R407 and suction accumulator

[nike123]

Could someone explain, how is that that suction accumulators are not advisable, or even forbidden with R407 in some literature (Danfoss-Maneurop scroll compressors manual) and again, rotary compressors are equipped with them regardless of refrigerant.

A suction accumulator must not be
used in systems with zeotropic refrigerant
mixtures.

Page 23:
http://ra.danfoss.com/TechnicalInfo/....004.A3.22.pdf

http://www.klimaxsystem.it/inglese/r...razione/1.html

[SteinarN]

I suppose the reason is someone is thinking the separation of different components of the refrigerant that would happen when the refrigerant boil off in the accumulator is unfortunate.

I cant see why that should be unfortunate. If some low wapour pressure components is "stuck" in the accumulator, then the rest of the refrigerant get a higher wapour pressure causing a higher actual superheat of the gas from the evaporator. Without constantly addition of liquid refrigerant, the liquid in the accumulator should be removed relatively quicly through the hole in the suction pipe in the bottom of the accumulator.

The conclusion is I dont see any reason to forbid a suction accumulator on systems with zeotropic refrigerants.

I dont understand why to install p-traps before vertical runs on suction or discharge pipes either. But that is another discussion.

[Thermatech]

Some VRF manufacturers had major high pressure problems with R407c due to fractionation.

When the compressor runs for very long period at very slow speed then lots of R134a tends to stay in the accumulator & the compressor sucks R32 & R125 out from the top of the accumulator & then the discharge pressure is much too high because very low R134a content.
At low load / slow compressor speed a small volume of refrigerant is required in the system & the accumulator will be quite full effectively storing the excess refrigerant.

Mitsubishi Electric City Multi VRF systems had counter measure for this. They made a composition sensing circuit which took a very small volume of liquid & flashed off through a tiny heat exhanger & back into suction. The temperature each side of the heat exchanger & pressure data allowed circuit board to constantly estimate the R32 content.
If the composition was not within limits the outdoor unit could do stuf like inject hot gas into accumulator to force the R134 out untill composition restored.

Thank goodness we have now moved on to R410a because the hated R407c was a complete dead end waist of time for air conditioning applications. ( in my opinion )