Good Refrigeration Tech Tip 5

 

     It is obvious that good boat refrigeration is viewed differently by each of us. There are those that are satisfied with only refrigeration enough to produce box temperatures equal to that of ice 45 to 60 degrees F. Other boaters want refrigerator box temperatures just low enough to slow down growth of bacteria. Cruising and live aboard boats prefer to have the ability to make ice and store frozen food. Since preferred box temperature figures are always arbitrary the buyer should be aware of their preferred temperature needs as the seller might be happy with a box temperature in the 45-60 degree range. In one Four Star rating system on the web +21 degrees F with pre frozen food in freezer will maintain food quality in your boat’s box for a week.

  •  Minimum temperature = −6 °C (21 °F). Maximum storage time for (pre-frozen) food is 1 week
  •  Minimum temperature = −12 °C (10 °F). Maximum storage time for (pre-frozen) food is 1 month
  • Minimum temperature = −18 °C (−0 °F). Maximum storage time for (pre-frozen) food is between 3 and 12 months depending on type (meat, vegetables, fish, etc.)

     There are a lot of theories in the application of mobile refrigeration for pleasure boats but little quantified data on performance. The reason for this is that there are no two boats identically the same. Nor are there standards in reporting performance data like box size ambient temperatures, box interior temperatures through out areas of boxes, equipment configurations, and daily power consumption data.   

 

     Anyone trying to recommend icebox conversion refrigeration for mobile refrigeration needs to study each application and intended use by the boat owner while disregarding creative advertising by sales people. You will not easily find in Mechanical Engineering Hand Books or ASHRAY folders information on pleasure boat refrigeration.  The ice box charts used to calculate insulation versus Btu conductivity that we all have used for document fillers are worthless. A pleasure boat’s cruising refrigerator’s daily heat load increases as the crew’s demand for cold liquid and ice increases so daily product in and out of the box must be added when calculating daily Btu power requirements.  If present insulation is free of moisture an R value of 20 is OK for a refrigerator in a cool climate and R30 is OK in the warm tropics. With 12 volt icebox conversions in tropical climates it is difficult to have freezer temperatures low enough for long extended cruises.

 

     When I look at an installation application concept for a cruising boat’s refrigerator I use a basic guide:

  • Determine worst case planed cruising climate for this refrigerator’s application.
  • Smart engineering with new variable speed BD compressors are designed for their running time to be less than 50% duty cycle. Optimum energy efficiency is achieved when desired box temperature can be maintained at the slowest compressor speed and still have less than a 50% compressor running duty cycle.
  • Box size and planed use will define size of a refrigeration system. Boxes smaller than eight cu ft can be efficiently cooled to refrigerator temperatures in warm climates with Danfoss BD compressor/condensing units. Refrigeration capacity larger than eight cu ft boxes in tropical climates will require compressors larger than a single Danfoss BD50.
  • Freezers in tropical climates with Danfoss BD compressors are limited to 4 cu ft. Even in winter a BD50 compressor system in tropics can be expected to run most of the day at max Rpm to maintain desired freezing temperatures below +10 degrees F.

 

      Most of the sail boaters I meet have plans, at some point, to cruise into tropical waters south of Latitude 26N. Twenty eight years in boat refrigeration has convinced me to develop a program of how to select refrigeration system capacity for pleasure boats. This program designs for worst case conditions then allows anyone to reduce projected Btu and amperage worst case for their cruising area conditions by a simple formula. After following the exercise on my wed site slide show the worst case Amp and Btu totals can be lowered by 2% for each degree of seawater temperature below 89 degrees. Example using my worst case water temperature always of 89 degrees and actual water temperature where the boat is to be operated say at Latitude 30N in the spring with a water temp of 55 degrees there is a reduction of 34 degrees times 2% = a refrigeration energy reduction of 68 % from the worst case calculation.

 

     Of course desired box temperatures will depend on many things:

 

Equipment selected

Avoiding experimental unproven components

Simplicity

Satisfied costumer Experience with this design.

If there is enough variable power to support this refrigeration.

Reasonable amount of Box Insulation

Selecting Manufacturer based on a dependable design at the best price.