On Tue, 26 Aug 2008 05:04:22 -0700 (PDT), ignator
wrote:
On Aug 25, 1:51 pm, "Steve Lusardi" wrote:

I wish to use a single large compressor to service 3 cold plates and 3 air
conditioning evaporators, but I am not a refrigeration expert, so if my
thinking is incorrect please let me know.

Not necessarily incorrect, but new technologies available might
require a rethink on how you accomplish the task. There's always more
than one way to do it, and the alternatives may be better than "that's
how we always did it."

"Don't raise the drawbridge, lower the river."

Each one of these (users) will
have their own mechanical temperature controlled evaporator valve. I assume
the compressor should be cycled by head pressure with an overtemp sensor for
safety. I would like to use a salt water cooled condensor on the high
pressure side and a freon gas that changes state around -40 C. Please
advise. Do I need an accumulator of some description on either side? If so,
what? Each one of the users have 3/8" in/out tubes. How do I size the
compressor? Any tips or books on this subject would be welcome.
Steve

No, the compressor should NOT be cycled on head pressure, only safety
devices for over pressure and under pressure should be used for head
pressure.
Also it can be done mixing air conditioning and refrigeration/freezer
evaporators in one system, but you will need many more controls and
mechanical systems. You want to have 2 compressors and keep the air
conditioning separate from your cold plate application. This is
because what ever working fluid you use (R22/502/134A) the evaporator
boiling point is a function of suction pressure. For freezer
applications, you will cause the air condition coils to freeze, or for
air conditioning you will never get the cold plates to freeze.

I've seen these big built-up "one compressor" refrigeration systems
at supermarkets, and they certainly can be made to work. But consider
what happens when that one compressor blows up, or that one massive
refrigerant system springs a leak - now you have no refrigeration for
the entire boat, and the Chief starts sweating.

Supermarkets can call the local refrigeration service company and
have a tech on site in an hour who knows that system inside and out,
and with full access to parts and supplies from the local supply
house. If you are out in mid ocean...

And supermarkets usually put all the freezers on one combined system
and refrigerators on the other, just to simplify the controls.
Otherwise they have to start adding check valves and suction
regulation everywhere. And it's best to unload the compressor and let
it run rather than constant starts and stops - but then you get into
run-time wear, and since you have to make your own the power is a lot
more expensive on a boat than a building...

Consider that these big systems always have a problem keeping the
refrigeration oil in the compressor - it gets carried out into the big
accumulator and all the low spots of the lines, and unless you put oil
traps and return lines everywhere it never gets back. Then the
compressor dies from oil starvation - see "What happens when the one
compressor fails" above.

You will be fighting this forever with oil traps and oil makeup
tanks and crankcase controls. And trying to keep the velocities in
all the return lines right, and getting the slopes and falls right,
placing P-traps everywhere to get the oil moving back - and then the
boat goes up on plane and all the working angles change...

You do need at least a small accumulator on the compressor suction
side as a trap to protect against slugging - gases compress, liquids
don't, and you don't want to test the anti-slugging feature on the
compressor head too often or it can break the springs. Then refer to
"What happens when the one compressor fails" above.

If you are hell bent on this, you will need to control the input
refrigerant to the evaporators with electric solenoid valves, I assume
you have a 12 volt battery system on your boat, I've seen 12 volt
valves. Also I assume you are connecting the compressor(s) to a
diesel engine. The output of the condenser will go into a receiver
(not an accumulator). There are suction accumulators, which are used
to prevent liquid slugging into the compressor. To control air
conditioning suction pressure on such a system (one compressor) you
will need to use pilot operated suction valves for the air
conditioning evaporators, in the output of the evaporator so the
suction pressure will not be at the same pressure as the cold plates.

The last time I was sailing on a Moorings bare boat charter, the cold
plate system was replaced with a +12 compressor, and this worked so
much better then the cold plate of previous sails, it would keep ice
cream frozen, where the cold plate would totally defrost between
engine recharge cycles, or be one frozen mass of sh*t.
ignator

This was my thought too - Danfoss (http://compressors.danfoss.com)
makes the BD Series 1/20 HP and 1/12 HP hermetic brushless motor
compressors that run on 12V or 24V internally, and they have several
control boxes to run them on 12/24V battery systems OR variable
voltage solar or wind systems OR auto-changeover between 120VAC/240VAC
world voltages and 12/24 VDC. And the control boxes have integrated
temperature controls - just add a thermistor in the conditioned space
and an adjustment rheostat.

Even one control box specifically to control EMI/RFI radiation that
could rip the navigation and radar systems on a ship to heck.

And if there is enough demand, I'll bet they could build them bigger
without too much trouble. 1/2 HP or even 1 HP isn't out of the
question for 12VDC battery supplied systems.

It will be a HECK of a lot simpler to make separate systems that run
continually from a deep-cycle battery bank, and each one optimized to
it's task - freezer, refrigerator, ice-maker, live-well chiller, etc.

And a whole lot simpler to circulate low pressure condenser water
around the ship to separate small heat exchangers at each
refrigeration device, rather than high pressure refrigerant.

And it WINS hands down on KISS simplicity and redundancy - if one
refrigerator fails, you just move everything to the three others that
still work. If your battery bank goes flat, start the generator set
and they all switch to 120V.

For space cooling in the cabins, consider either a "Mini-Split"
refrigeration based heat-pump separate from the freezers (if you plan
to run the gen-set or inverters 24/7), or ducted fan coil units and
circulating chilled/heated water from the engine room. Keep all the
refrigeration in one compact spot, and you could use a large ice-bank
tank low in the ship (they are heavy!) to shut down the gen-set at
night.

Heat is easy, I'll bet you have lots of excess heat when those two
big main propulsion diesels are running - and when they aren't use the
same tankless hot water heater you have for the showers. Or an
RV-style forced air propane furnace. Many ways, all depends.

-- Bruce --