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Old 26-11-2015, 16:57   #31
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Re: 120V Reefer off Inverter?

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Originally Posted by newhaul View Post
Stu actually most of then domestic fridges stateside have the coils either inside the back skin or more often a smaller unit in the bottom with the compressor
^^^This.

On my magic chef fridge the condenser tubing is bonded to the left and right sides of the fridge panels, located outside the insulation. Very common least wise in the US. Larger residential fridges generally have the coil at the bottom, underside of the fridge.
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Old 26-11-2015, 19:15   #32
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Re: 120V Reefer off Inverter?

Sc we are in agreement you said basicly the same thing I said that's Scarry now if stuM agrees with us its time to buy a lottery ticket.
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Old 26-11-2015, 22:26   #33
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Re: 120V Reefer off Inverter?

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Originally Posted by sailorchic34 View Post

Edit: OK BTU is energy and btuh (common U.S. engineering term for btu-hr) is power. Yet I was calculating BTU's and not btuh. That is Btu's required per door opening. Then converted to watts for 10 door openings. So is it a watt or a watt-hr in that case? If it is watt-hr, where did the time unit come into play in the calculations?



It's Wh (Watt hours)

BTU, Joules, Watt hrs = Energy

BTUH (actually not "btu-hr" is BTU per hour or BTU/hr - imperial units are not consistent in using the "/"), Watts, Horse Power = Power

So use Watt hours where you would use BTU.
Use Watts where you would use BTUH or HP.


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Old 27-11-2015, 10:35   #34
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Re: 120V Reefer off Inverter?

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Originally Posted by StuM View Post
It's Wh (Watt hours)

BTU, Joules, Watt hrs = Energy

BTUH (actually not "btu-hr" is BTU per hour or BTU/hr - imperial units are not consistent in using the "/"), Watts, Horse Power = Power

So use Watt hours where you would use BTU.
Use Watts where you would use BTUH or HP.


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Thank you StuM that was clear. I use BTUH and MBH (1 MBH=1000 BTUH) all the time with boilers and such, but most folks don't know what a BTU is, let alone BTUH. Of course to me, everything is energy, but I'm a little weird that way.
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Old 27-11-2015, 15:26   #35
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pirate Re: 120V Reefer off Inverter?

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Originally Posted by sailorchic34 View Post
Thank you StuM that was clear. I use BTUH and MBH (1 MBH=1000 BTUH) all the time with boilers and such, but most folks don't know what a BTU is, let alone BTUH. Of course to me, everything is energy, but I'm a little weird that way.

The hell it was!

In any case this discussion has given my 12v AB another stay of execution. Thanks.
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Old 28-11-2015, 13:00   #36
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Re: 120V Reefer off Inverter?

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That was part of it. My previous boat had a 12V fridge in it and it used about 3.5 amps/hr an hour for a 6-7 CF box. It was not insulated very well and it was Florida, so was a pig.

Myself I know that energy use would generally be close to the same for a given box size. Yes having a well insulated box will greatly reduce heat transfer.

It was partly to save money and partly to see if it could be done. Everyone said you cound not do it. But being an engineer and blonde I figured for $200, lets find out.

Really the daily energy load of the 120V fridge is not much higher then a average 12V fridge. Yes having a well insulated box will greatly reduce heat transfer.

But I do not think I use appreciatively more energy at 120V then a 12V compressor would on average. Yes some folks only use 30 amps/hr a day, but that's more a factor of box insulation.

I should note that I have a total of 320 watts in solar panels and a PWM solar controller, Just a basic system. I use roughly 50 amp/hr a day for the fridge more at 100 degrees less in 50 degree weather. The rest for my cad laptop, wifi thingy and lights.

Put it another way, a water heater can have a 1000 watt or 4000 watt heating element. One uses more power then the other. But the daily energy load or cost will be very close to the same for the same size tank. One just runs a bit longer. It's similar with fridge compressors.

Ok that's a simplification as some compressor systems dol have better efficiencies, but it's ballpark anyway.
You are right on the money!

Basically those dorm fridges / home fridges / and boat fridges are all using the same Danfoss (or equivalent) compressor. The efficiency of these compressors is basically related to the evaporator temperature which means that the colder the evaporator the less efficient they get. But for the same temperature evaporator the efficiency is fixed.

The measure on the compressor websites is the COP (coefficient of performance) and it measures the number of watts the compressor consumes to remove a specific number of watts from the fridge / freezer. Looking this up for a given compressor and desired temperature gives the expected efficiency of removing heat from the fridge.

Just for fun, this number can be above 1 and often ranges from 1.1 to about 1.5. The reason for this is that for a small compressor it might consume 65 watts of power to remove 75 watts of heat (COP of 1.15) at a given temperature evaporator. This is NOT more than 100 percent efficient, the condenser coil (and the compressor body) must reject 65+75=140 watts of heat to make the cycle go round and round. This is exactly why people use heat pumps in the winter rather than resistor heating as it is easier to move heat from one place to another than it is to make heat. (but not always!)

