T&T: A/C Pump Problems

[Dave Cooper]

Mike, that's a lot of plumbing and "head" for one centrifugal pump, IMHO.

Several things:
1. Is the pump breaker popping or is it the AC high pressure cutout that is
shutting down the pump and compressor(s)? The Marine Air units do have a HP
cutout I hope!
2. Centrifugal pumps run with the inlet completely blocked without any
increased load on the motor so blockage shouldn't be an issue with the motor
load.
3. Unless there is a physical restriction in the impellor casing the shaft
should turn easily.
3. You mentioned a "brass" impellor. If this is the case I would expect a
short life for it. Bronze is a far better material and even that will waste
away in warm sea water after a few years. Nothing beats a high quality
plastic pump for this application, IMHO. I have seen both brass and bronze
pump vanes broken off and lodged in the housing or some that have scored the
housing so that output was greatly reduced.

I would suggest that you shut off all but one AC unit, remove the outlet
hose of the pump, put a temporary hose on it and put it in a bucket or
bilge. The turn on the AC unit and see what you get as output. If it is a
very force full output of better than 20 GPM then the pump is OK. If it is
less then there is a problem. I say this GPM as with 4 units and the
attendant amount of hose, fittings and other restrictions you'd need at
least this amount in the warm sea water that you are in to avoid a high head
pressure cutout. If you can't get the AC to cycle long enough without
cutting out then you'll need to remove the motor leads from the relay box
and attach them to the supply voltage.

I would change the system to a more robust system and use 2 units per pump
and make sure that the pumps are sized taking care to account for all of the
hose, fittings and head that the pump sees. It takes surprising little
"plumbing" to reduce the flow of a centrifugal pump as they are very
sensitive to total head. An overboard out flow of 5 GPM is nominally
required for every 12-15K BTU of capacity in 85-90 degree seawater on a
clean system. If the AC heat exchangers are a bit dirty then you need more
unless you keep clean with an annual back flush.

Using any brass in a seawater system isn't a good idea. We have pumped out
many a boat where the brass components have failed. Some had to brought up
on airbags before they were pumped out :-(

Talk to DEPCO and the folks they should be able to get you on the road to
cool air and comfort :-)

Raw water heat exchangers just need lots of water. The warmer the water is
the more of it they need. Many AC manufactures don't assume hot raw water so
tend to be conservative in the flow requirements. However sizing a system
for 100 degree intake water will give you a system that is bullet proof even
if you neglect to keep the heat exchangers clean. Using two pumps with a
spare will keep you running even when one pump dies. Using plastic pumps
will last longer than bronze from our experience. Good mag drive pumps in
the order of 15-20 GPM each for each pair of AC unit should do the job
unless you have lots of head in which case the size needs to go up.

A pet peeve I have is that the AC makers always use a "short" heat exchanger
loop. Making the loop twice as long would reduce the flow requirements and
thereby reduce the size and power required to pump all this sea water. We
pump about 15,000 gallons a day with our three! That's a heap of water to be
flowing and it takes power to do so which all comes from the genset and
fuel. So genset time goes up and so does the fuel cost. All to save a $100
or so in the initial cost of the AC unit :-(

As always YMMV......

Dave & Nancy
Swan Song
Roughwater 58
Caribbean Cruise '07