Monday, March 28, 2011

Refrigerator magnets

No, not the kind that stick on your refrigerator, holding up the kids drawings or the grocery list.

I am talking here about magnetic drive pumps for the refrigerator.  Wait, what?

Let me back up a step.  Your household refrigerator discharges the heat it pulls from its interior into the surrounding air.  If you are old enough, you'll remember the "coils" on the back of the refrigerator, but now virtually all refrigerators have those coils concealed in the base.  Nevertheless, they are there, busy heating air with the heat removed from your food.

Here on Eolian, the refrigerator is water-cooled.  Other than the hassle of dealing with circulating seawater, this makes a lot of sense - the seawater is 48° right now, so it serves wonderfully to carry away the heat.  But there is still that circulating seawater thing.

Originally, Eolian had a very small Johnson brand conventional centrifugal pump for the refrigerator.  But not long after I moved aboard, that pump failed the way most such pumps fail:  the seal gave out and seawater dribbled along the shaft into the pump motor interior, destroying it.  That seal is the weak link in a pump.

There is an alternative, however:  the magnetic drive pump.  In these pumps, the rotor (the thing that impels the water into motion) has an embedded magnet, and is totally sealed off from the motor.  The motor drive shaft has a magnet on its end, and drives the impeller thru magnetic coupling alone.  There is *no* possibility of leakage at the seal - because there is no seal.

When that original pump failed, I sought high and low for a magnetic drive replacement pump, starting of course with Johnson.  The best I could do tho was a Sureflo baitwell pump.  It was oversized for the service, but it was indeed magnetic drive.  I bought it and installed it... and was disappointed because, tho it served well as a pump, it was terribly noisy.

So when I was visiting with my nephew Mark, who is National Sales Manager for SPX (the parent company for Johnson pumps), I brought this subject up.  Well, it turns out that Johnson does make a magnetic drive pump suitable for this service, but that their catalog incorrectly lists it as suitable only for freshwater or ethylene glycol service.  Mark told me that these pumps are used extensively for circulating brine - much tougher service than seawater.  He then arranged for Johnson to ship me a suitable pump for a beta test in my system.

After opening up the bilge space under the floorboards, I removed the old Sureflo pump.  As you can see, the Johnson pump is considerably more compact. 

Next, some plumbing changes were required, and tho I thought I had everything on hand to do the job, Reality checked in and I was short of the correct-sized hose clamps, despite my on-hand collection.  So a quick walk up to West Marine (thankfully close to the marina) and I was set.

The final installation - the pump is almost completely silent in operation - what a relief!  And because it is a magnetic drive pump, it should last as long as the motor.

Oh blessed silence!


Us Svletitgo said...

Thank you for the tips we have the same system and sure the pump will die one day. Now we will need to remember this post

jumager said...

I have these Johnson pump for the coolant in my generator. Works fine most of the time, but I always carry a spare as I had already three of these fail on me. Various reasons, for example one had the upper motor part going into small pieces, another one had motor still working but apparently the magnets came off the motor shaft so the motor still made a running noise without pumping anything.

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