Danfoss PL50F

[SoddemFX]

Sorry I meant you can't compare their cooling capacity and output power as they are two different compressors.

I'd say it's fair to compare two compressors to find which best suits your conditions. If you have the liberty of buying new compressors it's something you should do. Is this what you mean?

In which conditions?? I've though it was the max power their motor can consume on atmospheric conditions (without any systems, so with air density @ 1bar).

It's just the motor, like you could go into an engineering shop and say "i want a 300W motor". It the output power rating of the motor, just the motor.

I've asked a friend of mine who has been studying Air Conditionning and Refrigeration (at school) and he told me the main difference between LBP and MBP/HBP compressor is the motor coupling (in his opinion motors are the same between, for example, SC12g and SC12FT) :
- One has 1 pole per phase (don't know how to explain) --> low torque, high speed
- The other 2 : higher torque, lower speed
Including frictional losses that may have more important effects on the LBP because of its torque

The motors are the same in those two compressors, no they are standard 2 pole motors in both. As far as i know all Danfoss recips are 2 pole, i thought they were 4 pole a few months ago but i was wrong.

I've ever had 2 explaination : the motor coupling, and the piston surface that is different, a wide one on MBP compressors (higher surface --> higher torque needed), and a long one on LBP ones. But how to explain such difference of capacity in low evaporating temperature, I don't realy know so I think it might be the first solution? (I've already heard about "dead point" (end of piston runs) that may be higher with wide piston (Volume = distance*Surface :p) but I don't think it involve such differences).

Think about it - Which involves more work, to compress 30PSI of X volume to 200PSI or to compress 10PSI of X volume to 200PSI?

The higher suction pressure causes more work to be done by the piston per stroke, work done / time taken(fixed by rpm) - more power is needed.

The main (possibly only) difference between a LBP and HBP compressor is the motor. Use a LBP compressor in a HBP application and the tiny motor will burn out, use a HBP compressor in a LBP application and the huge motor will overheat as the suction gas flow is not enough to cool it.

That's what I've always been saying, and displacement neither (case nl11f/SC12G). You can just "compare" 2 compressors that belong to the same range with power rating and displacement.

Displacement, rpm and volumetric efficiency are the main things i'd look at. But for a quick check - as long as the test conditions are the same you can compare two compressors just by looking at the datasheets.

Tom

[SoddemFX]

So, is it a yes or no to the 1st post?

It's "why and why not" to the first post

Tom

[gosmeyer]

I'll take that as a no

[Clemmaster]

Sorry tom I'm ill and confused.
What I really meant is that we can't compare 2 compressors with their rating if they don't belong to the same range, I didn't answere the right thing xD

But LBP and HBP compressor "seem" to have same motors according to danfoss spec . What else than coupling should change between them? I know that valve play a role too, maybe that's the main difference?? I haven't got any answer for the mail I sent to danfoss, they could explain us what really change

And LBP compressor can run in the MBP range too, and do perform better than similar MBP compressor (motor power, displacement) until the 3/4 of the range (around -5° the 2 curves cross). if danfoss gives us these datas we can assume that they won't be killed running at these temps (moreover danfoss advise us for compressor cooling in both 3 ranges for each compressor). They would advise us not to run at these temps if it were dangerous for the compressor, but they don't... That's the same for MBP ones, they just advise use fan cooling but that's all...

[SoddemFX]

But LBP and HBP compressor "seem" to have same motors according to danfoss spec . What else than coupling should change between them? I know that valve play a role too, maybe that's the main difference?? I haven't got any answer for the mail I sent to danfoss, they could explain us what really change

And LBP compressor can run in the MBP range too, and do perform better than similar MBP compressor (motor power, displacement) until the 3/4 of the range (around -5° the 2 curves cross). if danfoss gives us these datas we can assume that they won't be killed running at these temps (moreover danfoss advise us for compressor cooling in both 3 ranges for each compressor). They would advise us not to run at these temps if it were dangerous for the compressor, but they don't... That's the same for MBP ones, they just advise use fan cooling but that's all...

As far as i know the valves are just plain spring steel plates, perhaps some use "stiffer" steel than others but only Danfoss know that. I think because of economies of scale i'd guess the same compressor basic "block" is used +/- between HBP and LBP compressors of the same family and displacement with just the motor changing.

If you look at the power consumption of the SC12G varient (LBP/HBP), it uses less power than the SC12FT (LBP) at low suction pressures, i.e. the motor in the FT is more heavilly loaded by "something".

This extra "something" might be from closer tolerences on machined parts, or maybe more stiff valves etc etc but either way, the less load on the motor of the G varient is probably what makes it suitable for HBP as well as LBP.

This is what makes the FT great for LBP (higher volumetric efficiency) but means that the motor would be overloaded in HBP, whereas the more "sloppy" G varient wont do so well in LBP (lower volumetric efficiency) but can work in HBP with the same motor.

As far as i can tell it's basically Danfoss doing a trade off...

Tom