Refrigeration - Evaporator vs. Holding Plate

[Richard Kollmann]

Originally Posted by OzePete

Regards eutectic tanks. Don't use fresh water, use a brine or better still a premixed auto coolant with approx 5% glycol for a fridge and approx 28% for freezer tanks. Water freezes solid suddenly at a narrow temp range while the glycol mix will see the mix swell while still fluid and mobile as it freezes. This prevents the 'hydraulic' effect of freezing straight water.

Cheers, OzePete Ozefridge

WARNING
Brine solutions require special additives to prevent internal plate failures.

If Glycol is used it must be food grade, not auto coolant. Check MSDS for toxicity of solution as holdover plates have been known to leak.

[RobMinton]

[QUOTE=Richard Kollmann;2434410]
Danfoss BD80 variable speed compressor equipped with AEO control module in this thread will become energy efficient only after thermostat stops and restarts compressor several times.
Danfoss BD35 and BD50 compressors with standard control modules are much more efficient when manually programmed to operate at slow speeds.
The object of newly designed compressors is to improve energy consumption matching volumetric efficiency (System Coefficient Of Performance) to a complete system balance.

Each of the above Danfoss BD compressors must have a condenser capable disposing maximum process heat into a controlled cooling medium regardless of climate conditions. Danfoss only recognizes fan air condenser cooled systems in there application engineering data.

.....

Richard - Could you please explain the difference between the BD35 and BD50 compressors and how one may be more efficient than the other? The Adler Barbour Cold Machine CU-100 uses the BD50 and the Dometic CU-85 uses the BD35. Either are designed for 9 CuFt box with appropriate evaporator units.
Next question - would oversizing the evaporator plate improve efficiency?

Thanks in advance for continued input on this bit of research.

[savoir]

I don't know your data sources but I do know that a BD35 based cooling system cannot handle a 9 cu ft box without water cooling assistance. The claim by Dometic is not correct.

[john61ct]

Well bigger is better for sure at that size.

But it's all about the box, 8-10" really tight chest style not opened that often, not tropical weather, fridge only rather than spillover freezer, dual Cool-Blue style condensers, water cooling does NOT improve things at that level.

But would be cycling at least 70% of the time, likely more, using dual eutectic plates, say 100% for 14+ divided up to two runs a day, maybe excess from gennie charging run in the morning then (excess) solar in the afternoon. Done that way would be very very efficient for 10 cu ft.

[Richard Kollmann]

Rob, Prior to 1995 all Danfoss BD 2, BD2.5 and BD3 compressors were fixed speed running at 2500 Rpm. This meant one size compressor and the option of two or three evaporators fits all sizes of boxes regardless of climate ambient conditions. The Btu of cooling of each of these earlier compressors was much the same around 300 Btu per hour maintaining a +10 degree F evaporator temperature.

At peak performance the fixed speed BD compressors would draw around 5 amps. For these older compressors to be considered efficient the design Btu capacity of compressor, evaporator and condenser must be equal to size of box that needs refrigeration. It is the thermostat’s job to stop and run compressor based on evaporator temperature and not box temperature to improve daily energy consumption. When a system’s components are out of balance with the box’s Btu demand energy is wasted. Example of an out of balance system is when it is installed in too small a box for climate conditions, this will result in compressor cycling OFF an ON too often. Energy is wasted pumping down refrigerant (repositioning refrigerant) each time compressor starts. There are applications where the older fixed speed compressors in the larger ice box conversions can perform as well as the new variable speed systems.

The new variable speed BD35 and BD50 can be set to run at any speed from 2000 Rpm to 3500 Rpm to permit best system performance. Daily energy consumption depends on a System Coefficient Of Performance (SCOP). You may never know what the true SCOP is of the icebox conversion you purchased so you are depending on the manufacturers engineer’s recommendation, not the sales person. Danfoss the compressor manufacturer does provide the Compressor Coefficient Of Performance (COP) based on compressor speed and evaporator temperature. These compressors at their lowest speed deliver the highest COP. At 2000 Rpm the BD50 compressor with evaporator temp of +10 degrees F is at 1.4 COP. When operated at 3500 Rpm COP is down to 1.3.

If you are interested in comparing the variations between Danfoss BD compressor performance go to Danfoss web site Application engineering specifications.

When purchasing a new variable speed compressor for an ice box conversion the manufacturer will offer different size evaporators. It is common practice for evaporator manufacturer to attach correct Btu capillary tube refrigerant flow control device to evaporators. If the box is large enough to support a larger evaporator one recommended for that size compressor. I do not recommend AB power plates or Fan cooled evaporators unless the box is just a drink cooler.

The average pleasure boat ice box to be converted to refrigerator is small enough and not expected to be operated in continuous tropical conditions will normally use the BD35 compressor running at lowest speed. At high speed the BD35 could use twice the daily power. To take advantage of best COP and lowest amperage run compressor at low speed producing enough hourly Btu to satisfy desired thermostat setting keeping compressor run time to less than 50%.

I always compare compressor Btu capacities from manufactures specs using reported +10 degrees F evaporator temperatures as this is normal evaporator operating temperature.

