coldest ambient temps to run a cascade?

Just curious as to what temps it will operate after initial startup,

Depends how it's tuned to a certain extent.

Anything below 5c ambient and I found that the first stage cooldown was really long, sometimes just not happening. Once it's actually cooled down, and under load, and working, it's usually 'normal' operation and actually colder overall as long as the first stage is still condensing.

If a cascade has a first stage with an expansion valve it's usually not as big a change, as long as it's able to work and condense. You get cold enough, and the high pressure can get so low that it struggles to start that condensing effect. No condensing, no cooling.

That's where the AC/Heatpump units get problems, but they get around them by using warmed (inside) air, or recycling the condensor air, or just using a switch for the fan to stay off until a certain pressure is reached.

So it may not want to work at all, or just can't condense enough at very low ambient to cool the second stage properly if the ambient is just too low.

However, if the ambient is just a little lower, and the condensing temp is just a bit lower, you can get a better temp on the HX, and end up with slightly lower evap temps.

So I wouldn't try to stick it outside at -10c since a lot of the time, the high pressure of the first stage is just too low to maintain condensing. For that, if a person installs a pressure switch for say, 200psi? on the first stage high side, and uses it to control the fan, you can maintain a higher pressure and the ambient temp won't affect it as badly.

It's not really a bad idea generally, though it adds some cost to the units being made, but a consistent high side pressure on the first stage will speed up it's condensing and cooling effect. I suppose when you tune it, and find the best high side result in tuning, that's the pressure switch you should use, whether 150psi or 250psi.

Carrier uses a 300psi switch for the fan clutch, some as high as 350psi, to keep the high pressure higher for efficiency and faster cooldown.

So yeah, for 'coldest' ambient, maybe 5c at the lowest, but it really depends on your specific cascade. You can only (carefully) try it, and if the HX temps rise on you, or the evap temp under load 'falls off', you know it's too cold and the first stage can't keep up because it's unable to condense enough refrigerant for the heatload demand.


Gray

It will be hard to start when its really cold but after it runs you could run the cascade when its well below freezing.
One thing which will help on cold days is a hardstart capacitor and a heating the first stage shell with a blow dryer.

Monitor the 2nd stage discharge pressures. That is the biggest problem, massflow plummets as the pressures drop and kill performance. If you have a DSH on the 2nd stage it might help to not run the fan in order to keep the temperatures/pressures up.

I posted this question because I tried it this morning with some of the problems described above. The ambient temp was around 38F, I put a small space heater inside the cart the cascate sits in and covered it on 3 sides with foam insulation ( I was monitoring it the entire time )
When both stages were running I let it cool to operating temps as normal, removed the foam, turned off the heater and started the pc.
The cascade was starting to crash so I shut off the pc. The cascade recovered quickly and began to crash again when I started the pc. I then placed the heater by the compressors and eventually things ran normally. After an hour I could still place my hand on the 2nd stage compressor, it was hot but it didn't burn me. I have a heated room inside a non heated metal building with the pc and cascade on one side of a window and monitors, keyboard etc on the other in the warm with me. I'd like to keep it that way this winter, it's much quieter in the room and the water cooled components are much cooler, so i guess I'll have to make an enclosure with a heater on a timer with a thermostat. I have the timer and thermostat and all I need now is to make an enclosure. Thanks for your help everyone and I'd appreciate any advice you have for accomplishing this