Generally, the home fridge / dorm fridge does not have as much insulation so it will be let heat leak in a bit faster but at the dock this is not significant and one can add additional insulation. The compressor is attached to the fridge so you do have to make sure the coils on the back and the compressor can reject the heat so don't restrict the airflow.

Also of interest is that these small compressors are actually AC devices, the control module in the small DC compressor systems is actually a small square wave inverter (they are more efficient than a nice sinewave inverter).

A really useful device is a "kill-a-watt" meter which can be picked up at many hardware stores. It measures power use over time and so you can plug the fridge into the meter, plug the meter into the wall and see how much power it uses per day or per week. These are less than $20 and really useful for testing computers, monitors, and other devices to decide which way to go.

David
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Old 28-11-2015, 13:10   #37
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Re: 120V Reefer off Inverter?

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Originally Posted by botanybay View Post
You are right on the money!

Basically those dorm fridges / home fridges / and boat fridges are all using the same Danfoss (or equivalent) compressor. The efficiency of these compressors is basically related to the evaporator temperature which means that the colder the evaporator the less efficient they get. But for the same temperature evaporator the efficiency is fixed.

The measure on the compressor websites is the COP (coefficient of performance) and it measures the number of watts the compressor consumes to remove a specific number of watts from the fridge / freezer. Looking this up for a given compressor and desired temperature gives the expected efficiency of removing heat from the fridge.

Just for fun, this number can be above 1 and often ranges from 1.1 to about 1.5. The reason for this is that for a small compressor it might consume 65 watts of power to remove 75 watts of heat (COP of 1.15) at a given temperature evaporator. This is NOT more than 100 percent efficient, the condenser coil (and the compressor body) must reject 65+75=140 watts of heat to make the cycle go round and round. This is exactly why people use heat pumps in the winter rather than resistor heating as it is easier to move heat from one place to another than it is to make heat. (but not always!)

Generally, the home fridge / dorm fridge does not have as much insulation so it will be let heat leak in a bit faster but at the dock this is not significant and one can add additional insulation. The compressor is attached to the fridge so you do have to make sure the coils on the back and the compressor can reject the heat so don't restrict the airflow.

Also of interest is that these small compressors are actually AC devices, the control module in the small DC compressor systems is actually a small square wave inverter (they are more efficient than a nice sinewave inverter).

A really useful device is a "kill-a-watt" meter which can be picked up at many hardware stores. It measures power use over time and so you can plug the fridge into the meter, plug the meter into the wall and see how much power it uses per day or per week. These are less than $20 and really useful for testing computers, monitors, and other devices to decide which way to go.

David
No such thing as "Watts of heat". If you replace Watts with Watt hrs in your explanation above, you are correct. Hint - you could also have said BTU but not BTUH (which is equivalent to Watts)

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Old 28-11-2015, 17:50   #38
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Re: 120V Reefer off Inverter?

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Originally Posted by botanybay View Post
Also of interest is that these small compressors are actually AC devices, the control module in the small DC compressor systems is actually a small square wave inverter (they are more efficient than a nice sinewave inverter).

A really useful device is a "kill-a-watt" meter which can be picked up at many hardware stores. It measures power use over time and so you can plug the fridge into the meter, plug the meter into the wall and see how much power it uses per day or per week. These are less than $20 and really useful for testing computers, monitors, and other devices to decide which way to go.

David
(they are more efficient than a nice sinewave inverter).

A square wave inverter is not as efficient as a sine wave inverter.

Rather than going into the math as to why this is so;
Consider why a sine wave inverter will cost 5 to 10 times more $$ than an equivalent power level square wave inverter?

Also the square wave inverter will have a much higher no-load current than a corresponding power level sine wave inverter. This means reduced efficiency in the DC system.

The square wave inverter produces much more AC harmonic voltages than a sine wave inverter, which causes additional losses in the compressor motor in operation, correspondingly reducing the efficiency of the reefer process.

Check the no load DC current of various square wave, modified square wave and sine wave inverters.
Check the quoted Total Harmonic Distortion percent figure quoted in the specifications for various inverters. (THD)

A $20 meter is not sufficiently accurately able to measure, when there are harmonics present in the system.
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Old 28-11-2015, 18:14   #39
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Re: 120V Reefer off Inverter?

What would concern me the most is the possibility of fire or shock from using a 110-volt fridge not designed for a boat.

Some have solid wires, which can fracture, and others have connections made with twist caps, which can fall off. The metals are vulnerable to corrosion, which can cause fires.

Anyone who owns an old boat knows how devastating a moist, salty marine environment can be to untinned wire. Well, the entire fridge is wired like that.

Lastly, any problems with ground wiring means you're subject to electrocution. Using a cheap, non-marine inverter with a cheap non-marine fridge magnifies the problem.

