MapisM
Well-Known Member
+1.
Out of curiosity, can you turn on/off each side?
While obviously it ain't rocket science to wire them separately, it's convenient to run only the side less exposed to salt spray, depending on sea conditions.
Engine Room Fans. Suck or Blow. ?
- Thread starter oldgit
- Start date
Out of curiosity, can you turn on/off each side?
While obviously it ain't rocket science to wire them separately, it's convenient to run only the side less exposed to salt spray, depending on sea conditions.
MapisM
Well-Known Member
I understand overlooking volumetric efficiency, which is neither here nor there, in this context.
But either you were thinking to have DD engines rather than Cat, or you must cruise at 4krpm...?
jfm
Well-Known Member
My maths was wrong - should have said 5mx5mx5m - sorry!I understand overlooking volumetric efficiency, which is neither here nor there, in this context.
But either you were thinking to have DD engines rather than Cat, or you must cruise at 4krpm...?
The factor of 2 that is behind your question is because I assumed 1 bar of turbo boost. I allowed for 4 stroke, 2000rpm, 2 engines
That gives 128,000 litres of air per minute (at atmospheric pressure) which is a 5x5x5 cube (not 50) - that's my mistake, though think you didn't spot it!
No worries!
jfm
Well-Known Member
Absolutely. I have modded my boat to shut off the intake on the upwind/upspray side of the e/room in heavy seas.
sunquest
Well-Known Member
Back to basics, I only use fans after a cruise to help cool and ventilate the engine room. Should they run when the engines are running? They are not switched automatically but, the engine room door feels as if a vacuum is present when opening.
the engines are 350hp cats - good donkeys!
the engines are 350hp cats - good donkeys!
TwoHooter
Well-Known Member
Here's a discussion of this question relating to a 54 ft trawler: http://mvdirona.com/2015/05/engine-room-cooling/
nigelmercier
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TwoHooter
Well-Known Member
I ought perhaps to keep my mouth shut because fluid dynamics ain't my bag, but engine room cooling is of great interest to me, for purely practical reasons (I have an engine room which I think is running a bit hot).
Let's say that "space" means "engine room". When you say 'any fan trying to extract air from this space is going to lose', well I just plain disagree with that bald statement. Whether the fan loses or not is going to depend not just on the demands for air which the engine is making but also on the supply of external air to the space (which in turn depends on the differential between the external and internal air pressures & the volume and shape of the passages through which the air has to pass on its way in), the configuration, power and efficiency of the extractor fan, and the passages through which the extracted air is being driven. And, as illustrated by the blog post I referenced above, you can alter the differential by having fans pressurising the space.
Reductio ad absurdum: (1) A space the size of the Albert Hall, (2) a 2 litre 4 stroke diesel engine hammering away in the middle at full chat with its exhaust going up the usual pipe to the atmosphere (3) all the windows and doors are open, and (4) somewhere in the upper parts of the building there is a Jean St. Germain fan (indoor skydiving vertical windtunnel) blasting air up and out. I think that fan is going to win.
If I'm right, the statement 'any fan trying to extract air from this space is going to lose' needs to be modified to something like: 'any fan trying to extract air from this space is going to need to be very powerful with a clear path to atmosphere, and there is going to need to be a good supply of air to the space at positive pressure'.
EDIT: My answer to Old Git who started all this is: You most likely need two fans in an engine room, one to blow in and one to suck out, if what you want is a cooling airflow. But you didn't say you wanted a cooling airflow. You just asked whether to suck or blow. There's a very rude answer to that question, old chap.
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oldgit
Well-Known Member
Mine suck.Think i will leave it that way. 
TwoHooter
Well-Known Member
MapisM
Well-Known Member
LOL, yeah, funnily I didn't at all.
My train of thoughts (all based on quick mental math, if that counts as a sort of excuse...
) was that 32x2 litres @ 2000 rpm is 128k litres, which sounds pretty similar to the 125k of your original cube.Therefore, since you mentioned atmospheric pressure, I assumed that you didn't consider any boost but forgot to consider the 4 stroke cycle - as you probably guessed from my joke about DD and 4k rpm.
But in doing that, I completely missed the fact that the 125k were cubic meters, not liters, doh...!
Memo to self: never forget the lessons of Dr.Hannibal Lecter: first principles, Clarice. Simplicity.
jfm
Well-Known Member
:encouragement::encouragement:
MapisM
Well-Known Member
Just a thought, 2H:
My old tub, while very far from any Nordie in terms of construction, equipment, etc., is similar in some respects: full D hull, big tank capacity, engines spinning slowly (hence "moving" nowhere near the air volumes of planing boats, at any given length).
