Determine approximate R-value of fridge box?

[Davidhoy]

As part of a planned refrigeration upgrade, I'm trying to determine, as best possible, what the R-values of my existing fridge and freezer boxes are. Does anyone have any suggestions on how to measure this, without disassembling the boxes?

Thanks,
David.

[a64pilot]

There is a way I believe maybe putting known weight of ice in and measuring melt rate?
Post is more of a bump for help more than anything

[estarzinger]

Get a look at Nigel Caulder's book - he goes into this in detail.

[Typhoon]

Hi David .

Other then destructive means, the only other real way to tell how well your insulation is stopping the heat influx into your box is to measure the delta T or the difference in temperature of the outside surfaces of your box with the ambient air temperature .

This of coarse is measured when you box is at its working temperature, be it a freezer or a fridge. You should have had it held at that temperature long enough for any heat to have left the insulation , usually around 48 hrs .

A delta T of 5 degrees or more and you will get sweating of the surface (indicates poor insulation) . The lower the delta T the better.

Regards John.

[UNCIVILIZED]

Maybe measure (or you know) the thickness of the insulation. Then guess at it's density, & look up the R-value for similar foams. That should give you a ballpark idea.

Otherwise, look up the math to calculate it, based on how long it takes a block of ice of X dimensions to melt, given an average temp. differential between the icebox & the ambient air of Y degrees. Which, that'll be a good bit more precise. Though far from perfect.

Especially as neither of these allows for any temperature "leaks" in the box in various locations. Which such leaks can be the biggest issues with keeping a reefer chilled to the proper temp's as anything.

Though if you're replanning to rebuild the unit, why is the current overall R-value important? Were it me, I'd just look up the ballpark range I'd like to achieve, & compare it to the tradeoffs of more insulation vs. icebox size, & then get to building.

Keep in mind too that the amount of insulation in the box needn't be fixed. As you can build removable, moisture sealed, insulation panels which you put in or remove, based on how much food's in the fridge/freezer, & what kinds of temp's you're trying to maintain.

That plus you can alter the cycle times of when & for how long the compressor has to run by adding or removing bottles filled with water, or a eutetic (sp?) solution. And this technique has been covered in quite a number of threads.

So, basically you look at your goal/ideal fridge size, & cycle times, & work backwards from there. Well, assuming you don't mind doing the math.

[Typhoon]

In my experieance , an "Ice Melt" test is inconclusive to how your insulation will work at resisting heat influx into your box.

Lets face it ,

If you are planning to sail in the tropics as most of us dream of doing, and you now have the boat that you want to do it on and that boat is 20 or 30 years old, like most ours here are . Then your insulation is more then likely not serving you at all . You have two options, there is no way around this . You can re-insulate your box to R30 for the fridge and R40 in the freezer and enjoy ice cream on solar power .

Or you can leave the box as it is and increase the cooling capacity of your refrigeration system . By using multi units or large compressors and then figure out a way of powering the appetites of those large cooling units .

Thermal dynamics is not a suggestion its a law .

I choose the first option , its a horrible job but once it's done , it's done. Ice cream in the tropics and never a thought to your refrigeration systems . Running silently on a couple of solar panels.

Regards John

[a64pilot]

I chose number 2, and that BD80 is a hungry mongrel. I just had too much going on to tackle a box re-insulate. I intend to insulate though, it is the right way to do things.

[Davidhoy]

My refrigeration upgrade is aimed at replacing a big 110VAC compressor with dual 12VDC compressors, one for the freezer and one for the fridge, all running off solar. But I need to know if it's even feasible, given the size of my boxes and their condition. Finding an approximate R value will give me some clues as to whether I can use them as is, add some insulation on the interior, or go the whole hog and re-insulate. Obviously the last option is the least desirable in terms of cost and time. Clearly, if the boxes are not appropriately insulated, any compressor upgrade is unlikely to yield a satisfactory outcome, both in terms of cold beer and ice cream, and in terms of having the power to drive it all.

While I'd prefer a non-destructive way of evaluating the current boxes, coring them is not out of the question. I'd like to exhaust other options first. I was wondering, is it possible to use ultrasound to probe the insulation? Not even sure if that is at all feasible...

John, when you re-insulate to R30 or R40, do you base this simply off the thickness and material of the insulation installed, or is there some other way of figuring this out. Adding insulation on the inside of my boxes is actually not unreasonable, considering their size, but how much to add is the big question. You also mentioned "sweating" as a sign that your insulation is not up to snuff. I do see a little of this, but only around the lid seal on the freezer box, and that is likely due to the seal itself, and easy to fix issue. But, in all fairness, I only have access to two sides of the boxes, the top and one side - whether there is any "sweating" in the inaccessible areas is a mystery (or perhaps a misery?).

Appreciate all the comments! This is a really great forum :-)

[Davidhoy]

Another thought. Could I estimate the effectiveness of the insulation by measuring the power consumption of the compressor over an extended period? How much current it draws to hold the fridge and freezer at a steady temperature should have a fairly direct relationship to the insulating properties of the box, right?

