Freezer Performance in Compressor Fridge

[peteco]

In the post on boosters Isotherm fridge he shows freezer and fridge temperatures. He has his freezer set at 20-deg and is working to get consistent fridge temps.

https://www.classbforum.com/forums/f...tml#post141725

In previous discussions I recall that typically the compressor unit freezer temps are in the mid 10s-20s degree range. Of course there are differences among the compressor units, with Novakool units seeming to only give the higher range of freezer temps as I recall.

I ask this as, for my needs, I would like low freezer temps. That was one reason I stayed with the same absorption fridge (Dometic RM2554) when I needed a replacement. When this one fails I will likely replace with a compressor unit, even though I now know how to get the absorption unit to work OK, though it is a hassle at times. (Sometimes it is better to stay with the devil you know).

My question is, are the compressor units able to achieve freezer temps in the low single digit range, say 0-7 degrees or so? What does it take to achieve this freezer performance?

[booster]

Originally Posted by peteco

In the post on boosters Isotherm fridge he shows freezer and fridge temperatures. He has his freezer set at 20-deg and is working to get consistent fridge temps.

https://www.classbforum.com/forums/f...tml#post141725

In previous discussions I recall that typically the compressor unit freezer temps are in the mid 10s-20s degree range. Of course there are differences among the compressor units, with Novakool units seeming to only give the higher range of freezer temps as I recall.

I ask this as, for my needs, I would like low freezer temps. That was one reason I stayed with the same absorption fridge (Dometic RM2554) when I needed a replacement. When this one fails I will likely replace with a compressor unit, even though I now know how to get the absorption unit to work OK, though it is a hassle at times. (Sometimes it is better to stay with the devil you know).

My question is, are the compressor units able to achieve freezer temps in the low single digit range, say 0-7 degrees or so? What does it take to achieve this freezer performance?

In my original tests of our new Isotherm the frig ran a 6*F for the most part if we wanted to hold the frig temp at 37-39*F approximately.


In many frigs you should be able to make the freezer colder compared to frig temp by restricting the heat transfer from the evaporator to the frig if that is possible in the design. It is easy to do in the Isotherm like ours with the evaporator being the freezer box.


Colder evaporator temps can heavily increase energy use, however, as was shown in the testing I did in those early tests of our Isotherm. Efficiency drops quite quickly with freezer temps so if you get hot weather where the duty cycle has to go way up to maintain frig temp, the freezer temp gets lower and will eventually hit a balance point of heat removed to heat gain and loss in the frig. If you don't get to that point because you ran out of duty cycle, you would have to increase compressor speed if you can, but you also can run out of speed eventually.


I think with our Isotherm we would have to have variable covering of the evaporator and also use the compressor speed control if we wanted to hold 6* without going a lot colder in hot weather or when frosting up.


We have no issue with 20-25* in the freezer as it is not long term storage of food. The 20* is this last testing is as warm as we can get freezer with the shelves totally blocked when using the fans and tubes. It would run colder than that to maintain frig temps with shelves blocked without the fans and have much more variation. With the shelves unblocked completely and in cool weather the freezer would probably get to 27* or so.


This all points out the huge benefit that Philip53 and soon Avanti will have with the dual compressor, dual evaporator, separate box frigs that have individual control.

[@Michael]

I suspect this is going to be highly dependent on the make/model of fridge.

For example, in my case (Nova Kool RM6000) the fridge and freezer compartments are completely separate, and each have separate evaporators fed by the same compressor. The thermostat is in the fridge compartment. The freezer compartment stays colder than the fridge compartment by an amount proportional to the duty cycle of the fridge, which is dependent on the outside air temp and contents of the fridge. Normally that's about 15F lower than fridge temperature, or about +20F.

But if I put warm food in the fridge or open to door a lot, the compressor will run until the fridge is cool, and because the compressor is running more, the freezer will get colder.

Other designs will behave differently.

Perhaps borrow from the marine world and get two appliances - a small freezer drawer and separate, larger fridge with no freezer compartment? Or perhaps as @booster suggests, a model with two compressors and two thermostats?

[Luv2Go]

My fridge is a Vitrifrigo C130RXD4-F. The sensor for the mechanical thermostat is located in the freezer as with many of this type. The temp as measured with a wireless thermometer sitting on the bottom of the freezer averages 7 degrees F with the control knob about 9 o'clock which is mid-range.

