Engine Room Ventilation.

[Driver]

Firstly, my apologies for confusing my "vent" with my "ins" and posting previously under the wrong heading. My post is - Twin diesel engine room with two large ventilation fans. No space for any more. What is best, both bringing fresh air in, one in and one out, or both out? Any suggestions, please?

 

[JOHNPEET]

Copied from your “insulation” post

Some initial information required in order to determine the purpose of the fans:

What size engines?
What size/volume engine room?
What other heat emitting equipment is in the engine room?
When do you perceive that you would need to run the fans?
Assume that the air inlet louvres to the engine room are adequately sized?

As an initial response, I would definitely rule out the “both out” option as this would create a slightly negative pressure in the engine room and adversely affect the engine induction.

 

[PCUK]

I was unable to increase the size of the inlets when I doubled the engine horsepower so installed three powerful inlet fans which were ony switched on when under way at cruising speed. These kept the engine happy at full power but needed to be switched off when on reduced power otherwise the wheelhouse filled with engine room fumes which was a good reminder to turn them off.
If you can adjust your inlet sizes there are various charts showing what sizes are needed depending on engine output.

 

[Driver]

The engines are Volvo D9 575hp each in a tight engine room. No other significant heat emitters. The fans come on automatically when the engines are running and stay on for a further 10 minutes. Impossible to alter the inlet sizes (or much else for that matter) without hacking at the the nice permanent furniture.

 

[ChromeDome]

.. the wheelhouse filled with engine room fumes which was a good reminder to turn them off.

The primary reason for actively sucking air out. Of course, supply to the engine room must be sufficient for the fans plus the engines' intake at WOT.

Edit: Engines can have direct air supply by ducting to dedicated intakes (won't have to be larger than the air filter intake).

 

[RAI]

The engines have a nice water-cooled temperature control system. The alternators only have engine compartment air for cooling. So blowing cold air in will help the alternators. But it may cause hot engine smells, if the engine compartment is not suitably sealed.
So maybe one in, blowing towards the alternator and one out, to reduce smells escaping to the cabins.

 

[JOHNPEET]

Diesel engines consume an incredible amount of air which varies only with RPM, not load unless turbocharged/supercharged

If the engine intakes are ducted to outside, this is obviously the most efficient option for the engines as the air will be cooler and therefore more dense. If this is the case, the fans will just need to be sized to remove the heat radiated from the engine and ancillaries. In this instance, I would suggest one fan sucking and one fan blowing in order to have a balanced throughput of air to the engine space.

If the engine intakes are from the engine space then the engine air consumption itself can play a big part in keeping the engine space cool. If the air intake louvres are undersized, then as has already been mentioned, if fans are used to pressurise the engine space, then there is a risk of leakage of air including exhaust fumes and engine smells into the accomodation area above (depending on the boat layout of course) Also as has been mentioned, the fans need to be sized for max RPM/load engine consumption plus the excess for cooling. Remember, you also need to provide a route for the excess cooling air to spill out of the engine space.

Overall, I would recommend liaising with the engine manufacturer on the sizing of air inlet louvres and the requirement for excess cooling air

 

[AndieMac]

A clean and vented engine room technically shouldn’t have smells emanating from that space either while the engines are running or not. If there is then the sources need sorting asap.
Dirty bilges, exhaust gases leaking from failed manifold gaskets or anywhere along the exhaust pipe system or diesel leaks from fuel tanks, supply and return lines etc. need priority attention.
Exhaust fumes from the system after it leaves the boat of course is beyond anybodies control.

 

[ChromeDome]

A clean and vented engine room technically shouldn’t have smells emanating from that space either while the engines are running or not. If there is then the sources need sorting asap.
Dirty bilges, exhaust gases leaking from failed manifold gaskets or anywhere along the exhaust pipe system or diesel leaks from fuel tanks, supply and return lines etc. need priority attention.
Exhaust fumes from the system after it leaves the boat of course is beyond anybodies control.

