compressor fridge efficiency

[GallenH]

There have been alot of discussions recently about compressor fridges and I have a few questions:

1. Owners of NovaKool fridges mention that you can't really have hard frozen ice cream. I'm wondering why. Is this only in extreme ambient temps? Always? Is it in the design of the fridge that it simply won't give a good freeze even in the most favorable ambient temps?

2. In these recent discussions someone mentioned that their fridge could get down to 50-deg below ambient. This may work if the highest temp is 90-deg but what about if it's 100? 110?

As someone contemplating replacing my existing...very old...3-way with a compressor the above observations are somewhat concerning to me. Can current compressor owners give me some kind of feedback regarding the above?

thx.glenn

[Rlum]

Originally Posted by GallenH

There have been alot of discussions recently about compressor fridges and I have a few questions:

1. Owners of NovaKool fridges mention that you can't really have hard frozen ice cream. I'm wondering why. Is this only in extreme ambient temps? Always? Is it in the design of the fridge that it simply won't give a good freeze even in the most favorable ambient temps?

2. In these recent discussions someone mentioned that their fridge could get down to 50-deg below ambient. This may work if the highest temp is 90-deg but what about if it's 100? 110?

As someone contemplating replacing my existing...very old...3-way with a compressor the above observations are somewhat concerning to me. Can current compressor owners give me some kind of feedback regarding the above?

thx.glenn

I do not yet have a compressor refrigerator yet. I had a Dometic 3 way. With multiple internal and vent fans. I have been able to down and stay at 38 F when camping in 90. When it got above 100, frig would struggle and stay about 48. I have removed the 3 way and existing cooling elements from the Dometic box. Added additional insulation where I could. Waiting on a Vitrifrigo cooling unit. Originally ordered the Nova Kool but it was too large for available space. The Vitrifrigo has a smaller foot print with the larger Danfoss (BD50). I believe this might be overkill since the cooling plate I have is also oversized. I suspect, I should be able to get below 38 F even if ambient temps are above 100 plus? In addition, I will not have to worry about the exact level of van.

[rvsprinterguy]

We've supplemented our Norcold 3-way with a chest-type compressor refrigerator/freezer. While I can't speak to the relative thermodynamic efficiencies of absorption vs. compressor refrigeration, I can state that the compressor refrigerator gets cold faster, has superior temperature control, and will make ice and keep ice cream hard. Our next Class B will have a compressor refrigerator and the necessary electrical capacity built in.

[avanti]

From the NovaKool FAQ:

TL;DR

Q: WHY IS ICE CREAM SO HARD TO KEEP FIRM?
A: As far as ice cream is concerned it is essentially a solution of milk (which contains water, lipids, proteins, and lactose) and refined sugar. With so many solid substances dissolved in water, we would expect the freezing point of ice cream to be below that of water. But, it's slightly more complicated than that.

Cow's milk naturally freezes at a temperature of approximately -.5°Centigrade, or 30° Fahrenheit which is not much lower than the freezing point of water. So how is it possible that ice cream can still feel "unfrozen" at temperatures far below -.5°C or 30° F?

The reason is that as the water component of the ice cream solution begin to freeze, it isolates itself from the rest of the solution by forming pure ice crystals (which are readily observable in ice cream). As a consequence, the relative concentration of the solid substances dissolved in the remaining liquid solution increases, simply because there is less liquid water left available for the solutes to dissolve in. The left-over water can then only freeze at a much lower temperature; when it does get cold enough to do so, the concentration of the solutes goes up even higher, again, because there is less liquid water left.

You can imagine that as the ice cream gets colder and colder, the concentration of the solutes continues to increase as water is progressively removed from the liquid solution as it freezes, thereby greatly depressing the freezing point of whatever amount of liquid is left. The ice cream eventually becomes a mixture of frozen crystals and a relatively smaller amount of unfrozen, liquid solution which gives it a soft feel. Below about -25°C, or -14°F ice cream is stable for indefinite periods without danger of ice crystal growth; however, above this temperature, ice crystal growth is possible and the rate of crystal growth is dependent upon the temperature of storage. This limits the shelf life of the ice cream.

