Smart ice box conversion refrigeration knowledge can insure satisfaction later

[Richard Kollmann]

When purchasing a power boat it usually is sold with a refrigerator. When purchasing a sailboat the boat hull’s interior cabin furnishing include one or two top opening insulated boxes. These insulated boxes can be used as iceboxes or can be converted to refrigerated coolers or refrigerator. Cooler refrigerated box temperatures range from 40 to 60 degrees F when used for beverages and food storage by lowering ambient warm climate temperatures. When converting sailboat top loading box’s not designed for multi temperature refrigerators there are some thermodynamic laws that need to be considered before selecting an icebox conversion refrigeration unit. The big problem in converting a top loading box’s available closed cavity space will be heat flow and air flow when you think of how refrigeration keeps things cold.:
1. Refrigeration unit is not making things cold it is absorbing heat energy and disposing of it.
2. Heat flows from something warm to something cooler. The greater the temperature difference the faster heat moves.
3. Heat moves from one object to another till temperatures are equal.
Most of the heat transfer in converted boxes is by convection natural air movement of warmer air to the refrigerant evaporator assembly as a result of variations in density. These variations are related to temperature and humidity causing warm less dense air to rise and cold more dense air to descend. Natural convection refrigerators require that the evaporator be installed high in the box. Air tumbling is sometimes supplemented by a cavity fan such as a battery powered Fridge Mate. For any form of air tumbling within a refrigerator to be efficient shelving or product restraints must allow air flow throughout box capacity, this is why older refrigerators have wire shelving.

Modern home refrigerators are much larger than most boat insulated iceboxes and no longer rely on natural air tumbling replacing it with forced fan air evaporators with self defrosting devices. By directing chilled fan air to different box cavity areas one refrigeration unit can support a freezer compartment, a separate refrigerator area, and drink cooler temperature area.

Most boaters but not all boaters think of refrigerator as a beverage cooler only and are satisfied with box temperatures of 40 to 60 degrees F as a replacement for carrying ice. A true refrigerator has a place to store food to prevent spoilage at 33 to 37 degrees F and small amounts of frozen food at temperatures below 22 degrees F. Many boaters like to freeze ice cubes and keep ice cream. solidly frozen. You know what size and shape boxes you have and what temperatures in refrigerator you prefer. It is obvious that everyone is not a vegetarian or needs only a drink cooler so you need to review the equipment hardware options available from each icebox conversion company before selecting a conversion unit.
All natural convection boat refrigerators must have an evaporator high in the insulated box cavity in order to prevent stratified layers of cooling. Spillover cooling by natural convection is how standard small refrigerators are made this is easy to design with a front opening door. Sailboat top opening lids boxes are difficult to create separate temperature
zones unless box is large enough to separate into at least two separate accessible areas.

EVAPORATORS
A refrigerator box must have an evaporator to collect and dispose of heat in order to lower temperature of box air and whatever is in the box.. For your box these are the evaporators available by group:
Group 1
The most common evaporators are known as thin roll bonded flat aluminum plates that come in many excellent different K value Btu capacity sizes. The lid opening and box cubic feet must be compatible with plate large dimensions and this will also help determine compressor capacity. Many manufactures allow plates to be bent where labeled as much as 90 Degrees. Engineering design data available for roll bond evaporators exceeds all information of other types of evaporators. Roll bond evaporators all should include accumulator chambers near their exit tube.
Group 2
Evaporators are evaporator refrigerant coils covered with aluminum or a stainless steel cover. They are ¾ inch thick and available in limited non bendable sizes. The refrigerant tubing size and length without restrictions in any evaporator is important to maintaining refrigerant flow velocity. Without adequate flow oil trapping deposits accumulate restricting K value performance and limiting compressor cooling. The low pressure side of refrigerant tubing that includes evaporator tubing should not be larger than the low pressure return refrigerant tube at compressor.
Group 3
Evaporator Bin freezing compartments ready to install either horizontal or vertical immediately converts an insulated box to a refrigerator with freezer area compartment. Systems with the number one favored evaporator are first choice by day sailors and blue water sailors. Bin evaporators are also a solution when normal size evaporators lack the surface area needed for box’s needing additional Btu capacity.
The lid open dimensions may be a problem for these bin evaporator. The bin evaporator is the best solution for a refrigerator with freezer on insulated box’s from 4 to 8 cubic feet. Several boat refrigeration companies offer different sizes of these freezer bins.
Group 4
Evaporator one inch or slightly thicker stainless steel plates contain a heat conductive material or a eutectic solution are normally limited to smaller capacity boxes do to their smaller heat conducting surface area. Such as Adler Barbour’s Power plate.
Group 5
Evaporators that store energy in ice known as Eutectic plates holding energy using it over a number of hours without refrigeration unit running or Cycling eutectic energy plates that perform like other types of evaporators. Some boaters believe eutectic plates are more desirable than standard evaporators, I do not agree as in most cases they lack the controlled heat transfer capacity equal to a conventional evaporator. It is difficult to convert the average size conversion box into a multi temperature refrigerator with a single bulky space consuming eutectic plate. The eutectic plate requires the same amount or more energy than a conventional evaporator and the energy source for both is from a 12/24 compressor smaller than 1/6 horse power. Most all eutectic plates are intended to store energy if and when it is available and small 12 volt refrigeration compressors only produce refrigeration when current is available from energy produced by engine alternator and stored in batteries. The primary justification reason for eutectic plates is if the boat is equipped with surplus electrical alternative energy from wind, solar or onboard generator. There are only two ways a eutectic plate can compete with a conventional evaporator is it can provide what is known as the flywheel effect when warm things are placed into an already cold box and store excess energy when available.