Agreed, in principle. Engines that run for hours get quite hot and can cause various materials to emit odours. Rubber, plastic and more.

If something suddenly isn't tight and clean, I would prefer air to be sucked out of the engine room.
Just like "all" boat manufacturers do.

 

[Momac]

The air intake on the engine is presumably a guide to the size of opening required in the boat hull to supply the engine with combustion air.
I am guessing the air intake on the boat exceeds this by some margin.

Fans are for extraction of fumes or excess heat. They are not for supply of combustion air. So fans should be extract only.

 

[PCUK]

Not in all cases. On many installations fan assistance is required. And as to engines not smelling at all, that's just silly!

 

[ChromeDome]

As per my input in #5 you can make a direct supply to the engines, and then use fan(s) for venting the room around them.

As an example, a 6000 ccm diesel engine with turbo at 3000 rpm needs 36000 liters of air for the combustion.
If the manufacturer installed an air filter with e.g. a 2½" inner diameter intake, that is what you need to provide air for the engine.

The room ventilation then is a matter in its own right... and should be sucked out.

 

[Paul1962]

You may also consider speaking with KPM Marine who specialise (amongst other things) in designing and providing Ventilation systems for Engine rooms.
I do know that they can alter the pitch of the fan blades to increase air flow without altering diameter of the fan.

No affiliation- just a satisfied commercial customer

 

[LittleSister]

As an example, a 6000 ccm diesel engine with turbo at 3000 rpm needs 36000 liters of air for the combustion.
If the manufacturer installed an air filter with e.g. a 2½" inner diameter intake, that is what you need to provide air for the engine.

The room ventilation then is a matter in its own right... and should be sucked out.

I broadly agree, but -

a) 36,000 litres in what time period?

b) It will need something larger than the 2 1/2" air intake (i) to avoid excessive noise, and (ii) if the 'something' is a tube of any significant length, as that will restrict the flow more than (and anyway be additional to) the restriction of the short air filter intake.

c) You will only effectively get the air out if there's somewhere for it to come in*. If the combustion air is separate, then the cooling/ventilation for the space needs an inlet and an outlet. If the ventilation is dependent on the same inlet as the combustion air, it must be capable of easily supplying (as I think you said before) enough for both the engine and the ventilation at WOT, but you could then have just an extractor (or two).

(*Many pub etc. landlords seem oblivious to this simple fact. I used to go to a busy large pub with maybe four or six big extractor fans blasting away full-time making a terrible racket (and probably costing a packet to run), yet the place was still always a fug (this was back in smoking days) because there was no significant air inlet air provided. When someone came in the front door you could hear the fans breathe a sigh of relief, quieten down (did they also speed up, I can't remember for sure) and very audibly start working more effectively. The inwards opening door was very reluctant to close because there was a huge draught of air rushing in through the doorway (the result of the extractor fans). So the landlord had 'solved the problem' by fixing a very strong door closer to that door, so strong that smaller people, e.g women, would struggle to open it. Had some of the fans pumped air in, and some out, the pub would have been much quieter, much less smoky/sweaty, cost less to run, and the front door would have opened and closed easily.)

 

[ChromeDome]

I broadly agree, but -

a) 36,000 litres in what time period?

b) It will need something larger than the 2 1/2" air intake (i) to avoid excessive noise, and (ii) if the 'something' is a tube of any significant length, as that will restrict the flow more than (and anyway be additional to) the restriction of the short air filter intake.

c) You will only effectively get the air out if there's somewhere for it to come in*. If the combustion air is separate, then the cooling/ventilation for the space needs an inlet and an outlet. If the ventilation is dependent on the same inlet as the combustion air, it must be capable of easily supplying (as I think you said before) enough for both the engine and the ventilation at WOT, but you could then have just an extractor (or two).

