Minus 70 freezer running in a vacuum

[ronaldo9]

Hi guys

I used to working on more commercial systems but I’ve come across some minus 70 degree Celsius freezers on a site. Im trying to look into the operation of these condensers (Refra) one system is running on 449a and the other on 407a (I think but I’m not sure) anyway both of these systems are running their suction in a vacuum, which I’ve just never come across before, I’ve tried to do some research but can’t find anything. Is this just as simple as it sounds or is there more to these systems than meets the eye? If someone could point me in the right direction I’d appreciate it.

Cheers

[Josip]

Hi, ronaldo9

vacuum is something relative ... speaking of point of view ...
on Earth, I think, it is not possible to obtain absolute vacuum ...

if we are speaking about absolute pressure ...
at -70*C (for R449A) there is still about 0,2 bar(absolute) pressure ...
what is our suction pressure even if we are 0,8 bar below
relative atmospheric pressure measured at sea level ...

all pressures below relative atmospheric pressure we call vacuum
because on open type compressors we can have a problem
(if shaft seal is not good) to suck atmospheric air into the system
causing a lot of problems .... due to air and moisture within that air ...

many refrigerants are working below atmospheric pressure i.e. in
"vacuum" but absolute pressure is still there ....


Maybe is not bad idea to download Danfoss Coolselector 2 ...
where you can find p-h diagrams for many refrigerants
There is also link to download user guide ... and all is free

Link is here:
https://coolselector.danfoss.com/Coolselector2/setup.exe

CoolPack is also good, unfortunately without p-h diagram for R449A.

Hope this is of some help to you, but if you have any more question ...


Best regards, Josip

[RANGER1]

On the practical side with these systems you have to be careful if connecting gauges etc, as air can enter.
Instruments like gauges have to be accurate & thermometers specialised for low temperature, as most things rated to -40 deg C.

[ronaldo9]

Cheers guys,

Danfoss are great for free info and apps thanks Josip. And thanks for the explanation, I guess everything is relative as regards a vacuum. When you hear vacuum you think you can’t run a compressor in a vacuum �� but obviously there is refrigerant present.

Thanks Ranger1 , the concern I was having was that there has been leaks on these systems which cannot be good as you say air and moisture can enter the system: can I ask would you recommend using digital gauges or would “analogue” ones be better suited ?

[al]

I'd go digital purely to ensure the compressor controller was reading correctly. Is there an inverter on the compressor?

[RANGER1]

ronaldo9,
Type of gauge does not matter, as long as accurate.
On one job we used a 100mm dial gauge that read vacuum only to get accuracy, but this was before digital was mainstream.
On that particular job they used 2 stage compressor.
Depending on refrigerant & design can be single stage, or 2 stage with intercooler.
Because of vacuum compressor get less cooling from smaller amount of refrigerant entering cylinders.