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Old 29-11-2018, 08:29   #1
Marine Service Provider

Join Date: Aug 2004
Location: Ft. Lauderdale
Posts: 1,315
Replacing 36 to 46 Foot Pleasure Boat’s Refrigeration

Remembering the 70s and 80s boat refrigeration was before the transition from low speed diesel engine power to the modern high speed diesel engines. Back then the larger boats 36 to 46 ft that needed refrigeration looked to Grunert and Crosby who adapted and modified proven supermarket condensing units and eutectic holdover plate evaporators from a company building refrigeration for trucks. If a boat was equipped with a generator 110 volt compressor refrigeration units with hold over plates was the answer to refrigerator and onboard boat freezers. Boats without a large electrical power grid used 12 volt DC belt drive motors to operate belt driven supermarket stile compressors. The alternative to large electrical driven refrigeration was a engine driven compressor with an electric clutch that allowed a refrigerator thermostat to control refrigerator box temperatures. There were advantages to the off the shelf refrigeration designs. It was not a problem to find a technician with experience on conventional refrigeration. Package units had safety switches, isolation valves and leak resistance line connections set a quality standard the still exists today.
The main disadvantages were% added weight as much as 300 pounds to boat and they all required an engine to run when refrigeration unit was running.

Simple Engine driven simple water refrigeration built by individuals for there own boats were in use as early as 1969. Cast iron automobile air conditioning compressor connected to a water cooled condenser with a variation of home made hold over evaporators becoming popular boat refrigeration systems. I new a man who built his boat with one of these basic systems and after 17 years he still had the same reliable refrigeration.

I ordered a new 32 ft sailboat in 1982 and spent time visiting most of the boat shows East of the Mississippi river looking at equipment for my new boat. Engine driven refrigeration components back them looked the same as automobile air conditioning except for the need for a water cooled condenser. I showed a picture of an engine driven system to my auto shop foreman. On Monday the foreman filled the trunk of my car with the complete AC system he removed from a wreaked 1978 Dodge Omni car, Swash plate Sanden SD508 compressor, expansion valve, evaporator and refrigerant hoses. I had a mechanic built a box out of 304 stainless with a removable top to put the Omni evaporator in. To complete my new boat’s refrigeration I only had to purchase a Seawater condenser, high and low pressure switches, New refrigerant receiver filter dryer, and a 60 minute timer to operate compressor clutch once a day. This refrigeration unit with all its original Omni car parts after 15 years was upgraded to a dual hybrid 12 volt/ engine drive system. When I sold boat the original compressor was still operating after 30 years with only drive belt replaced a few times.

There were a number of errors made in the earlier model designs of pleasure boat refrigeration prior to the year 1995:

• Automotive AC compressors came in several shapes and sizes and many failed the test of time when used as boat refrigerator compressors. The Tecumseh cast iron H1000 and 500 two cylinder compressors used a large ford vehicles were heavy and the aluminum small capacity two cylinder York all developed a pulse vibration do to longer belts as they were not mounted on engines. These two cylinder compressors had another design problem with drive shaft seal leakage. The clutch and compressor drive shaft was supported by an internal large wet bearing inside compressor caused shaft seal failures. Also the two cylinder shaft seal requires high crankcase refrigerant pressure to prevent leakage. When these first generation two cylinder AC compressors were used in low back pressure refrigeration they were installed on large heavy Low Rpm diesel engines where compressor Rpm normally operated a less than 1600 Rpm. In the 1970 and 1980s it was believed by many that if an AC compressor operated OK at 6000 rpm it could also operate at the same speed for refrigeration this proved to be a big mistake. After limiting compressor speed to less than 1600 Rpm earlier failures were eliminated.
Aluminum Swash Plate actual flow five cylinder compressors belt driven by high Rpm diesels boat engines became the reliable standard for reliable boat refrigeration. On a Yanmar 3600 Rpm engine a four inch engine drive pulley will keep compressor functioning for many years. Swash plate shaft seals do not rely on only refrigerant pressure to prevent seal leaks and the load on belt drive is transmitted from clutch bearing direct to compressor aluminum case and not to bearing inside compressor as was the problem with first generation compressors.
• Air cooled condensers are not recommended for engine driven refrigeration due to their high Btu refrigeration capacity, ten to twenty times more Btu per hour than small 12 volt systems. Engine driven refrigeration requires sea water cooled efficient cupro nickel condensers capable in tropical waters of disposing of from 6000 to 12000 btu per hour depending on compressor output. Boat engine driven refrigeration companies have been known to sell water cooled condensers that in warm seawater cause poor refrigeration performance. Source for condenser water is best provided by existing engine seawater cooling pump flow avoiding an extra electric pump.
• Expansion valve flow control valve size is important to prevent liquid slugging of compressor. For engine drive compressors that have capacities from 12000 to 24000 Btu capacity as AC units but now are used to control refrigerant limited flow in holding plate compressor start up situations flow must be restricted. Refrigerants are condensable and in a refrigeration system will change to a liquid in a cold enough area of the system like a partially frozen holding plate. Engine is almost always running when refrigeration compressor clutch is in gauged this fast clutch action will move liquid refrigerant from holding plate quickly possibly damaging compressor that only pumps gas vapor. Design mistakes of oversize flow control devices in the past have resulted in compressor failures. Suction line accumulators help resolve liquid slugging of compressor as well as long oversize suction tubing. I still recommend small tonnage expansion valve with max orifice of ½ to One ton for engine driven systems.
• Large boat refrigeration systems where sufficient energy is not available 24 hours a day generally use holdover evaporators containing large amounts of a eutectic solution to store surplus energy in the form of ice. All of these holding plates are not designed the same, besides size, shape the thermo energy efficiency designed inside plates is very important. The volume of eutectic solution will determine how long plate will continue to absorb heat. The preset phase change temperature of eutectic material will determine plate exterior temperatures where heat is collected. The lower the phase change temperature the shorter the holdover period. The exterior skin material of holdover plates varies from carbon steel coated with cold galvanizes to cast aluminum and stainless steel. Cold galvanized plates are not as attractive as stainless steel. The problem with welded stainless steel is the carbon added to weld areas by Tig welding can over time allow plate solution to leak out. Food stainless steel serving trays have been used by a few manufactures as holdover tanks with backs welded or bolted with gaskets to seal liquid in. Cast aluminum box plates also had bolt on plates with gasket material. The eutectic material inside plate needs to be non corrosive and food grade non toxic should material leak into refrigerated area. Inside all these holdover plates there is a refrigerant evaporator coil to collect the refrigerator heat. These special evaporator coils are not all as efficient as you might thank.
• Refrigerant hoses instead of metal tubing was a major mistake because of refrigerant leaks and moisture migration through flexible hoses. Moisture osmoses penetration into a hose when refrigerant did not leak through hose is hard to believe but it is true according to the hose manufacturer. Prior to 1995 boat refrigeration systems with rubber hoses were plagued with moisture freezing in expansion valves. Systems with all metal tubing did not experience moisture problems. Another trouble with hoses was their aluminum hose fittings would leak and even break into dumping all refrigerant.
• Copper nitrogen filled tubing is used throughout a system except near compressor where short vibration dampening copper line or very short section of hose is needed to prevent work hardening copper line failures.
• Refrigerant copper line connections on most of the older large pleasure boat refrigerator kits were hard silver brazed or soldered permanent connection. With the exception of evaporator plates and thermostat, all refrigerant components were mounted on one or two base plate assembles. When these major components were connected together Refrigeration grade flared brass connecters were used as they will remain leak proof under thermo expansion better than any other mechanical connection. Systems that used other light weight mechanical connections eventually had refrigerant leaks.


There are boats in the 36 to 50 ft range that still have these twenty to forty year old supermarket style refrigeration and Engine driven units in there boats. The trend seems to be to remove highly effective large refrigeration and replace them with very small 12 volt DC units. Experience has proven that without an onboard generator BD size 12/24 volt compressors in warm or hot climates are not adequate in Freezer boxes larger than four cubic feet or refrigerator boxes over six cubic feet. The real question in this post is how large a box can be refrigerated by a 12 volt DC hermetically sealed compressor? There are always comments suggesting the answer is to add insulation and not Btu capacity. Insulation has never been a solution if boat is to be a live aboard in the tropics unless adding insulation is to reduce size of area to be refrigerated.
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