Another project Am I nuts to try this??

[sailaway78]

OK, heres the project. I bought a small 4.5 cf frig. and an inverter when we started home with the 37. It did not have refrigeration. Now I think I am going to take the guts out of the little fridge. and build a frig. in the ice box on the new boat. The ice box is just a tad larger than the fridge, approx. 6 cf. The fridge was rather power hungry, and used approx. 160 AH per day. As we only had offshore runs of not much more than 24 hours on the way back to New Orleans, and were mostly motoring, this wasn't a problem. With my usual cruising habits, this will pose a problem. The ice box is much better insulated than the fridge, and the fridge has a very poor condensing system. Also with the box, the lid is smaller, and only opens the top, not the whole front like the fridge. Purely un scientific dead reconing, tells me that I should be able to cut the AH at least in half by using the same components in the ice box. The existing ice box has an ice sump in the bottom of it, which is almost a perfect fit for the evaporator, out of the fridge. I can also come up with a much more effective condensing unit. Added to these is the fact that I enjoy taking on this kind of project, I have all the parts, pieces and tools I should need, and last but not least, since I retired I have become a certifiable cheapskate. So, tell me why I should or should not give this a try. Am particularly interested in hearing opinions from the do it yourself guys, and from any refigeration experts on here.

 

[Stu Jackson]

As long as it's 12 volts

it seems OK. You mentioned an inverter. Going from 12 to 120 is pretty energy wasteful.

 

[Ralph18414]

Stu, is it wasteful going from 12 to 120v? I was

reading the specs on some inverters I was interested in and all of them had efficiency ratings in the 90s, I believe. I think the project is worthwhile.

 

[Donald6441]

for most small inverters you will see a

10-30% inefficiency depending on the quality of the design, plus the motor will have additional inefficiency from using a square wave. If you use a sine wave inverter it will be better, but then your into more cost, which can be better spent on a 12V compressor. I have a link somewhere to one, ill see if i can dig it up..

 

[Donald6441]

ah, here we go

Danfoss Brushless DC Hermetic Compressorshttp://www.rparts.com/Catalog/Major_Components/compressors/Danfoss/danfoss.asp

 

[Fred Ficarra]

And another inverter cost is 'standby drain'.

I have a Prosine 2.0. We leave it on and even use it for cabin lights and stuff like the microwave and home theater. When I installed a Xantrex battery monitor last summer I found that the inverter used 3 amps of 12 volt power just to stand by. After factoring in that extra draw and you still have ample power capacity, go for it. Sounds like a fun project.

 

[Paul H]

N 'n' E...it's just my $0.02

and I will let you know up front that I am no refrigeration expert. I bought one of those refrigerated coolers a few years back when my wife and I camped alot. I got this crazy idea to put in my lancer 28 (as I, too, only have an icebox that is hard to access sometimes). It's the best (and the easiest) thing that I've done so far. I installed it just like you would a fridge. I installed it under one of the settee's...the day before we push off, I plug it in and let it get cold; and it will keep items within cold for up to 5 days!! If we're out for longer, I just run the diesel for an hour or so, and it's ice cold again. There's no need to have it plugged in while under sail...I know that they're fairly inexpensive (mine was 95.00 CDN). Just a thought...

 

[sailaway78]

More on this

I have a Xantrex 1200 W. inverter on the boat now. It will run all the boat needs quite easily. I am aware of the power usage in the standby mode. Thought was to hook the fridge up direct to the inverter, and use the thermostat to switch the inverter, rather than the fridge. May end up adding a small inverter, or possibly some kind of by pass switch so I can use other appliances while at anchor. But at least no one has told me that it flat won't work, or that I am totally nuts. Guess you recognize the cheapskate in me.

 

[Monty Miller]

Archives

Do a search "dorm fridge" (exact phrase) and you will find some helpful information.

 

[Bill Roosa]

Can't be worse than 120 AH can it?

My 37.5 reefer draws around 85 AH and is a DC unit.I agree with Paul H, you can just run it when the motor is on so there should not be a battery storage problem. I would be concerned about the condenser being in a closed compartment and getting the heat out efficiently but you have probably already thought of that.

 

[sailaway78]

Interesting ideas

I checked the Danfoss site, and their units have almost the same current draw as my compressor. The one I have is rated at 190 watts, max. The Danfoss price is decent, but the module is as much as the compressor,which makes it kind of an expensive toy. Plus if I do this, I will be able to slap a new compressor on for about 100 bucks, if needed. So far no one has come with any major problems, that I hadn't thought of. As for running the engine at anchor, to run the fridge or charge batteries, I hate having to do that. Hopefully this thing will have a low enough power consumption that running the engine will not be necessary. Thank you all for the input. All responses are greatly appreciated.

 

[RAD]

Here's a link that might be helpful

I bought the book and it has all the info you need about refrigeration on a boat and DIY trouble shootinga must for the cheapskates and captains who will never ever let someone else work on there boat (like me)

 

[Ed]

cheap refer

I took the guts from a dorm size refrigerator a few years ago. I run it off a 750 watt inverter with a surge capacity of 1400 wattts. my box is 8 cu ft. The thing works great and will freeze items in the box if I am not careful. TO keep from having to recharge the system, I cut a hole in the top of the box to install the evaporator. Power drain has not been a problem, but my longest runs are 8 to 10 hours. Most inverters do not pull much current while on standby, certainly not the 3 amps that one person's does. Some of the dorm size units have condneser tubing in the sides of the unit rather than a coil -not the best for your purpose. This was one of my more sucessful handyman projects- go for it.

