View Full Version : 2-stage cascade

17-09-2007, 12:21 PM
Because finishing the 3-stager will take me more time, and because I hardly need a good unit for working; I've decided to quickly build a 2-stager unit.

Most of the parts have been shipped to me and I'm waiting for them. :)

Specifications of the unit.

1st stage
LG NP407P (23,400Btu/h)
1.9kW + Fan
Schrader HSP
Wigam ML60/18C4FA/A8 gauge
Frigomec 1.6l reciever
Copper filterdrier 50gr
HX 12 plates
Schrader LSP
Wigam ML60/38C4FA/A8 gauge
R290 or R404a

2nd stage
LG NP362P (21,800Btu/h)
Desuperheater + Fan
ShineYear automatic oil separator
Schrader HSP
Wigam ML60/18C4FA/A8 gauge
Copper filterdrier 50gr
Honeywell CPEV
Flaexible succion line
Schrader LSP
Wigam ML60/38C4FA/A8 gauge
R1150 or R1150/R50/R290 blended
(and of course something for safty)

This unit must be able of very cold temperatures and very good hold of load (up to 350w). Any suggestions welcomed. ;)

22-09-2007, 08:53 PM
I whised to build the cascade during the week end but GLS lost one of the packages with half of the parts.

BTW, I've started to place some parts over a wood board but I can only do basic stuff at the moment.

I've got a question for cascade freaks; I've read in another thread Vincent Yu saying the use of 2 compressors of same power is a good choice.

For two stage systems, it is a good choice to use 2 compressors having the same power. I am not sure if this principle still works for three stage systems.

I'm wondering why? I mean, isn't it better to use a stronger one for the 1st stage?

Thanks in advance. ;)

The Viking
23-09-2007, 12:27 AM
Let's think about this...

The low stage compressor needs to be selected on the duty required for the freezer.

The high stage compressor needs to be selected on the duty required to cool the low stage condensor, which would normally be around 3 times it's duty.

As the higher stage will have an higher evap temp, there will be a gain on compressor capacity.
If this gain will be enough to cope with the added load will vary from application to application.

Better do the proper calculations before you order the compressors.

23-09-2007, 01:09 AM
Ok; I think I get it.

Because the compressor power decreases relatively to the lowering of evaporating temperature; building a 2-stager unit with 2 compressors which are rated for the same power under standard conditions, means the first compressor will be more powerful than the second one. :)

23-09-2007, 10:09 AM
testing 123

23-09-2007, 02:32 PM
Here are some pictures of what I've done so far.

Few things to know before looking at the pics:

- Black condenser isn't use as a condenser but as a desuperheater for 2nd stage
- White shoes' box corresponds to HX place
- White bottle corresponds to receiver place





25-09-2007, 03:50 AM
If you are using POE oil in the comps, Then leaving them open to the air is ruining the oil by allowing mouisture (humidity) to be absorbed

25-09-2007, 09:33 AM
Yes :)

Second stage compressor is opened for taking pictures with copper pipes simply plugged in (not brazed yet). BTW compressor succion and discharge sides are both closed during the waiting for the remaining parts.

Anyway, those parts might have been delivered on Thursday/Friday but GLS lost the package (I would have love to build the cascade during the week end). Before end of last week, compressor suc and dis were sealed by manufacturer.

27-09-2007, 02:43 AM
It is recommended that POE not be exposed to air for more than 10 minutes. It is extremly hygroscopic. When the oil gets moisture trapped in it then heated, acid starts to form. This decreases the life of the compressor windings. Your system will run and operate it just want live as long, months instead of years.

27-09-2007, 02:59 AM
It is recommended that POE not be exposed to air for more than 10 minutes. It is extremly hygroscopic. When the oil gets moisture trapped in it then heated, acid starts to form. This decreases the life of the compressor windings. Your system will run and operate it just want live as long, months instead of years.

I'll quote this from another thread discussing this:

"You are another example of how the overblown fears of POE may not always be real. Yes, POE will absorb moisture from the atmosphere, but in real systems it may not be as rapid as some think (even in Florida). It takes a long time for moisture to migrate through the very small openings in a typical open system into the POE to load it up with a lot of water.

It's always good practice to keep any system sealed up, but POEs are not a disaster waiting to happen if the system is left open for an hour. Of course, if some people think an hour is OK, then two hours is probably OK, then 2 days may still be OK, so why not leave it open for 2 months and who needs to worry about putting in a new drier anyway.

The ten days you were planning before coming back is more than enough time to get the water out. The least amount of time I would give it is 48 hours, that makes sure the clean-up drier has had time to do its thing so the next drier going into the system will have as much of its available capacity as possible."

"The POE will saturate with moisture at about 1000-1500 ppm by weight when exposed to high humidity air. The final moisture level in the POE depends on the dew point of the air.