Nova kool freezers are design to keep frozen ice cream for a short period of time at temperature 10° to 20°F or -12° to -7°C.

https://www.novakool.com/full-faq

[jrobe]

We have a smaller Norcold compressor refrigerator that can certainly keep ice cream cold and hard although we would almost never do this. At 100 degree temps it would have to be set so that it was running almost continually especially if you opened the door on occasion. That would at least double the battery draw which even on a small refrigerator adds up quickly. I am just happy having cold food, pop and beer and can save the ice cream for the occasional rest stops. Storing ice cream on a hot summer day in Arizona is probably an unrealistic expectation.

[Davydd]

At 100 degree temperatures I would certainly seek out a shore power campground and plug in and not worry about the refrigerator's battery use. There are more important concerns like air conditioning.

[jrobe]

or better yet, think like a Class B'er. Just double the lithium battery bank size and don't worry anymore about the 25 cent ice cream bar melting.

[GallenH]

Originally Posted by avanti

From the NovaKool FAQ:

This makes sense. So my question is, if this is a scientific based explanation, wouldn't it also apply to, say, Isotherm? Anyone know?

thx.glenn

[GallenH]

Originally Posted by Davydd

At 100 degree temperatures I would certainly seek out a shore power campground and plug in and not worry about the refrigerator's battery use. There are more important concerns like air conditioning.

I agree. In part though my question has to do with the fridge's capability to cool regardless of the power source. Are some compressor fridge's incapable of adequate cooling when the ambient temp is high? This is, afterall, one of the faults with absorption cooling: inability to keep up with high ambients. Since you have been a long-time contributor with various past RVs, what have your experiences been?

[booster]

Originally Posted by GallenH

This makes sense. So my question is, if this is a scientific based explanation, wouldn't it also apply to, say, Isotherm? Anyone know?

thx.glenn

You can design, and usually modify, a compressor frig to have a colder or warmer freezer with the same temp frig as before. Just like on a home frig where the have basic cold thermostat control and usually another knob to open and close the path for cold from the freezer to get out to the frig section.


On the mechanical thermostat frigs, the temp sensing bulb is in the freezer box or on the cold plate which cools the freezer, in most cases. That means the knob you turn to control frig temp is really controlling the freezer temp. The freezer runs at whatever you have set it to, and the frig temp is whatever cold is allowed to get there from the freezer. In our new Isotherm that cold comes mostly off the bottom and sides of the freezer box and is blocked by the drip tray. If I remove the tray or it out from the back wall a ways, the frig will get colder while the freezer stays the same temp.


I thing the Novakool set up their frigs to have a warmer freezer at the correct frig temps by controlling the cold transfer to the frig.



Why would they do that? The efficiency of the frig is highly tied to the evaporator running temp, so making it warmer reduces energy use, when at the same frig temp. It is likely that the old school condenser coil setup they use is not quite as efficient so the raise the freezer temp to have similar power use as other brands do. It also lets them run the compressor at a slower speed and that is why you see them list the 2.1 amp or so running amps spec. Slower speeds are more efficient than faster, but have less capacity so slower to react and also may not cool as well in hot temps.

[avanti]

I am excited to see what life will be like with my new dual-compressor/dual-controller Isotherm. As we speak, it is on its way to my upfitter. I wish I had a van to put it in.

[@Michael]

Originally Posted by GallenH

2. In these recent discussions someone mentioned that their fridge could get down to 50-deg below ambient. This may work if the highest temp is 90-deg but what about if it's 100? 110?

I have a Novakool 6 cu. ft. fridge that when well vented, supplemented with fans, I had a 40F fridge compartment on a 100F day with the camper interior at least 100F.

