Help on cascade systems

[marc5180]

As above does anyone have any material that will help me understand the basics of cascade systems?
Im going through it at college now but i want to swat up before i go back next week.
p.s i have looked on the net but there isn't much help on it

[US Iceman]

A cascade refrigeration is just two refrigeration systems where each system uses a different refrigerant. The very low temperature system uses a refrigerant which has a very high vapor pressure at ambient temperatures. When the system operates at low temperatures, the vapor pressure is low enough for normal equipment, but high enough to stay out of a vacuum.

The other refrigeration system just acts as a condensing unit for the low temperature system. This happens due to the condenser for the low temperature system being the evaporator for the higher temperature system.

Whenever the low temperature system is shut down and allowed to reach normal ambient temperatures the refrigerant is allowed to expand in a larger vessel (just a like a receiver) to keep the vapor pressure of the high pressure refrigerant down to within normal pressures of the standard equipment.

Does that help?

[The MG Pony]

http://www.xtremesystems.org/forums/...d.php?t=144007 < Here is some seriously detailed and well explaind designing and building an autocascade.

[marc5180]

Thanks for the help guys but im afraid i still dont fully understand it? I think im going to have to do a lot of research but when i think i understand, i'll come back here and try and explain, then let you correct me if i get anything wrong.

[Gary]

Marc, if I'm not mistaken, I recall you ordering my books. The last two chapters in the ACR trouble shooting book are about basic cascades.

[Brian_UK]

Marc, there is some detailed explanation with line diagrams and PH charts here...

http://www.ishrae.in/journals/2002oct/article02.html

[marc5180]

Marc, if I'm not mistaken, I recall you ordering my books. The last two chapters in the ACR trouble shooting book are about basic cascades.

Yes i forgot about that, but to be honest i only flicked through the last few chapters because i don't work on ultra low temp equipment and i certainly didn't understand it. I'll have a look at it tomorrow because it's in my van, thanks for reminding me

[marc5180]

Thanks for that link Brian.

As i understand it in laymans terms but very basic;
Cascade systems are used for achieving ultra low temperatures. In order to get down to ultra low temp the first problem is the refrigerant, the lower the temperature the lower pressure is so we have to use a refrigerant that has high pressure at low temperatures such as R23 or R744 etc. We have to use 2 refrigeration systems to achieve this, the first refrigeration system theoretically could run on a refrigerant such as R22. This uses uses the heat exchanger as an evaporator the second refrigeration system uses the same heat exchanger as the condensor. If the required temperature is very low then more than one stage may be required as one compressor may not be able to achieve this, which is why you can see up to 3 stage cascades for temperatures down to -120.

Please correct me or add on to this if i've missed anything out.
thanks

[Brian_UK]

Nope, you've got it there Marc.

I used to work on -70°C cabinets which used R23 and R22; Lots of fun recharging, normally done to a pressure at a known ambient.

One thing to remember when working on them is that the second stage will not start until the first stage is down to temperature and able to take the load.

Good luck with your future studies.

[US Iceman]

As i understand it in laymans terms but very basic;
Cascade systems are used for achieving ultra low temperatures. In order to get down to ultra low temp the first problem is the refrigerant, the lower the temperature the lower pressure is so we have to use a refrigerant that has high pressure at low temperatures such as R23 or R744 etc. We have to use 2 refrigeration systems to achieve this, the first refrigeration system theoretically could run on a refrigerant such as R22. This uses uses the heat exchanger as an evaporator the second refrigeration system uses the same heat exchanger as the condensor.

Isn't that the same thing I said?

[marc5180]

Isn't that the same thing I said?

It is but you explained it in a very technical manner

[marc5180]

Nope, you've got it there Marc.

I used to work on -70°C cabinets which used R23 and R22; Lots of fun recharging, normally done to a pressure at a known ambient.

One thing to remember when working on them is that the second stage will not start until the first stage is down to temperature and able to take the load.

Good luck with your future studies.

I've never worked on a cascade system before but we are being taken to a place that manufacturers them in a few weeks in stafford. We are going with college on a saturday, to try and help us understand how they work and see them being built. I'm quite looking forward to it now that i have a bit more knowledge of how they work.

Now all i have to learn about is, compound, multi compressor packs, capacity control, pumped liquid recirculation systems and the list goes on...........

[Gary]

The cascade consists of two common everyday systems:

The high stage system is not unlike a water chiller, but instead of water running through its heat exchanger it has low stage refrigerant condensing in its heat exchanger.

The low stage is not unlike a water cooled refrigeration system, but instead of having water running through its condenser it has the high stage refrigerant evaporating in its condenser.

The evaporator for one is the condenser for the other.

No magic required.

[US Iceman]

Things are only magic when we can't explain how they work...

[The MG Pony]

Now to really mess him up lets start on auto cascades

Marc they are easy ones you look at enough, I been planning to make a 3 stage cascade out of bar fridge compresors at a evap temp of -100 or so., how? We start at the evap and work backwards as I go down the chain I need slightly more capacity to cool off the one above it.

stage 3 = -100c @ 50w
Stage 2 = -40c @ 60 to 70w
stage 1 = -25c @ 90 to 100w

each stage as we go down to the 1st has to handle all the load of the first one, the numbers are made up to make the example simple.

with some 1/2" tube and 1/4 tube you can make your coaxial heat exchangers, 1/2tube is the evap, the 1/4 tube is our condensor.

Now as you figured out as the temp range goes up it is easier to use good high capacity gasses, as it is with the final stages due to a narrow temp range we can select the best performing high pressure gases with out need for ultra high condensing pressures.

An autocascade does the same thing and has multiple stages of condensor/evaporators, how ever uses just 1 compressor!

[ozzy bill]

hey marc, all much the same, first stage will pull evaporator / heat exchange or condensor of 2nd stage down to -25'c to -35'c then 2nd stage will cycle on using first stage as a condensor, first stage should have the capacity to keep its evaporator at that temp to allow 2nd stage to cycle at -80'c or so ,,oil return on 2nd stage is sometimes a problem due to waxing in cappillary lines at low temps, on off cycle capillary heaters may be needed so prevent waxing depending on refrigeratant being used, dont know much about auto cascades but been told there to be phased out due to co2 ???

[pina]

I have one unit Bitzer IV can I used R744 + R404 making autocascade?

[pina]

Help me about cascade 2 stages , R 404 with R744, Can I used compressor bitzer IV ???