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Old 05-06-2010, 18:47   #1
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Dual Fridge Compressors

I have an engine driven compressor driving my onboard fridge/freezer. Is it possible to add a second compressor to the system that runs of the marina mains supply? This will allow me to use the fridge / freezer during weekend when we live on board. If possible, how would one connect the system in, in series with existing compressor or parallel with existing compressor?
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Old 05-06-2010, 19:52   #2
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There are four possible ways to convert your refrigeration to a hybrid without disturbing present engine drive refrigeration system. First I need to know dimensions of present box or boxes and location and size of holding plates. One option would be to use a second evaporator coil if one exists inside present holding plate or plates. Do your plates have a second evaporator coil inside? The other options will depend on box and plate configurations.
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Old 05-06-2010, 20:22   #3
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Hi, the fridge / freezer consist of two chambers with a stainless steel plate separating them The Fridge is approximately 75 litres and Freezer is approximately 110 litres. I’m unsure as to how the evaporation plates are connected but they both run off the same single compressor. The evaporation plates are mounted behind the stainless steel internal lining and are not visible. Ideally, I would like to use the existing evaporation plates but feed them from either the engine compressor of the mains power compressor.
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Old 05-06-2010, 20:32   #4
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You would need to have plates with two separate coils.If you have this, you could add a traditional type 110v. compressor with one or two thermostats. Will not be cheap.
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Old 06-06-2010, 19:16   #5
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I have address the problem of adding an additional refrigeration condensing unit for either redundancy or eliminating need to run engine for engine drive refrigeration when connected to shore power several times. It is never a good idea to disturb the existing refrigeration system when adding a second condensing unit.

As I understand your present refrigerator and separate freezer boxes the interior walls are clean stainless surfaces with present evaporator/holding plates behind the stainless boxes liners. The best solution for adding an electrically driven compressor condensing unit is to add a completely new system with thin plate evaporators attached to interior box liner, this concept work well with 12/24 volt DC or AC systems. It does not freeze present holdover plates but it will lower their solution temperature to box’s desired temperature. The only complaints I have had reported is the time required the first day when starting with a warm box in pulling box and plate solution down at the same time with a small compressor. The main reasons small capacity compressors are used is noise level at night as thin plate evaporation requires compressor cycling every hour off and on.

Selecting a system design and cost of this added refrigeration is the difficult part. Addressing temperature control of each box with two boxes and only one condensing unit is the main problem. Normally with two boxes as small as these if side by side a single unit with evaporator in freezer then a air spillover system to refrigerator would solve the problem.. There are several companies that sell 12/24 volt units that could handle your additional refrigeration needs if a spillover system is possible, Adler Barbour, Frigoboat, Seafrost and others. The only company in the US that has designs for three thermo thin evaporator plates one for refrigerator and two in freezer is Sea Frost. If your freezer were larger I would have recommended two separate refrigeration units one for each.

I guess this is as far as I can go with the info you provided. Post any questions you may have and I will try to answer them.
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Old 07-06-2010, 01:43   #6
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As the saying goes, no such thing as a free lunch. The idea is not to install a second system and actually use the existing installed system. The problem with a second system is the installation issues. Getting the evaporator plate through the small freezer opening and try to secure it will be a mission. The alternative is to dismantle all of the cabinetry but that will destroy the interior and I do not want to go there. It will also require a hole through the side wall for pipes etc. Sorting all this after manufacturing is not an easy task.
Given that the engine driven system works like a dream make sense to keep it. The alternative solution is to mount an electric motor in the engine bay to drive the compressor. I will still have to manage the raw water cooling for the condenser. Given that the compressor is a Blissfield (Tecumseh) and the specifications state it’s a ¾ - 1 – 1½ Hp it will require a chunky motor, at least a 1½ Hp. I do have sufficient space to mount the hardware. Somehow it would appear that an additional unit is my only option.



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Old 07-06-2010, 05:30   #7
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Using an electrical motor to drive Tecumseh HG1000 is an option used by a few boater’s, one boater has used his system off and on for 15 years. The electric motor used is a one HP Capacitor start 1700 rpm. The small pulley on motor is 1 ½ inch. Electric motor is mounted permanently. If condenser cooling pump is already electric there is no need to disturb engine drive electrical circuit. To activate shore power refrigeration the engine drive belt is removed and a belt to electrical motor is installed from compressor. To operate system the AC motor is powered manually and normal DC electric refrigeration switches are turned on. Thermostats and high and low safety pressure switches will still control and protect refrigeration unit by operating compressor clutch.
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