Engine room ventilation

Discussion in 'Boat Design' started by an2reir, Feb 8, 2018.

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  1. an2reir
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    an2reir fifty boat designs

    Hi there


    I am working on the design of an engine room . The diesel is
    Yanmar 4JH80 - RATED OUTPUT 58.8 kW / 80 mhp
    Volume E.R : 2.1 m3.
    The ventilation system of the engine room is forced exhaust and natural air intake .
    The exhaust section surface is 45.4 cm² ( recommended by Yanmar) and the exhaust blower is 12 cu meters @min ( recommended by Yanmar)
    The ventilation intake section surface is 340 cm²- recommended by Yanmar
    How to make sure the ventilator does not force in too much air out in regards of the air-in vent capacity : this would result in an excessive ER low pressure. The engine itself is sucking air for its internal combustion ( even if it's best to keep the ER at a slightly lower pressure than the surrounding accomodation spaces ) and it could be lacking air in an excessive low pressure situation.
    1.In this particular case of a forced exhaust and natural intake ventilation system does the airflow increase if I increase the intake ventilation surface from minimum recommended to double or triple ?
    2. Is there a way to calculate the airflow brought in via a range of increasing ventilation intake surfaces ( say 340 cm².; 500 cm²; 1000 cm²) by the forced extraction exhaust blower 12 cu meters @min + the diesel air consumption ? Thank you.
     
    Last edited: Feb 8, 2018
  2. TANSL
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    TANSL Senior Member

    The parameters that I have always used, in work boats, are:
    - trigger overpressure in the engine room, which may suggest forced intake of air, and
    - fan that allows a renewal of the air of the engine room of 60 times per hour + diesel air comsumption.
     
  3. Barry
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    Barry Senior Member

    While you could calculate the value if you knew the performance curve of the fan for various inlet pressures to the fan blade, assuming that the exhaust fan is bladed, the rpm of the engine will affect the amount of air consumed and also coming in the inlet vent.

    I cannot agree with Tansl that you want to have a positive pressure in the engine compartment. In order to have a positive pressure in the engine room, then the fan/blower must sit on the intake vent and pump air into the engine room. Additionally, the fan would have to be able to provide enough air to overcome the air venting out the back exhaust vent as well as the cubic meters per hour that the engine is consuming. Ie big numbers.

    While large (say over 20 meters) boat engine rooms may be completely isolated from crew quarters, living areas and even a cockpit, many pleasure boats may have engines under the cabin sole. These engine rooms most likely will not be 100% sealed to these habitable areas so a positive pressure in the engine room will be able to push dangerous fumes through cracks or poorly sealed doors etc.
    If you have positive pressure in the engine room when the engine is running ( which is pumping air out of the engine compartment as well) then the pressure will increase when the engine shuts off. Dangerous fumes could be, CO that has migrated into an engine room bilge, smoke from a engine room fire, steam from a ruptured water hose running over a hot engine component, overcharged battery h2s and even engine room bilge odors. You do not want any of these fumes etc being able to migrate into living quarters or even into the rear cockpit.

    I would follow Yanmars specifications. They have figured this out. You could increase the intake area, with no detrimental effects with the only change of a decreasing inlet speed over the
    vent area.

    I will check the Volvo Diesel installation/design book and see what they say. I believe that they calculate minimum areas for inlet and outlet, not optimum values.
    I also think that they have a notation of changes per hour of the engine compartment.





     
  4. gonzo
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    gonzo Senior Member

    The rule of thumb for natural engine room airflow is 1/2 square inch per HP. You need to calculate the extra area to accommodate the air flow for the fan.
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    All our boats have positive pressure in the ERs

    Then you're not building to Class/Flag rules. Since ER must be sealed to prevent the passage of gases/vapours.

    An ER that "could" allow the passage of vapours directly into an accommodation area is clearly highly dangerous.
     
  6. PAR
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    PAR Yacht Designer/Builder

    To solve negative pressure spaces starving induction and compartment supplies, it's often best to simply separate the two. A separate natural supply, vented directly to the induction intake, in addition to manufacture compartment recommendations, will insure two things. First is a cold (er) air supply to the induction, which is always best and free HP, plus the engine will have sufficient forced compartment exhaust. It would be nice if all compartments could be truly sealed, but in reality, particularly on smaller yachts and working craft, this isn't the case, so a positive pressurization routine seems to always push compartment smells and gases into these spaces. Unless building to a standard that can insure a true seal, I've found a negative pressure system to be much easier to live with on a yacht or work boat. Bleed cold air from the induction side of this divided system, can provide a level of redundancy, if the exhaust air side needs a little help occasionally. So, I guess I agree with both John and Barry, but in pleasure craft (as I and he have mentioned) negative is the way to go.
     
  7. Barry
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    Barry Senior Member

    As I mentioned above, "unless building to a standard that can ensure a true seal" a negative pressure in the engine room is better than one that is positive so as to exhaust the outside through the vented area so as to put a draw through those non sealed areas. I checked with ABYC as I was curious as to what they say about this wrt to diesels and their requirements are
    stated in H32-.5.3 notes 2 Due to the characteristics of diesel fuel, and the closed nature of the diesel engine fuel system, neither mechanical nor natural ventilation as prescribed for gasoline powered vessels is necessary to remove diesel fuel vapors.

