Boating Equipment & Product Review/Test; Marine Refrigeration

Marine Refrigeration

There are 2 types of Marine refrigeration for your boat. The first are self contained marine refrigeration units, which drop into a spot just like the fridge at home. They work better on larger powerboats that have an abundance of AC power and space.

The second type of refrigerators found on boats, are built in modular fridge systems that use available space in or near the galley. These modular systems use the same components as a self contained unit but can be mounted within 12 feet of the refrigeration box. This article will cover built in units

First decide what you need, is it to replace ice, so you will need a fridge temp between 34 and 39 degrees F, or do you need partial freeze or full time freeze? Freezer temperatures are from 10-20 F.

How does Marine refrigeration work?

The major parts of a DC refrigeration system include the refrigerant, a compressor, a condenser, a cooling system for condenser, and a plate or plates inside the refrigeration box.

The compressor forms part of a closed loop, pumping refrigerant through the system and through the evaporator plate in the ice box. The compressor has two sides the high side or discharge side. The discharge side pumps refrigerant under pressure to the condenser. The suction side or low side sucks refrigerant after it passes thru the evaporator plate back to the compressor.

More on how marine refrigeration works & refrigerants

Parts of a refrigeration system

The major parts of any refrigeration system include:

1 evaporator or holding plates in the box
2 a compressor to pump the refrigerant through the plates
3 a condenser to cool the refrigerant.
4 System size

Designing a boats refrigeration system requires us to make some choices on these major components and then how large a system you need.

1 Evaporator or holding plates

Evaporator plates are constant cycling and need constant power.
Holding plates or holdover plates only require periodic charging.

Flat Plate or bin evaporators are like the fridge in your home. They are also known as constant cycling DC refrigeration. This is due to the compressor cycling on and off several times an hour to maintain the desired temperature in the box. Therefore they need a constant supply of power due to the cycling.

Evaporators can get quite cold (thermostat setting) and many evaporators have the ability to make ice next to the evaporator plate or inside the evaporator box. Evaporators come in several shapes and sizes, they can be horizontal plates vertical plates and rolled plates. Image bin type evaporator from Dometic. More on evaporator plates

Marine Refrigerator Holding Plates act like large blocks of ice and the cold temperature of the holding plate sucks heat out of the boats refrigeration box. Holding plates can keep ice boxes cold for long periods. Holding plates can be made for fridge and freezer. This holding plate is from Rparts.

The main advantage of a holding plate over an evaporator is that it only needs to be recharged (temperature drawn down) 1 or 2 times per day. This charging can coincide with attaching to shore power, running the engine and so the refrigerator does not rely on the battery bank. When incorporated into a properly designed system, holdover plates can significantly reduce average energy consumption. DC holding plates are also possible.

The holding plate is filled with a solution that has a freezing point below 23 degrees F. As the compressor runs, the refrigerant passes through the holding Plates coil, freezing the holding plate solution. The compressor turns off, and as the holding plate thaws out, heat is removed from the box.

More on holding plates

2 Powering the Compressor

The power supply to the compressor is one of the key elements of the boats refrigeration system. Refrigeration is one of the largest energy consumers onboard, so the power supply is an important element of the system.

Power supply to a marine refrigeration system include, AC, DC, Shore Power, Engine power, and hybrid systems. Hybrids are combinations of say 12 volt and engine drive, or engine and shore power.

12 Volt

For most boats with a small box, a single 12V compressor, air cooled condenser, with evaporator type plate will be about the cheapest option.

The DC system combined with an evaporator plate that is thermostaticaly controlled gives flexibility over cooling requirements.

Many 12 volt systems use the Danfoss compressor. With the increased efficiency of the Danfoss compressor, DC refrigeration onboard is getting more efficient, but is still power hungry

The Adler Barbour Cold Machine (left) has been around for around 25 years and provides great refrigeration for small to medium size ice boxes.