Not worth saving a couple of hundred bucks, in my opinion. But you pays your money and takes your chance ...
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Old 29-11-2015, 09:44   #40
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Re: 120V Reefer off Inverter?

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(they are more efficient than a nice sinewave inverter).

A square wave inverter is not as efficient as a sine wave inverter.

Rather than going into the math as to why this is so;
Consider why a sine wave inverter will cost 5 to 10 times more $$ than an equivalent power level square wave inverter?

Also the square wave inverter will have a much higher no-load current than a corresponding power level sine wave inverter. This means reduced efficiency in the DC system.

The square wave inverter produces much more AC harmonic voltages than a sine wave inverter, which causes additional losses in the compressor motor in operation, correspondingly reducing the efficiency of the reefer process.

Check the no load DC current of various square wave, modified square wave and sine wave inverters.
Check the quoted Total Harmonic Distortion percent figure quoted in the specifications for various inverters. (THD)

A $20 meter is not sufficiently accurately able to measure, when there are harmonics present in the system.
If the inverter is turned off when the compressor is not running I am not sure that your statement is correct. The little module on an Danfoss is basically a small square wave inverter. I don't think they have gone to the trouble to make a sine wave version.

As for the accuracy... Well, it measures watts, volt amps, various other parameters.

However, I think I was not clear, I am NOT using a cheap meter to determine if the square wave or sine wave inverter takes more power or if a device takes more power when running on a sine wave or square wave (or more likely modified square wave - also known as modified sine wave) inverter. I use it when in the store or on shore power for a relative comparison.

I suspect that it's absolute accuracy is probably not great, however, it's relative repeat ability from one measurement to the next is pretty good and it provides a quick way to determine how much power a device like a fridge is going to use in a boaters average use.

I often leave the device behind on a friend's boat running the equipment they would like to use over a weekend of sailing during a weekend at the dock. At the end of the weekend we look at the kWh consumed and see if they are completely out of bed with their expectations or if it might work.

I am sorry that I was not absolutely clear in my remarks. I will endeavor to be far clearer in the future.
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Old 29-11-2015, 09:59   #41
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Re: 120V Reefer off Inverter?

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What would concern me the most is the possibility of fire or shock from using a 110-volt fridge not designed for a boat.

Some have solid wires, which can fracture, and others have connections made with twist caps, which can fall off. The metals are vulnerable to corrosion, which can cause fires.

Lastly, any problems with ground wiring means you're subject to electrocution. Using a cheap, non-marine inverter with a cheap non-marine fridge magnifies the problem.

Not worth saving a couple of hundred bucks, in my opinion. But you pays your money and takes your chance ...
Actually an inverter acts as an isolated power supply. That is there is only current flow between the two poles. It's not line to neutral but line to line. Oddly very safe as there isn't a hot to ground reference.

While the cord is non-tinned, that really is not a big issue inside the cabin.

My fridge has been installed for 8 years and has no rust on the frame or compressor. I had a old RV stove installed for 42 years and it also was mostly rust free, except where salt water boiled over on the stove top from heating dish water.
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Old 29-11-2015, 10:06   #42
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Re: 120V Reefer off Inverter?

As to efficiencies of the modified sine wave and full sine wave inverters. Modified sine wave inverters are 90% efficient. Pure sine waves are roughly 82% to 85% efficient, mainly due to the far more complex circuitry. A modified sine wave inverter is a pretty simple device.

So A cheaper modified sine wave inverter is more efficient then a pure sine wave inverter. Most equipment now a days works fine with a modified sine wave. Least wise my fridge works fine with one.
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Old 29-11-2015, 12:51   #43
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Re: 120V Reefer off Inverter?

Here is an idea is there a reasonable way to control the inverter with the thermostat in the refer unit ( ie: the stat calls for cooling and it turns on the inverter to power the compressor and when stat is satisfied it turns off the compressor and the inverter?)
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Old 29-11-2015, 13:36   #44
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Re: 120V Reefer off Inverter?

A small relay (or a FET) activated on the downstream side of a mechanical thermostat which would not draw enough extra current from the refrigerator electronics to damage them which would activate a larger relay capable of switching inverter DC input current.
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Old 29-11-2015, 18:19   #45
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Re: 120V Reefer off Inverter?

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As to efficiencies of the modified sine wave and full sine wave inverters. Modified sine wave inverters are 90% efficient. Pure sine waves are roughly 82% to 85% efficient, mainly due to the far more complex circuitry. A modified sine wave inverter is a pretty simple device.

So A cheaper modified sine wave inverter is more efficient then a pure sine wave inverter. Most equipment now a days works fine with a modified sine wave. Least wise my fridge works fine with one.
You state:
Modified sine wave inverters are 90% efficient. Pure sine waves are roughly 82% to 85% efficient,

This is counter intuitive and is at variance with the physics of the conversion.

It would be interesting if you could provide links to this assertion.

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