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Old 27-05-2019, 23:25   #1
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Invitation to curmudgeonly comment and constructive criticism

I happen to have a spare BD50F compressor from a previous boat lying about and being a keen follower of the cold plate evaporator versus eutectic threads in which the prime protagonists were Richard Collman and Ozepete, I admire them both greatly for their obvious extensive knowledge of boat refrigeration and deeply appreciate their willingness to share it on this forum.

A number of things I have noticed whilst fiddling about with refrigeration on boats.

In my first attempt at building a refrigerator on a boat I relied extensively on the assistance of a sailing mate who was a fridgey tech. His forst attempt at a condenser was a coild of 1/4" copper pipe in a length of 80 mm poly down pipe with caps on both ends and 1/2" thru hull fittings at either end.

I explained to him that it was an inefficient design because the water would not be in turbulent flow and it would require a water pump which would pull excessive amps. To increase the assault on his dignity I then took a hacksaw, cut the ends of and mounted it vertically in a cupboard with a computer fan blowing into the bottom end.

Some time later, after the fridge had been working fairly well for a couple of years the computer fan burnt out. I don't know how long it was like this as the fridge still worked quiet well. I only noticed when I opened the cupboard one day and realized that whilst I could hear the compressor running I could not hear the computer fan. On placing my hand at the top of the condenser I could feel a warm stream of air and realized that un-assisted convection was doing a fair job of air cooling the condenser coil even without the fan.

Years later, after I had rebuilt the fridge with a kit which included a fin and tube evaporator I accidentally built a eutectic system with the way I had designed the air flow through the evaporator and along a space in the bottom of the box and then backwards and forwards past shelves of sliding plastic containers which I was using to maximize storage whilst providing good access to the bottom layers in the box.

Condensation had filled about 3" of the bottom of the box and the flow of cold air over it had frozen it. I would have to shut down the compressor for a couple of days to let it unfreeze before I could clean it out.

Since at the time I had a very limited battery and solar panel installation I realized I could exploit the frozen water to keep the fridge cold overnight and only run the compressor during the daylight hours when the solar system provided plenty of amps. I fitted a 24 hour 7 day timer so that the compressor only ran from about 8 am to 4 pm, except that the compressor ran very hot. Which is why they have less than a 100% duty cycle.

I now have another boat which has a box about 5 times the volume of the old boat and a constant cycling BD50 condensing unit and evaporator plates which it appears will eat a few hundred amp hours of batteries every few years trying to keep the food and beer about half cold. In addition, since it is built into the aft end of the galley where the hull curves substantially the bottom of the box doesn't have much flat to it. It's pretty useless volume except for condensed water collection.

My intention is to build a condensing unit using the spare BD50 compressor, exploiting the convective effect noticed in the first DIY condenser assisted by a computer fan and using the air output to cool the compressor. Hence the prototype in the attached images (I realize a coat of paint would improve the appearance of the work so far but intend to go to the expense of a complete new unit with a bigger compressor with the base and coil housing built in alloy if the prototype works out.)

Keeping the commentary/derision down to speechless admiration/course profanity would be appreciated.

The first image is the condenser coil housing. The computer fan is not yet fitted but goes on the bottom end.

The second shows the coil and it's mounting, the white thing in the meddle is poly pipe with caps on both ends, hopefully it will serve to increase air flow velocity around the condensing coils.

The third image shows the compressor mounted on the top.
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Old 28-05-2019, 04:35   #2
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Re: Invitation to curmudgeonly comment and constructive criticism

Other than saying itís a nice piece of work, why is the compressor at the hot end of the condenser?
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Old 28-05-2019, 05:57   #3
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Invitation to curmudgeonly comment and constructive criticism

If you look a a cool blue System which is pretty well regraded, youíll notice that the compressor sits in front of the condenser fan. The compressor cooling is assisted with the air flow off of the heated air from the condenser.
It works fine, I believe the air really isnít all that warm, that the amount of air from the fan when compared to the small amount of heat that these compressors can move means the air isnít heated all that much.

The heat output of these small compressors is roughly about half the output of an average adult human, to put it in perspective.
I like your plan, I even think the pipe in the middle of the condenser is well thought out.

Some will argue quite correctly Iím sure that these compressors are cooled by the cold return refrigerant, but I think additional cooling can only be beneficial.
I put cheap finned heat sinks meant for stepper motors on mine, I figure it can hurt.
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