Duty cycle to achieve this is about 38% at 90F indoor temp and about 22% at 75F inside.

Note the remote compressor and condenser are located in the cabin with no venting to the outside hence the importance of indoor temp. The controls on this are stock, however I did add 1-2" of the pink foam insulation around the back, sides and top of the fridge. I'm sure I could get consistently lower freezer temps, however we like our ice cream at this temperature!

[peteco]

Originally Posted by Luv2Go

My fridge is a Vitrifrigo C130RXD4-F. The sensor for the mechanical thermostat is located in the freezer as with many of this type. The temp as measured with a wireless thermometer sitting on the bottom of the freezer averages 7 degrees F with the control knob about 9 o'clock which is mid-range.

Duty cycle to achieve this is about 38% at 90F indoor temp and about 22% at 75F inside.

Note the remote compressor and condenser are located in the cabin with no venting to the outside hence the importance of indoor temp. The controls on this are stock, however I did add 1-2" of the pink foam insulation around the back, sides and top of the fridge. I'm sure I could get consistently lower freezer temps, however we like our ice cream at this temperature!

That Vitrifrigo was my other choice. I would have gone that route if I had been up to doing the electric system mods I thought I should do. Sometimes I wish I had done the Vitrifrigo.

Could you describe your install please, with pictures if possible? Thanks.

[avanti]

Originally Posted by @Michael

Perhaps borrow from the marine world and get two appliances - a small freezer drawer and separate, larger fridge with no freezer compartment? Or perhaps as @booster suggests, a model with two compressors and two thermostats?

The Isotherm Cruise 195 is literally a stand-alone fridge and a stand-alone freezer bolted together. They are independent in every way. There are even two 120VAC plugs! (which I will not be using)

I am pretty sure that both units are available separately.

[Luv2Go]

Originally Posted by peteco

That Vitrifrigo was my other choice. I would have gone that route if I had been up to doing the electric system mods I thought I should do. Sometimes I wish I had done the Vitrifrigo.

Could you describe your install please, with pictures if possible? Thanks.

I'll try but unfortunately I have no pics.

Note this install is very specific for a 2006 Roadtrek 210 Versatile with the aisle bath. The fridge, which is on the driver's side, rests almost on the floor under the counter and the cabinet that holds the microwave, which is under the sink and stove, is just to the left of it. Under the microwave are the water heater and part of driver side rear wheel well.

The old fridge cavity was sealed off with a combo of insulating foam and Gorilla duct tape. A mostly square hole was cut in the left wall at the microwave floor level so the compressor/condenser module would fit through. The module fit in between back of the microwave and the driver side wall and is fastened to the shelf the microwave is mounted on.

The fridge unit was wrapped with 1-2" of the pink foam insulation (IIRC 2" on the top and back, 1" on the sides. The old opening was furred in with some prefinished cabinet material I had laying around that pretty much matched the Roadtrek cabinet color.

When sliding the fridge in, the module had to be fed through the hole in the left wall and carefully moved with the fridge to avoid kinking or otherwise damaging the refrigeration piping which is made of soft copper tubing. I've had it in and out three times for various reasons (not related to the fridge performance, etc) and have not broken any tubing yet, knock on wood.

The nice thing about this fridge is that it is shorter than the absorption fridge it replaced so I was able to build a ~4" drawer above it to fill the space between the fridge box and the counter. The not-so-nice thing is the freezer is not as practical as the old one, it fits less food than the old one even though it has a little more cu ft. Fortunately it does fit a 1.5qt ice cream container, just barely (who knew there was a benefit to the reduction in size of the half gallon container that used to be the standard)!

[Sailorguy7]

We are replacing our current Dometic RM8505 with a Isotherm Compressor Model- Cruise 115 Freeline, because it is not proving enough cooling when the Temps reach the mid 80's. We tried all the quick fixes and none worked! I just found out that the Dometic is a "Climate Class SN", which is intended for Temps 59-90 degrees, the Isotherm has a "Climate Class of ST" which is for temps 61-100+ degrees. No wonder we were having issues. BTW, you can find your Refridge's "Climate Class on the Data Sheet Decal inside your unit.