    In North America, the major volume manufacturers build to ABYC specifications for pleasure and work boats not needing a certification to carry paying passengers, and in boats up
    to say 50 feet, I don't recall seeing a completely sealed engine room. Certainly larger yachts have the room to make distinct and sealed engine rooms.
     
  8. Barry
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    Barry Senior Member

    An2reir
    Volvo D5 series Installation Manual (Page 175 of 212) https://www.manualslib.com/manual/1105814/Volvo-D5-Series.html?page=175#manual

    This section from Volvo's Marine installation manual includes the method of calculating air intakes and exhaust venting and discusses heat dissipation
    Note the wording regarding Engine Room Depression " a small negative pressure prevents gases from entering the boat"
    Take note that the Volvo manual contains a pretty concise source of information for installation of any marine diesel engine, wiring, controls, and fuel systems
     
  9. an2reir
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    an2reir fifty boat designs

    Thank you all for your messages. All of them are well received and given consideration and thought. In the meantime I did model a simplified model the engine room in Solidworks and I did three setting up tests with the Solidworks airflow simultation with intake ventilations of different surfaces. The FloXpress inbuilt Solidworks flow simulation module gives me the airflow visual trajectories and the speed of the airflow along the trajectory. It may be quite useful as I am trying to optimise as well the shape of thje intake vent and of the intake waterbox.
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    Very different regs from UK. Viz:-

    "14.1 General
    14.1.1 The boundary of the engine space should, with special consideration
    given to fire flaps, be arranged to contain the fire extinguishing
    medium, i.e. the engine space should be capable of being closed
    down in order that the fire extinguishing medium cannot escape. Any
    fans located within, or feeding a machinery space, together with fuel
    and oil transfer pumps, centrifuges etc… should be capable of being
    stopped from outside the space in the event of a fire. Systems
    comprising of automatic stopping of fans in the event of a fire should
    be supplemented with a manual override...."

    "
    5.6.1 A ventilator should be of efficient construction and, where situated on the
    weather deck and not complying with Section 5.6.3, should be provided with a
    readily available means of weathertight closure, consideration should be given
    to requirements of Fire Protection (Section 14).

    5.6.2 A ventilator should be kept as far inboard as practicable and the height above
    the deck of the ventilator opening should be sufficient to prevent the ready
    admission of water when the vessel is heeled (see Sections 11.3, 11.4, 11.6
    and 11.8)..

    5.6.3 A ventilator which must be kept open, e.g. for the supply of air to machinery
    or for the discharge of noxious or flammable gases, should be specially
    considered with respect to its location and height above deck having regard to
    Section 5.6.2 and the downflooding angle (see Sections 11.3, 11.4, 11.6 and
    11.7)...."

     
  11. an2reir
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    an2reir fifty boat designs

    Thanks Barry, I think the rules are quite similar with the RINA and the European Recreational Craft Directive. This is a high end 68 ft carbon composite Sail Catamaran therefore in my opinion to be compliant with the EU rules and as well with the US rules
     
  12. Barry
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    Barry Senior Member

    The ABYC standards include similar requirements or recommendations. I could not find a specific reference to a requirement that the engine room must be weather tight to all other onboard spaces that communicate with the engine room. There was a notation that said that bulkheads on engine rooms should not have more than a 2% openings wrt to the bulkhead area, obviously to minimize cross contamination of vapor, smoke, odors etc. A slight negative pressure ensures that vapors would not be pushed into other compartments while a higher pressure
    engine compartment will push vapors anywhere open to engine compartment. As 350 hp engine will require a lot of air for combustion and depending on the engine compartment perhaps
    more air flow for heat dissipation, a blower hooked to the intake of an engine compartment would require a large intake blower to provide a positive pressure in the engine room as it would have to provide enough air to overcome the volume required for engine combustion to create positive pressure.
    A concern that I would have is that if an engine room fire occurred and the engine shut down, a standard, then the large intake blower would be able to force smoke throughout any opening
    say a cockpit, through bulkheads and on small boats through the bilge area, from the bow to the stern to allow accumulated condensation water etc to flow to aft mounted bilge pumps.
    While with an exhaust fan/blower, these common portals would be under a slight negative pressure on these other compartments and push it out the exhaust side vent.
    Note that the ABYC standard does not apply to large commercial boats but provides standards for small craft. Unfortunately they don't define small craft but I have seen quite a few pleasure boat manufacturers who build boats to 65 feet, state that the boats are built to ABYC standards.
     
  13. an2reir
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    an2reir fifty boat designs

     
  14. an2reir
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    an2reir fifty boat designs

    Thank you for your messages, they are all well received. I did this above Solidworks CFD simulation of the engine room ventilation system in three test cases: 1 . with the inlet direct in the engine room, 2 with the inlet as the above video at the waterbox inlet, 3. with the inlet of larger or smaller areas. I only use the Solidworks FloXpress simulation tool therefore the only data I get is the velocity of the airflow at various points on the airflow. By comparing the speed of the airflow at the inlet and at the outlet in the three test cases I think I can draw some useful conclusion
     

  15. TANSL
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    TANSL Senior Member

    The video is very nice although I do not know what conclusions can be drawn from it. I can think of three comments:
    • the bulkheads in the engine room are watertight, right? That's right, imo.
    • If you do not leave any air outlet, there may be problems.
    • this simulation, assures you that there is enough air for the consumption of the engine and, in addition, that the temperature of the engine room will be kept within the proper limits?
     
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