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Mechanical Section Feature Articles
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Bruce is Owner and Senior Mechanic of Southeast Marine Services, Inc. in the
Palm Beach, Florida area. His extensive background, in Marine Mechanical
Systems, encompasses both gasoline and diesel engines, air conditioning, and
electrical and hydraulic systems.
"Bruce is one of the most 'In Demand' Systems Technicians we have in our area.
His expertise is exceptional!"
(Capt Patrick McCrary, webmaster)
Refrigeration
Marine refrigeration units come in two different power configurations. DC and AC.
First I'll touch on the DC units. These units are designed to run off your battery so that while on
the hook or under way, you can continue to run the refrigerator without firing up the generator.
Or you may not have gen set at all. There are 4 major players,
Norcold, Adelor Barbour,
Nova Kool, and Grunert.
Adelor Barbour and Nova Kool use a 12vdc-compressor fed through a module to control the start
and run functions. Norcold uses a 26vac compressor. In this system the dc is rectified and run
through an oscillator to produce 26vac. The Grunert system uses a separate dc motor to drive a
belt driven compressor. The advantage to the Grunert system is that you can get 12, 24 or 32-volt
motors depending on what your system requires and the pumping, or refrigerating capacity, is
greater than the other 3. The cost though is also much greater.
Most refrigerator boxes on boats up to 40' are the Norcold because they run on either AC or DC
and will switch over automatically so you don't have to think about it.
AC powered units can come in many manufacturers and size as most are used in commercial
applications. The main differences being that most marine units use a water-cooled condenser coil
or a combination air and water. Standard commercial units are air-cooled and if the compressor is mounted in the
engine room they do not perform at their best because of increased engine room ambient air.
Example, trying to cool a 130 degree condenser coil with 170 degree engine room air, it doesn't
work well. The advantage to ac powered units is that they will not run your batteries down and come in small
to very large capacity units.
The basic refrigeration system works like this, you have an evaporator (the part that gets
cold) a condenser (the part that gets warm) a compressor unit that pumps a gas through the
system and an expansion valve or capillary tube. The compressor pumps the gas in a liquid form
on what we call the high side through the condenser coil to cool it off. From there it goes through
a capillary tube or an expansion valve. These devises work like this, the liquid coolant is forced
through a small orifice, when it comes out the other end it expands into a gas and the temperature
drops considerably through the evaporator coil which cools the box and then is drawn back to the
compressor to start all over again.
The most important thing to any refrigerator/freezer box is insulation. The better the
insulation the less your compressor will run saving power. Also the door or lid gasket is an
important seal in the system. It will prevent moisture from entering the box and frosting up the
evaporator plate.
There are two basic evaporator designs, a holding plate and a flat plate. The holding plate
has a solution inside it. Usually a saline or antifreeze water combination. The solution is cooled
down and once the compressor switches off, the cold is released from the plate to continue
cooling the box allowing the compressor unit to run less. Once the surrounding air
temp. Rises, then the compressor kicks back on. This is ideal if power consumption is an issue.
The flat plate has no holding capacity to maintain box temperature, so the compressor may run more frequently.
In a poorly insulated box, with a warm outside air temperature, using a standard evaporator coil,
I have seen compressor units run almost continually. If
power is not a problem then okay.
Here's to keeping your ice cream frozen and your beer cold!
Bruce
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