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Q:  Do you work on refrigeration systems on commercial fishing vessels?
A:  No, we do not work on commercial fishing vessels.  Our scope of work only includes refrigeration systems on recreational powerboats and sailboats including deck freezers, galley fridges, cold plate systems, icebox conversion kits, ice makers, air conditioners and heat pumps.


Q:  How does refrigeration work?
A:  A refrigeration system consists of four main parts:  compressor, condenser, evaporator and a metering device.  A compressor can also be called a vapor pump, because its purpose is to pump vapor (liquid cannot be compressed, and can cause compressor damage if attempted).  It pumps high pressure vapor into the condenser where either a fan or pumped water is used to remove the heat, which causes the vapor temperature to drop below its dew point, causing the vapor to condense into a liquid.  As it changes state it releases a large amount of "latent" heat, which is the heat energy required for the refrigerant to change from vapor to liquid and from liquid to vapor .  This is how the refrigerant "carries" the heat from your refrigerator box to the location of your condenser.  At this point the refrigerant is in a high pressure liquid form and is pushed through the metering device, which separates the high pressure side from the low pressure side.  In the case of most refrigerators the metering device is a capillary tube (cap tube for short).  It is a long length of very small copper tubing with a tiny orifice bored through it.  After it passes through the cap tube the liquid refrigerant squirts directly into the evaporator, which is at a low pressure because the outlet of the evaporator connects to the suction side of the compressor.  At this low pressure the refrigerant quickly boils off into a vapor.  As it changes state from liquid to vapor it absorbs heat to facilitate the process.  This is similar to the way that boiling water requires heat to change from liquid to a vapor, while maintaining a constant 212 degree temperature.  Any additional heat added to the water is absorbed and used to accelerate the change of the water's state from liquid to vapor.  This is the latent heat that will later be rejected out of the condenser, and this absorption of heat is what cools your refrigerator box.  All of the refrigerant boils off into vapor and is sucked into the compressor to repeat the refrigeration cycle again and again.

Q:  What temperature should I keep my refrigerator and freezer at?
A:  Short answer: 35 degrees for the fridge, and 10-15 degrees for the freezer.  Long answer:  It is important to keep your fridge temps below 42 degrees to prevent potentially hazardous bacteria from being able to grow on your food.  To keep dairy and meat products from spoilage the closer to freezing the better, however you don't want fresh produce to freeze, so 35 degrees is usually a good compromise.  Keep in mind that all thermostats  have a differential between the temperature they shut off and the temperature they come back on.  Most mechanical thermostats have a differential of 5-7 degrees.  Digital thermostats allow you to choose the differential.  If you want to keep ice cream hard in the freezer, it will need to be about 5 degrees.  The reason we recommend 10-15 degrees is because you will use 20-25% less power by keeping it at that temperature.  Considering that the fridge/freezer is usually the largest draw on a boat's battery bank, 25% less power consumption seems like a no brainer... unless you REALLY love ice cream.

Q:  I had somebody come look at my boat refrigerator and they told me my compressor is bad, but its only 5 years old, why did it fail so quickly?
A:  There is a 95% chance there is nothing wrong with your compressor.  The 12 volt Danfoss compressors that almost all manufacturers use are very robust and rugged, and should give you 15 years or more of reliable service.  It is FAR more common for the compressor control module to fail due to water damage or power supply irregularities.  To an inexperienced technician it will look like the compressor has failed, and they know if they replace the whole compressor sled it will probably solve the problem (even though they don't actually know what the problem is), plus they get to charge you four times as much for a new compressor than a control module.  A compressor can be damaged in a few ways, such as a massive refrigerant overcharge where the compressor has to try to pump liquid, which doesn't compress and can warp the valve plate, or by running a keel cooled system or water cooled condenser system with the boat out of the water, which can cause overheating and potentially cause head gasket failure.  To verify that the motor windings haven't burned out (which is unlikely), check them with an ohm meter.  You should read between 1.5 and 3 ohms between any two of the three pins.

Q:  How often should my compressor cycle on?
A:  You can expect your compressor to cycle on 40-60% of the time.  This is called the duty cycle.  This is determined by many factors such as ambient temperature, insulation thickness, compressor capacity, door seals, how often the door is opened, condenser airflow, warm product being added to the fridge, and so on and so on.

Q:  What is a Cold Plate System?
A:  A cold plate (also called a holdover plate) is a large tank inside your refrigerator or freezer box filled with a solution that freezes and melts out at a temperature lower than water (20 degrees for a refrigerator, -10 degrees for a freezer).  Copper refrigerant tubing is coiled inside the tank so that the as the refrigerator runs the tubing is cold enough to freeze the solution solid.  After the solution is frozen solid, the compressor shuts off and the cold plate acts as a large ice block, slowly melting back to a liquid as it removes heat from the box.   Depending on the size of the cold plate and the heat load of the box, the cold plate can provide cooling for 12 to 24 hours or more.  Basically it is a thermal battery, storing cold when excess power is available, and providing cooling when power is not available.  To be able to freeze the cold plate solid in a few hours, a large capacity high amp draw 120V compressor is usually used (although some high capacity 12V compressor options do exist).  Engine drive compressors are often found on older sailboats as well.

This differs from a standard evaporator type system, where a low amp draw DC compressor cycles on and off several times an hour, pulling power from your batteries to maintain box temperatures.

Example: A small 12-volt compressor removes 200 BTUs of heat per hour. In a cold plate system, a large compressor can remove as much as 4000 BTUs per hour. The energy stored in cold plates then provides cooling for 12 hours or more.

The purpose of installing a cold plate system over a standard evaporator is to take advantage of surplus power when it is available.  When you are motoring out, running your generator, generating solar power during midday or plugged in to shore power, there is more power available than is being used to charge your batteries and power your boat.  Depending on your generator, you may be using only 20% of its electrical capacity when running it to charge your batteries.  This means there is surplus power that would be otherwise wasted, and can instead be used to power a high capacity compressor, which will draw a high level of current to freeze the cold plate solid within a few hours.  Then once you've shut your engines off to sail, or when you drop anchor to spend the night, your refrigerator or freezer stays cold without any draw from your battery bank.

Cold plate systems will not be the best option for everyone.  There are some disadvantages to cold plate systems, depending on your specific application and situation.  You will need to have surplus power available for 1 to 1.5 hours twice a day.  You will need to have a large enough box to fit the large cold plate tank inside.  Usually freezers require two cold plates, which can take up a significant amount of space in the freezer.  On many boats the top load access hatch may be too small to fit the cold plate tank through, making installation difficult or impossible.  Cold Plate systems are much more expensive than traditional evaporator type systems to purchase, you should expect to pay about two or three times as much.  Installation of Cold Plates can also be more difficult and costly than evaporator systems.  Cold plates can be the best choice if you have a limited amount of amp hours in your battery bank, although spending the money to add more batteries or solar panels may be more advantageous in the long run.

Any other questions?

Please email us and ask!

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