And when I bought her, I was also a bit disappointed by how hot the e/r became even after relatively short cruises.
But it didn't take a lot to understand that, more than the engines as such, the problem came from the tanks, progressively getting warmer and warmer because of the diesel in excess flowing back to them.
After fitting an heat exchanger on the fuel return lines, the situation improved dramatically.
And of course, feeding colder fuel to the engines is a good thing regardless of e/r temp.
As I said, just a thought - maybe your return lines are already cooled (that's an embedded feature, in some engines).
But if you feel that the tanks are getting warmer and warmer while cruising, it might be worth checking that.
superheat6k
Well-Known Member
Fans on a small engine compartment whether sucking or blowing won't affect the massive volumes the engines will be using, so for most boats the engine compartment will be at a slightly negative pressure, which means the engine itself will swallow any fumes or smells.
When the engine shuts down having the air drawn out rather than blown in will mean any smells from oil weeps, or stale bilge water will mean these are less likely to permeate into the occupied areas of the boat.
However, and of primary importance is the possibility of forcing leaked exhaust fumes into the occupied areas which could be positively lethal, therefore IMHO drawing out is essential if like most boats you only have one fan provided.
When the engine shuts down having the air drawn out rather than blown in will mean any smells from oil weeps, or stale bilge water will mean these are less likely to permeate into the occupied areas of the boat.
However, and of primary importance is the possibility of forcing leaked exhaust fumes into the occupied areas which could be positively lethal, therefore IMHO drawing out is essential if like most boats you only have one fan provided.
jfm
Well-Known Member
@mapism, yep +1 re return fuel coolers. I have them, integral in c32 and I think they're importnat. My brother didn't have them on his new John Deere 6068s and got a v hot e/room (he has side fuel tanks and a keel tank, all in e/room). He therefore fitted return fuel coolers last winter, and it is much better. DougH also retrofitted coolers to his sskr 66, and got a big improvement
rafiki_
Well-Known Member
I have the same donks as you. My fans run all the time the engines ar keyed on.
Momac
Well-Known Member
Pretty sure the fan extracts on my boat .
Whether it sucks or blows depends on which side of the fan you happen to be standing.
Whether it sucks or blows depends on which side of the fan you happen to be standing.
TwoHooter
Well-Known Member
Good point, and on Dirona (the boat blog I mentioned above) they have fitted heat exchangers on the fuel lines: http://mvdirona.com/trips/Australia2014/Australia11.html?bleat=3/28/2015:+Gateway+Hydraulics
At the moment my problem lies with the exhaust lagging, I'll get that fixed first, possibly introduce a fan blowing air into the engine room after that, then wait until we are doing long passages in warmer climates before doing much else.
Hurricane
Well-Known Member
I thought that my fans supplied air (blow) like JFMs
But I've just finished a full service yesterday so I thought I'd check.
And, in fact, mine extract.
So it got me thinking - I used to be (a long time ago) a heating and vent engineer.
It probably doesn't matter as long as the air change rate is high.
Think about it - the air pressure difference isn't that high between outside and inside the engine room - most of us have huge natural vents in the side of the engine room.
So, for combustion reasons, it doesn't matter as long as the engine room air change rate is high.
However, extracting fans have an advantage when the plant shuts down - overrunning the fans should then take out nasty fumes and make it safer.
I really thought that mine blew into the engine room but it "kind of" makes sense that they extract.
But I've just finished a full service yesterday so I thought I'd check.
And, in fact, mine extract.
So it got me thinking - I used to be (a long time ago) a heating and vent engineer.
It probably doesn't matter as long as the air change rate is high.
Think about it - the air pressure difference isn't that high between outside and inside the engine room - most of us have huge natural vents in the side of the engine room.
So, for combustion reasons, it doesn't matter as long as the engine room air change rate is high.
However, extracting fans have an advantage when the plant shuts down - overrunning the fans should then take out nasty fumes and make it safer.
I really thought that mine blew into the engine room but it "kind of" makes sense that they extract.
MapisM
Well-Known Member
That's a good point M, but I'm not sure your conclusion (extract-only fans) is the best possible answer.
I mean, with the engines off, there's not anymore an obvious path for the air flow (as opposed to engines running, where the flow is bound to be from the vents to the air filters, and eventually out through the exhaust).
In this respect, the setup that sunquest described, which is pretty much the same of my boat, should work even better than extractors-only, in terms of exchanging most of the e/r air volume.
But I'm saying this just intuitively, and with zero specific competence, so I might well be missing something...
