Air Conditioning Pump Woes

[capta]

Refrigeration (not A/C) Pump Woes

It seems when we're beating on a port tack the refer s/w cooling pump comes above the waterline and, being a centrifugal pump, it stops pumping.
There is no way to put it lower in the bilge so I am seeking another solution. Could an impeller pump be added in parallel to the centrifugal pump so that underway it could turned on to constantly prime the centrifugal pump? The centrifugal pump does not have the power to pass water through an impeller pump so they cannot be inline. I'm thinking that the system would be pretty much automatic; when not in use, the impeller pump would not affect the operation of the centrifugal pump and no valves should be needed.
We run this pump about 1200 hours a year. No impeller pump I can afford will do this job, so we must primarily use a centrifugal pump.
Any thoughts would be appreciated.

 

[transmitterdan]

It's hard to visualize how a pump down low in the bilge can get above the water line. Does the pump get it's water supply from a dedicated through hull or does it connect through a tee and share with other cooling systems? Is the thru hull getting out of the water when heeled?

It probably will not work to put two pumps in parallel because the centrifugal pump will short circuit and be the path of least resistance. You might try a check valve at the input of the pump. That might prevent losing prime.

 

[Bob J.]

Like Transmitterdan suggested you're losing the prime when on a port tack. Having another pump won't fix that. Relocated your thru hull so the suction side of your pump isn't sucking air.

 

[jeb21851]

you are running air conditioning while sailing???

 

[Bonzai]

Maybe a hose extending from the outer thru hull opening into deeper water would be a simpler fix?

 

[Jackdaw]

you are running air conditioning while sailing???

The boat is 53 feet long. He probably has the dishwasher going as well. I would.

 

[kloudie1]

Your thru-hull discharge should be low at the waterline.. if it is there, try a high loop up under the deck before going down to the thru hull..??

 

[capta]

Sorry all, it's the refrigeration pump not the a/c pump that we're having the problem with. No, we do not run a/c underway, often.
It is a dedicated thru-hull, with a grill on the outside, so no hose possible outside. Relocation of thru-hull is also not an option.
With the positioning of the thru-hull I don't believe it's actually coming out of the water, but it's quite possible the pump is getting above the heeled waterline, causing it to lose prime. Centrifugal pumps seem to have no suction, only push.

 

[Gunni]

A small in-line valved spigot just downstream of your s/w pump will allow you to burp the air from the upstream side of the pump and get flow re-started. Intake, pump and spigot all need to be below water level for this to work.

 

[Jackdaw]

Sorry all, it's the refrigeration pump not the a/c pump that we're having the problem with. No, we do not run a/c underway, often.
It is a dedicated thru-hull, with a grill on the outside, so no hose possible outside. Relocation of thru-hull is also not an option.
With the positioning of the thru-hull I don't believe it's actually coming out of the water, but it's quite possible the pump is getting above the heeled waterline, causing it to lose prime. Centrifugal pumps seem to have no suction, only push.

Right.... This happens to us with our AC cooling water intake. We have to close the seacock to keep water from draining out and losing prime when sailing aggressively. But for us no big deal because AC is a dock-side only treat on a 35 footer. Losing your refrig prime is a bummer, and sounds like a questionable install.

 

[capta]

A small in-line valved spigot just downstream of your s/w pump will allow you to burp the air from the upstream side of the pump and get flow re-started. Intake, pump and spigot all need to be below water level for this to work.

We have a bleed system, but I can't be running to the engine room every 2 minutes when we're sailing. The thru-hull is at the same depth as the main engine and generator intakes, but they are both impeller pumps.
This only happens when we're beating on sails of over 12 hours or so, and we need to run the refer, so it probably never came up before. It works just fine sailing on any other point of sail or tack.

 

[RichH]

The easiest solution is to relocate the pump into the absolute lowest altitude in the boat, even at the bottom of the bilge if necessary and using 'large diameter' in/out hose sizes. This will keep the pump 'flooded' and due to drain back (from either direction) will keep the pump cool and available to 'restart' on its own prime. If the pump is at the absolute lowest position with respect to all other components, any air that is trapped will gravitate 'upwards' and the pump will then 'auto-prime' if the pump is kept well below the dynamic waterline. Of course this may be expensive as all 'hoses' below the waterline should be reinforced/armoured, not some cheapy nylon reinforced 'water hose'.

