Suction pressure below ambient pressure

[kay86]

Hello! I know that most of the refrigeration system should not run at sub-atmospheric pressure on the suction side. Let's take for example a R290 blast chiller which is designed to have a -40°C so the pressure is slightly above atmospheric. Is there a risk that if the blast chiller is placed in a very cold room, say +5°C, the condenser doesnt get hot enough and consequently the evaporator will be colder, the suction pressure can drop below atmospheric?

[seanf]

I think it probably would run at a lower pressure on the high and low side.
It might be that the evaporator is at a lower pressure, but you get a lower cooling capacity and not as much of the evap surface is at the colder temp?

You could always control the condenser fan or restrict the air flow onto the condenser to regulate the high side pressure/temp.

[kay86]

Thanks Sean. Can I ask tho why is it a big deal for the R290 to run at sub-atmospheric pressure while for R600A is perfectly normal? Here in europe we have many freezers with R600A that can reach -20°C and below, which is impossible to obtain if the pressure at the evaporator is above 0 psig.

[seanf]

Not sure, I've never heard anyone say not to run it in a vacuum. But then I've not done much work with R290.

If you run any system in a vacuum you might pull in air, so if you can youd pick a refrigerant that runs in a positive pressure for the temp you need.

If you pull in air you might get towards the explosive limits, that might not be good.

If you pull in air/moisture its going to affect your systems performance, puts up discharge pressure ect.

Maybe oil return is poor running R290 at low pressures/temps.
Maybe there isnt enough refrigerant vapour cooling for the compressor.
Maybe the discharge temperatures go too high with R290 at higher compression ratios.

[Huston]

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