Marine Refrigeration Sizing. How big a Refrigeration Compressor & plate Combination. Before we pick a size of marine refrigerator we need to understand what factors are involved in keeping the refrigeration box cool. They 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.
After this basic size issue we have things like, Front opening or top opening. Front opening lets cold air out quickly but does allow you to get to the bottom of the box.
Drains, if you had a drain for you ice box plug it. You will not need to drain water out of the ice box and this will only let cold air out and heat in.
Gaskets, these are a must and must be properly sealed. A trick to identify if there are gaps in the gasket is to put a piece of paper in between the lid or door and the cabinet and close it. Pull on the paper and you should feel some drag if the gaskets are sealing properly. It it comes out easily there is a gap. Get new or better gasket material.
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.
There are manufacturers of vacuum panel thermal insulation. Both Glacier Bay and technautics make these panels. The Glacier bay Barrier Ultra-Râ„¢ super-insulation at R-50 per inch provides lots of insulation without taking up valuable volume. These panels are vacuum panels and are sealed to work. It is very important that you do not drill through or puncture these panels. These panels are custom made, so you would need to provide the manufacturer, exact sizes with locations for copper plate tubes to enter the box. These are built into the panels.
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.
The evaporator plate temperature is set by the thermostat. Dial the box temperature down and the system will have to work harder.
Number of people aboard
More people means the box gets opened more and the heat build up from more people adds to the ambient temperature.
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
Lets look at how this works for the 4 cu ft refrigeration system, using the above
4 cu. ft. times 600 = 2,400 Btu. Two additional people on board = 2,000 Btu.
Total required per day = 4,400 Btu
Choosing 12 volt unit with Evaporator
Match this number to the compressor capabilities, then calculate amps needed to power the system, then work on the battery bank capability
Start by using the 4,400 BTU form the above example
The Adler Barbour Cold Machine uses the Danfoss BD50 Compressor is rated at 650BTU / hr based on 25F evaporator temp.
This is well above the 4,400/day we need for the 4 cu ft fridge, using only about 1/3 of the power. We could easily go down the the Danfoss DB35.
BTUs can be converted to amps with this formula, Using an assumption of 5 BTUs per watt hr of energy used. Reference Refrigeration for Pleasureboats Calder
so say we have 4400 BTUS and 12 volts
4,400/5/12=74 amp hours/day
The Amps needed to power the compressor should be 1/4 of the capacity of the house bank. So for the above 74 amp hours needed multiply by 4 to get recommended house battery capacity = 296 amp hrs