BD Compressor Btu Capacities:
BD compressors prior to 1995 Fixed speed at 2500 RPM = 300 Btu
BD 35 at 2000 Rpm = 235 btu, at 3500 Rpm = 390 Btu.
BD 50 at 2000 Rpm = 300 btu, at 3500 Rpm = 500 Btu
General rule:
• Refrigerators under 4 cu ft can use a BD35 compressor in tropical conditions.
• Short term storage Freezers under three cu ft with R 20 insulation can use a BD35 compressor in tropical conditions.
• Refrigerator boxes in tropical climates of 4 to 6 cu ft are good candidates for the BD50 compressor.
• Production boat ice box conversion freezers for tropical climates require size BD 50 capacity compressors.
As far as I know when you order a icebox conversion kit with condensing unit and evaporator it will come with resistor for correct compressor speed. Adler Barbour thermostat and correct speed resistor is on wire attached to thermostat and is programmed to match compressor output to evaporator capacity.

Once you understand that all these systems use the basic components then you can avoid the non productive high dollar gadgets and go for price.

[a64pilot]

Just me, but I would want a compressor sized larger than minimum, oversized so to speak, or excess cooling capacity if you will for a couple of reasons, assuming of course the other components are correctly sized, it's a matched system of course.
First the newer ones are variable speed and I think they are more efficient at the lower speeds, secondly often you need excess cooling, lie when you put a couple liters of soft drinks and a case of beer in the fridge or several lbs of meat in the freezer.

[sailorboy1]

What good is having an oversized compressor supplying a correctly sized evaporator? I would say it is none!

[savoir]

You would expect less running time per day.

[Richard Kollmann]

Over sizing and reducing running time of compressor defeats energy saving purpose of variable speed compressors.

[a64pilot]

Originally Posted by Richard Kollmann

Over sizing and reducing running time of compressor defeats energy saving purpose of variable speed compressors.

Does it, or does it mean the compressor runs at is lowest speed more often?
You also have the option of easily changing the speed yourself, do you not?

And no, not just oversized compressor, the whole system of course. Although I feel sure you can have a bigger condenser than is min necessary and that is not detrimental, and many times may be beneficial.

Often people spec items like refrigeration based on it being able to just cover min needs. What I am saying is you want enough cooling to well exceed min needs, that way when you are in the tropics and are opening the door a lot and are often placing warm drinks in the fridge as your drinking a lot, you have excess capacity to cover that as likely your at least doubling your cooling needs.

[jhulmer]

Originally Posted by RobMinton

So I'm looking for the most efficient icebox conversion kit for the most reasonable cost. We'll be leaving after storm season this year for the Caribbean, Panama, and beyond so our primary cruising areas will be tropical/very warm. I've just started this bit of research and open to suggestions on the direction I should take our refrigeration, of which we don't require very much. But our existing icebox is about 9 CuFt. We don't require freezer space but evaporators do allow just enough space to freeze some cubes or whatever. We're a lower energy demand boat and hoping to keep it that way. I think a water cooled system is the way to go in the tropics. The cost of the evaporator systems vs. holding plate systems is very attractive. Can anyone lend advice or input on this? Thanks in advance!

I have followed all the replies with great interest and just a little confusion, but I think we have strayed on the rest of the OPs question.

I'm very curious as to your answers to what would you recommend for an off the shelf system for his 9cuft box?

I have concluded that although counter-intuitive water cool is way to much maintenance and air cooled is the way to go.

And, there is a split on holding plate vs coil plate. Have we concluded that the holding plate system is the most efficient?

I guess looking for the KISS answer.

[a64pilot]

I don't think there is any real efficiency advantage on cold plates, beyond their ability to store energy during times when it's plentiful for use when it's not, like a boat with Solar etc.

I'd also go so far as to say that knowing his R value is as important as knowing the box size.

And then as I said, I wanted to be a little oversized, if your a little oversized life is good, a little undersized and it's not, nobody likes soupy ice cream or half frozen meat, or warm drinks, spoiled milk etc.

My vote would be for a Coolblue, but not because of its efficiency so much as it's product support, reputation and rugged build. I don't want a thin rolled aluminum evaporator not because of any efficiency difference cause I don't think there is, I don't want one, cause they are fragile.

But that is my opinion

[a64pilot]

Actually as the frozen eutectic medium (frozen stuff inside of a cold plate) is an insulator, I'd bet that if everything is identical a cold plate is less efficient .

However that's meaningless, what matters is the efficiency of the system, and the Coolblue does well there I think.
Remember your not buying pieces parts, but a system, so pieces parts efficiency is irrelevant, it's how the whole thing works as one that is relevant

[jhulmer]

Originally Posted by a64pilot

I don't think there is any real efficiency advantage on cold plates, beyond their ability to store energy during times when it's plentiful for use when it's not, like a boat with Solar etc.

I'd also go so far as to say that knowing his R value is as important as knowing the box size.

And then as I said, I wanted to be a little oversized, if your a little oversized life is good, a little undersized and it's not, nobody likes soupy ice cream or half frozen meat, or warm drinks, spoiled milk etc.

My vote would be for a Coolblue, but not because of its efficiency so much as it's product support, reputation and rugged build. I don't want a thin rolled aluminum evaporator not because of any efficiency difference cause I don't think there is, I don't want one, cause they are fragile.

But that is my opinion

I like the plate coil because it fits well in my 7 cuft box. However, I agree they are fragile.
I remember as a numbnut college student, defrosting my dorm room frig with a steak knife and letting all the 'freezezone' out! I guess that's why my parents sent me to college ...hum... anyway, I wish they made the plate coils in stainless...or do they?

[Typhoon]

Yes we do .

https://www.youtube.com/watch?v=qcQskT8xXzY&app=desktop

Regards John