Cooling paranoia or common sense?

[Cheechako]

Many pumps suck much worse than they push. The cooler creates resistance, so it should be after. Especially Rubber impellor or centrifugals...

[a64pilot]

Age old argument.
When I was putting together the fuel system for the S2R-H80 aircraft, I put the pump first. Weldon, the pump manufacturer didn't like that, they didn't want their pump pumping dirty fuel, however GE, the engine manufacturer was dead set that the filter had to go after the pump, they didn't want their engine high pressure fuel pump and fuel control possibly getting particles from a worn or damaged pump.
I put the pump first as I wasn't sure the pump could pull enough vacuum to put the filter into bypass, and or that the vacuum at higher altitude wouldn't vapor lock the pump.

[Djarraluda]

Originally Posted by sailorchic34

As a general Rule all heat exchangers should be down stream of the water pump">raw water pump and not on the suction side. Reason being is there is a pressure drop across any heat exchanger and that will effect inlet suction pressure in what is essentially a unpressurized system.

As A64Pilot indicated, pump inlet suction pressure available is always lower then discharge head pressure.

While the typical raw water impeller is positive displacement, it is a low head pump and it works better with all the major head losses (from heat exchanger pressure drops) on the discharge side.

This is a much larger issue with centrifugal pumps which have generally very low NPSH (Net Positive Suction Head) Many an engineer has been caught, not understanding that very issue.

Just to add the technical bit (simplified) in case folks are interested:
Boiling point of water depends on pressure. Low pressure on inlet side of pump therefore risks cavitation (essentially, low pressure boiling). Lower the pressure too much and thus risk problems of cavitation. This can be mild and un-noticed, until the erosion caused by the air bubbles coming out of suspension eats through the metal.
Roger

[GILow]

Ok, I am convinced.

Now the next problem....

Transmission cooler before or after the engine heat exchanger? Transmission is a Velvet Drive 71C and the engine is a 96HP Nissan ED33.

Currently the transmission cooler is on the input side of the raw water pump, so clearly before the engine heat exchanger.

Matt


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[a64pilot]

I'd cool tranny first

[GILow]

Originally Posted by a64pilot

I'd cool tranny first

Yes, makes sense on basic thermodynamics principles, the transmission fluid SHOULD be cooler than the engine water. But, of course, that option is the hardest to plumb in properly.

Sigh.

[Oceanride007]

I'd agree cool the transmission first. Going back a bit, my pump experience was Friction losses on Suction side are twice as detrimental than on discharge side of the pump. However, unless your having a problem, I wouldn't bother.
My setup has a Engine oil cooler on suction side of pump, I'm nervouse of Heat exchanger replacement availability and no provision for Anode, due to the above mentioned problem of vapour pressure issues. I'll change it, thanks for bringing it to my attention.

And as a aside pumps need to be protected by a course filter on suction side, Fine filters are after the pump, that is for oil and water.

[GILow]

Originally Posted by Oceanride007

I'd agree cool the transmission first. Going back a bit, my pump experience was Friction losses on Suction side are twice as detrimental than on discharge side of the pump. However, unless your having a problem, I wouldn't bother.
.

I am not having problems as such. But I have never been satisfied with the total cooling flow. When I saw a 20 HP kobota conversion yesterday that had nearly double our cooling flow I decided it was time for me to revisit the whole cooling setup. I found a reference to needing a flow of around 80 litres a minute for our engine size. We would be less than half that, probably a lot less than half.


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[knockabout]

if it's not broken don't fix it / air compresses liquids don't / if enough air bleeds into the system and makes it to the pump the pump then compresses air and the check valve opens and shuts and makes a screaming noise but doesn't pump fluid / if your system has been working fine for years don't change it

[GILow]

Originally Posted by knockabout

if it's not broken don't fix it / air compresses liquids don't / if enough air bleeds into the system and makes it to the pump the pump then compresses air and the check valve opens and shuts and makes a screaming noise but doesn't pump fluid / if your system has been working fine for years don't change it

Sorry, but I don't really agree with that philosophy. There is no room for systems with no safety margin on a boat IMHO. The cooling system copes, but that is all. I don't want to be caught out when it really matters because some component such as the impeller was slightly worn. As I mentioned, the system flow rate is well below recommendations. I would rather it was in line with best practices.


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[svseachange]

Originally Posted by NorthCoastJoe

One 90 degree elbow creates a large friction loss, anything disturbing the flow will make the pump work harder. I would place everything down stream of the pump and use high quality hose and fittings.

Thanks for this. I put a 90 degree bend on my new strainer. Wasn't thinking about friction loss. Will reduce the bend to at least 60 degrees I'd possible.

Has anybody added an additional electric raw water pump to assist the mechanical pump? Would almost be nice to have one inline as a backup.

[Gts]

Two other factors to consider:
1) If any part of the cooler is above the pump suction, you could have an air bubble in the top of the cooler reducing its efficiency.
2) Consider a small HX tube leak. Do you want raw water entering the oil (if raw water pump discharge pressure greater than the oil pressure)?

[GILow]

Originally Posted by Gts

Two other factors to consider:
1) If any part of the cooler is above the pump suction, you could have an air bubble in the top of the cooler reducing its efficiency.
2) Consider a small HX tube leak. Do you want raw water entering the oil (if raw water pump discharge pressure greater than the oil pressure)?

Very interesting points, both of them. Neither is likely to help me sleep better at night, thank you.

But good points.

Sigh... boats... I'm starting to think the guys in camper vans are the smart ones.

[Gts]

Originally Posted by GILow

Very interesting points, both of them. Neither is likely to help me sleep better at night, thank you.



But good points.



Sigh... boats... I'm starting to think the guys in camper vans are the smart ones.

Matt,

Ensure the tranny cooler (shell side) is rated for the raw water pump's discharge pressure before thinking about relocating it.

Gerry

[GILow]

Originally Posted by Gts

Matt,

Ensure the tranny cooler (shell side) is rated for the raw water pump's discharge pressure before thinking about relocating it.

Gerry

Interesting. No idea the make, model or age. At least it is the sort that does not have any seals or end caps so there's not a lot to go wrong with it or to fail under pressure. Also I can't see any reason for it to be under significant pressure as the cooling flow is pretty well unrestricted after the pump. At least in theory.


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