All types of evaporators are mounted on vertical walls are mounted with standoff brackets to take advantage of take advantage of walls boundary layer air natural movement tumbling.

FREEZER
Freezer boxes or freezer chambers require better heat convection natural air flow towards evaporator than refrigerators. Freezers preserve food longer when they are surrounded by the evaporator so a single eutectic plate or evaporator that covers only one box side is not the best freezer. To improve on natural air tumbling in freezer ¾ hard wood rails vertical on the evaporator side of box and extending across box bottom and up the other side provides a natural flow of air. There is an interesting thing that happens on refrigerated box vertical walls as they are warmer than the center of box. Warmer lighter less density boundary layer air rises up the wall assisted by colder more dense air descending from evaporator through the rail system on the other side naturally move heat towards evaporator.
Knowledge of thermal and hydrodynamic boundary layers in refrigerator as well as near the evaporator and box liner walls is also important. If the product is too close to the evaporator wall, freezing can occur, and if it is too close to the other walls, there may be food quality changes or health risks.

SPILLOVER
Before fan forest cooled frost free evaporator home refrigerators all refrigerators relied on some type of spillover cooling. In a top opening boat boxes the evaporator must be in the freezer side with a spillover system midway up in refrigeration side to add cold air to refrigerator area while air is returned high in the separating insulated divider. For a spillover box to have two efficient temperature areas there needs to be at least a 25 degree temperature difference (Delta T) from one side of divider to the other. These dividers should have at least an insulation R value of 10.
Early sailboat icebox conversion spillover dividers used a series of holes top and bottom, Now holes with removable corks can be used to regulate refrigerator area temperatures. My recommendation for a spillover refrigerator is this. Most boat spillover boxes are side by side requiring pressurized air movement to maintain individual box temperature control. One thermostat controls freezer temperature by cycling compressor on and off. A second thermostat controls a small fan in refrigerator box producing 10 to 15 cfm into refrigerator section. Return air to freezer is provided by an equal size hole at top of the divider. The divider fan thermostat needs fuse limited power from refrigerator circuit breaker so if needed it will run even when compressor is not running.

OTHER BOAT REFRIGERATION KNOWLEDGE that can be useful
When it comes to Ice Box Conversion Refrigeration the best way to evaluate each system offered is first determine the actual performance of those system you are considering by past customer responses. With a dozen manufacturers and over one hundred equipment options to select from do not let price or advertised mumbo jumbo energy efficiency be a major consideration. Twelve volt direct current fixed speed compressors of 1980 in most cases wasted energy in small boxes and in cool climates because one size compressor had to fits all boxes. Salesman relate energy wasted by compressor cycling too often ignoring the fact that variable speed compressors sold after 1996 under the right condition can cut energy use by up to 50% if correctly installed with balanced refrigerant capacity components. As you compare conversion systems remember the only positive energy conversion savings from electric energy watts to Btu energy occurs at the compressor. Ounce you have compared at least two well designed units with at least five years in active boats consider if it is simple enough to be maintained by the boat’s captain or a local repair person. Simple systems are maintainable while systems with unnecessary gadgets require knowledge and parts not available locally.

If you really want to know what the daily power consumption will be to operate your boat’s refrigeration in worst case climate conditions and then correct for present climate location performance. Then refer to compressor manufactures performance profiles watts per hour into and Btu out of compressor. Other factual performance charts show power consumption and coefficient of Performance based on various compressor speeds. No engineer should recommend a compressor for your specific box without consulting these charts.

Water cooling refrigeration is necessary on compressors larger than one 1/6 HP but unfriendly on smaller compressors.

When ever there is an option of TXV or capillary tube refrigerant flow control consider the benefits of a TXV when boat moves yearly from cold climates in winter to warmer climates and back north in summer.

Always carry on board manufactures documents on typical repairs and troubleshooting procedures in addition to refrigeration maintenance manual.

[nhschneider]

Excellent primer on refrigeration for sail boats. Thank you for posting it.

Fair winds and calm seas.

[foojin]

Planning to install a icebox conversion unit (Nova Kool LT201) over Easter to replace a 40-year-old Adler-Barbour.

Just realized there is a much better location for the compressor if it can be installed standing up vertically with the fan at the bottom and tank at the top. There will be lots of circulation.

Love to hear from refrigeration pros.

Thanks

[Bowdrie]

Originally Posted by foojin

Just realized there is a much better location for the compressor if it can be installed standing up vertically with the fan at the bottom and tank at the top.

It must be mounted horizontally.

[robert sailor]

Richard
Who makes the most efficient small 12 volt refrigerators around 4.5-5.0 cu feet. Will be used in tropics most of the time. Thanks

[foojin]

Originally Posted by Bowdrie

It must be mounted horizontally.

Thanks Bowdrie,

Since posting, I have found a few statements online that suggested that was the case, but nothing that stated it clearly. Makes me wonder why a manual would not just say this is the way it must be oriented, whether or not they tell you why.