 

[Benny]

Ice box makes perfect dry storage comparment.

Nice and Easy you seem to be familiar with refrigeration so I will not bore with details but just want to offer info on what we do. As the ice box on our h27 Cherubini was poorly insulated we opted for keeping the dorm refrigerator in its original box and converted the ice box to dry storage. We run a 1500 watt Xantress which adequately powers the fridge froma single deep cycle group 27 wet cell. At dockside I keep the refrigerator full of beer but when we go on a trip we move the beer to a beach cooler and fill the fridge with food. A full fridge runs more efficiently than an empty one. As preparation we run the fridge on high for a couple of days prior to departure and then take the thermostat up when switching to the inverter. With the beer out there is hardly a need to open the fridge except for lunch or at dinner time. The first few hours it hardly cycles at all and it has been our experience that after a full day at sail when we get to our anchorage we usually still have power to turn the engine. At anchorage we switch to a generator, a Honda eu2000, to recharge batteries and for 120V electrical needs. By morning batteries are recharged and we are ready to switch back to the inverter. During the summer we also run a portable A/C unit which requires to turn the throtle ECO control off to handle the cycling loads. With increased RPMs the run time was cut almost in half to 3 1/2 hours but we have solved the problem by installing an external six gallon tank. After the first two days our ice is usually gone so we either move the beer back into the refrigerator or find a marina to get ice. We use this boat mostly for weekend outings so the ability to run refrigeration for an extended period of time was not a necessity. We find that we have a good working setup for our needs.

 

[Stu Jackson]

Refrigeration & Inverters DO THE MATH

Ralph, the reason I mentioned th importance of the voltage is addressed in Calder's 2nd Edition, page 172.a 1,200 watt, 120 V microwave (watts = volts X amps) will consume 1,200 watts divided by 120V = 10.0 ampsThat same microwave connected to an inverter will do this: 1,200 watts divided by 12V = 100 amps. Yikes!Essentially, that means that for ANY load in watts, the load on a DC (i.e., 12V battery) system will be 10 times the load in amps as the load on a 120V system.And that's before you factor in the efficiency of the inverter even if it is 90%.It is NOT the efficiency of the inverter that is the issue, it is the conversion of AC to DC loads with the same wattage that increases the amp draw TENFOLD.That's why I asked if 'Nice' was doing it with 12V or using the inverter.It is still not clear to me from these discussions just how he's planning on doing it.

 

[sailaway78]

More answers

Stu, this is a 110 volt compressor. 1900 watts. Amp draw on the batts when using the inverter are approx. 20 amps. The start surge is 30, for maybe a second. As Ed, who has done this says, for a condenser the fridge uses copper tubing taped to the outer skin, which I cannot imagine being very efficient. In it's origional condition, the little fridge ran about 1/3 of the time, which is where I got the dead reconing figure of 160AH day. This is a lot of power usage when you are planning on extended times away from shore power. I am thinking that for a condenser, I can run about 20' of tubing in the bilge. The bilge on the 37 does not have a sump, and it is virtually impossible to get all the water out of there, with a bilge pump. I really don't like this much, but why not at least make use of it. I don't think there will be enough heat to make any difference inside the boat, the fridge sides when running are barely warm to the touch, but it should do a pretty good job as a condenser. This will eliminate the need for a cooling pump, and the associated plumbing and wiring for it. What does the brain trust on here think about using the thermostat to switch the inverter on and off, which would eliminate the current draw when the compressor isn't running.

 

[Ralph18414]

Stu, I'm aware of the relationship between power,

voltage, and current. The physics you describe can't be denied. What I am fuzzy on, and what is relavent to Nice N Easy's original question, will the current draw be dramatically less using a 12 v compressor vs a 120 v compressor? From my old college courses I know a 1 hp electric motor will, more-or-less, translate to a fixed power consumption so that at 12 v the current draw will be significant. If the current draw for a 12 v compressor is going to be the same as a 120 v compressor working through a 100% efficient inverter then, for the kind of sailing I do (boat is always connected to shore power except during the weekends of sailing), I'd be willing to live with a 20% loss in power due to an 80% efficiency rating. Is my premise wrong?

 

[ruedi ross]

Another consideration is

that DC motors are inherently more effecient than single phase AC motors. Also, running an AC motor on a modified square wave inverter further increases its ineffeciency and shortens it life expectancy. In the long run it may be worth converting the fridge to a DC motor and powering it from the charger when on shore power.

 

[Funtime]

1900 Watts fridge?

"The one I have is rated at 190 watts, max.""this is a 110 volt compressor. 1900 watts."Which one is to be installed on boat?

 

[Ed]

Inverter for 110V frig

Thought I would check the amp draw on my inverter today. The unit is a Vector 700 watt with 1400 watt surge capacity. The results: With no load draw was 0.94 amps With a 50 watt lamp plugged in, draw was 4.9 amps, which is exactly what the calculated value is when the no load .94 is added to the 4 amp load (50/120 X 10= 4amps DC) With a dorm size refrigerator compressor plugged in draw was 8.5 amps. The compressor was open to the atmosphere so the working load would be slightly higher. If you assume 9 amps on 33% of the time, you have 72 amp hours in 24hrs, a heavy load. Ok if you are at a dock at night, but...Looks to me that the inverter is efficient other than for the additional 0.94 amp draw.It is true that the DC amps are very high when running an item like a microwave, but the length of time it is on is usually short so the amp hours is not excessive. My inverter struggles, but it runs my small microwave.