R134 refrigeration systems

[umakantvh]

Hi everybody,
This one's reg R134a systems used in freezers.
I wish to know how low one can go suction pressure. If the suction pressure is too low, it may cause leakage of out-side air in to the system. But if one is working for cabinet temp of say (-) 30°C, the suction pressure has to be quite low. Can some-one enlighten me how low (suction pressure ) is safe over long term ??

[thermo prince]

Hi there UMAKANTVH, marhabba, shlonek!
Presumably you are talking of regular 1 stage vapour compression system as you posted under 'domestic'.

For not alone reasons of the vaccum condition but also summing up as 'practical and efficiency' reasons you would not, could not use 134a for low temp freezer where you desire a cabin temp of -30C ( perhaps evaporating then @-35C ~).

Taking a look at 134a properties, at O psig (gauge) or 14.7psia it has a SST of -15deg F ( converts to approx -26degC).

here is an extract ... for 134a

Saturation
Temperature Pressure
(°F) (psia)
-50 5.51
-45 6.41
-40 7.43
-35 8.58
-30 9.86
-25 11.30
-20 12.89
-15 14.67 *** corresponds -26C SST in evap, 0 psig
-10 16.62
-5 18.78
0 21.16
5 23.76
10 26.61
15 29.71
20 33.09




This would tend to suggest ( and we know to be true) that a likely 'cabin temp' for you ( in transport we call "box temp" = RA temp and should be equal to SP setpoint temp) would be in the -18C~-20C neighbourhood at 0psig gauge.
Same was true in approx values for its predecessor R12 despite the many myths of trucks going to -30C box in " the ould days" ....

404a is a common refrigerant for low temp nowadays. In transport , rated down to -29C @38C ambient. Magnum Reefer using Copeland Digital scroll rated to -35C on ISO boxes. There are other refrigerants too.

You sort of answer your own concerns about risk of atmospheric air ingress at vaccum conditions on system low side. Gaskets, o rings and the crank shaft seal are vulnerable.
Many people forget shaft seal is " double acting" - it keeps HP gas in, at low side system positive pressures ( above atmospheric) but is also intended to keep air out once sump area runs below 0 psi gauge.
Your system must be by and large hermetic so your risks are less than open drive, splits, etc.

Hope this helps a bit ....

regards
T-P

[umakantvh]

Thanks, thermo prince. That was quite useful. We are considering R404 compressors.