Engine Drive

The idea behind an engine driven compressor system is that the engine gets used anyway for at least one or two tomes per trip. If you are Powerboating this makes sense. If you are sailing, calculate how much time you use the engine, to see if you can draw down the refrigerator temperature in this time.

If this is is the case an engine drive with a holding plate can draw down the ice box in a short period and after that it can be left for 12 plus hours. If you have engine drive with shore assist you can maintain temperatures at the Marina.

The compressor is run directly off the engine. Belt driven or direct compressor

The picture is of SeaFrosts engine driven holding plate refrigeration. The image shows all the parts except the sea water intake for cooling.

More on powering the refrigeration system

3 Cooling the Condenser

The marine refrigeration systems, condenser needs cooling. This is how the refrigerant gets cooled and turned into a liquid. There are 3 ways to cool the condenser;

Air cooling simplifies installation plus it does not rely on water or adding thru hulls. It is therefore the cheapest installation.

Water cooled compressors will work better in higher ambient temperatures and can be 30% more efficient. Water cooling may be best and is better for larger installs especially if freezer is concerned. Water cooling needs a thru hull and a pump to get the water to the condenser.

The keel cooler or keel condenser requires no thru hull fitting and will not be subject to clogging. The keel cooler is a 3" x 7" bronze plate that mounts on the outside of the hull and it is the condenser heat exchanger.

More on cooling Click here

4 Sizing a Refrigeration System

A leaky badly insulated refrigeration box, will require more BTUs than a well designed box. The factors involved in keeping the refrigeration box cool, are mainly box size, insulation and cooling water temperature, number of people aboard and the temperature you are setting the plates for.

Refrigerator Box Volume

This is obvious, the larger the box the more heat removal is necessary. A larger box will need more BTUs of heat removal.

Insulation

Typical insulation to a fridge or freezer is foam insulation like Dow Blue board. The recommendation is for 3-4 inches for refrigeration and 4-6 inches for freezer for medium sized boxes. Foam has an R value of 5 per inch thickness, R being a thermal unit. This means in terms of thermal units 3-4 inches represents 15-20R value for the refrigeration unit, and 20-30 for the freezer.

Water temperature

In the tropics water temperature is a lot warmer then northern climates. For every degree water temperature increases a corresponding 2 % increase in required BTU. If you are in the Atlantic portions of the east coast US, you have some cool sea water temps, but of you then cruise down to the Caribbean you may strain your refrigeration system.

How big a Refrigeration System is required

To calculate how big a refrigeration or marine freezing unit needed, you will need to start with an estimation of the BTU requirements of the box. A simple rule of thumb for estimating the BTUs is based on the box volume.

These BTU estimations are based on these assumptions

Insulation has an R value of 30 no leaks.
Water temperatures are tropical in the mid 80s F.
2 people aboard, For each extra person add an additional 1,000 daily BTU
Top opening box, for a front opening door add 15 BTU/inch of door

BTU estimate on refrigeration volume;

Refrigerator daily heat load	box temperature between 34 - 39 F	600 Btu per cu. ft.
Freezer daily heat load	box temperature between 10 - 20F	1200 Btu per cu. ft.

More on sizing refrigeration equipment

Refrigerator Freezer combinations

There are a couple of way to combine freezing and refrigeration needs onboard. One is to have separate boxes, another is to have one box and a a divider to separate the temperatures called spill over and then there is the evaporator plate which will freeze next the plate and have warmer temperatures away from the plate.

Refrigerator - Freezer

If space allows have separate refrigeration and freezer boxes, outfitted with separate compressors, evaporators and holding plates. Possibly a holding plate system for the fridge and an evaporator system for the freezer.

Separate compressors also provide redundancy in the system system, so you have a backup and still have cold food storage.

Spill Over

A box that is divided into two separate sections with a partition diving the box into 1/3rd freezer section and 2/3 refrigerator that is cooled by air spilt over from the freezer section. This concept provides both a refrigerator and freezer from one compressor. Factors for a spill over design;

More on combination fridge freezer