Question for Capta:
Most such water cooled 12vdc refrig. units with Danfoss compressors are 'dual cooled' and can operate solely without the water cooling. If yours is such an arrangement, simply switch off the mag. drive centrifugal pump and cool solely with air when the sea-state is exceptionally 'rough'. I do as it saves the price of a replacement March mag.drive centrifugal that has lost all it 'prime', and kept on running dry. If youre sailing in HOT tropical waters, the water cooling probably isnt doing a damn bit of good anyway!!!

Another possibility (but perhaps not ready for 'prime time') ... many outboard and small boat engine manufacturers are in the beginning stages of the application/use of quite small 12vdc, constant duty rated, constant displacement but 'speed/demand' controlled, cooling pumps to replace or 'assist' the typical engine driven vane pumps ... some of the proposals Ive seen are for such pumps to be as low as into the 1GPM range. I have seen, but cant remember where (probably Johnson Pumps - Sweden), some already being offered as retofits to existing engine systems. Youll have to do your own websearch if interested.

 

[Gunni]

We have a bleed system, but I can't be running to the engine room every 2 minutes when we're sailing. The thru-hull is at the same depth as the main engine and generator intakes, but they are both impeller pumps.
This only happens when we're beating on sails of over 12 hours or so, and we need to run the refer, so it probably never came up before. It works just fine sailing on any other point of sail or tack.

I was just talking about this issue with a fellow boater, he had replaced his raw water frig cooling system with a heat sink that fit into the thru-hull. Think it was vitrifrigo. It was a closed system. My solution has been to keep the freezer full of ice, shut down the compressor for the day, and let the ice keep the refrigerator cold. Restart the compressor when I get to anchor. Saves me about 100 Ah / day.

 

[jeb21851]

better solution

I have a better solution! On my Hunter 35.5 Legend, the P/O installed a line from a tee off of the fresh water line (before the freshwater pressure pump) to the reefer water cooling pump with a return line from the pump back to the fresh water tank thereby creating a closed system for the cooling water to the reefer. Very ingenuous. It runs great and haven't had any problems with it. You might want to consider such an arrangement.

 

[Patrick S/V Shangra-La]

I had the same problem..

With my AC pump. If I forgot to turn off the sea cock before sailing and I heeled too much I lost the prime.

Not being a contortionist...It was too much work to try to get to either the pump or the cooling line below the v berth. I placed a T in the cooling water line. Ran the line to a stringer which supports the V berth and placed a gate valve in the end. If I loose my prime I open the gate valve for a second and water has again filled the pump. Problem solved.

Though it would be a lot easier to remember to close the sea cock. Some times I just forget.

 

[Bill Roosa]

had the exact same problem on my Hunter 37.5. Moved the pump toward the center-line of the boat and a little lower and the problem went away.
I do like the fresh water solution too.

 

[capta]

It probably will not work to put two pumps in parallel because the centrifugal pump will short circuit and be the path of least resistance. You might try a check valve at the input of the pump. That might prevent losing prime.[/quote]

The two pumps would be parallel hydraulically, not electrically. As a matter of fact the centrifugal pump will be 110vac while the impeller pump will have to be 12vdc.
Wouldn't the impeller pump force the centrifugal pump to prime, just by bringing water to the T where the lines separate for paralleling the pumps?
I can't see why it wouldn't work, but if it won't, I'd sure like to know before I go to all the trouble of replumbing the system.
Thanks.

 

[Bill Roosa]

I think you will find that while two pumps can be made to work you are going to really bust your energy budget with the impeller pump. A centrifugal only draws a large fraction of an amp, an impeller will draw as much as the reefer compressor. That effectively halfs your endurance if the reefer is the "big load" on your batteries.

 

[kloudie1]

Probably would work if you are talking just pulsing the "impeller" pump.. (Flexible impeller) The centrifugal would have to have a check valve in its discharge side between the pump and the discharge of the flex impeller pump. The 110V pump has an impeller that wouldn't restrict flow, so the 12 V pump would pick up air and the 110V pump would circulate the air back to the 12V pump suction.. Would work with a check valve in the 110V pump discharge.. Thing is, with that valve there, you may not have to put the 12V pump in.. so ya may want to try that first.

 

[Bill Roosa]

A check valve in the centrifugal pump output would keep the pump primed as water could not drain backward and let air into the pump when it is not running. There is no need for a "priming pump".
course you are going to get some serious flow reduction from the check valve unless you get one with a VERY LOW pop off pressure.