Things that might help:

• Size: Small fridge, large compressor. Mine is the largest fridge that Novakool makes that still has a single compressor. If it were smaller, or if it had two compressors, I'm sure it would do better.
  
• Configuration: Mine has two doors with freezer and fridge in completely separate boxes, separated by an air gap. A single door might be more efficient, as there would be less external surface area. (I.E. I have two evaporators but only one compressor.)
  
• Insulation: On mine there isn't room to add polyiso on the top, bottom or sides. If I could add insulation, I'm sure it would do better.

[GeorgeRa]

Originally Posted by booster

.........................
Why would they do that? The efficiency of the frig is highly tied to the evaporator running temp, so making it warmer reduces energy use, when at the same frig temp. It is likely that the old school condenser coil setup they use is not quite as efficient so the raise the freezer temp to have similar power use as other brands do. It also lets them run the compressor at a slower speed and that is why you see them list the 2.1 amp or so running amps spec. Slower speeds are more efficient than faster, but have less capacity so slower to react and also may not cool as well in hot temps.

Do you think that your ICT test had lower than capillary efficiency because of this fridge vs freezer control? I finely got my parts, should start testing soon.

[booster]

Originally Posted by GeorgeRa

Do you think that your ICT test had lower than capillary efficiency because of this fridge vs freezer control? I finely got my parts, should start testing soon.

I think it probably does overall, but not necessarily at any particular point in time. It certainly does if you control the freezer temp with a control and set it for a warmer freezer. The compressor efficiency tables plainly show that correlation. The ITC also uses variable compressor speeds when in auto mode so has an average speed that is quite high and less efficient. That makes the freezer temp vary along with the compressor speed so variable efficiency, I think.


The ITC could be much more efficient, IMO. First off they could make the ECO mode actually to minimum compressor speed of 2000 rpm. They say they run slowest speed, but don't it appears, as other literature said it runs at 2500 rpm. The factory, resistor based, speed for the Cruise 85 Elegance that we got recently was at about 2800 so nearly as slow as the ECO mode of the ITC. We had a previous Cruise 85 and we ran 2000 rpm essentially all the time without having the frig temp climb even at 100+*F but the ITC is for all their frig sizes and the bigger ones probably need the extra speed to keep up. Not good for those with small frigs trying to save battery capacity.


You saw our final test numbers on our frig after turning up the freezer temp to about 20*F (from 6*) and turning down the compressor speed to 2000 rpm. IIRC we were in the 30-40% more efficient at those settings than the ITC even on ECO, but I would have to look to make certain.


Controlling off of the frig temp has one big advantage and that is the frig temp stays more constant with varying outdoor and indoor temp swings. If you control in the freezer, that is the place that is constant. The frig will run colder in cold outside temps, if you have outdoor venting, and warmer if it is hot out. That is less pronounced, I would think, if you use indoor cooling air but still there to some degree. I marked a scale on our drip tray so I can move it in and out some to control frig temp, so it will be interesting to see how well that works for fine tuning for outdoor temp with constantly changing the freezer temp.

[Ken Allan]

I will admit that the subject of keeping ice cream frozen caught my attention. I can only offer my experience with our compressor Norcold DE0788B (3.1 cu ft, 2.8a @ 12v) this past summer.

The end of July, we took an 18 day trip from OH to WY mountains. Fridge would be packed full. We knew it would be a warm ride out. And then a heat bubble set in. 103° to 106° for four days. I was worried and added a 1/2" thick foam sheet to door as that is the warmest area of fridge. Purchased a wireless thermometer (2 sensors) so we could monitor temp without opening. Kept a drawer above fridge open as that allowed more venting.

Results... The fridge did better than I expected. With some adjustments, the freezer box kept well frozen. (Did not have ice cream though). The fridge held 40-44° mid-day with the door area being a bit warmer. The fridge was nearly full so probably did not allow for good circulation?