(*Many pub etc. landlords seem oblivious to this simple fact. I used to go to a busy large pub with maybe four or six big extractor fans blasting away full-time making a terrible racket (and probably costing a packet to run), yet the place was still always a fug (this was back in smoking days) because there was no significant air inlet air provided. When someone came in the front door you could hear the fans breathe a sigh of relief, quieten down (did they also speed up, I can't remember for sure) and very audibly start working more effectively. The inwards opening door was very reluctant to close because there was a huge draught of air rushing in through the doorway (the result of the extractor fans). So the landlord had 'solved the problem' by fixing a very strong door closer to that door, so strong that smaller people, e.g women, would struggle to open it. Had some of the fans pumped air in, and some out, the pub would have been much quieter, much less smoky/sweaty, cost less to run, and the front door would have opened and closed easily.)

a) 36000 to one liter of diesel.
b) 2½ was an example. If you find a 3 or 4" then that is it.
c) Logically the amount of air removed needs to be replaced. Not necessarily by a blower, though - can be passive. And the amount used by the engine is not to be factored in as it is delivered on its own accord.

Your reference to pubs and women is beyond my knowledge

 

[LittleSister]

a) 36000 to one liter of diesel.
b) 2½ was an example. If you find a 3 or 4" then that is it.
c) Logically the amount of air removed needs to be replaced. Not necessarily by a blower, though - can be passive. And the amount used by the engine is not to be factored in as it is delivered on its own accord.

Your reference to pubs and women is beyond my knowledge

a)
b) My point being that if an inlet is in the form of a tube or passage it needs to be larger than the mouth of the air filter inlet to avoid restricting the flow (it takes puff to brow/suck air along a tube, and so reduces the flow), and even if not a tube, a larger opening will be quieter (not something that is an issue on top of an engine in an engine room/compartment, but may be if the air inlet is in earshot of the cockpit or whatever.
c) The OP said he had two openings. In such a case he wants one inlet and one outlet, and either just the outlet or, preferably, both powered, otherwise he will be trying to draw air through gaps in doors and bulkheads etc.

You could ask for guidance and advice on pubs and women in the Lounge, should you wish to, er, bone up on those topics.

 

[penberth3]

a) 36000 to one liter of diesel.
b) 2½ was an example. If you find a 3 or 4" then that is it.
c) Logically the amount of air removed needs to be replaced. Not necessarily by a blower, though - can be passive. And the amount used by the engine is not to be factored in as it is delivered on its own accord.

Your reference to pubs and women is beyond my knowledge

36,000 litres of air to combust one litre of diesel? Are you sure that's right?

 

[AndieMac]

……And as to engines not smelling at all, that's just silly!

We obviously have different standards ;-)

 

[ChromeDome]

36,000 litres of air to combust one litre of diesel? Are you sure that's right?

Had to check, and unfortunately, I made a typo in the rpm (3000 vs. 4000, admittedly higher than most 6-litre diesels will go). For 3000 rpm the figure is 27.000 liters - and it is per minute at max rpm.

It seemed high to me too, but if checking by estimating the engine will need 6 liters to fill the cylinders, 3000 times per minute, you get the result 18.000 litres - completely passive. Then add the boost and HP produced.
27 m3 might be fairly accurate.

I relied on the calculator here: Air Flow (CFM) | Widman International SRL where you'll find an explanation as well.

On the intake ducting dimension it depends on the internal resistance. Hoses, tube, bends and total length. Oversizing could be relevant.

 

[JOHNPEET]

Had to check, and unfortunately, I made a typo in the rpm (3000 vs. 4000, admittedly higher than most 6-litre diesels will go). For 3000 rpm the figure is 27.000 liters - and it is per minute at max rpm.

It seemed high to me too, but if checking by estimating the engine will need 6 liters to fill the cylinders, 3000 times per minute, you get the result 18.000 litres - completely passive. Then add the boost and HP produced.
27 m3 might be fairly accurate.

I relied on the calculator here: Air Flow (CFM) | Widman International SRL where you'll find an explanation as well.

On the intake ducting dimension it depends on the internal resistance. Hoses, tube, bends and total length. Oversizing could be relevant.

Does your estimate allow for only filling the cylinders on every other stroke - assuming 4 stroke?