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Old 07-27-2019, 03:06 PM   #1
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Default Compressor frigs and specs

We have been getting a lot of discussion of compressor frigs lately as replacement for failing or poor performing gas frigs, which IMO is a really good thing as long as you have the power to run them.


On one of the discussions, the whole thing about how the frigs are speced by the manufacturers came up as part of the "which compressor frig" decision process.


The big problem is that many use as a primary point the running amps of the unit, which can be an indicator but will really tell you nothing about the total energy use of the frig as it does not include anything that ties that amperage to duty cycle under specific test conditions or to how much heat is actually removed by those amps.


The Danfoss compressors used in most of the frigs have a wide range of speeds they can be set at, which change the amount of amp draw they pull. Put on top of that the fact that the amp draw changes a lot based on the evaporator temperature and that change is opposite than many of us would expect, but the efficiency moves opposite of amps. Evaporator temp is generally lower than the freezer temp by quite a bit but is directly related to the freezer temp. The freezer temp is kind of settable by how much cold you let out of it to frig where the temp setting is normally controlled.


Obviously, this make it all pretty complex to try to figure out for most of us.


For illustration, below is the compressor specs chart for a Danfoss BD35F, which may not be the exact model that was in the other discussion but should be typical and very close to it. All we know from the spec given for the one in the discussion is that it uses 2.2 running amps under unknown conditions.





Looking at the chart, you can get 2.1 amps at two different places on the chart. They are different compressor speeds and evaporator temps from each other. They also have different heat removal capacities by 19% and efficiencies by 19.5%. The logical choice would be the higher efficiency one which is slower speed and warmer evap temp, but only if that would give an acceptable freezer temp.


Of interest to me was that the amps go down with evaporator getting colder, but you suffer big time in cooling capacity and efficiency at over 15% difference at the same compressor speed.


As we all have heard, slower compressor speed improves efficiency and lower heat removal capacity. It is also likely that speeding up the compressor will also lower the evaporator temp and hurt efficiency even more, even though the amps drop.


I couldn't find a similar chart for condenser temp changes, but it is widely published that higher condenser temps (mostly from ambient being higher) will decrease capacity and efficiency plus raising the amp draw. This is not something the frig makers have control of so not part of the selection process, I think.


LET ME SAY THIS VERY PLAINLY.....NOBODY EXCEPT US CRAZY SPEC CHECKERS AND TESTERS SHOULD HAVE TO LOOK AT A CHART LIKE THIS TO TRY TO FIGURE OUT HOW MUCH VARIOUS FRIGS USE IN COMPARISON TO EACH OTHER.


I am currently looking for a good example of a frig manufacturers product chart that shows how the information can be put out there in an accurate, standardized to industry standard testing, way so it is easy to use.
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Old 07-27-2019, 03:29 PM   #2
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Found a good product chart. This is from Isotherm website marine catalog





This chart plainly give you everything you need to know, size, freezer temp, and most of all how much power it uses under the listed conditions. All the manufacturers have this information, but many are choosing to not put it all out there and put up only what they think will help them sell. For instance that 2.2 amps from the other discussion may look really good compared to other units running amps, but based on a lot of other things in the units, may have lower efficiency and/or cooling capacity.


As example, our older Isotherm Cruise 85 used about 3.5 amps from the factory as they run fairly high at 41* frig setpoints and the freezer goes up similarly. I slowed down the compressor, which dropped the amps to 2.4amps or so but we didn't see any decrease in power use and maybe even a slight increase. It also took longer to cool initially and recover. We still run the lowest speed most of the time (I made it switch selectable) but let more cold out of the freezer to raise it's temp. This increased the amps as the chart above shows it would, but it did reduce our energy use, even with the frig temp at 37*. We currently have running amps at over 3amps in most conditions, but we have lower power use than when we had 2.5amps.
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Old 07-27-2019, 03:58 PM   #3
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Here are the NovaKool specs from 2013:

novakool specifications 2013_001.pdf
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Old 07-27-2019, 04:03 PM   #4
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Quote:
Originally Posted by avanti View Post
Here are the NovaKool specs from 2013:

Attachment 7882

Thanks Avanti, but that one only shows running amps and then tell you to use your own run time % for power use. It doesn't reference any standardized conditions so doesn't do much good for comparisons because all you have is the running amps and they don't even give you the conditions needed to get those amps.
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Old 07-27-2019, 04:13 PM   #5
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Quote:
Originally Posted by booster View Post
Thanks Avanti, but that one only shows running amps and then tell you to use your own run time % for power use. It doesn't reference any standardized conditions so doesn't do much good for comparisons because all you have is the running amps and they don't even give you the conditions needed to get those amps.
booster, can you at least compute the max amps in an hour from the attachment?

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Old 07-27-2019, 04:29 PM   #6
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Quote:
Originally Posted by Bud View Post
booster, can you at least compute the max amps in an hour from the attachment?

Bud

Only sort of based on 100% duty cycle (but the answer is in amp hours per hour not amps ) , which nobody really will ever have with most compressor frigs, but yes you can. The result of that calculation can be very badly misleading, also, because you don't know how much energy it is taking out compared to other frigs when it is running. If one company sets up the frig to run at 95btu/hr removal to get their amps, and another sets theirs up at 113btu/hr (these are the numbers from the chart example) they could have the same amps, but quite different duty cycles and efficiencies. That makes it very possible, or even likely, that the frig with the lowest max amp hours per day based on 100% run time will have higher actual max amp hours than the higher running amp unit if used under the same conditions.



The problem with that calculation is that you don't know what conditions it was run under for frig, freezer, ambient temp etc so based on the charts you can see that all those can all make a big difference in running amps and efficiency.


The chart that give you the results in BTUremvoed per watt of used energy is the one that really says exactly what is happening under the given test conditions and you don't need duty cycle or anything else to use it, as long as the conditions are standarized.
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Old 08-04-2019, 10:19 PM   #7
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Engel provides performance data on some of its models. The MT45 used to have performance data listed, but since they went to the "Platinum Edition" (digital controls instead of mechanical, unfortunately) they don't have it for that model anymore. They still have data for the MT60:

http://www.engelcoolers.com/pub/medi...0f%20curve.pdf


Hope this helps!
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