-David

[Typhoon]

Hi David , yes if you calculate how many watts it is taking to keep your units at its desired temperature you can estimate the systems power requirement .

As for insulation . You can do it all through the opening , I have done many boxes this way . You don't even have to remove the counter top .
The last one I did had a stainless steel liner like the ones on my Bristol . Got in there with a cutoff saw and took it out piece by piece and the built a new liner out of Fiberglass . Client was very pleased . As for the amount or r value . I go with the first inch with XPS or blue board and then follow up with polyiso. You calculate the r value by the thickness you build up too.
It's really not a terrible job you just have to dive in and get started.
Regards John

[Davidhoy]

Originally Posted by ColdEh Marine

Hi David , yes if you calculate how many watts it is taking to keep your units at its desired temperature you can estimate the systems power requirement .

As for insulation . You can do it all through the opening , I have done many boxes this way . You don't even have to remove the counter top .
The last one I did had a stainless steel liner like the ones on my Bristol . Got in there with a cutoff saw and took it out piece by piece and the built a new liner out of Fiberglass . Client was very pleased . As for the amount or r value . I go with the first inch with XPS or blue board and then follow up with polyiso. You calculate the r value by the thickness you build up too.
It's really not a terrible job you just have to dive in and get started.
Regards John

Hi John,

I'm going to rig up one of those "Kill A Watt" monitors in the power supply lines to the compressor (and cooling water pump) so that I can measure the watts consumed by the system. I'll also get a temperature sensor so that I can verify that the temp stays in a reasonable range. If I get the fridge/freezer down to it's setpoint first, let it sit there for a couple of days, and then start recording temperature and watts over the course of at least a week, I should have a good understanding of the power requirements of the current system. Whatever watts go in must also be coming out, right? Even if I don't know the R-value of the box insulation, I will know how much power will be needed to keep it cold. From that I can determine whether the combination of new compressor(s) and the available solar power will meet that requirement. Clearly, if the total watts going in are beyond what my new panel can provide, then I need to figure out ways to reduce the demand, most likely in the form of new/additional insulation.

Right now I'm looking at installing about 720 watts of solar (2x SunPower 360 watts panels), charging a 420 Ahr bank of FLA batteries. My goal is to have enough solar that, assuming reasonable sun, the batteries will recharge by noon, and then the "excess" solar in the afternoon will be used to make water and chill down the fridge/freezer holding plates. Ideally the compressor(s) would not need to run again until the following afternoon. Basically I'd be storing the excess solar power in the holding plates instead of larger batteries. Does that sound reasonable?

-David

[a64pilot]

Once you get your measurement, at least double the cooling capacity
Reason is often we are adding food and beverages that are at 90F, and cooling required to drop lbs of liquid 50 or more degrees F is considerable, and for the freezer, it takes a LOT of power to both remove the heat and the phase change from a liquid to a solid of course, and then there is the opening of the lids too.

My reasoning is that if your cooling capacity is inadequate or just barely so, your fridge duty cycle is 100% and things spoil and your not happy.
However say for example if you have twice the cooling than necessary, your duty cycle is 50% and your happy as the temp is almost always controlled, there will be times of course like if you throw a full case of hot beer in that temps will rise for awhile, that is inevitable.
And you may one day be in an area where it's hotter than planned, and of course as systems age they lose capacity a little, maybe as simple as dust accumulation on the evaporator.

Oh, and I don't care if you have 10KW of Solar, your batteries will not be fully charged by noon, but if you have sufficient Solar, there will be an excess of power available by noon due to battery acceptance.
If your Solar controller says your batteries are fully charged by noon, your murdering your batteries, need to keep them at absorption voltage longer.
I have 1 KW and a 660 AH bank and find that at anchor that if I keep the charger at absorption voltage, most days I do not get to 100% charged and I use approx 25% of my banks capacity overnight.

However your plan on storing excess Solar power in the cold plates is good and valid, and I believe is the strength of cold plates, to store excess power, Solar or maybe when you motor

[Typhoon]

Sounds like a good plan , just note that the cop (coefficient of performance ) of a Danfoss compressor is going to be much better then the the 110 compressor you have now . Also if you have access solar to run the fridges wether you store it in a holding plate or the batteries is not much different . You want your fridge to have a stable temperature range and not fluctuate 10 degrees or more.

Keep us posted
Regards John

[a64pilot]

Only real difference if you store in a battery bank, is you need a bigger bank of course, however disadvantage of a cold plate in my opinion for it to work well it has to be quite large, several gallons, and of course that reduces you fridge / freezer capacity by several gallons
There just ain't no such thing as a free lunch

[Davidhoy]

Originally Posted by a64pilot

Only real difference if you store in a battery bank, is you need a bigger bank of course, however disadvantage of a cold plate in my opinion for it to work well it has to be quite large, several gallons, and of course that reduces you fridge / freezer capacity by several gallons
There just ain't no such thing as a free lunch

My freezer and fridge boxes already have holding plates, in fact the freezer has 3 large ones. I should just start looking at them as an extension of the house battery bank ;-).