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.