Ultra low temperature

[A.Mortezania]

Dear All,
Do friends have any experiences about ultra low temperature ?
There is cascade system that using R404A in high temp. cycle and R23/R13/R508A/R508B in low temp. cycle.
Also there is R290 in one stage cycle (ultra low temp. freezers) to reach to lower that -80.

Are there any other ways and cycles ? Do you have any other experiences to share it with me and others ?

[Rob White]

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The most common way is to use a cascade system
and mixing the refrigerants is the way to establish the
correct pressures and temperatures.

But one method I have seen on one ultra low temp system,
down to about -120 C was to use one compressor and then
blended refrigerants that evaporate at different stages inside
the evaporator and the result is an evaporation temperature
down to about -120 C.

The refrigerants were blended by a consultant and it was used
to dehumidify instrument lens and was critically charged.

Rob

.

[A.Mortezania]

Dear Rob,
Thanks for your reply, It was interesting way ! I have not seen cycle like that !

[Rob White]

.

I had not either until this.

My company were asked to design and build an ultra low temperature machine
and they worked with a very intelligent consultant who mixed his own blend
of 3 refrigerants.

It had a 30 or 40 kW compressor with an air cooled condenser, it had one receiver
but a multiple pass evaporator and the refrigerant blend then boiled at different
temperatures and the result was a final temperature of about -120 C.

Very interesting, even from my limited knowledge of it, but they made about 8
units and they were exported out.

Rob

.

[A.Mortezania]

​Dear Rob,

Would it be possible to explain with a sample, What actually happen when 2 or 3 refrigerant in a blend evaporates in different temperature and their boiling temperature be different ?
As you know in Zeotropic refrigerants such as R407C or R410A , Refrigerant is blend but boiling point of elements in blend is near to each-other.
Now what happen when boiling point of elements of blend be different ?