This means that every pound of POE can hold a maximum of 1,500 / 1,000,000 pounds of moisture, or 0.0015 pounds of moisture. In grams that is 0.68 grams of water per pound of refrigerant. Each drop of moisture weighs 0.05 grams, so a pound of POE can hold about 14 drops of moisture. POE weighs about 8 pounds per gallon, so each gallon of POE can hold about 110 drops of moisture. Each ounce of POE can hold 110/128 drops, or just under one drop per ounce.

That should let you size the drier appropriately. Even a small drier has the capacity to dry a reasonable amount of POE.

I have used commercial filter driers to dry pure POE during charging of systems, and a single pass through the driers knocked down the moisture concentration in half. A four core molecular sieve drier (Z48 at the time) would dry over 1000 gallons of POE from 50 ppm down to 25 ppm moisture as it went from the oil tank to the charging board. The customer loved the *******ized compressed air driven diaphram pump oil drying rig we built them."

:Name removed on purpose:

27-09-2007, 01:27 PM
Thank you for the info BigJon. I did not realize that you could make the moisture content that much lower with a pass through the filter drier.
I am sure that moisture content of the POE is not that critical in regular refrigeration, but when dealing with cascades it does become more critical. The moisture collects in the in the cap tubes and forms a restriction just as in a regular system, but the cabinet cannot be warmed up to allow the ice to melt. Also the cap tubes are extremely long on these systems which adds to the moisture sensitivity. The biggest problem is that moisture contributes to acid level in the oil. With the system running in a vacuum already the acidic level of the oil doesnot have to be as high to start causing problems with the windings. Therefore the life of the compressor is shortened. The manufacturer that we get our compressors from processes their own oil to get the moisture content below 7ppm. The reason again is to increase the life of the compressors.
I am not trying to change your mind on the subject iam just giving the point of view and reasoning behind my previous statements.

14-10-2007, 08:24 PM
I've recieved most of the parts now (it's still missing me 2 flare nuts for the Alco TXV, and 5 capillary tubes with 1/4" flare nuts for the 4 gauges and the link between oil separator and 2nd stage suc line).

So, here are some pictures of what I've done during the week end. Work consisted in pipes adjustment.







15-10-2007, 06:08 PM
Today, I've painted the condenser in blue and brazed few things as well.










17-10-2007, 01:03 AM
I've got a problem to tight the receiver screw (with the Teflon O-ring)...

Is it required to heat it before being able to tight it?

17-10-2007, 08:58 PM
Cascade as it looks today:






The MG Pony
18-10-2007, 03:25 AM
First off the ring isn't teflon it is a type of nylone, secondno do not heat it!

Thirdly why do you have two access points at the same place, the schrader then rotolock valve?

18-10-2007, 09:42 AM
I've finally realized the ring must be installed between the rotalock valve and the reciever. I was wrong when trying the put it between the screw and the reciever.

I've brazed several schraders because some will be used for service while others will be used for installing captubes for the gauges and another one will be used for oil return into 2nd stage. :)

The MG Pony
18-10-2007, 03:04 PM
Ah Ok, A tip of advice, just use brazed in T's and aviod relying on shraders unles it really has to come off, looks better and adds to reliability and system tightness!

The Nylone washer should be inside a groove on the rotolock valves mating surfate to the receiver, ie when you tighten it, it compresses between the valve and receiver forming a solid gas tight seal.

18-10-2007, 04:49 PM
Thank you ;)

18-10-2007, 05:30 PM
ATM I'm hesitating about the way to connect the HX... As an Evaporator? Or a Condenser? Does it really matter?

The one installed between 1st and 2nd stages of my 3 stager is connected as a condenser and it works fairly good.

In fact, I think the HX is still an evaporator for the high stage and a condenser for the low one.

BTW actual HX has 2 different sizes for the connections while the other having 4 equivalent connections...

18-10-2007, 09:28 PM
My question was a bit stupid as first stage refrigerant needs to go from bottom to top (liquid to gaz) while second stage refrigerant needing to go from top to bottom (gaz to liquid)... Additionaly, tighter connections are obviously to be used for high side pressure. Contrarily, larger connections are to be used for low side pressure. http://www.xtremesystems.org/forums/images/smilies/smile.gif

Shame on me for not thinking before writing. http://www.xtremesystems.org/forums/images/smilies/biggrin.gif

19-10-2007, 05:38 PM
CPEV has been brazed to captube side and liquid line from HX is ready for brazing. :)





24-10-2007, 09:45 PM
Brazing work is finished. It remains leak testing, wiring and insulating. :)






25-10-2007, 06:37 PM
Apparently there's no leak so I've started insulating. :) Did few tests with R290, and both stages look to work.









26-10-2007, 10:46 PM
Insulation isn't finished yet... But I couldn't resist to test 1st stage a bit.

T1 = evap
T2 = suction

Difference between 2 temps is a bit high in my opinion; by the way, this TXV is to be used with R404a instead of R290 and insulation is messy.



Quick movie http://www.wideo.fr/video/iLyROoaftqXk.html...

31-10-2007, 09:37 PM
My friend its not posible to use the termostatic valve (R404) for R209 because the refrigerant in the termostatic bulb its R404 a and the regulation its not the same, I recomend to you to use the properly termostatic valve

02-11-2007, 08:41 PM
Yes, you are right señorfrio ;) By the way, I've ran out of R404a.

