Bruce in Bangkok wrote in
:

On Sat, 22 Mar 2008 09:19:58 -0400, "Roger Long"
wrote:

With a small engine like mine and modest electric loads otherwise,
wouldn't it me more efficient to put the largest compressor that a
toothed belt pully and engine PTO limitations would permit on the
front end of the engine? Then have a large cold plate in the box and
draw it down as quickly as possible keeping the engine under maximum
load. It wouldn't give you the fine temperature control of a
constantly running 12 V. fridge system but would be as good as ice.
Drawing down the cold plate would be like buying ice (and probably
nearly as expensive at next year's fuel costs).

The conversion from fuel to electricity to charging and back to motive
force has got to use up more fuel than just running the compressor
directly.

Most of the boats I see here use exactly that system. The more
up-market ones have a duel system with both engine driven and A.C.
powered compressors for use at sea and in the marina.

If you really want to get into it find a copy of Nigel Calder's book
on refrigeration systems (he is a fellow Mainiac. It is down to earth
and really teaches you how to build a system.


Bruce-in-Bangkok
(correct email address for reply)

I used to have a dual-coil system and ripped it out because it had
terrible performance. It started out having both an engine drive
compressor and an AC compressor. I replaced the AC compressor with a DC
driven compressor (1/2 HP motor driving a Bitzer IIY compressor) which
is basically what Glacier Bay utilized.

I wasn't happy at all with the performance. I then realized that the
problem was with the dual coils in the cold plate. Imagine a cold plate
that's about 2" thick. The front half of the plate had coils cooled by
the engine compressor and the back half had coils cooled by the DC
compressor.

In order to freeze the entire plate, either compressor has to freeze the
eutectic solution in the other half of the plate where it doesn't have
coils. This leads to very long run times.

The engine drive system did a great job of freezing the plates, but that
was rarely used. I suppose that if I ran the engine to charge the
batteries, this would be acceptable, but I rely on wind/solar and a
generator when the first 2 don't produce enough.

I ripped out the engine drive system and combined the 2 coils into 1
loop and after a lot of fine tuning, got the system to where it utilizes
about 85 Ah in the Caribbean. I don't have the measurements here, but
this includes a 3 cf freezer with it's own plate that's kept at about 4F
and a 4-5 cf refrigerator that maintains 40F. The addition of
electronic controls was very important also as it allowed me to
precisely control the on/off set-points.

I also installed run hour meters on the both the refrigeration and
freezer sides of the system, so I can closely monitor the system for
performance issues. I log the results in a spreadsheet on a regular
basis to make sure that everything is working OK.

I extensively used the Calder book and to say that "It is down to earth
and really teaches you how to build a system." is being very kind. It
provides you with a lot of information and you really have to study it
and other sources to determine how you really design a system. It's a
much better debugging guide than a design guide.

I will also point out that Richard Kollmann, the OP, has 2 good books on
marine refrigeration. See http://www.kollmann-marine.com/

-- Geoff
www.GeoffSchultz.org