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Old 03-05-2009
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Refrigeration - Engine Driven or DC Compressor?

I have an engine driven refrigerator but I was wondering whether it might not be more efficient to replace the compressor with a high output alternator and redo the refrigeration to run off one of those off the shelf units.

Probably also safer in the long run, i.e. the temp in the refer is automatically kept in the right range?

At present I have a condensing unit that uses the raw water from the engine.

For an electric compressor the choice is (i) air cooled, (ii) water cooled with (iii) additional pump or I think some sort of external heat exchanger on the outside of the hull.

I don't like (ii) (iii) because they will not work on the hard and (iii) is a bit bulky anyway.

What about the efficiency of air cooled in the tropics?

Would a combo condensing system make any sense?
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To make my reply easier, most of this reply is from my posting on Anything-Sailing.

Dump the engine driven system!!

When I bought my Freedom 32 it had a Seafrost engine driven holding block system, it was an R12 system and had lost it's refrigerant charge due to lack of use. The holding block had 2 circuits so I had considered a 110v compressor on the spare circuit while at dock and repairing and upgrading the engine driven system to an R134a system.

Well, speaking with Cleave at Seafrost he proposed replacing the engine driven system with one of their 12volt BD systems. I was nervous about this as Freedom 32's only have space for 3 group 24 batteries and so I only have about 160 to 170 amps of house capacity and to increase it required loosing storage space and the starting another project, neither of which I wanted at the time. As the boat was only used to day sails and weekends away, I decided to go with the advice and give it a try, it was going to work out cheaper than my original plan anyway!!

So I ordered a 12volt pre-charged BD system along with a evaporator plate style freezer bin that was sized to fit my reefer.

Installation was complete within a couple of hours, the hardest part was removing the old holding block, damn that thing was awkward and heavy, installing the new freezer bin also took as much time again. The installation, plumbing and wiring of the compressor unit was easy and took probably 30 to 35 mins.

I installed this system in September/October last year and to date I am very pleased with the performance. The boat is now being used as a live aboard during the week and I have really cold beers and can can store frozen meals for the week. The system runs 24/7 off the batteries which also have sufficient capacity for day sails so far and I think may also be ok for weekends based on how the unit has performed so far. However, the heat of summer may have me reviewing my battery capacity, but I have a well insulated reefer and remain hopeful based on current performance plus I have eliminated several items from my engine room and adjacent lockers with the removal of the engine driven, sea water cooled system.

During the winter months while maintaining cabin temperature at 68 degrees the unit is operating about 40% of the time, I am expecting this to 70% / 75% during the summer. Maybe the addition of the variable speed controller for the Danfoss BD compressor could reduce the runtime!

Disclosure, I do not work for or have any commercial interest in Seafrost, I do however work for a company that does on occasions supply Seafrost equipment, when requested, for customer use with our products.

Bob
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If you have a seafrost system, call Cleve at seafrost. He'll talk you through all the variations and help you out. Excellent technical support.
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Did you go for air-cooled or water-cooled?

Thanks for that info.

I guess most off the shelf units are air cooled, but in the tropics this could actually heat up the cabin and decrease the effectiveness of the refer.

So ideally some way to have the condenser air cooled sometimes and water cooled at other times might be best. But does this exist, is it worth it?
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matt
seafrost has done work with various types of systems, have you looked at their site?

Have you considered keeping the engine drive system and add a dc system?

since you already have the engine system, it becomes 'free' to operate when you do run the engine, and it adds redundancy
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Quote:
Originally Posted by xort View Post
seafrost has done work with various types of systems, have you looked at their site?
Looked at them all at one time and another, but will look again.

Quote:
Originally Posted by xort View Post
Have you considered keeping the engine drive system and add a dc system?
Yes, but there is not a lot of room. If I was to have an electric version I would need more alternator output to charge the batteries quickly. Any high output alternator would go on the bracket where the engine driven compressor is. Wish I could fit both but not possible.

Quote:
Originally Posted by xort View Post
since you already have the engine system, it becomes 'free' to operate when you do run the engine, and it adds redundancy
Yes it would. It all actually works now but I need the run the engine for about three hours to cool it down then one hour a day. It seems a shame to break down a working system. I'll think about it, but then where will I put the hi output alternator?
Thanks for your thoughts.
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What is the current alternator?

You can get up to about 100 amps on a small frame alternator. That could replace the current alternator, if it's too low powered.

Do you have alternate sources of power?

If you are going to run the engine to charge batteries and power the refer, it would be more efficient to run an engine driven refer directly.

With a high power alternator, how log will you need to run the engine to charge the batteries?

I'm thinking you could run the engine with the 100 amp alternator that charges the batteries AND the engine driven compressor chills the cold plates. The cold plates should be able to stay cold for 24 hours on one hour of engine time. After that, the 12 volt system kicks in to extend the refer chill time. depending on the battery bank, you could get another 24 hours. Add solar and win and get another 24 hours before running the engine.

I wouldn't do all this from scratch but since you already have the engine system, perhaps it works to have a dual system. I love the redundancy too.
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Don't you currently have an alternator in addition to the engine-driven compressor? You could swap out the existing alternator for a high output one.

I plan on adding an electric compressor and keeping (recharging) the existing Sea Frost engine-driven system. Not sure how many amp hours it will use per day. I'm adding a wind generator to keep the fridge going so we don't ever have to run the engine for cooling.

Helpful thread. Thanks.
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Remove the old system, regardless of which replacement system you choose. I understand that the current system has been fully amortized, but to retain it implies 2x the weight, complexity,and upkeep.

Glacier Bay makes a DC driven unit that will charge the plates when it senses excess capacity available from the alternator. I'm sure the sea frost alternatives are as good too.
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I think a good DC-based system is a much better idea. If you plan the size and have proper electrical setup, you can often use it without having to run the engine to charge the batteries, especially if you have a decent passive charging setup.
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