Attention to Correct Thermodynamics for Best Refrigeration Performance.

[Richard Kollmann]

When you purchase and install an icebox conversion refrigeration you are responsible for its future performance not the manufacturer of the system. Only you know what temperature levels inside the box you will be satisfied with and what cruising climate temperatures refrigeration system will need to operate at. Manufacturers of these systems generally list performance of their systems based on standard day 70 degree F temperatures. If you plan on operating this refrigeration in warmer climates than 70 degrees add 4% for each degree warmer to sales person’s standard energy estimate.

There are a few basic thermodynamic rules a boater needs to understand about boat refrigeration:
1. Cooling a refrigerated box is the process of removing box heat and process mechanical heat and then disposing it as far away as possible. Some of these icebox conversion system designs ignore the fact that compressor heat must also be disposed of leaving this up to the person installing or owning unit.
2. The first law of conservation of energy in a refrigeration system states that energy can not be created or destroyed in an isolated refrigeration system like an ice box conversion 12 volt compressor system. The source of energy for a12 volt system comes from an onboard alternator, a generator or alternative electrical power, wind, solar, or shore power electric. Only the compressor converts electrical power to energy for this refrigeration system. Regardless of what the sale person may say other components of this mechanical refrigeration system can not create energy, this includes eutectic holding plate evaporators.
3. Thermodynamics of air within a refrigerator normally is not considered a conductor of heat but under the right density air rises or descends. All refrigeration systems use the rise and falling air principle inside refrigerators to create a natural tumbling convection air flow to cool refrigerator box temperatures evenly. Air that is blocked from tumbling top to bottom by closed shelves will stratified temperature layers within box. Efficient refrigerators have evaporators mounted high in the box to create natural air movement from top to bottom of refrigerated area. Freezer sections need air space surrounding frozen product to allow air movement. Air flow in freezer can best be achieved by installing wood rails in bottom of compartment connected to vertical rails at each end.

Selecting and Installing Icebox Conversion Refrigeration for Best Performance.

The condensing unit that includes compressor and electronic controller is where the process and electromotive heat of refrigeration is collected for disposal. When purchasing the system’s condensing unit it should be designed to remove heat to another area of the boat. In other words air that has passed through condenser and absorbed heat should not pass through condenser with the added heat a second time. Most condensing units have a fan that forces fresh air through condenser refrigerant coils and over compressor including electronic module to help cool them.
A visual check of the condensing unit will let you know if it is suitable for heat disposal where you plan to locate it. Many condensing units are equipped with air duct adaptor flanges like SeaFrost, Frigoboat’s Capre 50, Isotherm has one model with hose adapter. AdlerBarbour sells a adapter hose kit with adapter for better heat disposal. Most all condensing units for boxes larger than 3 cubic feet will have fan cooled condensing units although they may be with open un-shrouded forced air fans. On systems where condensing unit is located in a compartment under 100 cubic feet in warm climates without adequate air exhausting an extra compartment fan to ventilate condenser warm air somewhere else is recommended. Water cooled hermetically sealed compressors without cooling fans need a compressor and module fan to keep compressor under 130 degrees F.
Many condensing units on these 12 volt systems have condenser coils open on one side and can be mounted against an open area. Best solution when a new system is installed is a lover vent with condenser unit air intake pushed and sealed against open lover allowing ambient cabin air to be drawn through condenser. The warm air pressurized by fan in compartment is then allowed to be removed by any other opening in cabinet area. Selecting a condensing unit without the ability to move heat away to another area may only be 50% energy efficient, the first law of refrigerator thermodynamics. Attempts to increase condenser fan Cfm will not move heat to another area. Adding additional water cooling band aids as several companies have attempted only compromises future system reliability.

The heat absorbing evaporator is the other half of a icebox conversion unit, it is located inside the insulated box to be refrigerated. Other than selecting a compressor with matching capacity condensing unit selecting the correct evaporator is the key to converting a useable insulated box into a full functioning refrigerator. Box size and shape will determine evaporator size before selecting condensing/compressor unit.
Manufacturers of thin plate roll bond evaporators like Alcoa Aluminum of Canada defines Btu ratings for their evaporators based on operating evaporator temperature range. Once evaporator capacity is established and refrigerant flow controlled by capillary tube a compressor/ condensing unit can be selected to balance output of compressor to match evaporator capacity.
A full function refrigerator’s temperatures are more than just a drink cooler if possible it should be capable of making ice and keeping ice cream frozen solid. Other eatable materials not in freezing area will need a slightly higher temperature area. The problem with ice box refrigeration conversions is they generally have lid openings on the top making it difficult to have separate temperature zones in most converted boxes. There are many types of evaporators available to separate multiple temperature zones in refrigerator. If converted box is to have a freezer section the evaporator needs to surround frozen product. A straight thin plate evaporator or a single eutectic plate may perform well in drink coolers but not in a true refrigerator with a freezing section. Square boxes under six cubic feet normally use bin shaped evaporators to provide a freezer and refrigerator section. Large or rectangular shaped boxes are mandates for a short term storage spillover freezer.

To select the right icebox conversion hardware I recommend watching my slide show at:
Kollmann Marine