Cascade Explanation needed!

[Fhqwhgads6680]

Good morning All,

I've been looking aroud for a while now, and while I think I understand the basic principles of a cascade I still have some confusion. Here is what I *think* understand so far but please feel free to correct me as im very noobish to cascades:

1. Basic idea is using one refrigerant to cool another so that it can condense easier at lower pressures?

2. a heat exchanger is used to cool the 2nd stage refrigerant.

3. An oil seperator is used to return oil to the compressor so it doesn't sludge or get too cold.

4. some use capillary and some use CPEV ( I believe I have also seen TEV and AEV dunno what the differences are. from what I understand it is used to adjust for heat load by limiting/expanding amount of liquid refrigerant to the evap)

Thats pretty much it...I've looked at chilly1's design drawings but still am unclear as to the flow of each stage of refrigerant and how exactly it works. I'd REALLY appreciate it if someone here could do a break down of the basic (haha) 2 stage cascade and go through it part by part. It'll help me out ALOT and im sure it'll help others trying to learn aswell!

Thanks in advance guys!

[Nyx]

u are not ready.....And have you build a chiller or SS yet?

But still i will try to answer some of your doubts.Any mistakes someone pls correct me.

1. It is to condense at a lower temperature. Different refrigerants have a different boiling point so therefore different pressures at different temperature.

2.it is not to cool it is to strip the heat off the second stage refrigerant so it acts like a condensor only thing is it is to condense lower boiling point refrigerant at a temperature below ambient.

3. oil seperators are use to prevent oil from slugging at any part of the system like in the evap. and the oil is acutally use to lubricate parts in the compressor, etc and such so an oil sep is needed for applications below -60C cause oil doesnt flow well at this temps so an oil sep is needed to bring it back to the compressor.

4. I think u have to read even more.... TXV is not an AEV txv uses superheat from evap oulet to calculate the amount of refrigerant entering the evap. AEV or CPEV is used to regulate pressure.

Better get a book or start reading more.. And Build a SS to to understand more or a TXV waterchiller which will sure give you a challange.

[Fhqwhgads6680]

Yeah I know im a noob at this, and I am trying to learn...I never said I was ready yet....but I'm not the type to learn slow. I want to learn as much as I can as fast as possible.

I also have just completed my first SS you can read about it here (http://www.xtremesystems.org/forums/showthread.php?t=84972)

I'd just like some guidance on this as I really would like to take the next step, but I want to be sure that I am ready. It wouldn't be very xtreme to build a SS and be satisfied with it now would it. And last time I check this IS Xtreme Systems!

Thanks for the insights, more would be very much appreciated!

[SexyMF]

My understanding is that the second stage is more dangerous because the gas will have high pressure at ambient temperature (ie when off). This is why you need to run the first stage for a while first, to pull down the HX temp so the second stage compressor doesn't die under stress. Also why you need good brazing.

As Nyx said in his 2nd point, the first stage evporator becomes the HX acting as condensor for the second stage.

[kayl]

1. Basic idea is using one refrigerant to cool another so that it can condense easier at lower pressures?!

sort of. the idea of a second stage refrigerant is that to condense it at a reasonable pressure it needs to be at a certain temperature. a first stage refrigerant can be condensed at roughly room temp.
to condense a second stage refrigerant it needs to be around -40c so that the condensing pressure is with safe limits.
ie ya can condense ethyle at around 25c room temp but ya need over 1000psig to do so (ruff figures here not exact temps/pressure) which is no way a safe and shouldn’t be tried unless ya got a death wish.


2. a heat exchanger is used to cool the 2nd stage refrigerant.?!
yes :toast:

3. An oil seperator is used to return oil to the compressor so it doesn't sludge or get too cold.?!
sort of. the main reason of an oil separator is to remove the oil from the refrigerant in the second stage. most oils will freeze at cascade temps causing a restriction in the capillary line, which can be dangerous, a high pressure cut off should be used as a safety mechanism;)

this pic from the sticky at the top should explain how a cascade works.
1 pic is a thousand words.:D

[Fhqwhgads6680]

Ya I saw that pic in the stickies, but I'm having trouble understanding what is what in this pic and the flow of each stage of refrigerant.

[Bergo]

Well, here's a simplified exlanation of the picture:

The picture is of a two stage cascade. The first stage compresor is the one to the right. This compressor is pumping refrigerant from the top left line to the top right line flowing from the compressor (blue to red).
The coil which goes from dark red (signifying high temp) to a softer red (signifying the refrigerant is coling (condensing)).
Follownig the line the refrigerant is sent through a filter/dried (little black thing) to take out any impurities from the first stage refrigerant (I.E. watter or oxidation left i lines after brazing)
From the filter drier the liquid refrigerant flows to a metering device (the apilary tube) which supplies liquid refrigerant to the heat exchanger (commonly refered to here as an HX)
In the heat exchanger the second stage refrigerant condenses as it is cooled by the first stage refrigerant evaporating. there are several types of heat exchanges that are used around here, the most common are the "tube in tube" where one refrigerant sheds ehat to another within a double walled tube, the other is a trombone style which utilized a "twisted" tube inside of a tube to increase surface area in a smaller space.
The second stage is built similarily to the first stage, however it uses an oil seperator after the conpressor on the high side to eliminate oil entering the loop which will clog the ssytem when it gets cold, the small line cming from the oil seperator on the bottom left corner is the oil return to the compressor. The evaporator works as it would on a single stage using a metering device (capilary tube or often a temperature expansion valve (TXV)) to deliver the refrigerant.
On the suction line close to the compressor the brown colored device is a suction line accumulator which boils off any excess refrigerant before it gets to the compressor to avoid unneeded strain on the compressor.

Hopefuly that helps a bit, sorry if there are mistakes, just quickly typed this up while @ work.

[kayl]

Ya I saw that pic in the stickies, but I'm having trouble understanding what is what in this pic and the flow of each stage of refrigerant.
study it and study it
basically first stage cools secondstage.
look at it long enough and it should click:stick:
one thing about cascades, ya really need to do alot of ya own research to understand. Unless ya understand ya self ya not ready to have a crack at it.

if ya want to see some cascade builds have a look at TP
cascade factory, best cascade in the world live here

http://teampuss.com/forums/viewforum.php?f=16


and the gallery.

http://teampuss.com/forums/viewtopic.php?t=62

Speaking of cascades im just going to the shed to have a play:D

[Bergo]

lol, or, what he said, Kayl knows more than me ^_^

[yngndrw]

Ya I saw that pic in the stickies, but I'm having trouble understanding what is what in this pic and the flow of each stage of refrigerant.

Labels & Arrows.

[Shyfty]

uh oh...now I have enough information to be dangerous! ;)

[Fhqwhgads6680]

Ok guys thanks a BUNCH that really helps alot. Yeah, im still doing alot of research and learning, I hope to try for it this summer when I have the time/money.

Also, Never seen suction line HX's used, and what exactly are they cooling? Are they just for improving subcooling of the captube of the 2nd stage or the Discharge line in the case of the first stage?

Also what are passable 2nd stage refrigerants, I know that R1150 is the most common from what I have seen, and a few using CO2. Also when building the 2nd stage what special tools do you need? from what I understand the 2nd stage refrigereant is under very high pressure right?

Another quick question, On the second stage the oil returns to the compressor through the service port correct? what if you are running a Rotary on the 2nd stage that has only 2 ports?

Oh and thanks Yngndrw for the labels! thats exactly what I needed!