How fine a filter needed for raw water cooled intake?

[JonnyQuest]

have a couple of different filters and both seem to have larger sized screens than I had expected. I expected needing a fine screen mesh to filter out whatever size the critters are that turn into barnacles or whatever inside an unftered raw water cooled engine.

Any pointers?

Thanks!

 

[RichH]

The typical filtration engineer's 'rule of thumb' is that the 'retention size' should be 4 to 5 times smaller than the smallest 'orfice' in the circuit. If larger than 4-5X then the probability increases that the 'particles' will *bridge* across the 'orfice' and thus block it.

Also ....
When choosing 'smaller retention size' one must always consider the 'total effective (open) surface area' of the filter/screen/strainer so as to not unduly increase the 'pressure drop' (gallons per minute per psid) across the 'filter'. Choosing 'smaller' removal rating size should always be coupled with 'larger' effective surface area. You 'size' such solely on 'pressure drop VERSUS flow rate'.
;-)

 

[Ralph Johnstone]

Or to Put it in Simpler Words ..........................

............... for an engine as would be found in an O'day 27, a 3/4" (pipe size) filter with an SS wire screen opening of about 1/16" opening would be just fine.

That's what was factory installed for my Yanmar 2GM20F and it has no problems

Interesting what you say about about preventing the "seeds" of barnacles getting into the raw water cooling system of an engine. I've never even thought about them getting in there and causing problems but I've been into the engine and attached cooling hoses and never seen signs of any growth.

Maybe the 1/16" screen is keeping the "seeds" out of the cooling paths or the sea water is too oxygen deficient to allow any growth in there.

Any biological types out there able to explain this ?

 

[JonnyQuest]

The local marine supplier says the typical large mesh strainers will be fine and I ordered the one from boaterbits that some here have also used. Still curious though.

 

[Merlin Clark]

Barnacles are not going to grow in the darkness inside the engine block.
There are many tiny, tiny beasties that get sucked into and thru an engine and if the screen was small enough to stop them all you'd spend more time cleaning the screen than motoring.
The off the shelf strainer like Groco protect an engine just fine.

 

[JonnyQuest]

Thanks!

Barnacles are not going to grow in the darkness inside the engine block.
There are many tiny, tiny beasties that get sucked into and thru an engine and if the screen was small enough to stop them all you'd spend more time cleaning the screen than motoring.
The off the shelf strainer like Groco protect an engine just fine.

A friend of a friend (I know, I know...) told me how his friend had opened up his engine to find an assortment of what looked like oysters inside of it that they had to attack with chisels and hammers to remove. Not sure how accurate the story is, but I'm relieved to hear that I won't have an artificial reef growing inside my cooling passageways!

Thanks again for the comments.

 

[Merlin Clark]

Salt builds up and can block passages (not to mention impeller blades) in the HE in a freshwater cooled or the engine in raw water cooled but I've never heard of anything like that. Doesn't mean it didn't happen though but I think that unless there is a constant water current carrying food thru the engine (blocked by seacock and water pump when shut down) the filter feeders will soon perish. These things also are fond of at least some sunlight.
Some people bag thier running gear to keep crap from growing on it and it works.

 

[RichH]

I sail often to the caribbean, etc. and have had both my refrigeration cooling intake and engine intake (both 'downstream' of each strainer) fouled/restricted with 'hard/shell' growth. The fouling in both cases was FAR inside the hosing/piping and also in 'the dark' .... very surprising. I now run RydLyme through these circuits to keep them 'free flowing' when I return 'home'.

The characteristic size of the larval stage of most barnacles and like is 0,5 millimeter or 500 µM .... easily handled by a 'tight' strainer screen. but needs careful selection so to keep the GPM/psid values appropriate.

 

[JonnyQuest]

Here's the filter I have [almost] ready to install

http://www.moyermarine.com/forums/showpost.php?p=39966&postcount=2

(Thanks to Shawn for posting the photo earlier!)

Same as in the thread above. You can see the size of the mesh--probably not in the .5 mm range! This does however appear to be typical of what many people have for raw water straining.

Tuns out I ordered the wrong diameter and need to get a replacement so this part of my latest project is on hold pending a suitable replacement.

More thoughts?