I've performed some tests with R290 into 1st stage and R1150 into the second one.

2nd stage went down to -112C unloaded with CPEV almost shut... Not really good at the moment. http://www.xtremesystems.org/forums/images/smilies/rolleyes.gif

By the way insulation is still in its early stages and thus it might explains why the temp is so "hot".

Anyway, I don't know if the actual captube (2mm; 1.5m) is good or not to use after the CPEV.

05-11-2007, 03:33 AM
What was the temp differance of evap in and evap out of the first stage when the second was running?

06-11-2007, 09:54 AM
As far as I know, 1st stage can do -67C HX inlet to -60C outlet unloaded. This is useless because after turning 2nd stage on, the HX cannot hold load very well. HX inlet climbs to about -50C and the outlet to -40C and both temps fluctuate a lot. It looks like a lack of refrigerant, but HSP is good in my opinion and adding more R290 doesn't help.

07-11-2007, 12:32 PM
Wanna see how fast 2nd stage goes down? :D


08-11-2007, 10:01 PM
As far as I know, 1st stage can do -67C HX inlet to -60C outlet unloaded. This is useless because after turning 2nd stage on, the HX cannot hold load very well. HX inlet climbs to about -50C and the outlet to -40C and both temps fluctuate a lot. It looks like a lack of refrigerant, but HSP is good in my opinion and adding more R290 doesn't help.
I am guessing that you are seeing the fluctuations because you are using a TXV, it appears to be "hunting". Try removing the bulb/phial from the suction line and letting it hang in the air to see if the fluctuations stop.
While you have the bulb off of the suction line, try to get the temp difference between 1st HX inlet and outlet down to 2-3 degree differance while it is unloaded. This will ensure that the 1st evap is flooded. Be careful not to flood back to the compressor. Once you have the frost line within 6 inches of the 1st stage comp, start the second. Do not load the second yet. Run it unloaded and see how cold it gets. Measure the temp difference between the 2nd evap inlet and outlet. Add gas until there is only a 5-8 degree differnce between the two.

Oh, when you start the second watch for flood back on the first. If ice starts to form on the 1st comp let a hose full of gas out at a time. If you hear "pop corn" in the first comp turn off the second.

We will figure out what to do with the TXV bulb later. I am use to charging cap tube systems.

The MG Pony
09-11-2007, 03:07 PM
To use R-290 you need a TXV rated for R-22 to get proper evap refrigerant flow.

15-11-2007, 09:30 AM
Thank you guys for your replies. :)

I've switched to R404a and temps are now quite stables. BTW they are very very cold; I mean HX inlet temp is about -70C loaded!

I don't have exact HX outlet temp in mind, but delta between both is a bit high in my opinion.

What delta should I expect? (Approximatively)

17-11-2007, 03:21 PM
In the ULT freezers we use a first stage TD of 1-2c between inlet and outlet while it is unloaded. This ensures a flooded evap.

2nd stage is a TD of 5-8c between inlet and outlet of the evap while running a static load.

25-04-2008, 08:41 AM
Thanks ultralo1 :)

I've decided to change the 2nd stage expansion metric... CPEV has been removed and replaced by captube.

In fact, I've performed a quick extrapolation to get a correspondance table for captube adjustment relativily to compressor power (rotary R407C compressor). Dunno if it makes sense or not...

BTW, I've brazed a piece of 3.8m. Here are the first results. :)


Windows idle: -114.1C evap ; -62.4C HX in

http://www.ixtremtek.com/images/AsusCommando/-114,1Ca.jpg (http://www.ixtremtek.com/images/AsusCommando/-114,1C.jpg)

Loading Spi 32M at 5535MHz (582.6FSBx9.5) @1.82v: -108.2C evap ;-56.1C HX out; HP = 7bars

http://www.ixtremtek.com/images/AsusCommando/-108,2Ca.jpg (http://www.ixtremtek.com/images/AsusCommando/-108,2C.jpg)
http://www.ixtremtek.com/images/AsusCommando/7barsHPa.jpg (http://www.ixtremtek.com/images/AsusCommando/7barsHP.jpg)

25-04-2008, 05:10 PM
Adjusting R1150 amount, and retuning TXV.

Unloaded: -118.0C evap (-57.1C HX out)

http://www.ixtremtek.com/images/AsusCommando/-118Ca.jpg (http://www.ixtremtek.com/images/AsusCommando/-118C.jpg)

-118C ins't the coldest; but the pic I've taken. Guess -120C or so is highly probable with a bit more time. Ambient temp is 20C.

Also; I'm not looking for the absolute unloaded/unuseable coldest temp to take a pic, but for the right balance between 1st and 2nd stages.

20-12-2008, 10:06 PM
Another impressive system. But why so big on the compressors? Ultimately you could easily run with smaller compressors, or if your willing to try, an autocascading system will result in much better compressor life and even better temperatures if you invest the time into it.