On the ride home it was 85-93°. With the freezer mostly empty, I bought some ice cream bars. While the meat items stayed frozen the ice cream bars were a bit soft... we still ate them, Ha! I think the chemistry of ice cream mentioned above comes into play here. So the bottom line is that for really warm weather...expect soft serve.

fwiw... Fridge only runs off battery but solar panels more than carry it all day. The lithium batteries easily run it all night. I did not know then but latter on discovered it had never been plugged into the AC circuit. For these extremely hot days (and nights) we camped with shore power running the Air Con. --KenA

[booster]

Originally Posted by Ken Allan

I will admit that the subject of keeping ice cream frozen caught my attention. I can only offer my experience with our compressor Norcold DE0788B (3.1 cu ft, 2.8a @ 12v) this past summer.

The end of July, we took an 18 day trip from OH to WY mountains. Fridge would be packed full. We knew it would be a warm ride out. And then a heat bubble set in. 103° to 106° for four days. I was worried and added a 1/2" thick foam sheet to door as that is the warmest area of fridge. Purchased a wireless thermometer (2 sensors) so we could monitor temp without opening. Kept a drawer above fridge open as that allowed more venting.

Results... The fridge did better than I expected. With some adjustments, the freezer box kept well frozen. (Did not have ice cream though). The fridge held 40-44° mid-day with the door area being a bit warmer. The fridge was nearly full so probably did not allow for good circulation?

On the ride home it was 85-93°. With the freezer mostly empty, I bought some ice cream bars. While the meat items stayed frozen the ice cream bars were a bit soft... we still ate them, Ha! I think the chemistry of ice cream mentioned above comes into play here. So the bottom line is that for really warm weather...expect soft serve.

fwiw... Fridge only runs off battery but solar panels more than carry it all day. The lithium batteries easily run it all night. I did not know then but latter on discovered it had never been plugged into the AC circuit. For these extremely hot days (and nights) we camped with shore power running the Air Con. --KenA

That sounds pretty typical of what we have heard of the Novakools also, but a bit surprised it didn't run below 40* if that is what you wanted. We try to stay under 40*, usually targeting 38* in the frig. Don't know for sure on the new Isotherm yet in 100+ degrees, but the old Isotherm handled 100*+ with no issues running at 38*.

[Ken Allan]

The 40-44 was for 95-106° temps. Normally we would regulate for 37-38 in the lower compartment without much trouble. These simple fridges with an upper freezer box aren't much different than a cooler with an ice block on top. No recirculating air and pretty thin on the insulation. There has to be performance limits. We were pleased it did as well as it did. Expecting better performance in exceedingly hot weather would take a more highly engineered design. And at a higher cost. --KenA

[booster]

Originally Posted by Ken Allan

The 40-44 was for 95-106° temps. Normally we would regulate for 37-38 in the lower compartment without much trouble. These simple fridges with an upper freezer box aren't much different than a cooler with an ice block on top. No recirculating air and pretty thin on the insulation. There has to be performance limits. We were pleased it did as well as it did. Expecting better performance in exceedingly hot weather would take a more highly engineered design. And at a higher cost. --KenA

If you have a removable drip tray or anything else that can be removed or opened up a bit somehow, you might be able to get more cold air into the frig section. When our old Isotherm lost a lot of capacity it would not get below about 45* in about 85* weather so I removed the drip tray and got down to right around 40*. Just enough benefit to not wreck a two week trip as it happened on the second day.

[Ken Allan]

booster...
Remove drip tray... I took note of that when you first mentioned it. It will for sure be removed in the future. It should allow more cold to flow downward AND now I added a battery fan for a bit more air flow. The tray only needs to be there after a shutdown and thaw anyway.

Now I can hardly wait to take another 106° trip to try it out!

Up to about 90° this fridge works pretty well. See'in as how we mainly travel to hike or kayak, we generally avoid the heat anyway.

[themexicandoctor]

I keep my Novakool R3800 at 29% average.

Fridge works perfectly...