating factor if the engine room temp or the ambient temp the engine will be operating. Engine power is derated accordingly. You can see this in the engine manual/specs.

    It is recommended to use negative pressure to ensure all volatiles or unpleasant odor is sucked out of the engine room. A small suction fan can do the job.

    What is not shown here, and for good practice, is to increase the vent area by 10% everytime there is a second bend. There is also an increase in vent size if the pipes are doubled (as shown).

    I am attaching also an Excel file I downloaded from the net about engine room ventilation. I have not used this.

    If you are savvy with the math, Catterpilar has a mathematical approach to the problem. You need to know the heat rejection rate of the engine though.
     

    Attached Files:


  12. PAR
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    PAR Yacht Designer/Builder

    In a lot of engine rooms, I try to use a dedicated intake supply, which isolates heat and fumes from the induction as well. It's not pressurized, simply a ducting specifically for the induction side.
     
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